Pn Velocity Distribution Beneath the New Madrid Seismic Zone, Central USA, Using Seismic Arrays

Regional earthquake data recorded by the Cooperative New Madrid Seismic Zone (NMSZ) network were analyzed to investigate spatial variation of Pn velocity beneath the NMSZ. Shallow regional earthquakes at epicenter distances from 220 km to 1100 km from different directions were selected to ensure a successful identification of Pn arrivals and a proper azimuthal coverage of seismic rays. Regional seismic network stations are sorted spatially into many overlapped small aperture seismic arrays. Apparent azimuth and apparent velocity of the incoming Pn waves for each array can thus be determined from array analysis of the Pn arrivals. The apparent Pn velocity determined for an array depicts the upper mantle Pn velocity for a region outside of the array along the incoming ray direction immediately beneath the earliest arrival station. Using regional events from the north, east, and west, the resultant Pn velocities beneath the NMSZ vary in the ranges from 7.91 ± 0.03km/sec to 8.22 ± 0.03km/sec. These observations are slightly different from the previous studies in the area using long seismic refraction lines by Mooney (1983) and Catchings (1999), which reported a constant Pn velocity of 8.1 km/sec and 8.25 km/sec, respectively, for the same region. In addition, Pn travel time residuals during least square fitting to determine apparent velocity reveal systematic spatial patterns that support further the observed lateral variations of Pn velocities in the region. Using the events from the south, the resultant apparent velocities of the first arrivals are ranging from 6.0 ± 0.05 km/sec to 7.0 ± 0.05 km/sec suggesting that the first arrivals are traveling first through a small section of oceanic crust before entering to mid to lower continental crust. The observed lateral Pn velocity variations will provide important constraints for receiver function structural study and other studies in the area.