During earthquakes, the most intense shaking is generally experienced near the rupturing fault and decreases with distance away from it. However, the shaking at one site may be entirely different from another site even though they are at the same distance from the rupture. It is now well known, and widely accepted amongst the earthquake engineering community, that the effect of local surface geology on seismic motion exist and can be large. The damage due to earthquakes is generally large over soft sediments than on firm bedrock outcrops. It is also a well known fact that in most cases site amplification/ shaking is stronger in low shear wave velocity areas. The sediment fill valleys are normally flat terrain with fertile soils, and therefore are attractive locations for human settlements. The National Earthquake Hazards Reduction Program (NEHRP, 1997) of US has defined five soil types based on shear wave velocity (Vs) which in turn help to evaluate the earthquake hazard.
The intensity of seismic ground motion is a function of earthquake magnitude and distance from the seismic source, as well as local soil condition, topography, geological condition, etc. Seismic zonation using Probabilistic Seismic Hazard Assessments (PSHA) are routinely used to estimate seismic intensity, characterized as Peak Ground Acceleration (PGA), Peak Ground Velocity (PGV) or spectral response ordinates for firm ground using the earthquake magnitude and distance from source to site. However, PSHA do not include consideration of site specific conditions such as surface topography, bedrock geometry and soil condition, which can affect local seismic intensity.
Amplification of ground motions at soft sites above those predicted by PSHA for firm ground has long been recognized from the damage patterns of historical earthquakes. In the last over three decades, comparisons of accelerographs have demonstrated that amplification of seismic motions on soft soil occur relative to adjacent rock sites. Therefore, the effects of local soil conditions on seismic ground motions should be considered for seismic design, damage estimates for future earthquakes and land use planning.
Thus, the two most important factors influencing the level of earthquake ground motion at a site are the magnitude and distance of the earthquake in addition to the influence of a third important factor called the site effect. It is the conditions at a particular location which can increase (amplify) or decrease the level of shaking that is otherwise expected for a given magnitude and distance. Combining this information with where and how often earthquakes of various magnitudes are likely to occur should provide improved assessments of seismic hazard. The site effect is briefly discussed hereunder.
Review on Site Effect Studies
The first step in forecasting the spatial location and the type of losses is to map the amplification of seismic waves due to soft near surface deposits. One alternative approach in evaluating site effects involves determining the physical properties of local soil setting by conducting borehole and seismic studies. Measured parameters can then be used in theoretical models to predict site response functions. The main disadvantage of this method is relatively the high cost of conducting the necessary geotechnical or geophysical studies that, in many cases, proved to be problematic. Although theoretical approaches are instructive, conducting the necessary sensitivity tests (with respect to different locations and sizes of possible events) and incorporating the inherent uncertainty (with respect to our limited knowledge of the Earth’s structure), is usually impractical.
The site response functions, by definition are equivalent to the spectral ratio of horizontal component of strong ground motion record with respect to a reference site located on rock (Rogers, et al., 1984, Singh, et al., 1988, Su, et al., 1998, Beresnev, et al., 1998, Hartzell, 1998, Reinoso and Ordaz, 1999, Zaslavsky and Shapira, 2000). In regions where the seismic activity is relatively low, as in Israel, this type of analysis is usually impracticable. Site response estimation was done using strong motion records (Nath et al., 2000) in Sikkim and week motion records (Nath et al., 2003) in Delhi region. In loose top soil situation seismometry data may not be good. Many investigators evaluated site response functions from moderate to weak motions of earthquakes (Tucker and King, 1984, King and Tucker, 1984, McGarr, et al., 1989, Field, et al., 1992, Steidl, et al., 1996, Toshinawa, et al., 1997, and Zaslavsky, et al., 2000). The main drawback of the reference site technique is the choice of reference motion, which represents the true input motion to the soil site. According to Steidl (1993), when it is possible, the reference ground motion should be calculated by averaging several rock sites. Steidl, et al., (1996) and Zaslavsky, et al.,(2002) show that the assumption of a flat response rock site is often false, mainly due to weathering. Use of these surface rock sites with flat response as reference sites often leads to underestimation of the amplification by a factor of 2 to 4 in the frequency range of 2–7 Hz. Lermo and Garcia, (1993) drew significant results from a nonreference technique, i.e., the receiver function technique, using the horizontal to vertical spectral ratios of shear wave.
Many studies report that the frequency dependence of site response can thus be obtained from measurements made at only one station (Lermo and Garcia 1994, Theodulidis, et al., 1996, Seekins, et al., 1996, Malagnini, et al., 1996 and Zaslavsky, et al., 1995). The implementation of this approach, however, still requires rather frequent occurrence of earthquakes. The idea of evaluating site characteristics from microtremor records originated from the pioneering work by Kanai and Tanaka (1961). They pointed out that predominant frequency of horizontal spectra of microtremor is related to shallow and local geological conditions. Since then, it has been reported that this technique has proved to be effective in estimating fundamental frequency (Tanaka, 1966, Tanaka, et al., 1968, Katz, 1976, Katz and Bellon, 1978, Kagami, et al., 1982, Ohta, et al., 1978, Zaslavsky, 1984 and 1987). However, in most cases, due to the influence of artificial sources from dense population, high traffic and various industries, the resonance frequency cannot be directly identified in the microtremor spectra (Zaslavsky, et al., 2001 and 2002).
Kagami, et al., (1982) proposed that the ratio of the horizontal components of the velocity spectra at the sediment site to those at the rock site could be used as a measure of microseismic ground motion amplification. This technique is widely used for estimating site response (Rovelli, et al., 1991, Field, et al., 1990 and 1992, Hough, et al., 1992, Malagnini, et al., 1996, Gutierrez and Singh, 1992, Dravinski, et al., 1995, Gaull, et al., 1995, Zaslavsky, et al., 1995 and 2000 and Shapira, et al., 2001). The experiments (Zaslavsky, et al., 2000) show that bedrock ground motion can be considered as a good reference site when distances are as smaller as 0.5 km-1.0 km from the soil site.
Nakamura, (1989) hypothesized that site response could be estimated by simply evaluating spectral ratio of horizontal versus vertical component of noise observed at the same site. Most studies show that the H/V ratio obtained from microtremors coincide with response functions of near surface structures (Ohmachi, et al., 1991, Lermo and Chavez-Garcia, 1994, Zaslavsky, et al., 1995, Seekins, et al., 1996, Gitterman, et al., 1996, Konno and Ohmachi 1998, Mucciarelli and Monachesi, 1998, Chavez-Garcia and Cuenca, 1998, Toshinawa, et al., 1997 and Shapira, et al., 2001). There is yet another fact, according to Field and Jacob, (1995), Bonilla, et al., (1997), Horike, et al., (2001) and Satoh, et al., (2001) that the spectral ratio of horizontal to vertical component of microtremors is similar to those obtained from traditional sediment to bedrock spectral ratio of the frequency of the predominant peak earthquake records (Borcherdt, 1970). However the absolute level of site amplification does not correlate with the amplification obtained from this method.