Basin/Soil Effects

Study of different aspects of basin effects on the ground motion characteristics needs special attention since most of urbanized areas are generally settled along river valleys over young, soft, surficial soil deposits.

Impedance contrast

Seismic waves travel faster in hard rocks than in softer rocks and sediments. As the seismic waves pass from hard medium to soft medium, their celerity decrease, so they must get bigger in amplitude to carry the same amount of energy. If the effects of scattering and material TABLE 1.9 Classification of local geology in different category

LOCAL SITE EFFECTS

A. Basin/soil B. Topography

a. Impedance contrast a. Ridge

b. Resonance b. Valley

c. Trapping c. Slope/slope

d. Focusing variation

e. Basin-edge C. Strong Lateral

damping are neglected, the conservation of elastic wave energy requires that the flow of energy (energy flux, rVSv2) from depth to the ground surface be constant. Therefore, with decrease in density (r) and S-wave velocity (VS) of the medium, as waves approach the ground surface, the particle velocity (v), must increase. Thus, shaking tends to be stronger at sites with softer soil layers.

Resonance

Tremendous increase in ground motion amplification occurs when there is resonance of signal frequency with the fundamental frequency or higher harmonics of the soil layer. Various spectral peaks characterize resonance patterns. For one-layer 1D structures, this relation is very simple:

f0 = VS1/4h (fundamental mode) and fn = (2n + 1) f0 (harmonics)

where V_S1 is the S-wave velocity in the surficial soil layer, and h is the thickness. The amplitudes of these spectral peaks are related mainly to the impedance contrast and sediment damping. Damping in soil

Absorption of energy occurs due to imperfect elastic properties of medium in which the collision between neighbouring particles of the medium is not perfectly elastic and a part of the energy in the wave is lost instead of being transferred through the medium. This type of attenuation of the seismic waves is referred to as anelastic damping. The damping of seismic waves is described by a parameter called as quality factor (Q). It is defined as the fractional loss of

energy per cycle, 2p/Q = –DE/E, where DE is the energy lost in one cycle and E is the total

elastic energy stored in the wave. If we consider the damping of a seismic wave as a function of the distance and the amplitude of seismic wave, we have

A = A r

Q

0 exp -

F

HG

IKJ

where a = w/2QV is absorption coefficient. This relation implies that higher frequencies will be absorbed at a faster rate.

Basin edge

Intense concentrations of damage parallel to the basin-edge had been observed due to strong generation of surface waves near the edge, during recent earthquakes (Northridge earthquake, 1994; Kobe earthquake, 1995 and Dinar earthquake, 1995). The conclusion that basin-edge induces strong surface waves had been drawn in many studies by examining the phase and group velocities, polarity and arrival azimuth (Bard and Bouchan, 1980 a, b 1985; Hatayama et al., 1995; Kawase, 1996; Pitarka et al., 1998; Narayan, 2003a, 2004, 2005). Surface waves start generating near the edge of the basin when frequency content in the body wave exceeds the fundamental frequency of the soil and their amplitudes decrease with increase of edge-slope (Narayan, 2004, 2005).

Figure 1.13a shows the vertically exaggerated basin-edge models having different thickness of single soil layer over the bed-rock. Figure 1.13b depicts the vertical component of ground motion, computed for thickness of soil layer as 195 m using a double-couple source (dip = 45°,

rake = 60° and strike = 90°) just below the edge at a depth of 13.7 km with a dominant frequency 1.0 Hz. The P- and S-waves velocities and densities were taken as 1396.5 m/s, 400.0 m/s and 1.9 g/cm3 for soil and 3464.1 m/s, 2000.0 m/s and 2.5 g/cm3 for half space (hard rock). The ground response was computed at 26 equidistant (105 m apart) receiver points. Figure 1.13b reveals four well-separated wavelets at receiver points some distance away from the edge. The differential ground motion in north–south direction clearly depicts horizontally travelling surface waves since vertically travelling body waves are more or less removed (Figure 1.13c).

14.5° Vacuum R10 N Soil 195 m 150 m 105 m 60 m R1 Receiversp, ,l m ®0 p = 2.5 g/cm , = 10 GPa, = 10 Gpa3 _{l l} Hard rock (a) 3 6 9 12 15 18 21 24 Time (sec) (b) 3 6 9 12 15 18 21 24 Time (sec) (c)

FIGURE 1.13 (a) Vertically exaggerated basin-edge model, (b) vertical component of ground displacement, and (c) the differential ground displacement corresponding to the vertical compo- nent of ground motion at 26 receiver points (after Narayan, 2005).

The generation of surface waves near the edge was confirmed on the basis of the large coherence in recording stations, increase of travel time of later phases as we move away from the edge, estimated group velocity of later phases and the analysis of differential ground motion (Narayan, 2005). Both P-wave and S-wave have caused generation of Rayleigh waves.

The major conclusions drawn in papers of Bard and Bouchan (1980 a, b), Hatayama et al. (1995), Kawase (1996) Pitarka, et al. (1998) and Narayan (2003a, 2004, 2005) are listed below.

∑ Basin-edge induces strong surface waves near the edge.

∑ Edge-induced surface waves propagate normal to edge and towards the basin. ∑ Surface waves start generating near the edge of the basin when frequency content in

the body wave exceeds the fundamental frequency of the soil deposit.

∑ Surface wave amplitude decreases with increase of edge-slope.

∑ Damage caused by edge-induced surface waves is confined in a narrow zone (width

2.5–3.5 km) parallel to the edge, and at some distance (0.5–1.0 km).

∑ Surface wave amplitude increases with the decrease of propagation velocity in soil.

Further, their characteristics are highly variable with change in propagation velocity and thickness of soil deposit.

∑ The characteristics of edge-induced surface waves are also very much dependent on the

angle of incidence of body waves.

∑ Edge-induced surface waves develop significant differential ground, the main cause

of damage during earthquakes, in addition to amplification and prolongation of the signal.

Basement topography

The focusing and defocusing effects caused by basement topography are strongly dependent on the azimuth and angle of incidence of waves. Seismic waves traveling upward from depth may be redirected by subtle irregularities at geological interfaces, particularly the basement topography. The effects of focusing and defocusing are maximum for normal incidence of waves and it decreases with increase of angle of incidence. Similarly, azimuth also affects the focusing and defocusing effects. This effect reveals the importance of considering not only the surficial soil layer but also the basement topography for seismic microzonation.

Trapping of waves

The fundamental phenomenon responsible for the increase of duration of motion over soft sediments is the trapping and multiple reflections of seismic waves due to the large impedance contrast between soft sediments and underlying bedrock. Sometimes, when a wave enters a basin through its edge, it can become trapped within the basin if post-critical incidence angles develop, causing total internal reflection at the base of the layer. Waves that become trapped in deep sedimentary basins can therefore be potentially very damaging.