Anatomy of a strong and prolonged Rio Grande Rift earthquake swarm
— Alan R. Sanford and Hans E. Hartse

Between November 1989 and June 1991, a strong earthquake swarm near Bernardo, New Mexico occurred within the Rio Grande rift. Using data from their short-period seismograph network, temporarily deployed seismic stations, and other regional seismic data, researchers at New Mexico Tech observed four earthquakes of between duration magnitude 4.1 and 4.7 (all felt in Socorro), and many additional smaller earthquakes. The Gutenberg-Richter relationship between earthquake frequency and magnitude for the swarm has a b-value of 0.57, or fewer small events relative to the numbers of large events than is typically observed for tectonic areas (where b-values are close to 1). The best-constrained aftershock locations following the two largest events cover epicentral areas of between 4 and 4.5 km², with depths ranging between 4 and 6.5 km. Aftershock trends, focal mechanism solutions from first motions, and modeling of regional broadband data conducted by other researchers suggest the swarm occurred along adjacent, nearly parallel normal faults with strikes trending slightly east of north. The events occurred just above a west-to-east trending listric fault previously imaged on active-source crustal profiles obtained in the Bernardo area. Thus, a triggering mechanism for the Bernardo swarm may be aseismic movement on the east-trending listric fault stressing north-trending normal faults in the shallower crust above.