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
CONCLUSIONS
Aichi prefecture is regarded as one of the most vulnerable area to destructive earthquakes. TASSEM developed in Toyohashi, east part of Aichi, is an earthquake observation network and has the sufficient specifications as the high- fidelity observation system. [11]
The amplification characteristics in frequency domain around TASSEM are proved from analytical results and consequences in microtremor and strong motion observations. [11]
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.
ACKNOWLEDGEMENTS
This research was supported by a grant from subsidy of Science Research Fund, Ministry of Education, Science and Culture through 1989 to 1990 fiscal year: this support is gratefully acknowledged.
REFERENCES
1. Conference on Prevention of disasters in Aichi Prefecture, “Anti-disaster Plan in Aichi Prefecture”, 1990
2. Asada, T., 1988, “Earthquake Prediction Study in Japan”, Proc. 9th WCEE, Aug. 2–9, 1988 Tokyo-Kyoto, Japan, Vol 2, pp13–19
3. Usami, T., “Damage Caused by Major Earthquakes in Japan—New Edition”, University of Tokyo Press, 1987 4. Ishibashi, K., “Specification of Soon-to-occur Seismic Faulting in the Tokai District, Central Japan, Based upon Seismotectonics”, Earthquake Prediction, An International Preview, 4, 527–532, Amer. Geophys. Union, 1981, Washington D.C.
5. Yamashita, N., et al., “Tyubu Region II”, Geology of Japan 5, Kyoritsu Press Co. Ltd., 1988
6. Secretariat of Conference for Preservation of Subterranean Water in Toyohashi, “Subterranean Water of Toyohashi”, 1986
7. Kuroda, K., “Diluvium and Tectonics of Atsumi Pen.”, Earth Science Shizuhata, 16, pp38–45, 1958
8. Lysmer, J., et al., 1975, “FLUSH—A Computer Program for Approximate 3-D Analysis of Soil-Structure Interaction Problems”, EERC, Univ.of California, Barkley, Dec., 1972
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9. The Japan Road Association, “Standard Specifications for Highway Bridges Part 5, 1980
10. Jiang, T., and Kuribayashi, E., “The Three-Dimensional Resonance of Axisymmetric Sediment Filled Valleys”, Soil and Foundations, Vol. 28, No. 4, pp130–146, Dec.1988
11. Kuribayashi, E. et al., “Engineering Tactics on Lifelines Safety Against Earthquakes”, Proc. 3rd Conference on Lifeline Earthquake Engineering, ASCE, 1991
*A11 references are written in English except 1, 3, 5, 6, 7, 9
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Site-Response at Foster City and San Francisco Airport—Loma Prieta Studies
M.Çelebi, A.McGarr
U.S. Geological Survey, MS/977, Menlo Park, CA 94025, U.S.A.
ABSTRACT
Strong motions recorded during the Loma Prieta earthquake and aftershocks recorded thereafter are used to quantify the amplification at several locations within a 20-km strip of the bay side of mid-peninsula, in San Mateo County, south of San Francisco, California. The area, extending from San Francisco International Airport southward to Foster City and Redwood Shores, presents a unique opportunity to quantify the amplification of motions at soft soil sites as compared to hard rock sites and compare them with the recent code site factors and maps based on these factors. The amplifications are calculated for frequency ranges of engineering interest and are shown to exceed 2 (the maximum site factor in the code) in most locations.
INTRODUCTION
The peninsula south of San Francisco experienced significant variation of ground motions during the Loma Prieta earthquake of October 17, 1989 [Ms=7.1]. In this paper, we present evidence of such variation within a 20-km strip of the bay side of the mid-peninsula covering the San Francisco airport (SFO) and extending south to Foster City and Redwood Shores (Figure 1) within the boundaries of San Mateo County (at epicentral distance of 75km or more). The area includes an important lifeline such as SFO, the commercial strip south of SFO, and the urban areas of Foster City, Redwood Shores and vicinity.
