Using Low Frequency Earthquakes to Examine Slow Slip Beneath the Olympic Peninsula, WA

Abstract:

Using data from the Array of Arrays (AofA) and Cascadia Arrays for Earthscope (CAFE) experiments we found 61 families of low frequency earthquakes (LFEs) beneath the Olympic Peninsula, Washington. Detections within each family range from Mw1-2. We cross-correlated 6-second long windows within 99 separate hours of tremor data during the 2010 and 2011 Episodic Tremor and Slip (ETS) events. For each hour, we stacked the autocorrelation functions from a set of 7 three-component base stations chosen for their high signal-to-noise ratios. We extracted windows with correlation coefficients higher than 9 times the median absolute deviation. These time windows contain our preliminary LFE detections. We clustered the time windows and produced template waveforms by stacking the waveforms corresponding to a given cluster. By scanning templates through the entire time period when the AofA network was active (June 2009 – September 2010 and August 2011), we identify bursts of LFE activity lasting 2-10 days during times of known tremor [Wech, 2010], including the August 2010 and 2011 ETS events and three tremor swarms in November 2009, March 2010, and July 2010, each of which is assumed to signify a slow-slip event (SSE). During the larger ETS events, we can identify the passage of the main rupture front, rapid tremor reversals and/or dip-parallel streaks. These alternate propagation modes are less obvious during smaller SSEs. We examine the spatial extent of the SSEs by determining which families were active during each event. While all families were active during the 2010 and 2011 ETS events, fewer families were active during the smaller SSEs. The families active during smaller SSEs are located farther down dip. Finally, we are applying double-difference methods to obtain more robust LFE locations, which will shed light on the patchy structure of the plate interface.