Forearc versus Backarc Attenuation of Earthquake Ground Motion

Understanding of the attenuation of seismic wave amplitudes with distance is a crucial issue for ground motion prediction equations and seismic hazard analysis. In this study, we empirically evaluate the influence of regional geologic structure, in particular the attenuation effects of traveling through a volcanic arc region (forearc versus backarc attenuation) in northern Japan. We perform regression analysis of Fourier amplitude spectra (FAS) of five well-recorded Japanese events that occurred very close to the line formed (in map view) by the volcanic arc. This provides an approximately symmetrical distribution of stations for each event relative to the volcanic front; forearc stations lie to the east of the front, while backarc stations lie to the west. To compare the characteristics of shallow and deep events, we have included both crustal and in-slab events in our dataset. Based on inspection of the data, we assume a hinged bilinear geometric spreading model with a fixed slope of -1 out to a distance of 50 km, with slope of -0.5 thereafter (for all events). Our results show, with a very high level of confidence, that high-frequency attenuation is greater in the backarc direction. The Q values for forearc regions (Q similar to 240 at 1 Hz, increasing to Q similar to 3890 at 10 Hz) are about a factor of 2 larger than those for backarc regions (Q similar to 196 at 1 Hz, increasing to Q similar to 1669 at 10 Hz) at high frequencies, implying much stronger attenuation as waves travel through the volcanic crustal structure to the backarc stations. The separation of forearc and backarc travel paths results in a significant reduction in the standard deviation (sigma) of ground motion predictions (by as much as 0.05 log10 units), which has important implications for hazard analyses in subduction zone regions.