Continental Tectonics from Dense Array Seismic Imaging: Intraplate Seismicity in Virginia and a Steep Cratonic Margin in Idaho

Dense array seismic techniques can be applied to multiple types of seismic data to understand regional tectonic processes via analysis of crustal velocity structure, imaging reflection surfaces, and calculating high-resolution hypocenter locations. The two regions presented here include an intraplate seismogenic fault zone in Virginia and a steep cratonic margin in eastern Oregon and Idaho.

The intraplate seismicity study in Virginia consisted of using 201 short-period vertical-component seismographs, which recorded events as low as magnitude -2 during a period of 12 days. Dense array analysis revealed almost no variation in the seismic velocity within the hypocentral zone, indicating that the aftershock zone is confined to a single crystalline-rock terrane. The 1-2 km wide cloud of hypocenters is characterized by a 29° strike and 53° dip consistent with the focal mechanism of the main shock. A 5° bend along strike and a shallower dip angle below 6 km points toward a more complex concave shaped fault zone.

The seismic study in Idaho and Oregon was centered on the inversion of controlled-source wide-angle reflection and refraction seismic P- and S-wave traveltimes to determine a seismic velocity model of the crust beneath this part of the U.S. Cordillera. We imaged a narrow, steep velocity boundary within the crust that juxtaposes the Blue Mountains accreted terranes and the North American craton at the western Idaho shear zone. We found a 7 km offset in Moho depth, separating crust with different seismic velocities and Poisson's ratios. The crust beneath the Blue Mountains terranes is consistent with an intermediate lithology dominated by diorite. In the lower crust there is evidence of magmatic underplating which is consistent with the location of the feeder system of the Columbia River Basalts. The cratonic crust east of the WISZ is thicker and characterized by a felsic composition dominated by granite through most of the crust, with an intermediate composition layer in the lower crust. This sharp lithologic and rheologic boundary strongly influenced subsequent deformation and magmatic events in the region.