MONITORING ACTIVE CRUSTAL FAULTS IN WASHINGTON

The Washington Geological Survey installed portable broadband seismographs to record earthquakes near Maltby/Granite Falls in support of our geologic mapping in areas NE of Seattle region and near the Entiat earthquake cluster area in central Washington. We deployed 14 IRIS-PASSCAL instruments in the mapping areas in July 2015 and 12 instruments in Entiat area in September 2016.

We recorded a 20km deep M3.5 mainshock event on July 1^st, 2015, followed by multiple aftershocks very near our PASSCAL array in Granite Falls. Nearest and farthest stations of the portable array that recorded this cluster of earthquakes were at 1 km and 20 km distances, respectively, whereas the closest regional Pacific Northwest Seismic Network (PNSN) station was at 10 km. We analyzed first motions of these earthquakes, local magnitudes ranging from 3.5 to 2.0, to determine fault mechanisms compared to the PNSN solutions. Results show that some of the fault plane solutions of the M<3 events were significantly improved with the new portable station coverage.

We installed 12 PASSCAL rapid deployment instruments for monitoring the Entiat earthquake cluster area, Chelan County, which is the most seismically active region in Washington. We recorded a 2.1km deep M3.3 mainshock event on July 24^th, 2017, followed by a few aftershocks in the array.

The new PASSCAL instrument deployments help significantly improve focal mechanism solutions of the local and (M<=3) and regional (M>3) events. PNSN focal mechanism solutions are verified and recalculated based on our new additional stations; new focal mechanism solutions are determined. Our additional deployments of stations in the Entiat area made possible to determine new focal mechanisms for small events. These observations combined with regional network recordings can now be used for relocation of events (specifically in earthquake clustered areas), crustal velocity estimation, and additional focal mechanism studies. We will present local crustal earthquakes recorded by the regional and the new portable arrays, and suggest correlations to faults recently identified by geologic mapping and geophysical studies.