title : Soil Dynamics and Earthquake Engineering V author :
publisher : Taylor & Francis Routledge isbn10 | asin :
print isbn13 : 9780203293096 ebook isbn13 : 9780203215944
language : English
subject Soil dynamics--Congresses, Earthquake engineering--Congresses, Soils--Mechanics
publication date : 1991
lcc : TA711.A1I57 1991eb ddc : 624.1/5136
subject : Soil dynamics--Congresses, Earthquake engineering--Congresses, Soils--Mechanics
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Soil Dynamics and Earthquake Engineering V
FIFTH INTERNATIONAL CONFERENCE ON SOIL DYNAMICS AND EARTHQUAKE ENGINEERING SDEE 91 KARLSRUHE, GERMANY, SEPTEMBER 23–26, 1991
LOCAL SCIENTIFIC COMMITTEE, UNIVERSITY OF KARLSRUHE G.Borm J.Brauns J.Eibl K.Fuchs G.Gudehus E.Keintzel O.Natau E.Plate B.Prange R.Scherer P.Vielsack
INTERNATIONAL ADVISORY BOARD H.Antes C.A.Brebbia A.S.Cakmak W.D.L.Finn G.Gazetas D.V.Griffiths V.A.Ilyichev K.Ishihara J.M.Roësset F.J.Sánchez-Sesma S.Savidis G.Schmid G.Schneider G.Schuëller P.Spanos G.Waas R.V.Whitman J.P.Wolf R.W.Woods SPONSORING ORGANIZATIONS German Science Foundation (DFG)
International Society of Soil Mechanics and Foundation Engineering (ISSMFE) International Journal of Soil Dynamics and Earthquake Engineering (JSDEE) German Geophysical Society (DGG)
Alfred Wegener Foundation (AWS)
German Committee of the International Decade for Natural Disaster Reduction (IDNDR) Swiss Committee on Earthquake Engineering and Structural Dynamics (SGEB/SIA)
Acknowledgement is made to H.Takemiya et al. for the use of Figure 5.2 on p. 147, which appears on the front cover of this book.
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Soil Dynamics and Earthquake Engineering V
Edited by: IBF, Institut für Bodenmechanik und Felsmechanik, Universität Karlsruhe, Germany
Computational Mechanics Publications Southampton Boston
Co-published with
Elsevier Applied Science London New York
IBF Institut für Bodenmechanik und Felsmechanik Universität Karlsruhe W-7500 Karlsruhe 1 Germany
This edition published in the Taylor & Francis e-Library, 2006.
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ISBN 1-85312-153-3 (Print Edition) Computational Mechanics Publications, Southampton ISBN 1-56252-081-4 (Print Edition) Computational Mechanics Publications, Boston, USA Library of Congress Catalog Card Number 91-74078
No responsibility is assumed by the Publishers for any injury and/or damage to persons or property as a matter of products liability, negligence or otherwise, or from any use or operation of any methods, products, instructions or ideas contained in the material herein.
©Computational Mechanics Publications 1991
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PREFACE
Despite considerable advances having been made in the fields of Soil Dynamics and Earthquake Engineering during the last two decades, earthquakes still continue to cause loss of life and property. In addition, dynamic excitation due to heavy industry, construction machinery, pile driving, high speed traffic, etc. can cause severe damage to existing structures, especially to those of historical importance.
The 5th International Conference on Soil Dynamics and Earthquake Engineering (SDEE ’91) was aimed at a better understanding of the dynamic ground-structure-interaction, to exchange experience and knowledge of the participants and to enhance the efforts of geophysics, soil-, rock- and structural dynamics in the mitigation of risks to people and structures in civil and mining engineering. It provided a forum for the presentation and discussion of new ideas and innovative approaches in Soil Dynamics and Earthquake Engineering in theory and practice. The proceedings, in two volumes, contain selected papers from those submitted to SDEE ’91, and are intended to serve as a permanent reference and as a brief survey of the theoretical, experimental and applied methods and their predictive powers, which are
available at the present time to deal with dynamic problems in geotechniques.
The scope of the conference is reflected by the following topic areas covered in the proceedings: engineering
seismology, earthquake hazards, wave propagation, dynamic soil properties, liquefaction, dynamic response of dams and earth structures and of foundations and piles, earthquake engineering of structures, vibrations, impacts and rock
dynamics. The conference was further emphasized on contributions to the International Decade for Natural Disaster Reduction (IDNDR). It offered an opportunity for intensive discussions, particularly on the recent advances in European seismic standards which are relevant in view of the continuing integration of the European Community and the
progressive opening of the East European countries.
The organizers are grateful to the authors for their contributions and for having shared their knowledge and experience. Acknowledgement is also made to the support given by the German Science Foundation (DFG) and the University of Karlsruhe.
O.Natau
Head of the Institute for
Soil Mechanics and Rock Mechanics G.Borm
Coordinator of SDEE ’91 Karlsruhe, July 1991
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CONTENTS
SECTION 1: ENGINEERING SEISMOLOGY, EARTHQUAKE HAZARDS Urban Earthquake Hazards, Risk and Mitigation
M.Erdik
3 Probabilistic Method in Maximum Earthquake Assessment
V.Schenk, P.Kottnauer
15 Study of an Assessment for Site Effect of Seismic Strong Motion
E.Kuribayashi, T.Jiang, T.Niiro, H.Nagasaka, S.Kuroiwa, S.Nishioka
23 Site-Response at Foster City and San Francisco Airport—Loma Prieta Studies
M.Çelebi, A.McGarr
35 SECTION 2: STRONG GROUND MOTIONS
Effects of Earthquake Characteristics on Ground Response Spectra A.M.Ansal, A.M.Lav
49 The Artificial Wave in Earthquake Safety Analysis for Nuclear Plant Shield
X.Shen, J.Yu
61 Site Dependent Simulations of Earthquake Time Histories
O.Henseleit, M.Kostov
73 Spatial Coherency of the Strong Ground Motions on the SMART 1 Seismic Array
I.A.Beresnev
99 SECTION 3: WAVE PROPAGATION
Comparison of 2-D and 3-D Models for Analysis of Surface Wave Tests J.M.Roësset, D.-W.Chang, K.H.Stokoe, II
111
Inversion of Rayleigh Wave Dispersion Curve for SASW Test N.Gucunski, R.D.Woods
127 Transient Response of Certain Topographical Sites for SH-Wave Incidence
H.Takemiya, C.Y.Wang, A.Fujiwara
139 Surface Wave Propagation in Stiff Top Layer Half-Space
W.Haupt
151 Wave Transmission at a Multimedia Interface
R.S.Steedman, S.P.G.Madabhushi
163 SECTION 4: DYNAMIC SOIL PROPERTIES
In-Situ Dynamic Property Evaluation of Gravelly Soil T.Kokusho, Y.Tanaka, Y.Yoshida
177 Characterization of Material Damping of Soils Using Resonant Column and Torsional Shear Tests
D.-S.Kim, K.H.Stokoe, II, J.M.Roësset
189 Effect of Triaxial Stresses on Shear Wave Propagation
H.-C.Fei, F.E.Richart, Jr.
