Laboratory approach to measuring earthquake source parameters.

Doron Morad, Shahar Gvirtzman, Jay Fineberg, & Emily E. Brodsky

Published September 10, 2023, SCEC Contribution #13292, 2023 SCEC Annual Meeting Poster #116

At what time is the ultimate magnitude of a big earthquake determined and observable in the rupture dynamics? This is a major issue in both earthquake physics and its societal applications. Observationally, a common approach to answering the question is to analyze the early arrivals and the seismic wavefield as captured by the far-field source time function. This approach is theoretically well-founded, but relies on a series of necessary assumptions about both the source and the path in the heterogeneous Earth. Here we use the same approach to query laboratory earthquakes in transparent materials where independent information is available both on source finiteness and pre-conditions, which allows us to capture the full rupture process armed with linear elastic fracture mechanics theory guided by directed observations of the elastic strains.

In this study, we put a set of acoustic sensors on the upper boundary of a transparent (PMMA) block. We calibrated it to the displacement field, and designed the experiment so the acoustic sensor is located sufficiently far from the frictional source (i.e., seismic source) to be treated as a remote seismic station. Then we can measure the source parameters of laboratory earthquakes by applying a Brune’s fitting model to our data.

By assuming the maximum amplitude of the source time function at the first arrival wave, is a proxy for the lab quake magnitude, we can relate the initial stage of the rupture to its magnitude potential. We further show a good correlation between higher corner frequencies and maximum amplitude whereas the spectral domain is well fit with a ω^2 relationship.

We measure the initial rate of moment release and show that the final moment can be well-predicted. Both an instantaneous measure of the moment release and a more averaged measure over the first half of the source-time function perform well. We conclude for these experiments that there is a range of initial rupture growth speeds and those that grow quickly are more likely to generate a relatively large earthquake, thus creating a degree of magnitude predictability based on the source time wavefield.

Key Words
Fault rupture, Acoustic sensors, Source time function

Citation
Morad, D., Gvirtzman, S., Fineberg, J., & Brodsky, E. E. (2023, 09). Laboratory approach to measuring earthquake source parameters.. Poster Presentation at 2023 SCEC Annual Meeting.

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