Geodetic Measurements of Slow‐Slip Events Southeast of Parkfield, CA

Abstract

Tremor and low‐frequency earthquakes are presumed to be indicative of surrounding slow, aseismic slip that is often below geodetic detection thresholds. This study uses data from borehole seismometers and long‐baseline laser strainmeters to observe both the seismic and geodetic signatures of episodic tremor and slip on the Parkfield region of the San Andreas Fault near Cholame, CA. The observed occurrence rates of both the tremors and co‐located families of low‐frequency earthquakes are not steady but instead exhibit quasiperiodic bursts of increased activity. We show that these periods of elevated seismic activity correlate with statistically significant stacked strain signals consisting of 44 slow‐slip events. Modeled individual slow‐slip events and their total summed moment, which are constrained by seismic signals and stacked strain, respectively, indicate that the individual moment magnitudes of these events range from Mw 4.6–5.2. We find that the measured geodetic signal likely precedes the seismic signal by several hours, consistent with the aseismic slip preceding and driving the observed seismic tremor activity. We confirm that strike‐slip faults, in addition to subduction zones, are capable of producing episodic tremor and slip.

Complex Migration of Tremor Near Cholame, CA, Resolved by Seismic Array Analysis

“…(2019) revealed deep episodic fault slip beneath the Parkfield section with equivalent moment magnitude of 4.9. A similar magnitude was estimated from long‐baseline strainmeter data for SSEs occurring along the deep extension of the SAF south of Cholame (Delbridge et al., 2020). Given the estimated geodetic moment, the amount of slip is dependent on the dimensions of the slipping region, which were constrained from the distribution of LFE families.…”

“…(2019) and Delbridge et al. (2020), suggesting the Parkfield geodetic signal and LFE activity are contemporaneous, and we consider it as unlikely. Another possibility is that some tremor goes undetected.…”

“…One possibility, which is at odds with observations from Cascadia (e.g., Bartlow et al, 2011;Hawthorne & Rubin, 2013b;Wech et al, 2009), Japan (e.g., Obara, 2010), Alaska (Rousset et al, 2019), and Mexico (Frank, 2016), is that high rates of tremor in Cholame are simply not correlated with episodic deep fault slip. This possibility seems also to be at odds with the results of Rousset et al (2019) and Delbridge et al (2020), suggesting the Parkfield geodetic signal and LFE activity are contemporaneous, and we consider it as unlikely. Another possibility is that some tremor goes undetected.…”

Episodicity and Migration of Low Frequency Earthquakes modeled with Fast Fluid Pressure Transients in the Permeable Subduction Interface

“…These LFEs appear to be due to repetitive, impulsive and localized sources [15,33]. High rates of tremor activity are associated with geodetically observed slow deformation episodes called slow-slip events (SSEs) [e.g., 27,34,57,78]. The correlation between LFE and SSE activity gave rise to the term Episodic Tremor and Slip (ETS) [77].…”

Bayesian Estimation of Surface Strain Rates From Global Navigation Satellite System Measurements: Application to the Southwestern United States

“…These maps have a very broad range of applications, in long‐term tectonics (e.g., Flesch & Kreemer, 2010; Kreemer et al., 2003), seismic cycle (e.g., D'Agostino, 2014; Klein et al., 2019; H. Wang et al., 2019), or hydrology (e.g., Silverii et al., 2020). Recently, strain rates have been calculated on much shorter time‐spans in order to capture the surface deformation associated with phenomena such as ground water variations (Klein et al., 2019; Silverii et al., 2020), magmatic intrusions (Silverii et al., 2019) or slow‐slip events (e.g., Delbridge et al., 2020). Our interpolation method, with its ability to properly account for data errors, could prove useful in these cases where observations are associated with larger than usual uncertainties.…”