[S-SS08] Fault Rheology and Earthquake Physics

[SSS08-01] Fault frictional heterogeneities can explain earthquakes with no foreshocks, accelerating foreshocks and earthquake swarms observed in central Alaska

★Invited Papers

*Yoshihiro Kaneko¹, Nealey Sims², Carl Tape², Stephen Holtkamp², Andrea Perez³ (1.Kyoto University, 2.University of Alaska - Fairbanks, 3.Victoria University of Wellington)

Keywords:Accelerating foreshocks, Fault frictional heterogeneities, Earthquake cycle deformation

While most earthquakes start abruptly, with no evidence for a nucleation process, accelerating foreshock sequences within or in the vicinity of the eventual mainshock rupture zone for some moderate to large crustal earthquakes have been documented previously. In particular, Tape et al. (2018) reported unique observations of nucleation signals of crustal earthquakes in the Minto Flats fault zone in central Alaska, manifested by ~20 seconds of simultaneous high-frequency foreshocks and a very low-frequency earthquake. One potential explanation for such observations is a slow slip front propagating over the fault and triggering these foreshocks as it transitions into the mainshock rupture (e.g., Tape et al., 2018). Another explanation may be that accelerating foreshocks represent cascading sequences of fault ruptures due to static and/or dynamic stress changes, without underlying slow slip, as known as a cascading hypothesis (e.g., Ellsworth and Bulut, 2018). Here we show that a numerical fault model incorporating full inertial dynamics and rate-and-state friction laws with frictional heterogeneities reproduces ~20 seconds long, accelerating foreshock sequence that led to a mainshock as observed in the Minto Flats fault zone. We find that the time scale of accelerating foreshock sequence depends on the degree and size of frictional heterogeneities and tectonic loading rate. Furthermore, an accelerating foreshock sequence occurs in only a transitional regime between an earthquake swarm regime and the regime of mainshocks with no foreshocks. This may explain why the observations of accelerating foreshock sequences are relatively rare. We conclude that reproducing the observations of ~20 seconds long, accelerating foreshock sequence would require slow physical process, such as slow slip, fluid diffusion or rock damage, in between small-scale, seismogenic asperities.