ANOMALOUS Vp/Vs
RATIOS IN THE NEW MADRID SEISMIC ZONE; IMPLICATIONS FOR SEISMIC HAZARD MAPPING
POWELL, C., WITHERS, M., and DUNN, M., Center for Earthquake Research and Information, 3876 Central Ave., Suite 1, University of Memphis, Memphis, Tennessee 38152-3050, and VLAHOVIC, G., Department of Geography and Earth Sciences, North Carolina Central University, Durham, N.C., 27707, capowell@memphis.edu,
Anomalously low Vp/Vs ratios have been detected in the New Madrid seismic zone (NMSZ) that correlate with the major arms of seismicity. Vp and Vs models were constructed using 11,656 and 8,579 P and S arrival times, respectively, recorded by the New Madrid seismic network for the time period 2000 to 2004 and by PANDA. The inversion algorithm inverts simultaneously for both P and S velocities and hypocentral locations. Block size was 2 by 2 by 2 km. Model resolution was investigated using reconstruction tests and chessboard models. Vp/Vs ratios as low as 1.63 are clearly associated with the major arms of seismicity to depths of at least 9 km. The low Vp/Vs values are unusual and cannot be attributed to resolution differences in the P and S velocity solutions. Low Vp/Vs ratios are caused by anomalously low Vp values and anomalously high Vs values. Plausible explanations for the low Vp/Vs ratios include the presence of highly fractured rocks and fluid saturated rocks with high pore pressure. The striking coincidence of low Vp/Vs values and earthquake occurrence in the NMSZ suggests that detecting low Vp/Vs ratios in intraplate regions may be an important target for future seismic hazard mapping. Careful mapping of Vp/Vs may provide insights into why certain portions of intraplate failed rifts are seismically active while other portions are not. The passage of USArray and associated instrumentation can allow detailed 3D velocity models and Vp/Vs ratios to be determined even for relatively inactive regions such as the Wabash Valley.