GRAVITY ANALYSIS OF THE SPOKANE, WASHINGTON EARTHQUAKE ZONE

Spokane, Washington is not known for earthquakes but starting in 2001 with a series of shallow (< 2.0 km), small magnitude (< 4.0) events that occurred along a northwest-trending line parallel to the Spokane River and then additional events occurred east of this area. The first geophysical study was an InSar investigation that indicated a region of uplift just north of the Spokane River near Spokane’s downtown region. Modeling of the uplifted region suggested that a portion of the earthquakes occurred along a thrust fault. Later, two-dimensional seismic refraction and magnetic models also implied a shallow thrust fault may be the source of the earthquakes. Problems determining the exact source, is that the source region is covered by the Columbia River plateau basalts (CRPB) and the Pliocene Latah Formation. The actual geology of the source region is more complicated as it occurs at the margin between the CRPB, Cretaceous granitic intrusions and the Precambrian Belt Group which has been subjected to high grade metamorphism. Geological studies have suggested that the NW-trending events may have been related to relatively recent faulting in the area especially along the Hangman River valley. In order to determine the location of subsurface features possibly related to the earthquakes, a gravity survey was undertaken where over five hundred gravity stations have been collected and were merged with the sparse existing data. The merged data were processed and contoured to produce a complete Bouguer gravity anomaly map. This map indicates several short wavelength residual gravity anomalies in the vicinity of the NW-trending events and gravity minimum in the region where the InSAR analysis determined an uplift. The main loci of NW-trending epicenters are located on a short wavelength gravity minimum which may be associated with subsurface a granitic intrusion or thicker sediment within the Latah Formation. Analysis of the data using wavelength filtering and derivative methods indicates that the gravity minimum is located at the southern end of a more regional northwest-trending gravity minimum, which may be the source of increased strain in the region. Three-dimensional modeling will be performed to determine the density variations of the subsurface and will be related to earthquake epicenters