Spectra and Mechanics of Slow to Fast Contained Laboratory Earthquakes

Date: 4/24/2019

Time: 11:45 AM

Room: Cascade II

We report a spectrum of slow to fast contained laboratory earthquakes generated on a dry, nominally homogeneous 3-meter granite fault. Using a newly constructed biaxial friction apparatus at Cornell University, we study the slow events using measurements of local stress, slip, and ground motions, and we find many similarities to low frequency earthquakes (LFEs) or tremor sources. In our experiments, spontaneous fault rupture events at 4 to 12 MPa stress levels are similar to M -3 to M -2 earthquakes. Prior to these dynamic rupture events that propagate across the entire fault, we observe contained earthquakes that nucleate and arrest within the sample. Closer to natural earthquakes than standard stick-slip events, these contained events rupture a patch of the fault with length P = 0.4 to 2 m. Their slip speeds range from 0.07 - 100 mm/s. The fastest events are similar to regular M -2.5 earthquakes, with a single distinct corner frequency and w^-2 spectral falloff at high frequencies. They are well fit by the Brune earthquake source model with a 0.4 MPa stress drop. Events with intermediate slip speeds (2 – 10 mm/s) have 50 kPa to 100 kPa stress drops and weakly radiate tremor-like signals. Their source spectra are depleted near the corner frequency relative to a Brune model. The slowest events (<1 mm/s slip rates) have a w^-1 spectral shape, similar to slow slip events observed in nature. Our results show that a fault patch P can radiate in vastly different ways, just from small changes in the ratio of a critical nucleation length (Lc) to P. In the lab, the patch P is set up from stress conditions. In the field, patch P is likely set up by fault structure or rheology such as a velocity weakening patch surrounded by velocity strengthening fault. LFE "families" and tremor locations that are persistent over many years support this view.

Presenting Author: Bill Wu

Authors