Shear-Wave Splitting Study of Crustal Anisotropy in the New Madrid Seismic Zone

摘要

This study investigates crustal anisotropy in the New Madrid Seismic Zone (NMSZ) by analyzing shear-wave splitting measurements from local earthquake data. In addition to the waveforms provided by the Center for Earthquake Research and Information (CERI) for over 3000 events, seismograms recorded by the portable array for numerical data acquisition (PANDA) network were obtained for over 800 events. Data reduction led to a final data set of 168 and 43 events from the CERI and PANDA data, respectively. One-hundred and eighty-six pairs of measurements were produced from the CERI data set by means of the automated shear-wave splitting measurement program mfast and 49 from the PANDA data set. Two dominant directions, respectively striking northeast-southwest and west-northwest-east-southeast, are identified and interpreted to be due to stress-aligned microcracks. The northeast-southwest polarization direction is consistent with the maximum horizontal stress orientation of the region and has previously been observed in the NMSZ, whereas the west-northwest-east-southeast polarization direction has not. Path-normalized time delays range from 1 to 33 ms=km for the CERI network data and 2 to 31 ms=km for the PANDA data. These results produce a range of estimated differential shear-wave anisotropy between 1% and 8%. These values are higher than those previously determined in the region. The majority of large path-normalized time delays (>20 ms=km) are located along the Reelfoot fault segment. These high values are believed to be indicative of high crack densities and high pore fluid pressures, which agrees with previous results from local earthquake tomography and microseismic swarm analysis.