Tidal triggering of earthquakes

Abstract

Since tidal forces deform the Earth’s crust with completely predictable stresses, the role of the Earth tides in triggering seismicity has long been investigated. The results of these investigations have, however, been inconclusive. Tidal triggering has been found to be non-existent or small (still not statistically significant) for strike-slip faults. On the other hand, a tide-earthquake has been observed in hydrothermal or magma-affected areas and for reverse faults in the oceanic settings where the amplitude of tidal stresses is the largest. Here, we examine the effect of tidal stresses on earthquake occurrence in Southern California using three novel approaches. First, we analyse the difference in seismicity rates at times of positive and negative tidal Coulomb stresses/stress rates; for that, we resolve the tidal stresses/stress rates on failure planes assumed to be controlled by the orientation of their nearest geological structures. We find the earthquake behaviour with respect to tidal influence statistically non-random only in close proximity to tidal extremes. A particularly strong tidal signal is observed for reverse fault type events. Along with the significant increase in earthquake rates around tidal Coulomb stress maxima, we observe that the strength of the tidal correlation is closely related to the amplitude of the tidal stresses. Second, we look at the response of aftershocks to tidal stresses. Based on the 1992 Mw 7.3 Landers and 1999 Mw 7.1 aftershock activity, we demonstrate that the probability of a fault failure due to tidal oscillations is strongly affected by earthquake-induced static stress changes. Accordingly, earthquakes in regions of increased static stresses are more consistent with tidal triggering than events in areas of static stress decrease. Third, we examine the relationship between the effect of tidal maximum compressional stresses on earthquake timing and the orientation of regional tectonic stress. We find that the dominant azimuth of the tidal compressional stress aligns with the principal direction of tectonic compression suggesting that earthquakes are tidally triggered when tidal stresses act in the same direction as the background stress.

Date of Award	Nov 2015
Original language	English
Sponsors	Department of Education and Learning (NI)
Supervisor	Sandy Steacy (Supervisor) & Suleyman Nalbant (Supervisor)