Determining earthquake source properties, seismic attenuation and site amplification using the Indonesian strong-motion network

Abstract

Distinguishing source, path, and site responses from S-wave displacement spectra recorded by the Indonesian strong motion network is the objective of my Master's degree research. In this research, a well-established, nonlinear parameterization of the earthquake source and path components of the observed spectra was used, while site response is considered as an arbitrary function of frequency that multiples the source and path spectra. This model for the S-wave displacement spectra is fit to the data by combining a non-linear optimization for source and path parameters with a linear inversion for the site response spectra. Because it combines linear and nonlinear optimization, we refer to this as a 'hybrid' method. Before inverting actual observed spectra, the ability of such spectral inversions to separate source, path and site effects was explored by analyzing synthetic spectra using a grid search method. Three cases involving different types of data sets and different constraints on site response were considered, and it was determined that use of a reference site having known site response was critical to avoid strong trade-offs amongst source, path and site parameters. However, it was determined that when such a reference site is used, a simple optimization algorithm such as the Nelder-Mead Simplex algorithm could resolve the parameters reasonably well. S-wave displacement spectra of clustered earthquakes recorded by several stations were simultaneous fit by using the hybrid inversion method. Four earthquake clusters were considered: the 02 September 2009, Tasikmalaya, West Java intraslab earthquakes; the 14 June 2011, Tapanuli, North Sumatra crustal earthquakes; the 03 September 2012, Banyuwangi, East Java interslab earthquake; and the 13 June 2013, Tasikmalaya, West Java interslab earthquakes. I found that Q(f) is lower for crustal earthquakes compared with intraslab and megathrust earthquakes. From earthquake properties, the average stress drops are higher for crustal and intraslab earthquakes compared with the two megathrust earthquakes, 9.43 MPa with standard deviation 3.98, 13.31 MPa with standard deviation 0.29, 2.73 MPa with standard deviation 1.38, and 9.07 MPa with standard deviation 2.04, respectively. Finally, the site information for the 20 Indonesian Strong-Motion Network stations that recorded these earthquakes are mostly comparable with H/V measurements of dominant period. However, amplification factor from that spectral analyses are higher compared with the H/V measurements. Our study demonstrates that the hybrid approach to inversion of S-wave displacement spectra is a reliable method for simultaneous estimation of source, path, and site response. I have also obtained important new information about Indonesian earthquakes, seismic wave propagation in the Indonesian region, and site response selected stations of the Indonesian Strong Motion Network. Show more