ANALYSIS OF GRAVITY DATA WITHIN THE SPOKANE SEISMIC ZONE, EASTERN WASHINGTON

The Spokane seismic zone (SSZ) in eastern Washington is a relatively new geologic feature that first came to prominence in June, 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. The lack of detailed geophysical, geological and drillhole data and the fact that the source region is covered by the Columbia River plateau basalts (CRPB) and the Pliocene Latah Formation, and occurs at the margin between the CRRP, Cretaceous intrusions and the metamorphosed Precambrian Belt Group have made determining the source of the events difficult. Preliminary geological studies have suggested that there has been 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 preliminary gravity survey was undertaken where over five hundred gravity stations were collected at existing elevation points and merged with the sparse existing data. The merged data were processed and contoured to produce a Bouguer gravity anomaly map. This map indicates several short wavelength anomalies in the vicinity of the SSZ. The main loci of epicenters is 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 enhanced horizontal gradients 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.