Vs30 Mapping and Site Classification in Surakarta City Based on Multichannel Analysis of Surface Waves Method

Abstract

The Multichannel Analysis of Surface Waves (MASW) method is one of the noninvasive methods that can be used to determine the shear wave velocity. The shear wave velocity can be obtained from the inversion of dispersion curve of the Rayleigh wave. The surveys have done at 10 sites in Surakarta city. Active MASW measurements are performed by using the P.A.S.I. Seismograph Mod. 16S24-P. Geophone used as many as 24 channels with threshold frequency is 4.5 Hz of the vertical component and 8 lbs hammer as an active source. Each spread is given once shot point at 5 to 10 meters from the near geophone. The measurement data as a time domain is transformed to frequency domain. The average shear wave velocity up to 30 meters depth is referred as Vs30. The results show that the distribution Vs30 of Surakarta between 250 - 450 m/s. Based on the National Earthquake Hazard Reduction Program (NEHRP) classification, these values are classified as C and D class. The highest Vs30 values at the southwest part (Laweyan site) is 435.7 m/s and the northern part (Banjarsari site) is 411.7 m/s classified as site class C. Meanwhile, the lowest Vs30 values at the northwest and southeast part (Serengan and Pasar Kliwon sites) classified as D class.