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Group A, Poster #139, Fault and Rupture Mechanics (FARM)

Testing rheological models for creep events using their temporal evolution

Daniel B. Gittins, & Jessica C. Hawthorne
Poster Image: 

Poster Presentation

2022 SCEC Annual Meeting, Poster #139, SCEC Contribution #12208 VIEW PDF
Along the central creeping section of the San Andreas Fault, aseismic slip does not occur at a steady rate. Instead, bursts of slip known as creep events regularly punctuate a steady background slip rate and can accommodate upwards of 50% of the total slip observed at the surface (Gittins & Hawthorne, 2022). Creep events typically have a few mm of slip, last a few days to weeks, and extend a few kilometers along strike. However, despite observations of creep events using creepmeters dating back to the mid-1900s, there is still no consensus about their driving mechanism. A variety of physical models proposed have been proposed, including viscous or quasi-plastic rheologies (e.g., Crough &...amp; Burford, 1977; Wesson, 1988), two-layer kinematic models (e.g., Bilham & Behr, 1992), and conditionally stable velocity weakening layers (Wei et al., 2013), amongst others. Here we attempt to use the shape of the creep events recorded on USGS creepmeters to understand their driving mechanism.

Using creep events from the catalog of Gittins and Hawthorne (2022), we test various rheological models in both the frictional and viscous regimes, analyzing how well they describe the temporal evolution of a creep event. We assess whether the temporal evolution is best fitted by a linear/power-law viscous flow model (e.g., Crough & Burford 1977; Wesson., 1988) or one of a variety of rate & state frictional models, focussing on those that are predominantly velocity strengthening (e.g., Helmstetter & Shaw 2009), as this is the generally accepted rheology for the very upper few kilometers of the earth’s crust where these creep events occur. Investigating the shape of the creep events will give us insight into the driving mechanisms behind creep events.

We are also examining strainmeter data from the Central San Andreas Fault to constrain the depths at which creep events occur. Strainmeter data provides us with further insight into how slip occurs during creep events with multiple phases.