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Waveform Similarity and Differential Travel Times Illuminate a Spatial Coalescence of Foreshock Activity Prior to Fast Laboratory Earthquakes

Description: 

Earthquake nucleation is a key problem in earthquake science with rich implications for earthquake early warning systems, earthquake forecasting, and understanding foreshock properties. Foreshocks are often viewed as a byproduct of the nucleation phase, and thus, understanding properties of foreshocks and the factors that control their spatiotemporal behavior is key for determining how earthquakes get started and their connection to the impending mainshock. We report on a suite of well-controlled laboratory friction experiments instrumented with an array of acoustic emission (AE) transducers. We systematically modulated the stiffness of the loading apparatus in tandem with the fault zone normal stress to produce a spectrum of slow and fast slip behaviors. AE data were measured in parallel with fault zone properties, allowing for a more robust understanding of the casual processes driving foreshock activity. We measured waveform similarity and pair-wise differential travel-times using AE templates and tracked their spatiotemporal evolution throughout the seismic cycle. During slow stick-slip, the fault creeps continuously throughout the seismic cycle and differential travel-times and waveform similarity remain constant. In contrast, during fast laboratory earthquakes, the fault remains locked throughout most of the seismic cycle and begins to unlock and accelerate once the fault reaches ~ 80% of its peak frictional strength. Once the fault begins to unlock and creep, the data show a sharp increase in waveform similarity and a rapid decrease in differential travel times, indicating that the foreshocks are starting to coalesce in space and time immediately before failure. These observations point to key differences in the nucleation process of slow and fast lab earthquakes and suggest that the spatiotemporal evolution of laboratory foreshocks is linked to fault slip velocity. Our data indicate that high waveform similarity and low differential travel-times are a fingerprint of fault creep and could be useful tools for tacking precursory processes before tectonic earthquakes.

Session: Earthquake Preparation Across Scales: Reconciling Geophysical Observations With Laboratory and Theory

Type: Oral

Date: 4/20/2023

Presentation Time: 10:15 AM (local time)

Presenting Author: Chas Bolton

Student Presenter: No

Invited Presentation: 

Authors