GRAVITY ANALYSIS OF THE SPOKANE SEISMIC ZONE, EASTERN WASHINGTON

The Spokane seismic zone (SSZ) in eastern Washington is a 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 drill data and the fact that the source region is covered by the Columbia River plateau basalts and the Pliocene Latah Formation have made determining the source of the events impossible. Preliminary geological studies provide sketchy evidence that there has been relatively recent faulting in the area. However, to determine the exact structural features of the SSZ, additional studies including geophysics, drilling and trenching are needed. In order to determine subsurface features, a preliminary gravity survey was undertaken. Four hundred gravity stations were collected 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 anomalies are basically trending northwest but in the vicinity of the majority of the events a small amplitude gravity minimum cuts north-south across this trend. This may indicate a structural discontinuity that is responsible for the seismic events. Analysis of residual and edge enhancement anomaly maps indicate this north-south trending anomaly extends north of the known SSZ and may be a loci for future tremors.