Evidence of earthquake triggering by the solid earth tides

Clear evidence for earthquake triggering by the earth tides has remained elusive for more than a century. Using the largest global earthquake catalog available (the NEIC catalog with 442412 events), we observe a clear correlation (with similar to 99% confidence) between the phase of the solid Earth tide and the timing of seismic events: earthquakes occur slightly more often at the time of ground uplift by the Earth tide, i.e. when normal stresses are reduced within the lithosphere. We observe that this phase distribution anomaly is larger for smaller and shallower earthquakes. Although earthquakes in regions with dominantly normal and strike-slip faulting seem to exhibit more tidal triggering than regions dominated by thrust faulting, there is no statistically significant evidence for a focal mechanism-dependence on earthquake triggering. Finally, we show here that it is highly probable that the observed triggering is caused by the solid Earth tide, rather than by loading from the ocean or atmospheric tides. Although an additional impact due to loading from ocean tides is possible and probable, we cannot detect it here because the earthquake database is not sufficiently complete and homogeneous (more small magnitude earthquakes in oceanic areas are needed). Our results are consistent with the idea of a damped sensitivity of earthquake initiation to stress change-an event is slightly more probable (similar to 0.5 to 1.0%) when the tidal displacement is maximum, particularly for small and shallow events. (c) 2008 Elsevier B.V. All rights reserved.