Three‐dimensional S wave velocity structure and V p / V s ratios in the New Madrid Seismic Zone

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 V p / V s ratios. Most of the seismicity in the NW trending central arm of the NMSZ is associated with normal V p / V s values that border regions with high V p / V s . North of Ridgely, high V p / V s 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 V p / V s 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.