The Variability and Interpretation of Earthquake Source Mechanisms in The Geysers Geothermal Field From a Bayesian Standpoint Based on the Choice of a Noise Model

Abstract

Moment tensor (MT) inversion studies of events in The Geysers geothermal field mostly focused on microseismicity and found a large number of earthquakes with significant non‐double‐couple (non‐DC) seismic radiation. Here we concentrate on the largest events in the area in recent years using a hierarchical Bayesian MT inversion. Initially, we show that the non‐DC components of the MT can be reliably retrieved using regional waveform data from a small number of stations. Subsequently, we present results for a number of events and show that accounting for noise correlations can lead to retrieval of a lower isotropic (ISO) component and significantly different focal mechanisms. We compute the Bayesian evidence to compare solutions obtained with different assumptions of the noise covariance matrix. Although a diagonal covariance matrix produces a better waveform fit, inversions that account for noise correlations via an empirically estimated noise covariance matrix account for interdependences of data errors and are preferred from a Bayesian point of view. This implies that improper treatment of data noise in waveform inversions can result in fitting the noise and misinterpreting the non‐DC components. Finally, one of the analyzed events is characterized as predominantly DC, while the others still have significant non‐DC components, probably as a result of crack opening, which is a reasonable hypothesis for The Geysers geothermal field geological setting.

Seismotectonics of the 2018 northern Osaka M6.1 earthquake and its aftershocks: joint movements on strike-slip and reverse faults in inland Japan

“…These marginal PDFs are fitted with Gaussian functions defined by means and standard deviations σ. The reliable estimate of the uncertainty of CMT parameters proved to be beneficial in terms of interpretation of the results (e.g., Hallo et al 2017;Liu et al 2018;Ma et al 2018;Mustać et al 2018).…”

A composite seismic source model for the first major event during the 2022 Hunga (Tonga) volcanic eruption

“…Insights into the explosion source depth could be important to understand the exact mechanism of the eruptive explosion . Constraining the source depth for shallow events is challenging (e.g., Mustać et al, 2018;Mustać & Tkalčić, 2017). cavern, but the cone rims remain almost untouched (Mackay et al, 2022).…”

Point‐Source Inversion of Small and Moderate Earthquakes From P‐wave Polarities and P/S Amplitude Ratios Within a Hierarchical Bayesian Framework: Implications for the Geysers Earthquakes

“…Furthermore, the water injection and steam withdrawal can cause or induce small to moderate‐size earthquakes. The Geysers has produced thousands of earthquakes, and many researchers have documented the existence of non‐DC components in their source mechanisms (e.g., Boyd et al, , ; Gu et al, ; Guilhem et al, ; Johnson, ; Martínez‐Garzón et al, ; Mustać et al, ; Ross et al, , ).…”

“…There are several local broadband networks around the Geysers: Berkeley Digital Seismic Network (BDSN), USGS Northern California Network (NCSN) and USArray Transportable Array (EarthScope_TA), which have been used to analyze MT solutions (e.g., Boyd et al, ; Mustać & Tkalčić, ; Mustać et al, ; BSL catalog: http://www.ncedc.org/mt/). However, the waveform data is attenuated substantially for small to moderate earthquakes, which leads to considerable errors in MT inversion.…”

“…Mustać and Tkalčić () developed a full‐MT hierarchical Bayesian inversion which uses a MCMC to sample the MT parameters and noise parameters by means of the covariance matrices. They applied their method to the Long valley Caldera and the Geysers earthquakes (Mustać et al, ; Mustać & Tkalčić, ). Gu et al () extended the method of Mustać and Tkalčić () further by considering velocity model uncertainties.…”