Estimation of earthquake source parameters and site response in Anchorage, Alaska from strong-motion network data using generalized inversion method

Abstract Generalized inversion of the S-wave amplitude spectra from the strong-motion network data in Anchorage, Alaska, has been analyzed to estimate simultaneously source parameters, site response (SR) and the S-wave attenuation ( Q s −1 ). The data represented 60 earthquakes of magnitudes ( M L ) from 3.0 to 6.3 in the hypocentral distance range from 30 to 300 km. The source term obtained from inversion was analyzed to estimate various source parameters, namely, seismic moment ( M o ), seismic energy ( E s ), corner frequency ( f c ), apparent stress ( σ A ) and Brune’s stress drop (Δ σ ) for each event. It is shown that M o and E s varies from 10 14 to 2.1×10 18  N m and form 5.8×10 8 to 0.8×10 14  J, respectively, while Δ σ varies from 0.2 to 24 MPa. The regressional analysis of the source parameters shows the following: (1) E s / M o is approximately equal to 1.2×10 −5 and (2) M o is proportional to f c −3 . It is also shown that σ A and Δ σ approximately satisfies the relation, σ A =0.23 Δ σ . The SR values obtained for the Anchorage basin from the above inversion were averaged for two frequency bands, 0.5–2.5 and 3.0–6.3 Hz with geometric center frequencies of about 1.0 and 5.0 Hz, respectively. The spatial variation of SR at 1.0 Hz shows a gradual increase from 1.0 along the foothills of Chugach mountains to 3.5 in the western part of the basin. At 5.0 Hz, the trend of the variation of SR values is from north to south. Moreover at 1.0 Hz, the above trend of SR values is in agreement with that of shear-wave velocity of the uppermost 30 m of the sedimentary formations of the basin. The variation of Q s −1 in the region is found to be 0.0066 f −1.2 in the frequency range from 0.25 to 20 Hz.