Investigate the Spatiotemporal Relationship Between Slow Slip Transients and Tremor in Cascadia Subduction Zone

Abstract:

Episodic tremor and slow slip events (SSE) have been observed in many subduction zones such as southwest Japan and Cascadia. Despite their relatively well-observed nature, the physical mechanism that underlies slow slip and tremor remains unknown. Tremor has been thought either to be loaded directly by aseismic slip, or to represent the different manifestations of the same physical process as slow slip, or be modulated by the stressing and aseismic slip rate. In Cascadia, slow slip without tremor was observed only recently [Welch and Bartlow, 2014], suggesting a more complex relationship between episodic slip and tremor. We use the combined daily position time series from a NASA MEaSUREs project and a time-dependent Kalman filter to examine the spatiotemporal relationship between slow slip and tremor in Cascadia. The combined GPS position solutions are based on JPL and SIO independent solutions, using the GIPSY and GAMIT software, respectively, but with a consistent set of a priori epoch-date coordinates and metadata. We remove the outliers and common mode errors from the time series. We visually identify all transient slip events and fit the time series with a model that consists of inter-SSE velocity, offsets, annual and semi-annual variation, and slow slip transients. A grid-search is applied to estimate optimal duration and center time during the transient fit. The cleaned filtered time series with the inter-SSE velocity removed are then used in the slip inversion. Our application to some recent SSEs shows a spectrum of different slip and tremor behaviors. For example, despite good space-time correlation between high fault slip rate and tremor locations, we see the instances where the tremors locate at the downdip slip edge or slip front, suggesting possible stressing from the slip transient. We find that during the 2014 ETS event a period of high fault slip rate and tremor is followed by a short period of tremorless slip, indicating tremorless slip in Cascadia is more common than previously thought.