201 Stiffness Degradation of Weathered Marl in Cyclic Undrained Loading
J.A.Little, N.Hataf
215 Measurements of Material Anisotropy by Ultrasonic Technique
S.V.Jagannath, C.S.Desai, T.Kundu
223 Elastic Attenuation in Non-Homogeneous Porous Materials
B.Gurevich, S.Lopatnikov
235 SECTION 5: LIQUEFACTION
Liquefaction of Gravelly Soil at Pence Ranch During the 1983 Borah Peak, Idaho Earthquake R.D.Andrus, K.H.Stokoe, II, J.M.Roësset
251 Validation of Procedures for Analysis of Liquefaction of Sandy Soil Deposits
J.H.Prevost, C.M.Keane, N.Ohbo, K.Hayashi
263
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Liquefaction of Sands Under Undrained and Non-Undrained Conditions J.Chu
277 The Characteristics of Liquefaction of Silt Soil
H.-C.Fei
293 Evaluation of Liquefaction Susceptibility
A.M.Ansal
303 Post Initial Liquefaction Behaviour of Soils
K.Talaganov
313 Liquefaction Associated with Manjil Earthquake of June 20 1990, Iran
S.M.Haeri
325 Countermeasures Against the Permanent Ground Displacement due to Liquefaction
S.Yasuda, H.Nagase, H.Kiku, Y.Uchida
341 Soil-Pile Interaction in Liquefied Sand Layer
K.Kobayashi, S.Nakamura, K.Sato, N.Yoshida, S.Yao
351 SECTION 6: DYNAMIC RESPONSE OF DAMS AND EARTH STRUCTURES
Dynamic Behavior of Embankment on Locally Compacted Sand Deposits S.Yanagihara, M.Takeuchi, K.Ishihara
365 Three-Dimensional Finite Element Analyses of the Natural Frequencies of Non-Homogeneous Earth Dams
P.K.Woodward, D.V.Griffiths
377 Lumped-Parameter Model of Semi-Infinite Uniform Fluid Channel for Time-Domain Analysis of Dam-Reservoir
Interaction
J.P.Wolf, A.Paronesso
389
Earthquake Resistant Design of Earth Walls—A Probabilistic Approach D.Genske, H.Klapperich, T.Adachi, M.Sugito
403 Passive Earth Pressure Coefficients in Seismic Areas by the Limit Analysis Method
A.H.Soubra, R.Kastner
415
SECTION 7: SOIL-STRUCTURE-INTERACTION, FOUNDATIONS, PILES Dynamic Stiffness of Unbounded Soil by Finite-Element Multi-Cell Cloning
J.P.Wolf, C.Song
429 Application of the Hybrid Frequency-Time-Domain Procedure to the Soil-Structure Interaction Analysis of a Shear
Building with Multiple Nonlinearities G.R.Darbre
441
Dynamic Soil-Structure-Interaction of Nonlinear Shells of Revolution in the Time Domain W.Wunderlich, B.Schäpertöns, H.Springer, C.Temme
455 Dynamic Soil-Structure Interaction of Rigid and Flexible Foundations
L.Auersch
467 Experimentally Determined Impedance Functions of Surface Foundations
B.Verbi•, S.Meler
479 Stiffness and Damping of Closely Spaced Pile Groups
B.Boroomand, A.M.Kaynia
491 Chaotic Motions in Pile-Driving
M.Storz
503 SECTION 8: EARTHQUAKE ENGINEERING OF STRUCTURES
Seismic Damage Assessment for Reinforced Concrete Structures A.S.Cakmak, S.Rodriguez-Gomez, E.DiPasquale
515 Reduction of Linear Elastic Response Spectra due to Elastoplastic Behaviour of Systems
S.E.Ruiz, O.Díaz
545 Problems in the Determination of Input Data for the Seismic Design of Structures in Regions of Low Seismicity
J.Eibl, E.Keintzel
555
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Determination of the Behaviour Factors for Brick Masonry Panels Subjected to Earthquake Actions V.Vratsanou
565 Statistical Study of Nonlinear Response Spectra for Aseismic Design of Structures
E.Miranda
577 Shear Transfer and Friction across Cracks in Concrete under Monotonic and Alternate Loads
C.Karakoç
589 Tests on Upgrading Dynamic Properties of Existing Damaged Structures for a Better Seismic Performance
O.Yuzugullu
599 Helical Springs in Base Isolation Systems
G.K.Hueffmann
613 Damage Reduction with Controlled Seismic Pounding
S.Govil, A.Singhal
627 On-Line Hydraulic Servodrives to Protect Serviceability of Antiseismic Structures—Pre-Design Criteria
A.Carotti
639 SECTION 9: VIBRATIONS
Shielding of Structures from Soil Vibrations G.Schmid, N.Chouw, R.Le
651 Vertical Vibration of a Rigid Plate on a Continuously Nonhomogeneous Soil
S.Savidis, C.Vrettos, B.Faust
663 The Influence of Thickness Variation of Subway Walls on the Vibration Emission Generated by Subway Traffic
R.Thiede, H.G.Natke
673 Vibration Isolation by an Array of Piles
B.Boroomand, A.M.Kaynia
683 Numerical Modelling of Stability Cases for Caisson-Type Breakwaters without Through-flow
E.Stein, M.Lengnick
693 SECTION 10: ROCK DYNAMICS
Explosion Effects in Jointed Rocks—New Insights F.E.Heuzé, T.R.Butkovich, O.R.Walton, D.M.Maddix
707
Numerical Analysis and Measurements of the Seismic Response of Galleries H.-J.Alheid, K.-G.Hinzen
719 Dynamic Solution of Poroelastic Column and Borehole Problems of Soil and Rock Mechanics
D.E.Beskos, I.Vgenopoulou
731 Fundamentals of a Practical Classification of Mining Induced Seismicity (Rock Bursts)
P.Knoll
743
Authors’ Index 757
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SECTION 1:
ENGINEERING SEISMOLOGY, EARTHQUAKE HAZARDS
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Urban Earthquake Hazards, Risk and Mitigation
M.Erdik
Bo•aziçi University, Kandilli Observatory and Earthquake Research Institute 81220, Çengelköy, Istanbul, Turkey
ABSTRACT
The hazard assessment, microzonation, vulnerability, risk and mitigation issues involved with urban centers prone to earthquake disasters are covered. The mitigation efforts should concentrate in the preparedness phase. The
microzonation maps and the land use requlations are important long-term tools in the mitigation of earthquake risk. The weak areas in the urban infrastructure and the critical structures may need to be retrofitted. The treatment is supported with case studies from .
INTRODUCTION
With the recent 19.9.1985 Mexico (M8.1), 7.12.1988 Armenia (M7.0), 17.10.1989 Loma Prieta (M7.1), 21.6.1990 Iran (M7.7) and 16.7.1990 Philippines (M7.8) earthquakes the earthquake hazards and the attendant risk in urban areas gained focused attention. Urbanization in earthquake prone countries create an associated increase in the earthquake vulnerabilies and the risk.
Assessment of the earthquake hazard is one of the preliminary steps towards the mitigation of the risk. In urban centers the earthquake hazard is usually quantified and portrayed in terms of microzonation maps. The microzonation maps and the land use requlations are important long term tools in the mitigation of earthquake risk. The vulnerabilities and the damage statistics of lives, structures, systems and the socio-economic structure are the main factors influencing the earthquake risk in the urban areas. The mitigation efforts should concentrate in the preparedness phase with emphasis on awareness building and training. The weak areas in the urban infrastructure needs to be retrofitted. The earthquake performance of the critical structures and systems may also need strengthening. This report will review these critical issues with case studies from and suggest solutions.
ASSESSMENT OF THE EARTHQUAKE HAZARD IN URBAN AREAS
A rational earthquake hazard assessment methodology should provide for the uncertainties associated with the input parameters and be based on appropriate stochastic models. The purpose of such probabilistic earthquake hazard analysis is to provide a basis for decision making about the design basis ground motions applicable in a metropolis.
Probabilistic Assessment of Ground Motion
The earthquake hazard is usually depicted as annual probabilities of exceedance for given ground motion (or intensity) levels. The probabilistic earthquake hazard assessment, in a rigorous way, were probably first initiated by Cornell [9]. Although the basic elements of his methodology remained the same, in the recent decade several researchers have tried to improve by addressing to the issues associated with high uncertainties. The development of criteria for the
interpretation of alternative source zones and seismicities, and expert systems have been the focus of these efforts.
