Exploring seismological properties of asperity-type events in a rate-and-state fault model

Natalie Schaal, Nadia Lapusta, & Yen-Yu Lin

Submitted August 15, 2017, SCEC Contribution #7855, 2017 SCEC Annual Meeting Poster #182

Observations of foreshocks on both natural and laboratory faults suggest that the source of the foreshocks may be fault heterogeneities, such as bumps, that are driven to fail by aseismic processes of the mainshocks nucleation process [e.g., Dodge et al., 1995, 1996; Bouchon et al., 2013; McLaskey et al., 2014]. We explore foreshock-like asperity-type sources in rate-and-state fault models by numerically simulating their behavior over long-term sequences of seismic and aseismic slip. We seek to understand the rupture mechanics of these earthquake sources, as well as their seismologically derived characteristics.

To create the potential for smaller isolated earthquakes to occur during the nucleation of the mainshock, our model has circular patches of higher compression within the seismogenic (velocity-weakening) region of the fault. Such patches are both stronger in the frictional sense and have smaller nucleation size. To further encourage these smaller-scale seismic events to occur, we sometimes also consider smoother patches, via lower characteristic slip distance, corresponding to an even lower nucleation size. In the model, such events initiate on the patches, driven by the surrounding aseismic nucleation, and then rupture significantly into the surrounding area, with substantially different stress drops on and off of the patch. Despite the substantially higher and variable normal stress on the patches, this rupture process results in average stress drops within the reasonable range of 1 – 10 MPa.

The special character of these events may lead to unique seismological signatures. To study this possibility, we produce far-field synthetic seismograms and compare the source parameters calculated via three common source models (Brune, Sato-Hirasawa, and Madariaga) to those calculated directly from our long-term numerical simulations. Preliminary results show that the Madariaga model may be most suitable for describing asperity-type sources. Current work is directed towards producing a variety of types of earthquake sources, in addition to the asperity sources, to investigate whether the type of source can be ascertained from the spectral properties of the seismograms over the focal spheres. Discovering any unique observable features of foreshock-like asperity-type events could contribute to the possibility of physics-based earthquake forecasting.

Schaal, N., Lapusta, N., & Lin, Y. (2017, 08). Exploring seismological properties of asperity-type events in a rate-and-state fault model. Poster Presentation at 2017 SCEC Annual Meeting.

Related Projects & Working Groups
Fault and Rupture Mechanics (FARM)