Crustal studies, velocity inversions, and fault tectonics: Results from a microearthquake survey in the New Madrid Seismic Zone

Abstract

A dense network of portable seismographs was operated in the Ridgely area of western Tennessee in May of 1978. During 32 days of operation, 122 earthquakes were detected, of which 90 were located. The high quality P and S arrivals recorded exhibit travel time ratios (ts/tp) that decrease significantly with depth. This decrease is most rapid in the upper few kilometers and necessarily reflects similar changes in the velocity ratio (Vp/Vs). Based on laboratory and theoretical studies, such observations are consistent with a decrease in crack density with increasing confining pressure. A special study of Vp, Vs, and Vp/Vs revealed strong variations with depth that also correlate with rock type. Inversion of the data for both P and S velocity structures confirms the presence of a low‐velocity zone in the depth range 2 to 5 km; a high‐velocity lid, with high Vp/Vs and thus, high crack density; and a boundary at the near surface for S to P conversion, associated with the unconsolidated saturated sediments of the Mississippi embayment. Relocated hypocenters using independent velocity structures for P and S are systematically deeper and define a seismogenic layer between 4 and 13 km depth. The earthquakes near Ridgely show that most of the activity is occurring on a northwest striking reverse fault that dips to the southwest. Minor off‐fault activity within the hanging wall was also observed representing motion on a series of small normal faults dipping to the northeast. Other composite focal mechanisms were determined for nearby regions. These solutions are all in general agreement with a horizontal compressive stress field striking northeast to east, favoring right‐lateral strike‐slip motion on left‐stepping northeast trending faults, or reverse motion on faults trending northwest and north. These results are consistent with the stress field found in a large area interior to the United States, and imply a rotation of the regional stress field since the late Cretaceous when extensional faulting occurred in the area.