Unlocking the Inner Life of Cascadia Tremor with an Array of Arrays

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In the last 10 years, a strange way for faults to slip has been observed. Rather than regular earthquake fault slip in a matter of seconds or steady deformation, both of which have been known to occur for decades, large parts of deep faults move unsteadily over the course of days or weeks, then stop moving for months to years before lurching in slow motion again.This new mode of sporadic coupled deformation and weak seismic radiation is termed Episodic Tremor and Slip (ETS) has been discovered in several subduction zones. It recurs episodically down-dip from the locked portion of the Cascadia megathrust about every 14 months. The close relationship and likely interaction of the ETS zone with the locked and hazardous portions of subduction zones mandate better understanding of the ETS phenomena.The Array of Arrays project is a comprehensive study of many of the key questions surrounding ETS utilizing a temporary seismic deployment designed to image Cascadia Subduction Zone tremor in unprecedented detail. Specifically, a set of arrays are being deployed on the Olympic Peninsula to record the ETS events occurring during Summer 2009 and Fall 2010, comprising one 40-station array, six 20-station arrays, and a line of stations connecting two of the arrays. There is also a backbone set of 15 short-period and 7 broadband continuously recording seismometers.The Array of arrays is unveiling the spatio-temporal details of the tremor location and depth, its migration, its spectral content at high frequencies, as well as better resolution of tremor and low-frequency earthquake source character. Analysis of the data from this unique array will answer many key questions about tremor and further elucidate the physics of the ETS process, and provide answers to vexing issues of Cascadian earthquake hazard mitigation.