Searching for earthquake swarms and aseismic deformation in the Western U.S

Earthquake swarms often occur during periods of transient aseismic deformation and can be a sensitive indicator of the time periods when processes such as slow fault slip, magma intrusion, and hydrothermal circulation are happening. Recent studies have shown that these types of swarms can be identified as anomalies in earthquake catalogs relative to the Epidemic Type Aftershock Sequence (ETAS) stochastic seismicity model. The parameters of the ETAS model are optimized to remove the clustering in an earthquake catalog that results from normal aftershock activity as well as the overall background rate of seismicity. Often, the ETAS model can explain the seismicity-rate variations in a given region over a period of decades except for the swarms that are associated with known aseismic deformation events. We are systematically searching for these types of anomalies in earthquake catalogs that sample the region covered by the Plate Boundary Observatory in hopes of identifying more swarms triggered by aseismic deformation. Here we present results from catalogs derived from the Advanced National Seismic System (ANSS) covering two separate regions in the western continental United States. We studied the Yellowstone region (44 N to 45 N and 110 W to 111.05 W) over the period 23 June 1999 to 21 July 2010. The parameters of the ETAS model were optimized from 23 June 1999 to 18 April 2000. The largest anomalies relative to the ETAS prediction were (1) a cluster of 143 earthquakes with moment magnitudes (M) ranging from 0.1 to 2.6 beginning 31 July 2008 and ending 20 August 2008, and (2) a sequence of 28 earthquakes with 0.2 ≤ M ≤ 2.9 that began on 26 June 2010 and ended on 7 July 2010. We also studied the Pacific Northwest region, specifically Washington and Oregon, (117 W to 125 W and 42 N to 49 N) over the period 1 January 2000 to 29 July 2010. During that time period, the ANSS catalog contains approximately 16,500 events, with ten events of M ≥ 4.5, including one M = 6.8 on 28 February 2001. ETAS parameters were optimized from 1 Jan 2000 to 1 February 2004. The largest anomaly relative to ETAS was the Lakeview, Oregon, sequence of events which spanned June - July 2004 and included several earthquakes of 4.0 ≤ M ≤ 5.0.