INTEGRATION OF HIGH-RESOLUTION SEISMIC REFLECTION AND MICRO-GRAVITY TECHNIQUES TO IMPROVE INTERPRETATION OF SHALLOW SUBSURFACE STRUCTURE: NEW MADRID SEISMIC ZONE

Shallow high-resolution seismic reflection surveys have traditionally been restricted to either compressional (P) or horizontally polarized shear (SH) waves in order to produce 2-D images of subsurface structure. The northernmost Mississippi embayment and coincident New Madrid seismic zone (NMSZ) provide an ideal laboratory to study the experimental use of integrating P- and SH-wave seismic profiles. In this area, the relation between “deeper” deformation of Paleozoic bedrock associated with the formation of the Reelfoot rift and NMSZ seismicity and “shallower” deformation of overlying sediments has remained elusive but could be revealed using integrated P- and SH-wave reflection. Surface expressions of deformation are almost non-existent in this region, which makes seismic reflection the only means of detecting structures that are possibly pertinent to seismic hazard assessment. Since P- and SH-waves respond differently to the rock and fluid properties and travel at dissimilar speeds, the resulting seismic profiles provide complementary views of the subsurface based on different levels of resolution and imaging capability. P-wave profiles acquired in southwestern Illinois and western Kentucky (USA) detect faulting of deep, Paleozoic bedrock and Cretaceous reflectors while coincident SH-wave surveys show that this deformation propagates higher into overlying Tertiary and Quaternary strata. As a test, we also acquired high-resolution micro-gravity (accuracy to 5 microGal; stations spaced 25 m) along one of the seismic P- and SH-wave profiles, in the Ohio River bottoms where topographic variations are minimal. The purpose of the micro-gravity profile was to determine if faults observed from the seismic profiles were mappable as lateral density contrasts. The results of forward modeling indicate that the larger and deeper faults are expressed in the gravity signature, while the expression of the shallowest faults is debatable. The integration of the two seismic and the micro-gravity methods provides a more powerful strategy for shallow fault detection and mapping. Combining geophysical methods therefore increases the scope for investigating the relation between the older and younger deformation in an area of critical seismic hazard.