Effects of Surface Geology on Seismic Motion

Ten strong motion accelerometers have been operated by the Philippine Institute of Volcanology and Seismology (PHIVOLCS) and Tokyo Institute of Technology in Metro Manila, since 1998, to observe the strong ground motions and amplifications at different geological settings. In this study, we performed joint inversion of receiver function (RF) and surface-wave phase velocity to determine the deep sedimentary layers in the metropolis and to evaluate the site effects. Using the conventional receiver function method in the analysis of 12-year earthquake strong motion data, the observed RFs contained seismic phases that are generated from the P-S conversions at velocity discontinuities. Long-period microtremor array measurements were conducted within and near the strong motion stations to be able to perform efficiently the joint analysis with receiver function. For the long period microtremor exploration, we deployed temporary circular arrays consisting of 7 stations to measure surface waves and applied the frequency-wavenumber (FK) spectral analysis to obtain the phase velocity dispersion curves. We applied the hybrid genetic simulated annealing algorithm (HGSAA) in the joint inversion for the successful determination of S-wave velocity profiles. Amplification characteristics were also evaluated using the inferred S-wave velocity profiles. INTRODUCTION Records from strong motion accelerographs contain waves that come from the source, path and local geological setting. The first part of the waveform, usually on the P-wave, can yield abundant information about the subsurface structure directly beneath the strong motion station. One method that can be applied to strong motion records is the receiver function (RF) analysis to identify the shearwave velocity (Vs) structures underneath the seismic station. RF analysis has been applied extensively to broadband seismic records to estimate the fine scale Vs of the lithosphere by applying an inversion technique (e.g. Owens et al. 1984; Ammon et al. 1990). Moreover, deeper Vs structures can also be inferred using long period microtremor exploration by exploiting the surface-wave dispersion curves (e.g. Yamanaka et al. 1994). Recently, in crust and mantle studies, the joint inversion of RF and surface-wave group velocities is used to bridge the gaps associated with the two separate methods using RFs and surface wave dispersions (e.g. Ozalaybey et al. 1997; Du and Foulger 1999; Julia et al. 2000). The advantages of the two methods in Vs profiling can be jointly applied to have good estimates of Vs structures. The RFs are sensitive to Vs contrast and vertical travel times while surface wave dispersions are sensitive to vertical shear wave velocity averages (Julia et al. 2000). The importance of identifying the Vs profiles down to the basement is useful to provide better estimates in the simulation of earthquake ground motion and assessment of the site amplifications in the metropolis. Firm rock with Vs of 3 km/s is usually regarded as the basement. In this study, we performed joint inversion method using hybrid heuristic search method to estimate the Vs structure of deep sedimentary layers in Metro Manila. The 12-year records of strong motion were used in the RF analyses and the obtained data from the long period microtremors in the metropolis were utilized for the surface-wave dispersions.