Ground Motion Prediction for Thrust Earthquakes

by P. Somerville and N. Abrahamson

Somerville, P., and N. Abrahamson (1995). Ground Motion Prediction for Thrust Earthquakes. SMIP95 Seminar on Seismological and Engineering Implications of Recent Strong-Motion Data, p. 11 - 24 .

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In previous studies, the ground motion for reverse faults has been distinguished from the ground motion from strike-slip faults by a style-of-faulting factor in the attenuation relation. In most studies, this style-of-faulting factor has been assumed to be constant for all magnitudes, distances, and periods; however, some studies have examined these factors but not all together. The empirical ground motions for thrust faults are evaluated by developing a model for the magnitude, distance, and period dependence of the style-of-faulting factor. In developing the distance dependence, we distinguish between sites on the hanging wall from those on the foot wall, and from sites off the edge of the fault rupture. We find that there is a strong magnitude dependence of the style-of-faulting factor with smaller magnitude events producing a larger style-of-faulting factor. A strong distance dependence is found: sites over the hanging wall at distances of 8 to 18 km have an additional increase in ground motion of up to 50%; sites on the foot wall at distances of 12-30 km have a reduction of the ground motion of about 35%. No systematic period dependence is found over the period range of 0.03 to 5 seconds for sites off the ends of the rupture (not over the hanging wall or foot wall), but a strong period dependence is found for sites over the hanging wall and foot wall with smaller style-of- faulting factors at long periods (T> 1 sec).