Crustal Strain Patterns Associated With Normal, Drought, and Heavy Precipitation Years in California

Abstract

We invert continuously operating Global Positioning System (cGPS) data obtained between 2007 and 2019 to quantify non steady‐state horizontal strain anomalies in California. Our long‐wavelength transient strain model shows seasonal and multiannual variations in horizontal strain anomalies within the plate boundary zone. During the summer, in general, a zone of extensional dilatation develops along the San Andreas Fault zone and Sierra Nevada, whereas contractional dilatation develops along the Eastern California Shear Zone (ECSZ) north of 36.5°N. The patterns of dilatational strain are opposite during the winter. We find that these seasonal strain anomaly patterns vary in magnitude, depending on precipitation intensity in California. Investigating hydrologic loading models and their horizontal elastic responses reveal that water mass loads on the surface from the precipitation in California are the major sources of the observed long‐wavelength horizontal transient strains. We show, however, that a heavy damping in the inversion of the cGPS data is required for the long‐wavelength horizontal strain solutions to best match with the expected elastic response from hydrologic loading. Appropriate fitting of the horizontal cGPS yields amplified horizontal strain signals in the Sierra Nevada, along regions adjacent to the San Andreas Fault, and within the ECSZ. The larger‐than‐expected amplitudes may be associated with poroelastic responses or thermoelastic changes that are superimposed on the hydrologic response. We demonstrate that there is a persistent sharp boundary of horizontal dilatational strain domains at the transition between the High Sierra and Basin and Range Province, caused by the sharp gradient in hydrologic loading there.

Repeating Nontectonic Seasonal Stress Changes and a Possible Triggering Mechanism of the 2019 Ridgecrest Earthquake Sequence in California

“…The no-length-change directions (strikes of vertical strike-slip faults), upon which we calculate stress changes (King et al, 1994), are in accord with the orientations of strike-slip focal mechanism solutions (Hauksson et al, 2012;Yang et al, 2012) and the surface trace of strike-slip Quaternary faults (Jennings, 1994;Nicholson et al, 2017;Shaw et al, 2015) within the ECSZ-WL transition zone (see Figure S17 from Kim et al [2021]). A more complex geometry of the faults, however, will have an influence on the accuracy of our results.…”

“…Kim et al (2021) showed that it is possible using cGPS data to extract short-wavelength (tens of km), high-amplitude horizontal strain anomaly patterns that are superimposed on long-wavelength (hundreds of km) strain anomaly patterns in California and western Nevada. The long-wavelength horizontal nontectonic strain solution can be explained as the Earth's elastic response to the large-scale hydrologic loading in the plate boundary zone (Kim et al, 2021). We also find that the short-wavelength horizontal strain patterns (Figures S6a and S6b) across the ECSZ-WL transition zone are superimposed on the long-wavelength patterns (Figures S6c and S6d).…”

“…If a poroelastic effect is dominant for a station, the pattern is opposite: excessive water from the precipitation fills up pore spaces in sediments, causing the maximum uplift during the wet seasons. Thus, we eliminate stations that show maximum vertical positions during the wet season of November-April (Kim et al, 2021). We also omit stations that show subsidence rates larger than 1 mm/ yr during the severe drought (2015)(2016)(2017)(2018)(2019)(2020)(2021) in California, assuming that the high subsidence rates represent anthropogenic signatures captured by cGPS (e.g., groundwater extraction; Argus et al, 2014).…”

“…We here briefly discuss possible sources that may contribute to the nontectonic seasonal strain/stress changes in the ESCZ-WL transition zone. Kim et al (2021) showed that it is possible using cGPS data to extract short-wavelength (tens of km), high-amplitude horizontal strain anomaly patterns that are superimposed on long-wavelength (hundreds of km) strain anomaly patterns in California and western Nevada. The long-wavelength horizontal nontectonic strain solution can be explained as the Earth's elastic response to the large-scale hydrologic loading in the plate boundary zone (Kim et al, 2021).…”

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

“…Several major earthquakes occurred during the time period considered in the study area (notably the 1994 Northridge, 1999 Hector Mine, 2004 Parkfield, 2010 El Mayor‐Cucapah, and 2019 Ridgecrest earthquakes). There is also evidence that multi‐annual hydrological variation affects horizontal motion to some extent (e.g., Kim et al., 2021). However, we assume that the data set mostly captures the interseismic deformation in the area and we take into account the existence of nonlinear deformation effects by increasing the uncertainties on the MIDAS velocities.…”