Earthquake nucleation, seismic wave radiation, and termination of dynamic rupture in a 3 m rock experiment
Greg McLaskey, Cornell
Wednesday, May 29, 2019 at 10:30 AM
- Building 3, Rambo Auditorium
- Nick Beeler
I describe sequences of laboratory earthquakes generated on a 3-meter laboratory rock experiment that provide insights into how earthquakes initiate, radiate seismic waves, and terminate. The granite sample was loaded to 2-12 MPa stress levels and then sheared, while local shear stress, fault slip, and ground motions were measured at arrays of locations along the sample length L = 3 m. If the initial along-fault stress distribution was sufficiently heterogeneous, continued loading produced sequences of smaller, M -2.5 earthquakes that repeatedly ruptured the same subsection of the fault, with successive events increasing in rupture length and stress drop. These contained events initiate on fault sections with conditions favorable to rupture—of length p—but do not rupture the sample ends, since p < L. We study the stress drop and radiated energy of the contained events since they are less influenced by the machine stiffness and are more representative of natural earthquakes than standard complete-rupture stick-slip events. We observe a spectrum of slow to fast events depending on the ratio p/h*, where h* is a critical nucleation length scale. Slow events, with 0.5 mm/s slip rates and 50 kPa stress drops, are produced from p/h* < 1. These events radiate tremor-like signals and have w-1 spectral falloff, consistent with slow events observed in nature. Fast events occur if p/h* >5. They have 100 m/s slip speeds, 0.4 MPa stress drop, and source spectra consistent with a Brune model. Most of these laboratory earthquakes exhibit a meter-sized zone of slow slip (nucleation zone) that expands and accelerates until reaching seismic slip speeds (>0.1 m/s), therefore h* ~ 1 m. However, we find that loading rate and strength heterogeneity cause large variation in h*, which has implications for how natural earthquakes initiate, and the interpretation of foreshocks.