Tidal triggering of earthquakes in the Northeast Pacific Ocean

P>There have been many searches for evidence of tidal triggering in earthquake catalogues. With the exception of volcanically active regions, the more rigorous studies in continental settings tend to find no correlation or only a very weak correlation. In the oceans, the effect of loading by the ocean tides can increase tidal stresses by about an order of magnitude over continental settings. In recent years, several studies have reported evidence of tidal triggering in oceanic regions and such observations can represent a useful constraint on models of earthquake rupture. In this paper, I systematically search for a link between ocean tide height and the incidence of earthquakes in the Northeast Pacific Ocean, a region of high-amplitude open ocean tides. The focal mechanisms of most of the earthquakes in these catalogues are unknown but it can be shown that tidal stresses will in most instances promote failure at low tides. I investigate three declustered data sets comprising (1) earthquakes from 1980 to 2007 on the Juan de Fuca plate and in the Queen Charlotte Fault region from land based catalogues; (2) earthquakes from 1992 to 2001 on the Juan de Fuca plate located with the US Navy's Sound Surveillance System (SOSUS) hydrophone array and (3) earthquakes from 1980 to 2001 south of Alaska and the Aleutians located with land based networks. I look at the distributions of earthquakes with ocean tide phase, height, and tidal range and apply Schuster and binomial tests and Monte Carlo simulations to determine if they deviate significantly from random. The results show no evidence of triggering during intervals of increased tidal range but all three data sets show a significant increase in earthquake incidence at low tides. The signal is particularly strong in the land-based catalogue for the Juan de Fuca Plate and Queen Charlotte Fault regions where there is a 15 per cent increase in the rate of seismicity within 15 degrees of the lowest tides. The signal is weakest in the SOSUS data set, which may reflect the lower average tidal range at epicentres in this data set or an analysis that is influenced by gaps in the catalogue. The triggering signal in the Alaska/Aleutian may be partially obscured by earthquakes in the Aleutians where the total tidal stresses can be significantly out of phase with the ocean tide height. The increase in the rates of seismicity I observe at low tides is less than observed by local networks on mid-ocean ridges, similar to the prediction from an analysis of global thrust earthquakes and greater than inferred by extrapolating laboratory simulations of fault failure under tidal loading.