Shear-wave splitting and earthquake forecasting

Seismic shear-wave splitting (SWS) monitors the low-level deformation of fluid-saturated microcracked rock. We report evidence of systematic SWS changes, recorded above small earthquakes, monitoring the accumulation of stress before earthquakes that allows the time and magnitude of impending large earthquakes to be stress-forecast. The effects have been seen with hindsight before some 15 earthquakes ranging in magnitude from an M1.7 seismic swarm event in Iceland to the Ms7.7 Chi-Chi Earthquake in Taiwan, including a successfully stress-forecast of a M5.0 earthquake in SW Iceland. Characteristic increases in SWS time-delays are observed before large earthquakes, which abruptly change to deceases shortly before the earthquake occurs. There is a linear relationship between magnitudes and logarithms of durations of both increases and decreases in SWS time-delays before large impending earthquakes. However, suitably persistent swarms of small earthquakes are too scarce for routine stress-forecasting. Reliable forecasting requires controlled-source cross-hole seismics between neighbouring boreholes in stress-monitoring sites (SMS). It would be possible to stress-forecast damaging earthquakes worldwide by a global network of SMS in real time