Imaging the Active Faults of the Central New Madrid Seismic Zone Using Panda Array Data

More than 700 earthquakes have been located in the central New Madrid seismic zone during a two-year deployment of the PANDA array. Magnitudes range from < 0.0 to the mblg 4.6 Risco, Missouri earthquake of 4 May 1991. The entire data set is digital, three-component and on-scale. These data were inverted to obtain a new shallow crustal velocity model of the upper Mississippi embayment for both P- and S-waves. Initially, inversion convergence was hindered by extreme velocity contrasts between the soft, low-velocity surficial alluvial sediments and the underlying Paleozoic carbonate and clastic high-velocity rock. However, constraints from extensive well log data for the embayment, secondary phases (Sp and Ps), and abundant, high-quality shear-wave data have yielded a relatively robust inversion. This in turn has led to a hypocentral data set of unprecedented quality for the central New Madrid seismic zone. Contrary to previous studies that utilized more restricted data, the PANDA data clearly delineate planar concentrations of hypocenters that compel an interpretation as active faults. Our results corroborate the vertical (strike-slip) faulting of the the southwest (axial), north-northeast, and western arms and define two new dipping planes in the central segment. The seismicity of the left-step zone between the NE-trending vertical segments is concentrated about a plane that dips at ∼31°SW; a separate zone to the SE of the axial zone defines a plane that dips at ∼48°SW. The reason for this difference in dip, possibly defining segmentation of an active fault, is not dear. When these planes are projected up dip, they intersect the surface along the eastern boundary of the Lake County uplift (LCU) and the western portion of Reelfoot Lake. If these SW-dipping planes are thrust faults, then the LCU would be on the upthrown hanging wall and Reelfoot Lake on the downthrown footwall. If in turn these inferred thrust faults were involved in the 1811–12 and/or pre-1811 large earthquakes, they provide an internally consistent explanation for (1) the existence and location of the LCU, (2) the wide-to-the-north, narrow-to-the-south shape of the LCU, and (3) the subsidence and/or impoundment of Reelfoot Lake.