New Mexico Geological Society Annual Spring Meeting — Abstracts

Five hundred and thirteen well located microearthquakes were used in an analysis of focal depths in the central Rio Grande rift near Socorro, New Mexico. The seismogenic zone extends from approximately 4 to 12 km in depth with more than 60 percent of the events occurring between 8 and 12 km. A sharp decrease in the number of focal depths below 12 km is interpreted as the rapid transition from brittle to ductile crust. This essentially normal depth to the seismogenic zone occurs in a region where heat flows as high as 490 mW/m2 (11.7 HFU) have been observed and where a thin magma body is known to exist at a depth of approximately 20 km. Several distributions of focal depths along a profile within the area are bimodal, possibly indicating a semi-ductile zone from 8 to 10 km lying between layers of more brittle crust above and below. Relatively shallow seismicity, with focal depths occurring predominantly between 4 and 8 km, exists approximately 25 km north of Socorro near the apex of a region of surface uplift. A previous study of a major swarm in this region found some microearthquakes whose first motions were compressive in all but 10 to 25 percent of the focal sphere, implying that these earthquaakes may be related to injecction of magma, steam or hot water.