Intrusions and Anomalous Vp/Vs Ratios Associated with the New Madrid Seismic Zone
Abstract
We present detailed Vp and Vs models and Vp/Vs ratios for a major portion of the New Madrid Seismic Zone (NMSZ) based upon arrival times recorded by the New Madrid seismic network and by PANDA stations. We perform a simultaneous inversion for P and S wave velocities and hypocentral locations. Block size is reduced to 2x2x2 km, yielding the most detailed tomographic image of the upper portion of the seismogenic crust to date. Inversion results are tested using synthetic models and reconstruction techniques. Mafic intrusions along the northwest boundary of the Reelfoot graben are imaged in detail, revealing the presence of several small intrusive bodies that appear to influence the location of the northern strike-slip arms of seismicity. Low Vp and high Vs appear to be correlated with earthquake occurrence; this results in distinct regions of low Vp/Vs ratios that correspond to the major arms of seismicity north of the intersection of the Cottonwood Grove - Blytheville Arch fault with the Reelfoot fault. The unusual low Vp/Vs values can be attributed to the presence of quartz rich rocks. A high Vp, low Vs body is imaged at the southern termination of the Reelfoot fault but interpreting this feature is problematic because it lies at the edge of the inversion solution where resolution is diminished. Two regions contain anomalous Vp, Vs, and Vp/Vs ratios that cannot be attributed to variations in rock composition and are probably produced by overpressured fluids. One region is located on the hanging wall of the northern portion of the Reelfoot fault and is aseismic. The other region corresponds to a portion of the southern Reelfoot fault that experiences swarm activity. A distinct velocity contrast exists across the Cottonwood Grove - Blytheville Arch fault at depths exceeding 4.65 km; basement rocks SE of the fault have Vp values that are 4 to 6% slower than values for rocks located to the NW. The most logical explanation is that the fault follows a preexisting structural feature or lithologic change in basement rocks. Our results suggest interesting targets for USArray Flexible Array experiments within the NMSZ (e.g. the southern termination of the Reelfoot fault) and experiments outside of the NMSZ. Flexible Array experiments targeting the region just north of the well defined arms of NMSZ seismicity will help determine if the features found in our study (e.g. the low Vp/Vs ratios) are unique to the NMSZ or extend into other portions of the crust.
- Publication:
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AGU Fall Meeting Abstracts
- Pub Date:
- December 2009
- Bibcode:
- 2009AGUFM.T53D1616P
- Keywords:
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- 7205 SEISMOLOGY / Continental crust;
- 8110 TECTONOPHYSICS / Continental tectonics: general