Earthquake initiation and foreshocks informed by 3-meter rock experiments

Gregory C. McLaskey

Published August 2, 2020, SCEC Contribution #10235, 2020 SCEC Annual Meeting Talk on TBD

I describe the mechanics of foreshocks and the information they might provide about an impending larger earthquake. The work is informed by 3-meter laboratory rock experiments. I present two plausible scenarios which lead to different interpretations for foreshocks.

In the first scenario—likely representative of highly coupled faults—the critical nucleation length scale h* is small (<1 m) so dynamic rupture initiates rapidly in space and time, and M -2 events are earthquakes that met unfavorable conditions and stopped soon after initiating, while larger earthquakes continued to rupture and grow. In this scenario, if the regional stress approaches a critical state, earthquakes will begin to initiate at higher rates, but during this increase in seismicity rate, most of the fault remains essentially locked. Smaller events (foreshocks) may redistribute stress, and make the ratio of stress/strength more uniform in space which will allow a large event to eventually propagate without being stopped. In this case, foreshocks herald a larger earthquake.

In a second scenario—likely representative of many subduction zones, oceanic transform faults, and some mature faults such as sections of the San Andreas Fault—slow slip can occur without immediately initiating an earthquake. When considering the large-scale spatial average of fault properties, the effective nucleation length scale h* is large (~1-10 km), but heterogeneity of fault properties causes higher coupling on some fault patches which can rupture to produce “sub-h*” earthquakes or repeating earthquake sequences. These “sub-h*” foreshocks are driven by aseismic slip and can occur in swarms that may or may not eventually ignite a large earthquake.

Key Words
nucleation foreshocks

McLaskey, G. C. (2020, 08). Earthquake initiation and foreshocks informed by 3-meter rock experiments . Oral Presentation at 2020 SCEC Annual Meeting.

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