Earthquake Hazard in
The probabilistic hazard assessment methodology that will be employed for will be an updated version of the one incorporated in [13]. It involves: Acquisition of geotectonic and seismologic data and seismic source modeling; Construction of recurrence relationships; Development of intensity based local attenuation relationships; and Use of a proper stochastic model for recurrence forecasting. For the computational part a computer program entitled SEISRISK III [6] will be utilized. Figure 1 shows the active tectonic elements to be considered for the earthquake hazard assessment for . The epicentral distribution of earthquakes are indicated in Figure 2. For the attenuation of intensities the relationships of Erdik et al. [13] will be used. For the attenuation of peak ground acceleration (PGA) the relationship developed by Campbell [7] are found to be appropriate on the basis of comparisons with Turkish data [13]. Figures 3 and 4 provide the variation of MSK Intensity and the PGA (at competent rock outcrops) for the northern and southern
for different return periods.
Modification of the Ground Motion by Site Conditions:
Due to urbanization the reclaimed land near the coast has been spread rapidly. The earthquake response of the reclaimed land and the soft alluvium can be much more amplified than that of the consolidated deposits, as observed in 1985 Mexico (great damage in the lake bed region) and 1989 Loma Prieta (collapse of the I–880 Cypress viaduct at the north end founded on bay mud) earthquakes. Several researchers have shown that for layers of given thickness, the relative shaking response will be greatest where the surface layers have the lowest impedance values and where the impedance contrast between the surface layer and the underlying one is the greatest. Joyner and Fumal [18] have incorporated the local shear wave velocity of near surface geologic material in the assessment of site effects for the attenuation of peak ground acceleration (PGA) and velocity (PGV). Their results indicate that, contrary to PGV, the site dependence of the PGA is not statistically significant. However this finding is not shared by others. For example, Fukushima et al. [17] have computed, on the basis of a worldwide strong
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motion data set, the residuals between the observed and the predicted PGA’s and the mean values for each ground classification (rock, hard-, medium—and soft-soil ground). The observed PGA’s are about 40% lower at the rock site and about 40% higher at the soft soil site than those predicted, but the differences are insignificant for other soil sites. Roger et al. [23] using a collection of nuclear test explosion recordings, have correlated the gelogic structure with the mean spectral amplification of ground motion in the Los Angeles region. In the 0.3–2Hz frequency range the mean ground response on crystalline rock has the following values relative to:
Age of surficial material: 3.2 for Holocene/Pleistocene, 1.7 for Pliocene/Miocene and 1.4 for Mesozoic; Quaternary thickness: 1.6 for Om, 2.3 for <75m and 3.6 for >75m;
Depth to basement rock: 1.1 for Okm, 2.7 for <4km and 3.8 for >4km.
As evidenced by in several earthquakes, the immediate vicinity of lateral discontinuities and contact zones between highly contrasting formations are also usually the zones of amplification. Amplification due to topography has been identified in theoretical as well as empirical studies. Celebi [8] has observed the topographical amplification phenomena in 1985 Central Chile and in 1983 Coalinga earthquakes. The top of isolated hills, elongated crests, edges of plateaus and cliffs are usually zones of amplification due to diffraction and focusing.
Earthquake Induced Soil Failures and Terrain Movements:
The most important earthquake induced soil failures are liquefaction, loss of strength and densification. Liquefaction involves [10]: Flow Failures (massive displacement of completely liquefied soil), Lateral Spreads (lateral displacement of surficial soil layers over a liquefied layer), Slumps (in steep banks underlain by liquefied sediments), Loss of Shear Strength (tipping or bearing failure of above ground structures, buoyant rise of underground structures). Liquefaction tends to begin at an intensity threshold of about MMI V–VI. Soil liquefaction occurred and caused much damage in 1989 Loma Prieta earthquake (Marina district in San Francisco) and in 1990 Philippines earthquake. Techniques to evaluate the liquefaction potential are well established and generally involve the preparation of two types of maps: one showing the liquefaction susceptibility and the other expressing the opportunity for critical levels of shaking. These two maps are merged to depict the liquefaction potential [26]. Tokida [25] lists the following criteria for liquefaction susceptibility (a conglomerate of Japanese criteria for bridges, water supply, sewage and buildings): (1) Saturated alluvial sandy layers within 20m from ground surface, (2) Ground water level within 10m from ground surface, (3) D50 values between 0.02 and 2mm in grain size acccumulation curve, and (4) Standard penetration test blow count N≤30.
Earthquake induced terrain movements include landslides, rockfalls, and subsidence. Materials that are susceptible to earthquake induced landslides are: weakly cemented, weathered or fractured rocks; Loose unsaturated sands; saturated sand and gravel with layers sensitive clay. Experience show that most earthquake induced landslides involve
rials that have not previously failed, and existing landslides are seldomly reactivated [10]. The probability that a
landslide will occur on a particular slope during a particular earthquake is a function of both the pre-earthquake stability of the slope and the severity of the earthquake ground motion. According to Newmark [21] one of the measures of slope stability under seismic shaking is the acceleration required to initiate an irreversible displacement of the soil mass. Strength loss in sensitive clay in strong earthquakes may involve failures similar to liquefaction and specifically can initiate large landslides as is the case in 1964 Alaska earthquake. Urban area landslides (rock falls, soil slides, lateral spreads and slumps) can cause massive property damage. Transportations are blocked and lifelines are damaged disrupting the community services. Prudent siting, involving adequate setbacks from steep slopes, flattening cut slopes and avoidance of instability areas can mitigate the hazard. For massive landslide problems the risk can be accepted with appropriate emergency response preparedness.
Tectonic Ruptures (Surface Fault Ruptures):
The information about the movements and the surface expressions of possible active faults should be included in the microzonation maps to avoid (or to accomodate) their effects on structures and systems. It is generally regarded to be appropriate to consider faults that show evidence of Quaternary motion as active with a possibility of rupture.
MICROZONATION MAPS
Seismic microzonation maps can be defined as maps providing estimates of parameters needed for the siting and the earthquake resistant design of civil engineering structures and systems. The necessary information that should be conveyed by an earthquake hazard based microzonation map are: (1) Modification of the strong ground motion by site conditions; (2) Earthquake induced soil failures and terrain movement; and (3) Tectonic surface ruptures.
Site Specific Ground Motion
For the incorporation of the “Site-Specific” ground motion in the microzonation maps there exists several analytical, empirical and experimental approaches. Analytical procedures range from simple one-dimensional calculations to three-dimensional, linear/non-linear, time/frequency domain and finite difference/element computations. Microzonation maps have been prepared using one-dimensional non-linear analytical procedures. These maps yield the
input-motion-amplitude-dependent predominant periods and the peak surface accelerations [24].
From analyses of microtremor records obtained at over 1000 locations in a wide variety of soil conditions, Kanai et al. [19] discovered that the time and frequency domain wave shapes of microtremors are distinctly different in different soil conditions and proposed two methods for the purpose of microzoning. One method attempts to delineate the four soil zones on the basis of the largest period and the mean period of the microtremors. The other one does the same using the largest amplitude and the predominant period of the microtremor measurements. The critics of this method claim that the microtremors originate at shallower
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depths, may be nonstationary over different periods of the day and can provide information only on the low-strain behavior of the medium.
Medvedev [20] attempted to relate the increments of seismic intensity to seismic site rigidity (product of the longitudinal seismic wave velocity by the density of the geologic layer) and to the elevation of the water table. The intensity
increments for the basic categories of the ground with respect to bedrock (granite) is found to vary from 1 (firm ground) to 3 (loose fills). An additional intensity increment of 1 unit are considered for cases where the water table lies directly below the structural foundation level. The microzonation of Bucharest, Romania, which is based on the Medvedev’s method, had the opportunity of being tested by the 1977 (M 7.2) Romanian earthquake [4]. Erdik et al. [15] have compared the microzonation map of Bucharest with the intensity observations of this earthquake and found a negative correlation between the predicted and the observed intensities. Similar to the Mevedev’s method, in western USA tables providing changes of intensity has been used for microzonation purposes [16]. For California the relative intensity values for different ground characteristics vary from −3.0 for Granite and methamorphic rocks to 0.5 for saturated Quaternary sedimentary deposits.
