Repeating earthquakes detected by a new fast method reveal complex creep behavior in the northern San Francisco Bay Area

Nader Shakibay Senobari, & Gareth J. Funning

Submitted August 15, 2017, SCEC Contribution #7837, 2017 SCEC Annual Meeting Poster #069

Repeating earthquakes (REs) are families of seismic events generated by repeated loading and failure of a single fault asperity. They produce similar waveforms at common seismic stations, that can be identified by similarity search. Sequences of REs are commonly interpreted as slip on small locked patches surrounded by large areas of fault that are creeping (Nadeau and McEvilly, 1999). Successfully detected REs, therefore, are a signature of creep at depth, information that is typically a challenge to obtain using geodetic measurements/models.

Geodetic observations show that several faults in the northern San Francisco Bay Area (North Bay) creep, however the distribution of observations, such as alinement arrays and coherent InSAR pixels, is sparse (McFarland et al., 2013, Jin and Funning., 2017). As a result, the lateral and depth extents of creep on the faults of this region, such as the Rodgers Creek, Maacama, and Bartlett Springs faults, are uncertain. Here we use a new fast method to search for REs in this region. Unlike typical RE searching studies that target a small area using a few stations, in this study we include all event waveform data from all seismic stations with operation times of more than 10 years (i.e. 531 stations in total). In total, we process more than 600,000 archived waveforms from 43,000 events.

We successfully detect 39 periodic and 84 non-/semi-periodic RE families. In addition, we find 119 pairs of identical events with recurrence intervals more than 3 years. Our results are strongly in agreement with previously detected creep on the Rodgers Creek, Maacama and Bartlett Spring faults. The RE locations, however, illuminate the depth of creep on these faults, and also suggest the presence of creep in areas where it was not previously observed at the surface. We can also identify structural complexity from the REs on the Maacama fault, both in the stepover region with the Rodgers Creek fault, and in the northern segment of the fault close to Willits, potentially indicating parallel and/or down-dip branching creeping structures in both locations. Shallow depth of some REs on the Bartlett Springs fault near Lake Pillsbury and also on the Maacama fault near Cloverdale can allow us to test and/or recalibrate the empirical relationship between moment magnitude and fault slip for REs introduced by Nadeau and Johnson (1998).

Key Words
Repeating earthquakes, Creep, Similarity search

Shakibay Senobari, N., & Funning, G. J. (2017, 08). Repeating earthquakes detected by a new fast method reveal complex creep behavior in the northern San Francisco Bay Area. Poster Presentation at 2017 SCEC Annual Meeting.

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