Determination of the Site Characterization Properties in Eastern Segment of the North Anatolian Fault Zone in Turkey based on the MMSPAC Method

Abstract

Erzincan (a small city in Eastern Turkey) is located in the conjunction of three active faults: North Anatolian, North East Anatolian and East Anatolian Fault Zones. Erzincan city center is in a pull-apart basin underlain by soft sediments which significantly amplify the ground motions. Combination of the tectonic and geological behaviour of the region have led to destructive earthquakes such as the 27 December 1939 (Ms=8.0) and the 13 March 1992 (Mw=6.6) events resulting in extensive losses. In this study, in order to perform site characterization in the region, we employ the passive microtremor survey technique at nine sites on three sides of the Erzincan city center in order to gain quantitative estimates of 1D shear-wave velocity profiles for key parts of the basin. The sites are aligned as two profiles in the North-South and East-West directions allowing construction of approximate 2D profiles. At each site we performed surface wave dispersion curve analysis using the Multi-Mode Spatial Autocorrelation (MMSPAC) technique on either single or nested four-station arrays together with horizontal to vertical spectral ratio measurements at array centers. The combination allows resolution of Vs for typically the upper 4 m from high-frrequency data, and as deep as 480 m using low-frequency SPAC and HVSR data in combination. The survey showed that a rate of production of two sites per day is achievable. We present our results in the form of one and two dimensional velocity structures and fundamental frequencies obtained from the HVSR spectra, a combination suitable for input to earthquake ground-motion prediction studies.