Spatial and Temporal Relationships between Tremor and Slip in 2010 Cascadia ETS

Abstract:

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 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. As shown by Houston (AGU abstract, 2012) and Hall and Houston (AGU abstract, 2014), the slip inferred from GPS data extended updip of the seismically-detected tremor in the 2010 M6.8 ETS event, as well as in other events. While tremor appears to have a clear updip limit, the updip extent of slow slip is less clear. This also raises questions about spatial variations of the ratio of tremor activity to slip on the fault. We further explore the details of the 2010 event, using the Extended Network Inversion Filter (ENIF) (Segall and Matthews 1997) and tremor locations from the PNSN. Our preliminary results yield a M6.8 event that begins near Seattle on August 8th and propagates mainly to the north with some smaller slip to the south, following the propagation of the tremor. This is consistent with our static inversion results. The initial tremor begins at a depth of ~45 km and migrates up-dip before propagating along strike. There is no resolvable slip from GPS until the tremor has migrated up-dip. 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. We also find that GPS stations closer to the initiation of the ETS show higher deformation rates than the stations near the end. This suggests a clear progression of slip throughout the event from the initiation to propagation. Similarly, it has been suggested by Ulberg and Creager (AGU Abstract, 2013) that ETSs start with an initiation phase during which tremor amplitudes increase linearly and then a propagation phase where amplitude is highly variable. We plan to explore how this variation in amplitude corresponds to variations in slip. Our goal is to determine how closely tremor activity mimics slip activity, and understand where and why they may manifest differently.