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Constraining Source Parameters of Seismic Events Generated by Circular Gouge Patches on 4-meter-long Laboratory Fault

The mechanics of the dynamic earthquake ruptures are investigated with the friction law applied to the fault. The source size, the stress drop, and the energy budget of the ruptures help us to configure it. The source size is usually estimated with the corner frequency of the radiation via source models such as Brune (1970) or Madariaga (1976). However, the models assume the rupture propagation on the circular patch with a given rupture velocity and a shear wave velocity, which causes a bias in the estimation of the source parameters. In this study, therefore, we control the size and shape of the source by artificially setting thin circular gouge patches on the laboratory fault such that we constrain the energy budget and the friction law without using predefined source models.

We used the 4-meter-long biaxial rock friction apparatus to conduct the stick-slip experiments with the macroscopic normal stress of 2.0MPa. We set seven gouge patches with a diameter of 8mm on the fault. We preliminarily measured the enhanced normal stress on the gouge patch, showing 4.5MPa on average with the background stress of 2.0MPa. The seismic events are observed during the propagation of the preslip towards the mainshock, most of which correspond to the location of the gouge patches. We first fit the numerically computed waveforms to one of the seismic events and obtained its seismic moment as 0.31Nm (Mw=-6.4). Given the ruptured size is equivalent to the gouge patch, we obtained the average slip of 0.16μm, the stress drop of 2.1MPa, and the seismic efficiency of 0.17, which are in reasonable ranges concerning the amount of preslip on the simulated fault and the normal stress on the patch. However, the observed seismic efficiency was significantly smaller than the estimation from Brune's source model even though the model provides a better fit on the source size with the best-fit source duration to the gouge patch size. This study aims to contribute to clarifying such discrepancies by investigating the possible stress and frictional conditions on the controlled patch to cause the ruptures reproducing the observed AE waveforms.

Session: Understanding and Quantifying the Variability in Earthquake Source Parameter Measurements - III

Type: Oral

Room: Tubughnenq’ 3

Date: 5/3/2024

Presentation Time: 02:00 PM (local time)

Presenting Author: Kurama Okubo

Student Presenter: No

Additional Authors