Effects of Style of Faulting on Earthquake Ground Motions in NMSZ
OGWARI, P. O., University of Memphis, CERI, Memphis, TN, firstname.lastname@example.org; HORTON, P. S., University of Memphis, CERI, Memphis, TN, email@example.com; WITHERS, M., University of Memphis, CERI, Memphis, TN, firstname.lastname@example.org; CRAMER, C. H., University of Memphis, CERI, Memphis, TN, email@example.com
We investigate the effects of style of faulting on earthquake ground motion in the New Madrid Seismic Zone (NMSZ). Style of faulting has not been considered in previous ground-motion relations in Eastern North America (ENA) although it has been found to produce systematically different ground motion in recent western U.S. ground motion models that distinguish between reverse and strike-slip earthquakes. We undertake a detailed analysis of local waveform data using a multistage processing method (e.g. Raoof et al., 1999; and Malagnini et al., 2000) to produce empirical attenuation relations for frequencies between 1 and 16Hz. The dataset consists of 1350 local events of magnitude 1.6<=M<=4.3 recorded at an hypocentral-distance ranging from 5.6 to 145km. Earthquake focal mechanisms are from two recent studies (Johnson et al., 2008; and Horton, 2008). The modeling procedure yields geometrical spreading as a piecewise continuous function of distance, parameterized as ra, within a discrete number of distance ranges. Preliminary results show r-1 geometrical spreading with Q(f)=660f0.35 within the first 60 km hypocentral distance. Beyond 60 km distance, the geometrical spreading changes to r0.5. This is comparable to previous relations in ENA although the change in geometrical spreading typically occurs at 70 km instead of 60 km. This could be due to a proposed high velocity rift pillow, lying above the Moho within Mississippi Embayment, which could reduce the crustal thickness hence travel time of critically refracted rays producing pseudo-Moho post-critical reflections at a shorter distance. Of particular interest are preliminary results indicating a faster decay of the amplitude with distance for reverse fault events, and slower decay for strike-slip fault events, compared to the average attenuation of the region at frequencies greater than 8Hz. At lower frequencies, no difference is discerned for the two styles of faulting.