Spatiotemporal Correlation Between Seasonal Variations in Seismicity and Horizontal Dilatational Strain in California

Abstract

We extract significant spatially coherent strain variations from horizontal seasonal Global Positioning System (GPS) displacements in the American Southwest. The dilatational strain is largest in northern California with maximum margin-normal contraction and extension in spring and fall, respectively, consistent with the Earth's surface going down and up at those times. The northern California signal has a phase shift with respect to that in southern California and the Great Basin. For northern and southern California the proportion of larger earthquakes are in-phase and the aftershock productivity out of phase with the inferred Coulomb stress on the San Andreas fault system. The intensity of mainshocks is in-phase in the north as well but not in the south. This suggests that a seasonal increase in fault-normal extension may or may not trigger mainshocks, but when an earthquake happens at those times, they grow larger than they otherwise would, which would cause a larger stress reduction and result in fewer aftershocks.Plain Language Summary The changing amount of water and snow mass that lays on top of the Earth's surface is one possible explanation for observed seasonal variations in seismicity. This hydrological loading would change the state of stress inside the crust minutely with the seasons. We image the seasonal stress variation by using the horizontal seasonal displacements of GPS monuments in the southwestern United States. This reveals large-scale seasonal patterns of the crust contracting and extending in-phase with the Earth's surface going down and up, respectively, particularly in northern California which experiences a large excess of water and snow in late winter. The seasonal variations in horizontal deformation there correspond to variations in the number of mainshocks, with more earthquakes occurring when the crust is under extension. In southern California, we see no correlation with the number of mainshocks. In both regions, seasonal deformation correlates with the proportion of large earthquakes and shows an anticorrelation with the aftershock production. So even though seasonal deformation may not directly trigger earthquakes, if an earthquake happens during the right season, it seems to be able to grow a little larger, releasing a little more stress than it otherwise would and reducing the need for (more) aftershocks.

Seasonal and Long‐Term Groundwater Unloading in the Central Valley Modifies Crustal Stress

“…The San Andreas fault south of ~35.5° latitude is ~4–5 months out of phase with the rest of the fault, which might be due in part to its proximity to the irrigation‐controlled groundwater recharge cycle in the southern part of the Valley that is ~3 months out of phase with the rest of the Valley. Kreemer and Zaliapin () found a similar ~4‐month phase lag for peak Coulomb stress change between the northern and southern San Andreas fault, although that study was based on seasonal horizontal strain.…”

Drought‐Triggered Magmatic Inflation, Crustal Strain, and Seismicity Near the Long Valley Caldera, Central Walker Lane

“…Recent studies have identified a connection between loading‐induced strain and earthquake rates in Nevada and California (e.g., Johnson et al, ; Kreemer & Zaliapin, ). Here we investigate seismicity rates in our study area to determine if they are sensitive to the horizontal strain rate changes associated with the drought periods.…”

“…MAGNET data constrain rates of motion nearly as well as continuous stations but in some cases do not well constrain seasonal oscillations since they are only occupied part of each year. Continuous stations are more common on the Sierra Nevada and LVC (since they monitor the active volcanic system), while MAGNET dominates to the east in the Great Basin where time‐variable motion was thought to be less prevalent owing to generally drier conditions in the rain shadow of the Sierra Nevada (Amos et al, ; Kreemer & Zaliapin, ; Figure ) .…”

“…Recent studies have shown that active uplift of the Sierra Nevada Mountains in California and Nevada, western United States, is detectible in vertical component GPS measurements, with rates up to 2 mm/yr (Fay et al, ; Hammond et al, ). The geographic pattern and seasonality of the signals provide clear indications that surface mass unloading from loss of ground and surface water help drive the observed uplift (Amos et al, ; Argus et al, ; Kreemer & Zaliapin, ). As California entered severe drought conditions in late 2011, precipitation decreased and was followed by an intensification of the extraction of groundwater and reduction in water availability in California Central Valley aquifers (e.g., Faunt et al, ).…”

Hydrologically Induced Deformation in Long Valley Caldera and Adjacent Sierra Nevada

“…The SNR receives large amounts of precipitation concentrated in winter/spring, but with noticeable multiyear variability, alternating between periods of surplus precipitation and drought (Margulis et al, ). Several studies have highlighted the signature of these precipitation trends in deformation data, showing that observed subsidence/uplift trends at both seasonal and multiyear scales are driven mainly by surface loading/unloading from increasing/decreasing surface water and groundwater (Amos et al, ; Argus et al, ; Borsa et al, ; Kreemer & Zaliapin, ). These studies mostly focused on the vertical component of the deformation, since horizontal displacements induced by surface loading are typically much smaller (Wahr et al, ).…”