Comprehensive Analyses of Cascadia Tremor and Slow Slip

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Slow slip and tremor are key processes in tectonic plate motion in subduction zones, one of which is positioned along the Pacific Northwest coast from Cape Mendocino up to Vancouver Island in Canada. For the basic mechanics of subduction zones to be understood, slow slip must be made to yield its secrets. Progress in this puzzle in the past decade has been and will continue in the future to be transformative. To understand these processes, this project is analyzing the copious seismic and geodetic data that has been gathered over the past decade. Different data types and analysis approaches are being integrated to determine more precisely where and when strain is accumulating and the role of slow slip in loading and releasing plate-interface stress.The potential interaction of slow slip with the hazardous locked portions of subduction zones mandates better understanding of slow slip phenomena. For example, the next generation of Pacific Northwest seismic hazard maps will use our estimates of the updip limit of tremor as one way to estimate the downdip limit of megathrust rupture, refining estimates of the shaking across the west coast in the next great American earthquake. In addition, it is possible that the slow slip being observed and interpret may reveal changes in the seismic hazard over time as motion changes pattern, accelerates and decelerates.The investigators are involved in the operation of the regional seismic network and are active in various forms of outreach to educate the public about seismic hazard. Episodic tremor and slip events is providing a golden opportunity to advance public awareness of the regional geological environment and seismic hazard through blogs, TV and newspaper interviews and NOVA documentaries. The University of Washington has captured seven episodes of slow slip under northern Washington since 2004 with seismic arrays two with the EarthScope Array of (eight) Arrays, accumulating a rich dataset.Several approaches are being integrated to sift through the mountain of continuous recordings and our growing catalogs in search of key relationships. The foci in this work is (1) delineation of the space-time progression of tremor by both beam backprojection and envelope correlation methods, (2) precise tracking of slow slip with LFE locations and recurrence, (3) systematic determination of tremor amplitude and its relation to tidal stressing and amount of slip, (4) integration of GPS and strainmeter timeseries with spatio-temporal constraints from tremor, and (5) comparison of emerging ivory-tower slow-slip models with observation. These approaches allow refinement of the relation of tremor propagation patterns such as rapid tremor reversals and streaks to slip front propagation and low frequency earthquake generation, and thus yield constraints on the slip velocities, propagation velocities, and rise times of slow slip pulses. As well, this study is teasing out how much slow slip extends updip from tremor, with implications for seismic hazard, and to assess the importance of tremor amplitude in scaling tremor duration to slow slip moment release.Currently in hand are growing caches of several types of geophysical data: multiple catalogs of tremor ranging from Cascadia-wide with 210,000 tremor locations to high-resolution locations collected by the Array of Arrays, as well as a burgeoning catalog of low-frequency earthquakes. These provide views from the 1000 km to 100 m scales. In addition, GPS, strain, and tilt-meter time series record deformation more directly. Linking the above datasets is leading to characterization of the inferred propagation of slow slip fronts over a range of spatial and temporal scales.