Abnormal low-magnitude seismicity preceding large-magnitude earthquakes

Unraveling the precursory signals of potentially destructive earthquakes is crucial to understand the Earth's crust dynamics and to provide reliable seismic warnings. Earthquake precursors are ambiguous, but recent experimental studies suggest that robust warning signs may precede large seismic events in the short (day-to-months) term. Here, we show that the M6.4-M7.1 2019 Ridgecrest sequence (California) and the M7.1 2018 Anchorage earthquake (Alaska) were preceded by up to ~3 months of tectonic unrest on regional scales, as evidenced by abnormal low-magnitude seismicity spreading over the ~15-25% of Southern California and Southcentral Alaska. This precursory unrest has been discovered with an algorithm that integrates an innovative random forest machine learning approach and statistical features built from earthquake catalogs. Supported by a novel suite of finite element solid mechanics models, we propose that precursory, abnormal, low-magnitude seismicity arises if the pore fluid pressure within large fault segments escalates significantly as they approach failure, which leads to major uneven changes in the regional stress field. Our findings and method may open up new perspectives for surveillance agencies to anticipate when a region approaches an earthquake of great magnitude weeks to months before it occurs.