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Seasonal water storage, stress modulation, and California seismicity

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Science  16 Jun 2017:
Vol. 356, Issue 6343, pp. 1161-1164
DOI: 10.1126/science.aak9547

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Saving earthquakes for the wet season

Earthquakes can be triggered by changes in crustal stress, such as variations in fluid pore pressure. As a result, the alternating wet and dry cycles in earthquake-prone California should affect the earthquake rate. Johnson et al. asked whether this is indeed the case by combining detailed earthquake records with high-resolution GPS data from the past 9 years. Slight changes in stress did indeed influence the timing of earthquakes, which confirms that the annual hydrological loading cycle modulates microseismicity in California.

Science, this issue p. 1161

Abstract

Establishing what controls the timing of earthquakes is fundamental to understanding the nature of the earthquake cycle and critical to determining time-dependent earthquake hazard. Seasonal loading provides a natural laboratory to explore the crustal response to a quantifiable transient force. In California, water storage deforms the crust as snow and water accumulates during the wet winter months. We used 9 years of global positioning system (GPS) vertical deformation time series to constrain models of monthly hydrospheric loading and the resulting stress changes on fault planes of small earthquakes. The seasonal loading analysis reveals earthquakes occurring more frequently during stress conditions that favor earthquake rupture. We infer that California seismicity rates are modestly modulated by natural hydrological loading cycles.

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