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

Here we characterize the 13-year history of nontectonic horizontal strain anomalies across the regions surrounding Ridgecrest, CA, using cGPS data from January 2007. This time-dependent model reveals a seasonality in the nontectonic strain anomalies and the associated Coulomb stress changes of ∼±0.5-2 kPa. In the area surrounding the epicenters of the 2019 Ridgecrest earthquake sequence of July, we find that the seasonal preseismic Coulomb stress changes peaked every early summer (May and June) during the last 13 years including during June 2019, a month prior to the large events. In addition, our statistical tests confirm that more strike-slip earthquakes (Mw ≥ 2) occur during times when seasonal stress changes are increasing on right-lateral faults in comparison with times when stresses are decreasing. These results suggest that the timing of the 2019 Ridgecrest earthquakes may have been modulated by nontectonic seasonal stress changes. The dynamic source of the seasonal nontectonic strain/stress anomalies, however, remains enigmatic. We discuss a possible combination of driving forces that may be attributable for the seasonal variations in nontectonic strain/stress anomalies, which captured in cGPS measurements.

Water input into the Mariana subduction zone estimated from ocean-bottom seismic data.

The water cycle at subduction zones remains poorly understood, although subduction is the only mechanism for water transport deep into Earth. Previous estimates of water flux1-3 exhibit large variations in the amount of water that is subducted deeper than 100 kilometres. The main source of uncertainty in these calculations is the initial water content of the subducting uppermost mantle. Previous active-source seismic studies suggest that the subducting slab may be pervasively hydrated in the plate-bending region near the oceanic trench4-7. However, these studies do not constrain the depth extent of hydration and most investigate young incoming plates, leaving subduction-zone water budgets for old subducting plates uncertain. Here we present seismic images of the crust and uppermost mantle around the central Mariana trench derived from Rayleigh-wave analysis of broadband ocean-bottom seismic data. These images show that the low mantle velocities that result from mantle hydration extend roughly 24 kilometres beneath the Moho discontinuity. Combined with estimates of subducting crustal water, these results indicate that at least 4.3 times more water subducts than previously calculated for this region3. If other old, cold subducting slabs contain correspondingly thick layers of hydrous mantle, as suggested by the similarity of incoming plate faulting across old, cold subducting slabs, then estimates of the global water flux into the mantle at depths greater than 100 kilometres must be increased by a factor of about three compared to previous estimates3. Because a long-term net influx of water to the deep interior of Earth is inconsistent with the geological record8, estimates of water expelled at volcanic arcs and backarc basins probably also need to be revised upwards9.