Three‐dimensional S wave velocity structure and Vp/Vs ratios in the New Madrid Seismic Zone

Abstract

A three‐dimensional S wave velocity model for the New Madrid Seismic Zone (NMSZ) has been developed using nonlinear travel time tomography. The inversion utilized 5544 S wave arrival times from 720 earthquakes recorded by digital, three‐component stations deployed in the NMSZ over the time period 1989 through 1992. We imaged S wave velocity anomalies ranging from −5% to +8% relative to the starting one‐dimensional velocity model. Lowest S wave velocities are found south of Ridgley, Tennessee, in an area characterized by a high earthquake swarm rate and shallower than normal hypocenters. Two centers of higher than average S wave velocity are located west of the Mississippi river, north of Caruthersville, Missouri. The S wave model is similar to a P wave velocity model generated using the same earthquake data set. The similarity in ray coverage in both the P and S wave solutions allowed calculation of Vp/Vs ratios. Most of the seismicity in the NW trending central arm of the NMSZ is associated with normal Vp/Vs values that border regions with high Vp/Vs. North of Ridgely, high Vp/Vs values are associated with higher than average compressional and shear wave velocities and are interpreted to be due to mafic intrusions along the axis and edges of the Reelfoot rift. SE of Ridgley, the end of the central arm coincides with high Vp/Vs values that are due to a significant shear wave low‐velocity zone and are interpreted to be the result of highly fractured and fluid saturated crust.