Estimation of S-wave site response in Anchorage, Alaska, from weak-motion data using generalized inversion method

The generalized data inversion method has been used to estimate the spatial variation of site response (SR) in the Anchorage basin in south-central Al aska. The data analyzed represents weak motion of the horizontal components of S waves for local earthquakes. They were recorded by a temporary portab le 22-station network (IRIS-PASSCAL equipment) that was operated for about six months. Setting the path effect from independent sources, the inversion of the data was carried out to determine SR. The SR values thus obtained w ere averaged (logarithmically) for two frequency bands, namely, a low frequ ency band (LFB) and a high frequency band (HFB) from 0.5 to 2.5 Hz and 3.0 to 7.0 Hz, respectively, with center frequencies of about 1.0 Hz and 5.0 Hz . In LFB, SR increases from 1.0 along the foothills of Chugach Mountains in the east to about 3.5 in the west in and around Campbell Lake and Governme nt Hill. The areas with SR > 3.0 are associated with extensive ground failu re during the Prince William Sound earthquake (M-w = 9.2) of 1964. In HFB, there are two small areas adjoining Tumagain Arm in the southern side of An chorage with SR greater than 3.0. A comparison of the SR values obtained fr om the inversion with these reported by others for standard spectral ratio (SSR) and horizontal to vertical spectral ratio (HVR) showed (1:1) correspo ndence with the values of SSR but larger HVR values by a factor of about 2. Areas of soil class D in the study area are characterized by SR > 2.0, whi le those in C are characterized by SR less than or equal to 2.0 in LFB, but soil class and SR lack correlation in HFB.