Subsurface imaging across the 2001 Spokane, Washington earthquake swarm

We acquired 4 km of minivibe reflection seismic data in Spokane, Washington, to image subsurface deformation associated with the 2001 swarm of shallow (<2 km) earthquakes. These data were collected by USGS as part of ongoing earthquake hazards investigations in the area. In 2001 unexplained earthquake ground shaking as well as audible "booms" were reported over a span of six months (June to November) in the Emerson-Garfield and West Central neighborhoods of Spokane.; the area has since been seismically quiescent. Seismograph recordings of the earthquake swarm suggest shallow depths of hypocenters, yet the local subsurface geology is not well known. Although the source region of this swarm is poorly constrained within Spokane due to sparse seismic station coverage in the area at that time, recent InSAR data analysis has revealed a zone of surface deformation that may be related to the earthquake swarm. This surface deformation consists of an elliptical area about 3 km across that had as much as 15 mm of uplift during 2001. Preliminary processing of the two new seismic profiles provides the first subsurface images of the upper 500 m within the Spokane area across the inferred source region. One seismic profile through downtown Spokane shows a three-layer structure of Holocene valley fill and Quaternary Lake Missoula flood deposits underlain by Tertiary Columbia River basalts. We observe a Columbia River basalt bedrock high of 100 m located between seismic profiles and verified by geologic and aeromagnetic maps. The seismic data also image a paleochannel showing the migration of the Spokane River through time. An inflection within the Quaternary basin sediment reflections suggests uplift from faulting that is consistent with the sense of deformation observed in the InSAR data.