New Madrid Seismicity, Gravity Anomalies, and Interpreted Ancient Rift Structures

Abstract

Seismicity data for the New Madrid seismic zone recorded by a regional seismograph network since 1974 delineate prominent epicentral trends associated with the Reelfoot rift in northeastern Arkansas, northwestern Tennessee, and southeastern Missouri. Additional epicentral trends and more diffuse clusters of epicenters have also become visible during the 20-plus years of microearthquake monitoring. Several of these trends appear to be spatially correlated with geophysical anomalies, particularly gravity anomaly data. The geophysical anomalies are interpreted to be caused primarily by crustal structure variations, some of which are related to ancient rifting. The epicentral patterns and associated gravity anomalies extend considerably beyond the interpreted Reelfoot rift structure, particularly to the north. A substantial amount of new data, particularly the precise epicenter information, and recent geophysical studies have significantly improved our understanding of the causes of earthquakes in the Reelfoot segment of the New Madrid seismic zone. However, the geographical extent of the seismic zone, detailed information on the structure of the crust in the seismically active areas, and a refined seismotectonic model for the New Madrid area remain to be determined.