Foreshocks and aftershocks on geometrically complex faults

Camilla Cattania, Sebastian Hainzl, & Paul Segall

Published August 16, 2019, SCEC Contribution #9909, 2019 SCEC Annual Meeting Talk on Tue 1100

Faults are geometrically heterogeneous across a wide range of scales: faults systems present multiple orientations, and single faults are fractal surfaces. I will present numerical studies highlighting the role of geometrical complexity in two types of seismic sequences: aftershocks on a regional fault network, and foreshocks during nucleation on a single fault.

In spite of the popularity of the Coulomb stress hypothesis for aftershock triggering, its predictive power has been questioned. Coulomb-based forecasts have performed poorly compared to statistical ones, largely due to the occurrence of aftershocks in areas of negative stress changes (stress shadows). We consider models coupling static stress changes and the Dieterich (1994) seismicity-rate theory. We show that variability in receiver fault orientation plays a first order role in the spatio-temporal distribution of aftershocks, and virtually suppresses stress shadows in the early part of the sequence. Accounting for variable receiver fault orientation systematically improves model performance, as confirmed by formal testing carried out by the Collaboratory for the Study of Earthquake Predictability (CSEP). These results provide encouraging evidence for the predictive power of Coulomb-based models.

On the other hand, the physical mechanism driving foreshocks remains controversial, with two contrasting views: 1. foreshocks are driven by the aseismic nucleation process; 2. foreshocks occur as a cascade, with each event triggered by the previous ones, eventually triggering the mainshock. We consider seismic cycles on rate-state faults with fractal roughness, modeled with a 2-D pseudo-dynamic code. Roughness leads to a rich slip behavior, with widespread creep and microseismicity intensifying prior to the mainshock. These processes are well explained by spatial variations in normal stress due to roughness. Most microseismicity occurs during nucleation; in this phase, creep acceleration leads to seismicity rates increasing as 1/t, where t is the time to the mainshock, in agreement with the simulated catalog and with reported foreshocks. The relative location of the foreshocks is consistent with static stress triggering, and so is the presence of earthquake clusters deviating from the 1/t prediction. However, accelerating creep releases most of the moment, indicating that foreshocks aren't directly triggering the mainshock, but rather responding to a predominantly aseismic nucleation process.

Citation
Cattania, C., Hainzl, S., & Segall, P. (2019, 08). Foreshocks and aftershocks on geometrically complex faults. Oral Presentation at 2019 SCEC Annual Meeting.

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