New Mexico Geological Society Annual Spring Meeting — Abstracts

Between February 25 and March 16, 1983, an earthquake swarm consisting of approximately 300 recorded shocks (100 above magnitude 0.0) occurred 28 km north of Socorro. Included within the swarm was a magnitude 4.0 earthquake, the largest in the Socorro area since 1961. A temporary seismograph array surrounding the swarm area, in addition to the eight permanent stations in the Socorro network, allowed precise locations of many of the larger earthquakes. Most of the earthquakes occurred at depths between 5 and 8 km, but there appeared to be no systematic migration of hypocenters with time. The 30 earthquake hypocenters with small errors (<0.5 km) seem to define a planar area that strikes NNW and dips 50° to the NE. The orientation of this plane roughly agrees with the composite fault plane solution for the swarm, which indicates normal faulting on a plane that strikes in a northerly direction. The roughly N-S trends of the hypocenters and the fault plane solution also agree well with the extensional stress regime of the Rio Grande rift, including the strikes of recent fault scarps in the area.

The swarm is located at the center of a region of surface uplift defined by level-line data, and above an extensive (> 1700 km²) layer of magma at a depth of 19 ± 0.6 km. A possible cause for the swarm is that upward migration of magma from the mid-crustal magma body produced movement on pre-existing faults.