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MAPPING THE DETAILS OF EPISODIC, NON-VOLCANIC TREMOR
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The maps show different elements of spatio-temporal tremor distribution, positioned along the logarithm time scale to illustrate the typical duration of each element. the arrow in each map indicates slip direction of the Cascadia Subduction Zone and the black solid square marks the Big Skidder array. (a) Circles are tremor locations, colored to show rapid migration of slip-parallel tremor streaks. (b) Circles are tremor locations, colored to show the slip-parallel bands that migrate along strike over several hours. (Faint yellow locations fall outside the tremor bands.) (c) Relative band-limited tremor moment patches that release much of the seismic moment during an ETS event. (Figure courtesy of Abhijit Ghosh, University of Washington)
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Studies of non-volcanic seismic tremor are offering new insights into fault mechanics and are leading to the development of new approaches for deploying seismic and geodetic stations, and data processing. Techniques for computing the tremor source region have evolved from double-difference methods using relative arrival times based on cross correlation of waveform envelopes, to beam back projection from dense, small-aperture seismic arrays. Beam back projection is much more effective at detecting coherent tremor, greatly increases resolution in relative tremor location, and can track migration of a tremor source from minute to minute. The technique was used to discover that tremor sources can migrate continuously for several minutes parallel to the dip direction of the Cascadia interplate thrust at a speed of ~50 km/hr, form bands of sources that sweep along strike at a speed of ~10 km/day for several hours, and develop distinct moment patches that overlap with geodetic slip patches on the interface. These varied and intriguing observations challenge earth scientists to develop a unified view of tremor.</p>

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