An investigation of tidally triggered seismicity in the central Rio Grande Rift of New Mexico

In this study, the degree of correlation between earthquake timing and the phase and stress orientation of the calculated solid-earth tide is evaluated for selected earthquake swarms occurring in the central Rio Grande rift of New Mexico. These earthquake swarms lie over the Socorro mid-crustal magma body in central New Mexico and occur near Heron and El Vado reservoirs in north-central New Mexico. The Socorro swarms appear to be initiated by the injection of small volumes of magma into the upper crust along deep fracture zones. Earthquakes near Heron-El Vado are caused, in part, by elevated pore pressures at depth from reservoir loading.; To test the hypothesis that events in these fluid-driven seismic systems were tidally triggered, tidal phase values were calculated for each earthquake with reference to adjacent semidiurnal tidal maxima. The random probability of occurrence for the distribution of tidal phases in each swarm was determined using Schuster's test to evaluate the degree of tidal correlation. Earthquakes occurring in swarms 16 km southwest of Socorro, beneath the Socorro-Lemitar mountains, and in the vicinity of San Acacia at an average depth of 9.5 km tend to occur during times of peak and falling solid-earth tide (confidence level {dollar}>{dollar}95%). Earthquakes occurring near and beneath Heron-El Vado reservoirs in 1982 correlated with a rising solid-earth tide at a confidence level greater than 98%. No significant tidal correlation, however, could be found for the August 1977 earthquake swarm 16 km southwest of Socorro. Most active faults in the Socorro area strike NNW-SSE and exhibit predominantly normal and minor strike-slip components. Extensional tidal stresses oriented approximately east-west both reduce normal stresses and increase shear stresses on these faults. The earthquake-tide correlations in the Heron-El Vado area can be interpreted in a similar manner where tides enhanced tectonic stresses and possibly pore pressures.; The presence of tidal triggering in these areas indicates delicately balanced seismic systems very close to failure. Fluid pressures from magma, steam or water probably bring these volumes to the failure threshold. Correlations with different tidal phases may reflect the way that fluids in these systems interact with brittle rock. Magma moving upward intruding fractures has a different effect on stresses at depth than a wave of pore pressures propagating downward from a reservoir. This study may provide a basis for earthquake predictions based on tidal stresses if longer data sets and other earthquake prediction indicators can be utilized.