Microzonation for
The preliminary microzonation for the Central part of . the area within the ancient city walls, is based on the morphology, geology, the distribution of artificial fills and other geophysical and geotechnical data [15]. Figure 7 illustrates the four identified earthquake hazard zones. The stable rock zone (MSKI VIII) defines some part of the Carboniferious rock (where the artificial cover is little or none) and the late Miocene Mactra Limestone. The semi stable zone (MSKI VIII–IX) represents mostly late Miocene sand and gravel, and clay and marl. In this zone ground shaking hazard is somewhat increased and slopes are prone to land sliding. The zone encompassing the thick artificial cover (MSK IX) will be subjected to increase in the ground shaking. The zone of mud and fill (MSKI IX–X) delineates potential of ground failures such as liquefactions, fissuring and slumping. In Figure 7 the locations of potential earthquake induced landslides are also illustrated. It should be noted that, in the stable rock zone there might be local problems due to fracture planes versus slopes (e.g. fault and joints) and ground shaking may increase depending on the thickness of the weathering zone. A preliminary damage zone map for the 1509 and 1894 earthquakes is presented in Figure 8. The 1509 earthquake [3] was one of the largest in istanbul, killing about 5000 and injuring 10.000. Every single house had some degree of damage, in some places the ground opened up, sand ejected and a 6m high sea wave occurred. In the 1894 earthquake [22] (M=6.7, I=VIII) most damage took place on the Fatih-Beyazit ridge and slumping observed in Eminönü. The Grand Bazaar had heavy damage due to its “loose fill” type ground condition. Both
earthquakes were in the Marmara Sea and related to the movement of the North Anatolian Fault.
EARTHQUAKE VULNERNERABILITY AND RISK
In technical terms earthquake risk is the probability of expected earthquake losses (such as: lives, injuries, physical damage and socio-economic). Indirect damage due to the disruption of industry, commerce and services should also be considered as losses. Seismic risk analysis attempts to calculate the probability of adverse socio-economic effect of an earthquake or series of earthquakes in a given urban center. A probabilistic seismic risk analysis takes into account the uncertainties inherent in the earth sciences and the engineering information. The process of rapid urbanization, the attendant socio-economic development, large scale constructions and the provision of infra-structural services will expose larger populations and valuable elements to earthquake hazards and risks. In many developing countries with increasing populations and inadequate housing the increase in the number of such buildings will bound to aggravate the earthquake related casualties and economic losses over the coming years. Several developing countries spend about 2% of their gross national product for post-earthquake reconstruction [14] and, in some instances, the losses caused by earthquake disasters have completely cancelled out any growth in the GNP [11].
Earthquake vulnerability is defined as the degree of loss to a given element(s) at risk resulting from the occurrence of an earthquake. Vulnerabi1ity assessments are usually based on past earthquake damages (observed), on laboratory testing and, to a lesser degree, on analytical investigations (predicted). In addition to these physical vulnerability, the social vulnerability of urban population needs to be assessed for a comprehensive earthquake risk assessment. The past earthquake disaster experience indicate that single parent families, women, handicapped, children and the elderly constitute the most vulnerable social groups.
Vulnerability of Buildings
In urban areas the vulnerability assessments for engineered structures (e.g. residential, governmental and commercial buildings, bridges, dams, port and harbor structures, lifelines and utilities) and non-engineered structures (mainly squatter settlements) needs to be differentiated. The vulnerability of the engineered structures depends on the siting, design and construction essentials and defies generalizations. Generalizations can be made on the earthquake vulnerability of different building classes of non-engineered construction. Several researchers have provided
deterministic vulnerability functions for different structures [e.g. 1, 12]. Worldwide data [1] indicates that the average damage ratios (i.e., cost of repair divided by the cost of rebuilding) unreinforced adobe and brick masonry structures are at least 4 to 5 times more vulnerable to receive damage than properly designed reinforced concrete and steel structures under the same earthquake exposure.
Vulnerability of Turkish Buildings
In this century only a limited number earthquakes in Tükiye have effected urban areas. The following vulnerability functions for three different classes of urban buildings have been obtained on the basis of damage and casualty data obtained from these events [5]. For low rise (2–5 story) reinforced concrete buil-dings, the percentage of the building stock to experience damage in an MSKI VIII are: 28±12% (no damage), 32±6% (slight damage), 26±9% (medium damage), 10±5% (heavy damage), 3±3%(collapse). The death rate is 0.45 persons per building. For low-rise
unreinforced masonry buildings, the percentages are 36% (medium damage), 37% (heavy damage) and 27% (collapse). The death rate being 0.77 persons per building.
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Vulnerability of Other Structures
Many other engineered urban structures, infra-structures, life-lines and services are vulnerable to the effects of earthquakes. Landslides, rock falls and fault ruptures can block highways and railways or damage pipelines. Strong shaking can cause transmis-sion lines and bridges to fail. Liquefaction can cause failure of port and harbor structures. The earthquake vulnerabilities of these structures and systems are not generally known in explicit formats. In any case these vulnerabilities are highly case- and site- specific and defy the general use. However the following observations acquired from past urban earthquakes can be used as a guide to assess their earthquake performance [10].
Bridges
Slope instability, liquefaction. settlement can move the abutments of bridges and tilt the piers causing extensive damages to bridges. Liquefaction phenomena can start at intensities as low as MMI V–VI. The bridge girders can fall off of their supports. Seismic restrainers that tie the adjacent simple spans prevent the fall off of the girders. The continuous span bridges should be tied together at the expansion joints.
Building Foundations
Outside the zones of faulting, liquefaction and ground failure the foundation failure due to strong shaking is very rare, with the exception of friction piles set in soft clays. In 1985 Mexico earthquake the penetration and/or poll out of such piles caused the tilting of the pile cap and, consequently, the superstructure.
Retaining Structures
The increased lateral soil and water pressures, loss of bearing strength and liquefaction have seen to cause damage to the retaining structures.
Tunnels
Outside the zones of faulting and landslide, the tunnels generally perform well during the earthquakes. Damage to cut-and-cover type tunnels has been caused mainly by increased lateral pressure from the backfill.
Water Supply and Sewage
Greatest damage to pipeline occurs in zones of faulting, liquefaction and landslide. Ductile pipes and flexible connections have the best earthquake performance.
Electrical Transmission and Distribution Systems
High voltage porcelain insulators, bushings and supports are most vulnerable to earthquakes. Damage generally occurs in improperly anchored electrical equipment.
Telecommunications
The anchorage of switching and battery racks against lateral displacement and toppling is the essential measure to avoid earthquake damage.
Gas and Liquid Fuel Lifelines
These lifelines are vulnerable to large differential ground movements. Quality of weld is the important factor for the earthquake performance of steel pipes. Ruptured gas lines lead to leaks and fire hazard.
Ports and harbors
Large scale liquefaction and sliding of the soil (or between the blocks) can lead to damage in port and harbor structures.
Building Contents
Modern buildings can suffer major functional and economic loss by damage to the equipment and furniture it houses even though the structure experiences little damage. Especially in research laboratories, hospitals and offices the non-anchored equipment are highly vulnerable to earthquake damage. The same also applies to the exhibited pieces in museums and in art galleries.
