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Poster #134, Fault and Rupture Mechanics (FARM)

Are creep events big? Estimating their rupture extents.

Dan B. Gittins, & Jessica C. Hawthorne
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Poster Presentation

2021 SCEC Annual Meeting, Poster #134, SCEC Contribution #11225 VIEW PDF
Fault creep is observed along many faults worldwide (Titus et al., 2011). Aseismic slip does not always accumulate at a steady background rate; instead, it accumulates in bursts known as creep events (Gladwin et al., 1994). Despite observations of creep events dating back to the mid-1900s, the size and importance of creep events remain unclear. Some researchers treat them as short near-surface events (Gladwin et al., 1994); however, others suggest that creep events may reach 4km depth and connect the surface to the seismogenic zone (Bilham et al., 2016). Here we present ongoing work aiming to determine the typical rupture extent and depth of creep events along the central San Andreas Fault.<...br /> First, we detected creep events and determined their along-strike rupture extent using 18 USGS creepmeter records, each operating for at least nine years 1985 and 2020. We systematically detect creep events using a cross-correlation approach. We identified periods with a significant slip and signals with high similarity to a template creep event. This automated detection allowed us to produce a catalog of creep events that contains over 2000 creep events. Using this creep event catalog, we are able to determine the along strike length of creep events.
Furthermore, by comparing event detections between creepmeters, we are able to calculate the percentage of creep events that rupture more than one creepmeter along each segment of the fault. We have observed five different types of creep events: isolated events, small (<2-km) events recorded at two creepmeters, 5-km events recorded at multiple creepmeters, possible long (10 to 30-km) propagating events, and multi-strand ruptures. We have compared the timing of these behaviors to rainfall and have determined that rainfall does not significantly affect the majority of creep event behavior observed. Our results suggest that many creep events are large, kilometers long segment rupturing events.
In order to understand the true importance of creep events to the slip dynamics of the creeping section, along-strike rupture extent is not enough; rather, the depth at which they occur may even be more important. In order to determine the depth extent of creep events, we are currently working with strainmeter data from 3 USGS strainmeters and 6 PBO strainmeters located along the creeping section of the San Andreas Fault.