ALBERT

Description: The New Madrid seismic zone is one of the most seismically active regions of eastern North America. It is a small region, distant from any plate margin, and surrounded by the relatively uniform crust of central North America. For these reasons it provides an unusual opportunity to pursue earthquake prediction research in an intra-plate environment. A regional telemetered micro-earthquake network in operation since July, 1974, has yielded much new information on the nature of the seismicity in the New Madrid region. The spatial distribution of hypocenters has indicated the existence of linear seismically active zones, presumably corresponding to active faults. Teleseismic P-wave residuals recorded by the network have yielded a threedimensional velocity model for the crust and upper mantle beneath the New Madrid region, using an inversion method developed by Aki et al. (1977). The most interesting feature of this model is an anomalous region of low seismic velocities in the upper mantle and crust beneath the area of high seismic activity. The anomalous portion of the upper mantle extends to depths of at least 150 km and is characterized by velocities which are about 5% lower than those in surrounding regions. Observations of possible velocity precursors in the New Madrid region have been inconclusive. Other studies of precursory phenomena are currently being implemented. These include the installation of a network of tiltmeters, the instrumentation of a lake for tilt studies, and the observation of water level fluctuations in wells. © 1977, The Seismological Society of Japan, The Volcanological Society of Japan, The Geodetic Society of Japan. All rights reserved.