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

Abstract

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°50′N, 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.

Tidal Triggering of Microearthquakes Over an Eruption Cycle at 9°50'N East Pacific Rise

“…Studies on whether the periodic stress change of ∼1 kPa produced by tidal forcing modulates global earthquake timing have produced conflicting results (e.g., Emter, ). Nevertheless, studies have found that earthquake timing correlates with tides for certain environments such as mid‐ocean ridges (Bhatnagar et al, ; Stroup et al, ; Tolstoy et al, ; Wilcock, , ; Wilcock et al, ), shallow thrust faults (Cochran et al, ; Tanaka et al, ; Wilcock, ), and volcanic regions (McNutt & Beavan, ; Rydelek et al, ). In subduction zones, studies have also suggested that tidal triggering of earthquakes appears in the years preceding some large earthquakes and disappears after (Tanaka et al, , , , ).…”

“…Looking at microearthquakes recorded between October 2003 and January 2007, Bhatnagar et al () subsequently showed that microearthquake timing also correlates with fortnightly tides. While true fortnightly tides have amplitudes of only about 10% that of semidiurnal tides (Agnew, ), fortnightly modulations of diurnal and semidiurnal tides have amplitude ranges comparable to semidiurnal tides.…”

“…While true fortnightly tides have amplitudes of only about 10% that of semidiurnal tides (Agnew, ), fortnightly modulations of diurnal and semidiurnal tides have amplitude ranges comparable to semidiurnal tides. Bhatnagar et al () found that the microearthquakes occurred preferentially during times of increasing volumetric extensional stress before the eruption, and during times of decreasing volumetric extensional stress after the eruption.…”

Loading‐Induced Earth's Stress Change Over Time

“…These loading‐induced stress changes are also found to play an important role in the triggering or modulation of many tectonic events. For example, various earthquakes have been found to be triggered by tidal forces (Bhatnagar et al, ; Brinkman et al, ; Cochran et al, ; Delorey et al, ; Ide et al, ; Johnson et al, ; Royer et al, ; Tan et al, ; Tanaka et al, , ; Tsuruoka et al, ; van der Elst et al, ; Wilcock, ), hydrological loading (Audet & Schwartz, ; Bollinger et al, ; Brothers et al, ; Chanard et al, ; Craig et al, ; González et al, ; Johnson et al, ; Kundu et al, ; Saar & Manga, ; Yadav et al, ), atmospheric pressure (Johnson et al, ), and PGR (Arvidsson, ; Hetzel & Hampel, ); slow earthquakes be triggered by strong hydrological and atmospheric loading induced by typhoon (e.g., Liu et al, ); tremors be triggered and modulated by tidal forces (e.g., Nakata et al, ; Thomas et al, ; van der Elst et al, ); and slow slip events be modulated by tidal forces (Hawthorne & Rubin, ) and hydrological loading (Lowry, ). In addition, volcanoes have been found to be triggered by tidal loading (e.g., Emter, ; Kasahara, ; Mauk & Johnston, ), and landslides be triggered by atmospheric and hydrological loading (e.g., Schulz et al, ; Tsou et al, ).…”

Fortnightly modulation of San Andreas tremor and low-frequency earthquakes

“…Fortnightly modulation has been anticipated, but only recently found, for tectonic earthquakes (22,23), and it has not yet been investigated for LFEs. The fortnightly tidal cycle can be thought of as the beat frequency arising from the interference between solar and lunar tides.…”