The damage sustained in the study area during this earthquake, by most
standards, was moderate to minimal. A two-story structure within the airport grounds built according to pre-1960 codes was extensively damaged and later razed. Two hotels in Burlingame were damaged extensively. Structural damage occurred in the Fluor Building at Redwood Shores. A 22-story steel structure in Foster City was reported to have received minor structural damage. In the majority of cases, residential homes within the study zone were not damaged. Available strong-motion records from the main shock and records of two aftershocks are used to document and discuss the features of the varying ground motion within the described mid-peninsula strip. There are several other aftershock records from these stations; however, due to space limitations, only two are presented and utilized herein. It should be recognized that, as in this case, unless special dense arrays are deployed prior to an earthquake, strong-motion stations in general are sparsely deployed. Therefore, we have relied on temporarily deployed dense arrays to record aftershocks. Figure 1 shows the locations of temporary aftershock stations as well as the strong-motion stations discussed throughout this paper. The strong-motion stations SF1 and AP7 (Figure 1) are maintained by the California Division of Mines and Geology (Shakal and others, [4]) and the MAL (Foster City), AP2 and AP9 stations are maintained by the United States Geological Survey (Maley and others, [3]).
Epicentral distances, latitudes, longitudes, elevations, depths to bedrock and relevant descriptions of all stations (and for only the strong-motion stations, the recorded peak accelerations) are provided in Table 1. In addition, specific site factors (S1–S4) are assigned to each station using a recent zoning map by Hensolt and Brabb [2]. The factors S1 through S4 assigned in their map are adopted from Section 2312 (Table No. 23-J : “Site Coefficients” of the Uniform Building Code (UBC) [5]. The site factors are 1.0 for S1, 1.2 for S2, 1.5 for S3 to 2.0 for S4. These are intended to correspond to
amplification factors and are to be compared with the spectral ratios found in this work, Ramplification(ω)=A2j (ω)/A1j (ω) where Aij(ω) is the jth component Fourier amplitude spectrum at recording station i. This relationship is valid assuming the differences in distances can be neglected. The amplitude spectra are calculated using 25 seconds of each record (originally 200 samples per second, decimated to 50sps). The spectra and ratios are smoothed with a hanning window of 10 points.
The scope of this paper is limited to assessing the engineering implications of the variation of the recorded strong-motion main shock and its aftershocks. Particular attention is given to impact on structural shaking and therefore zonation as a result of the variation of motions within distances
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that are close to one another. Only the horizontal motions are quantified in this work. Table 1. Recording site information. (D epicentral distance)
Station D (km) Lat. Deg. N Long. Deg. W Elev. (m) Site Factor Depth to Bedrock (m)
Comments (* Co-sited Stations)
SF1* 79 37.62 122.39 2 S2 92 Engineering Building, San Francisco Airport Co-sited with CSMIP #58223 strong- motion recorder. [0.33, 0.05, 0.24].**
SF2 79 37.63 122.39 2 S1 0 On pavement 25 m east of
northeast corner of United Overhaul Shop. Situated close to bedrock outcrop, free field.
SF3 79 37.62 122.39 2 S3 37 Inside Butler Aviation
Building on concrete floor of hangar.
SF4 78 37.61 122.35 1 S4 107 Near east end of Runway 28R on concrete slab, free field.
SBM 80 37.69 122.45 100 S1 0 Near San Bruno Mountain
AMF 72 37.39 121.95 5 S3/S4 107 1492 Old Bayshore Highway, Burlingame. Across from Hyatt (Burlingame).[0.20, 0.12, 0.12].
MAL* 66 37.55 122.24 1 S4 107 Co-sited with USGS #1515 strong-motion recorder. 335 Menhaden Ct., Foster City. [0.12, 0.09, 0.11].
FOX 64 37.51 122.25 1 S2 50 Fox & Carskadon Office, 75 Shoreway Rd., San Carlos. AP2* 63 37.52 122.25 1 S3 117 Co-sited with USGS #1002
strong-motion recorder. Portside Park, Redwood Shores [0.23, 0.08, 0.28]. AP7* 63 37.48 122.31 108 S1 0 Co-sited with CSMIP #58378
strong-motion recorder. Canada Rd., rural San Mateo County.[0.09, 0.06, 0.16]. AP9 62 37.47 122.32 106 S1 0 USGS strong-motion station
(#1161). No aftershocks recorded. Crystal Springs Reservoir.[0.11, 0.06, 0.12].
CRA 66 37.55 122.24 1 S4 107 Same conditions as MAL.
*Note 1: Numbers in brackets are peak accelerations (horizontal (NS), vertical, horizontal (EW)—in g’s) recorded during the Loma Prieta earthquake.
**Note 2: Another strong motion station (AP1) in Redwood Shores recorded peak accelerations of [0.29, 0.11, 0.26],