Low-Frequency Earthquakes in Cascadia: Results from Array of Arrays

Low-frequency earthquakes (LFEs) are a recently identified class of earthquakes that have been observed to occur coincidentally with tectonic tremor in time and space. These LFEs also have a frequency spectra that is nearly identical to that of tectonic tremor—implying a common source for these two phenomena. As demonstrated by Shelly et al. (2006, 2007, Nature), tremor in Japan can be thought of as a superposition of many individual LFEs. Accordingly, LFEs have been used to constrain the location and focal mechanism of tremor. LFEs have been identified within tectonic tremor at several subduction zones around the world (Brown et al., 2009, GRL), including Cascadia, as well as strike-slip faults like the San Andreas Fault. In 2008 we identified two LFE clusters in Cascadia using a single dense seismic array. These clusters each contained approximately 100 repeating LFEs over the course of about an hour. The clusters were located on the plate interface and individual LFE locations within the clusters varied by no more than ~200m. Here we show the detection of LFEs in northern Cascadia using newly collected data from the Array of Arrays (AOA) experiment. The AOA consists of 8 dense seismic arrays—containing 10 3-component and 10 single-component instruments each—deployed along the northern Olympic Peninsula of western Washington State. These arrays are situated above the portion of the Cascadia subduction zone that sees regular episodic tremor and slip (ETS) events—approximately between the surface projections of the 30-45km depth contours of the subduction interface. The AOA recorded the August 2010 ETS event that began in southern Puget Sound and migrated along strike directly underneath our arrays—and is continuing as we write this abstract. We will use slowness values calculated using a beam-forming technique to slant stack the array data and enhance the tremor/LFE signal to noise. Identified LFEs will be used as templates that can be cross-correlated with stacked waveform data to find repeating events. The distribution of multiple dense arrays should allow us to locate LFEs with a high degree of accuracy. Well-located LFEs that occur within a favorable geometry of the AOA may yield focal mechanisms and estimation of seismic moment.