Spatial and temporal relationships between tremor and slip in large slow slip earthquakes on the Cascadia subduction zone

Heidi Houston, Kelley A. Hall, & David A. Schmidt

Submitted August 15, 2016, SCEC Contribution #7022, 2016 SCEC Annual Meeting Poster #259

Slow slip and tremor migrate in tandem along several subduction zones at about 8 km/day in large Episodic Tremor and Slip (ETS) events. These events occur downdip of the locked megathrust, and thus the updip limits of slow slip or tremor provide potential constraints on the slip budget of subduction zones and on the downdip edge of the locked zone, which inform hazard assessments for major cities including Seattle and Tacoma. Inversions of GPS displacements suggest that slow slip extended about 15 km updip of the seismically-detected tremor in the 2010 M6.8 and 2012 M6.6 ETS events (Houston, AGU abstract, 2012; Hall and Houston, AGU abstract, 2014). In inversions where slip is restricted to regions with tremor, the inversion forces large and unphysical slip at the updip edge of tremor, well over that which could accumulate during an interETS period. This also suggests that slip extends updip of tremor, implying spatial variations of the ratio of tremor activity to slip on the fault. While tremor appears to have a clear updip limit, the updip extent of slow slip is less clear. We explore the time evolution of slip during the 5-week-long 2010 event, using the Extended Network Inversion Filter and tremor locations from the PNSN. Our preliminary results yield a M6.8 event that begins near Seattle and propagates mainly to the north with some smaller slip to the south, following the propagation of the tremor. The size and the larger slip to the north are consistent with our static inversion results, as well as with the tremor density. The slip pulse nature of the ETS process is clearly imaged, with regions continuing to slip for several days after tremor has passed through, but not for the entire duration of the event. High concentrations of tremor propagate along with the highest slip rates, although some tremor can occur before the leading edge. The initial tremor begins at a depth of ~45 km and migrates up-dip before propagating along strike. The GPS data are not able resolve slip until the tremor has migrated up-dip. We also find that GPS stations closer to the initiation region of the ETS show higher deformation rates than the stations near the termination. We explore how variations in tremor amplitude correspond to variations in slip (amplitudes from C. Ulberg and K. Creager). Our goal is to determine how closely tremor activity mimics slip activity, and understand where and why they may manifest differently.

Key Words
slow slip, GPS, tremor

Houston, H., Hall, K. A., & Schmidt, D. A. (2016, 08). Spatial and temporal relationships between tremor and slip in large slow slip earthquakes on the Cascadia subduction zone. Poster Presentation at 2016 SCEC Annual Meeting.

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