MITIGATION OF URBAN EARTHQUAKE RISK
Earthquake risk in urban areas can be reduced by either not building or moving away from the hazardous areas, which in either case is unrealistic. What remains is then the reduction of vulnerability, in terms of casualties, material losses and socio-economic losses. In urban areas, the process of anticipating and planning for damage that a major earthquake would eventually create is termed as “earthquake disaster management”. It is an unbroken chain of concerted actions involving: disaster, response, relief, rehabilitation, reconstruction, risk reduction, mitigation, preparedness and (if possible) warning. The earthquake disaster preparedness and the mitigation constitute the two of the important activities of the earthquake disaster management. For any earthquake disaster management program, the public awareness
building, information dissemination and the training of personnel constitutes the fundamental ingredient of success [14]. Major losses of life in the past earthquakes in urban areas have occurred due to the collapse of buildings with insufficient earthquake resistance or with inappropriate siting considerations. The facilities provided by the metropolitan
governments that are essential for the operation of the socio-economic system (sanitary services, utilities and, health services etc.) should be designed with lowest vulnerabi1ity levels. For example: the collapse of the central fire station in 1972 Managua, Nicaragua earthquake endangered the fire fighting; and the much needed ambulances were damaged under the collapsed canopy in Olive View Hospital in 1971 San Fernando, California earthquake. Transportation facilities (highways, railroads, port and harbors, airports and bridges etc.) are vital for rescue and recovery efforts. Redundancy in transportation networks is essential for rapid restoration. Metropolitan governments should also be responsible and take necessary measures to protect the cultural heritage through the maintenance and retrofitting of monuments and museums. Most of these issues can be addressed through proper planning, microzonation and appropriate construction technologies
The pre-earthquake restrengthening and retrofitting of critical urban infrastructure, facilities and hazardous buildings is an important physical measure for earthquake risk mitigation. In this respect, the
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action taken by the city of Los Angeles should serve as a model. Noting that “the pre-1934 unreinforced masonry buildings represent the greatest single threat to the life and limb in Los Angeles in the event of a major earthquake”, in 1981, the Los Angeles City Council passed an ordinance (No:154, 807) requiring the strengthening or removal of pre1934 buildings that have bearing walls of unreinforced masonry.
For mitigation of urban earthquake disasters the necessary plans, programs and the activities can be listed as follows under the pre-, co- and post-earthquake phases:
Preparedness (Pre-earthquake) Planning and Activities
Installation of earthquake data acquisition and monitoring stations and services; Assessment of earthquake hazard (seismo-tectonic mapping); Development of earthquake resistant design codes and construction standards; Pre-disaster planning and management activities and techniques; Disaster awareness, public information, education and training; Development of methods for retrofitting hazardous buildings and facilities; Development of appropriate techniques for repair and strengthening of non-engineered low-strength constructions; Creation and strengthening of programs and organizations for the prevention of earthquake disasters; Hazardous material management; Legislative and regulatory measures; Response readiness; Logistical support; Resource management and stockpiling; Mobile command and communication operations.
Emergency (Mid-disaster) Planning and Activities
Emergency rescue, evacuation, transportation and communication; Damage assessment, demarcation and condemnation of dangerous buildings and zones; Debris removal; Recovery and disposal of dead bodies; Emergency provision of health care, shelter. food and utilities; Human response and information management; Law enforcement; Quick assessment socioeconomic losses; Planning and coordination of disaster assistance.
Post Earthquake Planning and Activities
Detailed surveys regarding repair, restoration and condemnation; Assessment of socioeconomic conditions, resources and needs; Measures and policies for relief, resettlement and rehabilitation; Re-establishment of government services; Institutional framework, implementing agencies; Hazard abatement; Disaster accounting; Planning and coordination of rehabilitation and reconstruction assistance; Siting of new settlements and communities; Retrofit of design codes and construction standards; Training and education programs; Reconstruction.
REFERENCES
1. Akkas, N. and M.Erdik (1984), Considerations on Assessment of Earthquake Resistance of Existing Buildings. Int. Jour. for Hous.Sci., v.8.
2. Algermissen, S.T., K.V.Stinbrugge, and H.J.Lagorio (1978),“Estimation of Earthquake Losses to Buildings”, USGS, Open File Report No: 78–441.
3. Ambraseys, N.N. and Finkel, C.F. (1990), The Marmara Sea earthquake of 1509. Terra Motae, 167–174. 4. Balan, S., V.Cristescu and I.Cornea (1982), Curemurul de pamint din Romania de la 4 martie 1977, Editura Academiiei, Bucharest.
5. Bayülke, N. (1982), Building Types in Bolu Turkey and Their Predicted Earthquake Damages, in Sismic Risk Assessment and Development of Model Code for Seismic Design, UNOP/UNESCO Project RER/79/014, Sofia. 6. Bender,B. and D.M.Perkins (1987), SEISRISK III: A Computer Program for Seismic Hazard Estimation, U.S.G.S., Bulletin 1772, 48p.
7. Campbell, K.W.(1981). Near-source Attenuation of the Peak Horizontal Acceleration, Bull. Seism. Soc. Am., 68, 828-843.
8. Celebi, M. (1988), Topographical Amplification - A Reality?, Proc. 9WCEE, Tokyo-Kyoto, Japan, pp 11-459.. 9. Cornell, C.A. (1971), Engineering Seismic Risk Analysis, Bull. Seism. Soc. Am., v.58, p.1583.
10. EERI (1986), Reducing Earthquake Hazards: Lessons Learned From Earthquakes, EERI Publ. No: 86-02, San Francisco, California
11. Einhaus, H. (1988), Emergency Planning and Management for Disaster Mitigation, Regional Development Dialogue, v.9, No.l, UNCRD, Japan.
12. Erdik, M. (1987), Training and Education for Disaster Preparedness, Regional Development Dialogue, v.9, No.l, UNCRD, Japan.
13. Erdik, M., V.Doyuran, N.Akkas, P.Gülkan (1985), A Probabilistic Assessment of the Seismic Hazard in Turkey, Tectonophysics, 117.
14. Erdik, M.(1990), Disaster Management Education on Earthquakes, Proc., IDNDR International Conference 1990, pp.383-388, Yokohama, Japan.
15. Erdik, M., A.Barka and B.Ücer(1991), Seismic Zonation Studies in Türkiye: an Overview, Submitted to 4th Conf. Seism. Zonation, San Francisco.
16. Evernden, J.F. and J.M.Thomson(1985), Predicting Seismic Intensities, in Evaluating Earthquake Hazards in the Los Angeles Region, USGS Professional Paper No:1360, US Government Printing Office, Washington.
17. Fukushima, Y.,T.Tanaka, and S.Kataoka(1988), A New Attenuation Relationship for Peak Ground Acceleration Derived from Strong Motion Accelerograms., Proc. 9th World Conf. on Earthq. Eng., Tokyo, Japan
18. Joyner,W.B. and T.E.Fumal (1985), Predictive Mapping of Earthquake Ground Motion, in Evaluating Earthquake Hazards in the Los Angeles Region, USGS Prof. Paper No:1360, US Gov. Printing Office, Washington.
19. Kanai.K., T.Tanaka, K.Osada and T.Suzuki (1966), On Microtremors-X, Bull. Earthq. Res. Inst., v.44, Tokyo, Japan. 20. Medvedev, S.S. (1965), Engineering Seismology, Translated from Russian, Israel Program for Scientific
Translations, Jerusalem, 1965.
21. Newmark, N.M. (1965), Effects of Earthquakes on Dams and Embankments, Geotechnique, v.15, pp.139,160. 22. and N.Bayülke (1990), Historical Earthquakes of , Kayseri and Elazig, General Directorate of Disaster Affairs, Ministry of Public Works and Settlement, Ankara, Turkey. 22pp.
23. Roger, A.M., J.C.Tinsley and R.D.Borcherdt (1985), Predicting Relative Ground Response, in Evaluating Earthquake Hazards in the Los Angeles Region, USGS Prof. Paper No:1360, US Gov. Printing Office, Washington. 24. Sugimura,Y. and I.Ohkawa, (1984), Seismic Microzonation of Tokyo Area, Proc. 8th.WCEE, v.2, pp.721-728, San Francisco, California.
