Stacking GPS observations on ETS and inter-ETS tremor times to improve geodetic observations of slip in Cascadia
Abstract
Observations of Episodic Tremor and Slip (ETS) events in Cascadia have been leveraged by researchers attempting to understand the nature of locking along the megathrust. Results from such studies have been used to inform seismic hazard discussions in Cascadia by providing constraints on the down-dip extent of seismogenic locking, and have provided insight into the physics of fault rupture styles throughout the transition zone between fully locked and stable sliding. Many key observations are poorly resolved: (1) Geodetic inversions for fault slip during ETS events place slip updip of tremor locations. The nature of this updip offset is important to understand, as this is the region immediately adjacent to the locked portion of the megathrust. The vertical component of geodetic observations is the most sensitive to the depth distribution along thrust faults (the location of the vertical hinge line, for example, is very sensitive to the up-dip extent of slip), but is not widely utilized for ETS events here because of significantly higher signal to noise ratios. (2) The nature of slip during inter-ETS events is poorly resolved. Currently, the only observations of slip during inter-ETS events comes from strainmeter data. We will utilize tremor time histories from a variety of seismic methods to quantify the timing of ETS and inter-ETS tremor episodes, then stack all available GPS observations of these respective events (for each station individually, not cross-station stacking) to produce geodetic observations of slow slip which have significantly higher signal to noise ratios. We have found that by stacking even a modest number of inter-ETS events ( 10), we are able to resolve surface displacements of 0.5 mm. For ETS events, GPS stacking produces significantly better vertical observations and observations from stations up to 200km away from the slipping patch. For the smaller inter-ETS events, which previously have only been observed geodetically in strainmeter time series, GPS stacking produces observations robust enough to invert for slip on the interface. We find that slip on the plate interface during inter-ETS events is an order of magnitude smaller and resolvably down-dip of slip during ETS events, with slip during inter-ETS events peaking near 40-50 km depth, while slip during ETS events peaks near 35 km depth.
- Publication:
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AGU Fall Meeting Abstracts
- Pub Date:
- December 2017
- Bibcode:
- 2017AGUFM.S41C0817H
- Keywords:
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- 7209 Earthquake dynamics;
- SEISMOLOGY;
- 7230 Seismicity and tectonics;
- SEISMOLOGY;
- 8118 Dynamics and mechanics of faulting;
- TECTONOPHYSICS;
- 8163 Rheology and friction of fault zones;
- TECTONOPHYSICS