Microtremor HVSR Study of Site Effects in Bursa City (Northern Marmara Region, Turkey)

Abstract

Local site effects are one of the most important aspects in the assessment of seismic hazard. Local site response can be investigated by empirical and theoretical methods. Theoretical methods allow a detail analysis of the parameters considered in the evaluation; however, they require information of the geological structure (Dravinski et al., 1996). Empirical methods are based on seismic records on sites with different geological condition from which relative amplitudes and dominant periods may be determined directly. This approach requires of a large number of earthquakes. In regions with low seismicity, it would be necessary to wait for a long time to obtain a complete data set. For this reason, the use of ambient seismic noise is becoming popular as an alternative (Bard, 1998). Recording and analyzing ambient noise is simple. A few minutes of microtremor data are usually sufficient. Microtremors are present continuously in time and space. A single threecomponent station is the only instrument required. Routine spectral techniques can be easily applied to estimate the dominant frequency of vibration of the sedimantary structure. These frequencies of vibration are closely related to the physical features of the site under study, i.e., layer thicknesses, densities and wave velocities. Estimates of these frequencies are useful to constrain the physical properties at a given site. The Nakamura technique (Nakamura, 1989), based on the horizontal to vertical spectral ratio (HVSR), has been commonly used to estimate the site effects. Later it has been extended to both weak motions (Ohmachi et al., 1991; Field & Jacob, 1993, 1995); and strong motions (Lermo & Chavez-Garcia, 1994; Theodulidis & Bard, 1995; Suzuki et al., 1995). Lermo & Chavez-Garcia (1993) applied this technique to estimate the empirical transfer function from the intense S-wave part of a small sample of earthquake records obtained in three cities of Mexico. Their results showed that the HVSR can estimate the dominant frequency at a site based on earthquake data. Suzuki et al. (1995), using both microtremor and strong motion data in Hokkaido, Japan, showed that the dominant frequency obtained from HVSR was in good agreement with the predominant frequency estimated from the thickness of an alluvial layer. Lermo & Chavez-Garcia (1993) compared transfer functions computed using the Haskell method agreement with the HVSR. Lermo & Chavez-Garcia (1994) verified that the underlying