25. Tokida, K. (1990), Earthquake Disaster and Approach to Damage Reduction, Proc. ESCAP/UNDRO Regional Symposium on the International Decade for Natural Disaster Reduction, Bangkok.
26. Youd, L.T., J.C.Tinsley, D.M.Perkins, E.J.King and R.F.Preston (1979), Liquefaction Potential Map of San Fernando, California, in Progress on Seismic Zonation in the San Francisco Bay Region, USGS Circ. No.807
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Figure 1. Active Fault Segments in the Marmara Region South of (After Barka and Kadinsky-Cade, 1988)
Figure 3. probabilistic MSK intensity for North and South
Figure 4 probabilistic PGA(Rock Outcrop) for North and South
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Probabilistic Method in Maximum Earthquake Assessment*
V.Schenk, P.Kottnauer
Department of Seismology, Geoph. Inst., Czechosl Acad. Sci., CS-141 31 Praha 4—Spo•ilov, Czechoslovakia
INTRODUCTION
The “maximum possible (or expected) earthquake” is one of the most important input parameters in seismic hazard calculation techniques. Among the different probabilistic approaches usually used in the determination of the “maximum possible earthquake” is the method of extreme values, frequently called the Gumbel method. The application of the third Gumbel distribution and a method of determining the accuracy of the maximum earthquake estimate are presented and discussed on a few examples.
APPLICATION OF THE THIRD GUMBEL DISTRIBUTION
An application of the method of extreme values [1] requires a relatively long sequence of observations of “extreme values”, in our case, e.g. a sequence of “maximum observed earthquakes”. The time series of observations of T years has to be divided into intervals of a certain duration (one year, 5 years, 10 years, 3 months, etc.). From each interval a single extreme value is taken into the calculation: that of the maximum earthquake having occurred in the interval. If there is a sufficient amount of intervals for which the maximum earthquake is known, then the obtained estimate of the maximum earthquake (the asymptotic value of the 3rd Gumbel distribution) is assumed to be a representative value.
* Contribution of the Geoph.Inst.,Czechosl.Acad.Sci., No. 21/91.
We do not doubt that the value is representative indeed, but in some cases it has to be proved and the accuracy of the maximum earthquake determination assessed. As mentioned above, for an application of the Gumbel statistics an interval of a certain duration (years, months, weeks, etc.) has to be defined. Each interval can be further subdivided into shorter and shorter time intervals. It is understandable that the subdivision cannot be applied without a restriction. It is obvious that the number of observed earthquakes and their frequency of occurrence in a given area act controversially. As an example let us take the time series of earthquake observations for the period of 1700–1985, i.e. 286 years, and apply a 10-year interval. It means that for the first interval of 1700–1709 we obtain one value of the maximum
earthquake, for the second interval of 1710–1719 the second value of the maximum earthquake, and similarly the other values of maximum earthquakes up to the last one the for interval of 1970–1979. For this case we shall have a sequence of 28 values of maximum earthquakes.
However, the original time series of earthquake observations can be divided into other nine possible combinations of 10-year intervals: we can start with the interval of 1701–1710 or with the interval of 1702–1711 and sequentially we can reach the last possible interval 1709–1718 (Fig.1). In this way we obtain ten different more or less similar sets of the maximum earthquake values: seven sets of 28 values of maximum earthquakes and three sets of 27 values of maximum earthquakes.
These combinations were created under the shifting interval of one year. Of course if the shifting interval is half a year, then we will obtain fourteen sets with 28 values of maximum earthquakes and six sets with 27 values of maximum earthquakes. It is evident that for one time series of observations there could be a great number of combinations of applied intervals, for which the maximum earthquake has to be found, their overlapping being given by the shifting interval. Such a combinatory approach extends the standard way of the application of the Gumbel statistics, especially its 3rd Gumbel distribution, to another dimension: for an assessment of the approximation of this distribution.
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Figure 1.
ACCURACY OF THE MAXIMUM EARTHQUAKE DETERMINATION
For the whole time series of observations let us introduce for example a shifting interval S equal to one year and divide the time series into one-year intervals D in which the values of maximum earthquakes are determined. We obtain only one Gumbel approximation. If the same (one-year) shifting interval S is applied to the time series which is divided into two-year intervals D, in which the values of maximum earthquakes are determined, then we obtain two Gumbel
approximations. Likewise, for the one-year shifting interval S applied to the time series which is divided into ten-year intervals D gives us ten different Gumbel approximations.
For a better explanation of this approach the following example is demonstrated. Using the catalogue of
Italian earthquakes [2], we selected the subcatalogue for the Friuli region (northern Italy). In this catalogue the earthquake size is given in epicentral intensity of MCS. The Friuli region under study was delineated by geographic coordinates from 45°50’N to 46°36’N and from 12°50’E to 13°50’E . The subcatalog of observed earthquakes contains 1764 events from the period of 1700–1980 with the maximum observed earthquake of 9.5°MCS. In our calculations we applied only one combination of shifting interval S equal to 1 year and interval D equal to 30 years. Figure 2 shows the distribution of all approximations (23 cases) for which the 3rd Gumbel extreme value distribution has a convergent character. The fact that for 7 cases the statistical process had not a convergent solution was rather surprising and has to be explained in the near
Figure 2.
future. This finding is very important from the point of view of practical applications. It gives evidence that our idea concerning the “representativeness” of the 3rd Gumbel approximation need not be necessarily always valid.
The values of the maximum possible earthquakes (MPE) of all 23 convergent Gumbel approximations were analyzed for three different return periods of 120, 1290 and 15500 years and then they were compared with their asymptotes. Figure 3 shows the changes of the MPE values in dependence on the thirty different positions of the beginnings of intervals D with respect to their first positions. We can see that for return period of 120 years minimum values of MPE were obtained for cases in which the beginning of interval
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D is situated approximately in the centre of the original interval D, i.e. for the total shift equal to 10≈18 interval S. This conclusion is not very important because the original beginning of the earthquake time series could be shifted and then we can obtain quite easily an opposite result. But what is extremely important is the fact that for the same cases we obtain quite opposite results for higher return periods, and consequently, for the asymptotes too, e.g. for these cases the maximum values of the MPE are determined. An explanation of this feature does not seem to be very simple and probably special tests have to be made in order to clarify it.
Figure 3.
For each set of the MPE values obtained for selected return periods the “mean value of the maximum possible earthquake MMPE” and its “standard deviation MSD” were determined. These quantities obtained for the Friuli
seismogenic region by the application of the 3rd Gumbel extreme value distribution are drawn in Figure 4. Such a chart informs us immediately about the representativeness of the 3rd Gumbel approximation for the given subcatalogue of earthquakes. We can see that the best approximations and thus the MMPE value with the highest degree of a
representativeness, because of the lowest values of the MSD, belongs to the return periods which are close to the middle of the observation period; in our case about 350 years. For higher return periods the standard deviations increase and the degree of the representativeness of the resulting MMPE values becomes slightly lower. Numerically, the MSD for the return period of 350 years is around 0.5% of the MMPE, but for the return period of 15500 years it is as much as 6.5%,
Figure 4.
taining 9% for the asymptote of the MMPE value. We assume that the obtained accuracy of the MMPE can be accepted as a general degree of representativeness for the maximum possible earthquakes determined by the 3rd Gumbel
approximations.
CONCLUSION
The described statistical approach allows us to estimate the accuracy of the approximation obtained by the 3rd Gumbel distribution in a prediction of the maximum earthquake for given return periods of earthquake occurrences. These estimates do not only define the resulting predicted values but also give their possible variance. Such characteristics are quite important from the economic point of view, because for example, in tasks of the earthquake hazard assessments the determination of the maximum possible earthquake directly affects the total cost of seismic resistant structures. Such predicted values of maximum earthquakes also help in calculations of the seismic risk and can make a contribution in some questions of earthquake mitigation.
