Abstract
Unequivocal evidence of tidal triggering is observed for microearthquakes (−0.4 to 2.0 ML) recorded between October 2003 to April 2004 near 9°50′N on the East Pacific Rise (EPR). Although semidiurnal tidal stress changes are small (<2 kPa), seismicity exhibits a significant (>99.9%) nonrandom temporal distribution, with events occurring preferentially near times of peak extension. Due to the proximity of this site to an ocean tidal node, where changes in sea surface height are minimal, periodic stress changes are dominated by the solid Earth tide. In contrast, previous studies on the Juan de Fuca Ridge have shown microearthquake triggering to be a response to seafloor unloading during times of low ocean tide. The modulation of 9°50′N microearthquakes by small‐amplitude periodic stresses is consistent with earthquake nucleation within a high stressing rate environment that is maintained near a critical state of failure by on‐axis magmatic and hydrothermal processes.
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