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Abstract: Mapping of Crustal Anisotropy in the New Madrid Seismic Zone with Shear Wave
Published: Wednesday, February 26, 2014

Philip Martin, Pierre Arroucau, Gordana Vlahovic

Crustal anisotropy in the New Madrid seismic zone (NMSZ) is investigated by analyzing shear wave splitting measurements from local earthquake data. For the initial data set, the Center for Earthquake Research and Information (CERI) provided over 3000 events, along with 900 seismograms recorded by the Portable Array for Numerical Data Acquisition (PANDA) network. Data reduction led to a final data set of 168 and 43 useable events from the CERI and PANDA data, respectively. From this, 186 pairs of measurements were produced from the CERI data set as well as 49 from the PANDA data set, by means of the automated shear wave splitting measurement program MFAST. Results from this study identified two dominant fast polarization directions, striking NE-SW and WNW-ESE. These are interpreted to be due to stress aligned microcracks in the upper crust. The NE-SW polarization direction is consistent with the maximum horizontal stress orientation of the region and has previously been observed in the NMSZ, while the WNW-ESE polarization direction has not. Path normalized time delays from this study range from 1-33 ms/km for the CERI network data, and 2-31 ms/km for the PANDA data, giving a range of estimated differential shear wave anisotropy between 1% and 8%, with the majority of large path normalized time delays (>20 ms/km) located along the Reelfoot fault segment. The estimated differential shear wave anisotropy values from this study are higher than those previously determined in the region, and are attributed to high crack densities and high pore fluid pressures, which agree with previous results from local earthquake tomography and microseismic swarm analysis in the NMSZ

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