Influence of fortnightly tides on earthquake triggering at the East Pacific Rise at 9 degrees 50N

摘要

The study of tidal forcing provides an opportunity to view the response of the solid Earth to known changes in stress. Earthquakes triggered by diurnal and semidiurnal tidal forces have been well documented for the deep ocean and some terrestrial environments. However, few studies examine the influence of fortnightly tides on earthquake triggering and only in terrestrial environments. We assess triggering of microearthquakes related to true fortnightly tides and fortnightly tidal modulations along the East Pacific Rise at 9 degrees 50N, where strong diurnal and semidiurnal triggering of earthquakes has been observed. An ocean bottom seismograph experiment collected microearthquake data between October 2003 and January 2007, a time period that included a well-documented submarine eruption culminating in January 2006. We examine how triggering changed through the build up to and aftermath of this seafloor-spreading event. Results show that earthquakes occur preferentially during times of increasing peak volumetric stress leading up to the eruption, with strong statistical significance (Schuster Test p values up to 2.22e-31). However, the correlation loses its strength immediately prior to and during the eruptive period (p value as high as 0.2837), when competing tectonic and magmatic stresses may occur. Post-eruption triggering reappears (p value 1.45e-26) but with opposite phase. Statistical significance is also explored using Monte Carlo method, highlighting limitations of swarm-dominated time series for the Schuster Test. Nevertheless, the observations are consistent with a shift in the dominating stress triggering the earthquakes from extensional tectonics to magmatic processes, followed by a post-eruption period of cooling, relaxation, and possibly magma chamber refilling.