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REFERENCES
1. Gumbel E.J., 1954: Statistical Theory of Extreme Values and Some Practical Applications. Nat.Bureau of Standards, Appl.Math.Series 33, U.S.Govt.Print. Office.
2. Postpischl D., Ed., 1985: Catalogo dei terremoti italiani dal’anno 1000 al 1980. CNR, PFG, Quad. Ric. Scie. 114–2B, Graficoop, Bologna.
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Study of an Assessment for Site Effect of Seismic Strong Motion
E.Kuribayashi (*), T.Jiang (**), T.Niiro (*), H.Nagasaka (***), S.Kuroiwa (****), S.Nishioka (*)
(*) Dept. of Civil Engineering/Regional Planning, Toyohashi University of Technology, Toyohashi, Japan (**) Dept. of Geotechnical Engineering, Tonji University, Shanghai, The Peoples Republic of China (***) Kumagai-gumi Co., Ltd., Toyokawa, Japan
(****) Nagano Prefectural Government, Japan
ABSTRACT
Effects of sediment-filled valley on seismic ground motions have become of major interest in earthquake engineering throughout this decade. This paper presents interesting phenomena of both analytical and experimental approaches.
INTRODOCTION
Disasters caused by earthquakes are generally complicated. The earthquake damage strongly depends on the subsoil conditions and topography from the past experience of severe earthquake damage.
In order to prove the effects of topographical and geological conditions in behavior of ground motions, a strong motion observation network so called TASSEM, Toyohashi University of Technology Array System for Strong Earthquake Motions, has been developed since 1989. They are located around Toyohashi city, east part of Aichi prefecture, that is regarded as one of the most vulnerable areas to destructive earthquakes designated by many seismologists.
Several records of the strong motion observation have brought a reasonable results among analytical results using one and two dimensional analyses and consequences in microtremor and strong motion observations. From analytical results, amplification would not be influenced very much by the direction and angle of incident wave, but by the topographical conditions.
In addition, it is clear that Boundary Element Method is an effective tool to estimate the behavior of responses in symmetric valleys subjected to incidental waves.
PAST EARTHQUAKES AND THEIR DAMAGE IN AICHI PREFECTURE [1] [2] [3] [4]
There are no large-scale mountains in Aichi prefecture. The crustal movement in the area is very complicated and active in Honsyu (the mainland of Japan). In this area, Median Tectonic Line that divide the south-west part of Japan into two areas called Inner Zone and Outer Zone, is lying from direction of N.E. to S.W. Many active faults exist in inland and off or along the ocean coast. This area has suffered great disasters many times caused by great earthquakes and is one of the most vulnerable regions to destructive earthquakes. Fig. 1 shows distribution of the epicenters of past major earthquakes. The earthquakes brought destructive damage to this area in historical time are classified into two types. One is the earthquakes off or along the Pacific coast. Another is the inland earthquakes. In recent decades, typical earthquakes which caused major damage are, the earthquake off or along the Pacific coast, 1944 Tonankai earthquake with magnitude of 8.0 in Richter scale, the inland earthquake, 1945 Mikawa earthquake with magnitude of 7.1 in Richter scale. In Tonankai earthquake, the damage caused by liquefaction was concentrated in alluvial plain. In Mikawa
earthquake, the damage was concentrated the south side of Fukouzu fault which continues for 28km from seabed of off Gamagouri in Mikawa Bay to Yahazu River as shown in Fig. 2. In these two earthquakes, severe damage was observed in the 5 or 6th degree on Japan Meteorological Agency scale in Aichi.
Fig. 1 Distribution of the of Past Major Earthquakes
In the area around the western half of Suruga Bay, or the Tokai area, there have been no earthquakes since the one which occurred in 1854. This is the only place along the Pacific coast where no large scale earthquake has occurred in the past 100 years. The Japanese Government has prescribed Large-scale Earthquake Countermeasures, and has designated the Tokai region as one of the Areas Under Intensified Measures Against Earthquake Disaster.
ARRAY SEISMIC MOTION OBSERVATION SYSTEM
In order to prove the effects of topographical and geological conditions in behavior of ground motions, a strong motion observation network so called TASSEM has been developed around Toyohashi city, Toyohashi University of
Technology as a center station, that is regarded as marginal area near one of the most vulnerable areas to destructive earthquakes.
Geological and Topographical Aspects [5] [6]
The object area of seismic observation is Toyohashi city located east part of Aichi. Topographical aspects of Toyohashi is generally classified into three areas; (1) the hilly land and the terrace area, (2) the alluvial plain and (3) high lands. The geological feature is made up of the Paleozoic, the Quaternary and the alluvium. The sedimentary layers are consisted of marines without any igneous and metamorphic rocks.
Paleozoic
This area is composed of the Paleozoic Chichibu zone. It is the base of the diluvial formation widely distributed most part of Toyohashi and reveal at the highland of east of Toyohashi. Paleozoic is composed of chart, mudstone, sandstone, etc. and runs generally in direction from east-north-east to west-south-west and has a tendency to incline towards the north or south vertically. Around the object area, it exists about 200m under sea level.
Diluvium
Diluvial formations mainly consist of gravel, sand and silt and form the hilly land and terrace distributed most part of Toyohashi. These are almost horizontally laid on. Caused by the crustal movement called “Atsumi upheaval movement” by Kuroda [7], they incline slightly from south to north.
Alluvium
Alluvium mainly consist of gravel, sand and silt, they have not harden enough yet, and is widely distributed the basin of river.
Location of the Observation System
The arrangement of the observation points is shown in Fig. 3 and 4. Fig. 5 shows the distribution of standard penetration values, N, at each observation point. Table 1 shows site information.
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Accelerometers were installed in December, 1989 ground surface, actually one meter below the surface, of three sites which geological and topographical features are different respectively. POINT 1 is at Hongo Junior High School located on the center of the valley, Umeda River runs east-west and the ground surface is covered with the soft alluvial deposit. Two points locate in the left side of the river, terrace area composed from diluvial layers and called Tempaku-hara Terrace, Tempaku Elementary School as POINT 2, Toyohashi University of Technology as POINT 3. Moreover, as a POINT 4, at Toyohashi Fire Department located in the right side of the river, terrace area composed from diluvial layers and called Takashi-hara Terrace, supplemental observation is being done. From Fig. 5, it is clear that the thickness of soft layer.
Fig. 3 Arrangement of the Observation Points
Fig. 4 Cross Section of the Observation Site
POINT 2 N34°42.5′ E137º24.9′ 21.0m Clayey Sand, Gravel GL-lm
POINT 3, 3B N34°41.9′ E137°24.7′ 39.7m Sand GL-1m, −60m POINT 4 N34°43.4′ E137°24.3′ 25.0m Clayey Sand, Gravel GL
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Observation of base rock motion has been carried out at the layer composed of gravely sand under ground -60m at T.U. T. as POINT 3B since January, 1991 in present system.
Specifications of the Observation System
By employing an advanced electronic technology, seismological recording techniques have made remarkable progress. TASSEM has been ordinarily composed to obtain the fairly distinctive data and easy data management. Table 2 shows the specifications of the system. As a practicable means, the observation center at T.U.T. controls the branch observation points by using of telephone line. Fig. 6 shows outline of TASSEM. TASSEM is provided with the following remarkable functions.
1) By an advanced technology, wide dynamic range and frequency range can be acquired in the system.
2) By watching the state of system operation constantly, the center can get the certainty and reliability of total system operation.
