GRAVITY ANALYSIS OF THE NATURE OF EARTHQUAKE ACTIVITY IN NORTHERN SPOKANE, WASHINGTON

A series of earthquakes occurred in northwest Spokane, Washington in 2001 with a series of shallow (< 2.0 km), small magnitude (< 4.0) events that first occurred along a northwest-trending line parallel to the Spokane River and then east of this area. An InSar study 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. Subsequent analysis of an airborne magnetic survey and a two-dimensional seismic refraction data also implied a thrust fault occurs in this area. The source region is covered by the Columbia River plateau basalts and the Pliocene Latah Formation, and occurs at the margin between the CRRP, Cretaceous intrusions and the metamorphosed Precambrian Belt Group and the lack of outcrops in the seismicity region have made determining the source of the events difficult. Geological studies have suggested that the NW-trending events may have been related to the 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 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 NW-trending events and gravity minimum in the region of the InSAR determined uplift. The main loci of NW-trending 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.