Tremor Depth Using Array of Arrays in Cascadia
GHOSH, A., University of Washington, Seattle, WA, firstname.lastname@example.org; VIDALE, J. E., University of Washington, Seattle, WA, email@example.com; CREAGER, K. C., University of Washington, Seattle, WA, firstname.lastname@example.org
We installed 8 small aperture seismic arrays in northern Washington to capture the intimate details of tremor activity in Cascadia. The Array of Arrays (AofA) focuses on the tremor-active megathrust in this region, including the area we previously imaged with a solo array in 2008 [Ghosh et al., GRL, 2009, 2010; Ghosh et al., G-cubed, 2010]. Since it became operational in June 2009, the AofA recorded several tremor episodes, including the recent Episodic Tremor and Slip (ETS) event in August 2010. We have developed a Multi-Beam Backprojection (MBBP) technique to image tremor in high resolution using data from multiple seismic arrays. We apply a beamforming technique at each array to stack the seismic energy at every 0.2 Hz from 2 to 20 Hz using 1-minute sliding time window. During tremor episodes, the arrays show stable slowness, and azimuth over the tremor frequency band (generally 3-8 Hz). We get the best slownesses from all the available arrays, and backproject to find the tremor source location. While P-wave energy stacks coherently up to 10 Hz and higher, S-wave extends only up to 6 Hz or so. The MBBP method is applied to the AofA data to detect and locate tremor. Initial results indicate that the MBBP technique is able to constrain tremor depth reasonably well. The majority of the tremor under the arrays is located near the plate interface, and aligns parallel to the dip of the interface. We are currently analyzing the entire ~15 months of data, which include the latest ETS event. The MBBP method reveals significantly longer duration of tremor activity, including the episodes that remain completely undetected by a conventional envelope cross-correlation method. Exploratory perusal of array stacks indicates a complex pattern of tremor activity. Oftentimes, multiple lively tremor sources appear to be active simultaneously. The AofA is providing a high-resolution probe of seismic activity during both ETS, and inter-ETS time period.