3) Recorded data can be sent to main computer at TUT through the telephone line directly and made visible easily. 4) All seismic observation parameters such as trigger level, record length, sampling frequency, delay time, and correction of time can be easily controlled from the center.
5) As a counterplan against the power failure, the observation can be continued for three hours or more by use of back-up battery.
Table 2 Specifications of the System
Accelerometer Triaxial Force Balance Servo Type Frequency Range 0.02–30 Hz overall
Measurement Range ±1000 gal Dynamic Range 84 dB overall Low Pass Filter 30 Hz, −18 dB/oct.
A-D Converter 14bits, Sampling Rate 100 Hz
Internal Memory IC memory: 1.25 Mbyte, Froppy Disk: 1.25 Mbyte Telemetering Public Telephone Line, Data Transfer Rate 4800bps
Fig. 6 Outline of the system
Observation Results
At present time, several ground motion records have been obtained. Max. acceleration of ground motions observed by TASSEM is shown in Table 3. As one of the largest records, 06:13:07 Sep. 24, 1990, Fig. 7 shows Fourier spectra
calculated and smoothed by using Hamming type window. From the Fig. 7, peculiar peaks are shown in each observation point. The Fourier spectra in POINT 1 and 2 which thickness of soft layer is similar have a flat and wide peak in a short period range. In POINT 3 which thickness of soft layer is comparatively thick, the spectra has the peculiar peak around 3Hz. Up to now, strong motion data have never been obtain at POINT 3B (GL-60m at TUT).
Spectral ratio between the surface and the base, POINT 3B, for microtremor data observed by TASSEM in each
observation point is shown in Fig. 8. In case of POINT 3/POINT 3B, peaks are shown in range from 2 to 3Hz and in case of POINT 1/POINT 3B and 2/3B, large amplification is shown in high frequency range more than 5Hz.
Table 3 Max.Acceleration Observed by TASSEM
(gal) Date Feb. 20, 1990 Apr. 13, 1990 May 17, 1990 Sep. 24, 1990 Sep. 24, 1990
Epicenter N34º46' N35º9' N34°45' N33º6' N33º8'
E139º14' E136º31' E137°37' E138º38' E138º36'
Depth 6km 40km 33km 60km 42km Magnitude 6.5 4.4 3.4 6.6 6.0 Direction EW NS UD EW NS UD EW NS UD EW NS UD EW NS UD POINT 1 – – – 4.1 4.2 1.7 4.3 10.5 2.6 11.2 15.1 3.9 2.3 3.2 1.1 POINT 2 5.5 5.2 2.5 3.7 3.7 2.0 7.1 6.5 4.4 14.9 14.7 6.7 4.1 3.7 1.5 POINT 3 – – – – – – 4.0 2.2 1.7 11.1 15.9 5.4 3.3 4.0 1.3
Fig. 7 Fourier Spectra of the Observation Points
Fig. 8 Spectral Ratio
ANALYSES OF LOCAL TOPOGRAPHY AND GEOGRAPHY [8]
In order to confirm the effects of topographical and geological conditions in amplification characteristics of ground motions, linear response analyses are carried out by one-dimensional Multiple Reflection Method and two-dimensional Finite Element Method.
Soil properties needed in one and two dimensional analyses are obtained from field tests, soil types and N values, of each observation point and shown in Table 4. Damping ratio 5% is used in analyses. S-wave velocity is induced by following equation.
[9] Two-dimensional model is shown in Fig. 9. The model is formed of 7000 nodal points. Viscous boundary as its side boundary and rigid base as its base are regarded to the boundary conditions of the model. Calculated transfer functions in each point of observation are shown in Fig. 10. All results between one and two dimension show difference obviously on account of influence of the surface configration. Peaks of natural frequency of two-dimensional analyses tend to be lower than the one-dimensional’s.
As compared with the spectral ratio of observed data, value of magnification is differ in absolute ordinate, similar tendency is however shown in frequency domain.
Besides, two-dimensional analyses are carried out with the different incident angles, θ=0°, ±5°, ±10°, ±15°, in order to prove the effects of the direction and angle of incident wave in amplification characteristics of ground motion. The sign +stands for the incident direction from left side of model. Fig. 11 shows the transfer function with different incident angles. From the Fig. 11, the amplification was not influenced very much by the direction and angle of incident wave in any of point within the incident angle ± 10°, but by the topographical conditions.
Fig. 9 Two-dimensional Analytical Model
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Table 4 Soil Properties Used in Analyses (a)POINT 1
Layer (m) Thickness Soil Type υ γ (t/m3) Vs (m/s) G (t/m2)
No.1 1.6 Silt 0.49 1.60 145 3450 2 2.2 0.49 1.65 130 2850 3 2.2 Gravel 0.46 2.10 300 19300 4 2.0 Sand 0.47 2.00 315 20250 5 2.3 0.47 2.00 315 20250 6 1.7 Gravel 0.46 2.10 350 26250 7 2.7 0.44 2.10 450 43400 8 0.8 Sand 0.48 1.85 350 23125 9 1.8 Gravel 0.44 2.10 450 43400 (b)POINT 2
Layer (m) Thickness Soil Type υ γ (t/m3) Vs (m/s) G (t/m2)
No.1 2.5 Sand 0.49 1.60 170 4700 2 0.6 0.49 1.95 220 9650 3 0.6 0.49 1.80 205 7700 4 1.3 Silt 0.49 1.75 220 8650 5 7.0 Gravel 0.45 2.10 400 34300 6 7.0 0.45 2.10 400 34300 7 7.1 0.45 2.10 400 34300 8 5.5 0.44 2.10 450 43400 (c)POINT 3
Layer (m) Thickness Soil Type υ (t/m3) Vs (m/s) G (t/m2)
No.1 2.5 Sand 0.48 1.75 180 5800 2 2.5 0.48 1.75 190 6450 3 1.9 0.49 1.60 145 3450 4 1.9 0.49 1.60 155 3900 5 1.9 0.49 1.70 180 5600 6 2.0 0.48 2.00 255 13250 7 1.9 0.48 2.00 210 9000 8 2.0 0.48 2.00 235 11250 9 0.7 Clay 0.48 1.80 200 7350 10 1.5 Gravel 0.46 2.10 350 26250 11 1.8 Sand 0.48 1.95 280 15600 12 1.1 Gravel 0.44 2.10 450 43400 13 1.3 Sand 0.49 1.80 200 7350 14 0.6 Gravel 0.46 2.10 270 15600 15 0.7 Clay 0.49 1.75 200 7150
18 7.1 0.45 2.10 400 34300
19 5.5 0.44 2.10 450 43400
Fig. 10 Result of Response Analyses
Fig.11 Results of Response Analyses with Different Incident Angles
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ANALYSES OF SIMPLE MODEL OF MEXICO VALLEY BY BOUNDARY ELEMENT METHOD
Based on Ref. [10] (Kuribayashi et al.), the boundary element method using the half space fundamental solution is used for the response analyses of symmetric valleys subjected to incident SH waves and the vibration amplification
characteristics. Analytical simple model took the case of Mexico Valley and adopted parameter are shown in Fig. 12 and Table 5. Response analyses are carried out with the different incident angles, 0°, 30°, 60°, in both cases that the soft layer exists or not. Fig. 13 shows analysed amplitude ratio between surface and base.
As results; (1) because of being with the soft layer, amplitude ratio is distinctly larger than the case without the soft layer, (2) in both sides of valley, amplitude ratio is larger than the other parts of valley, (3) effects of different incidental angle are only a little. These results are equivalent to actual disaster in Mexico Valley in 1985.
Fig. 12 Analytical Model
SL: Soft Layer ML: Middle Layer BL: Bed Layer Table 5 Soil Parameters
Layer γ (t/m3) Vs (m/s) h (%)
SL 1.40 70 5
ML 1.80 500 0
BL 2.16 1250 0
Fig. 13 Amplitude Ratio
