Seismological studies at Cerro Prieto

Abstract

During the period 14 February — 11 March 1978, a radio-telemetered 12-station seismographic network was operated in a dense-array configuration over the Cerro Prieto field as shown in Fig. 1. Horizontal and vertical 4–5-Hz geophones were deployed with interstation distances of order 1 km in a multi-purpose survey for microearthquake occurrence and possible anomalies in P-and S-wave velocities and attenuation associated with the field. Seventeen sites were occupied on NE-SW and NW-SE lines crossing the production zone. Seventy-four earthquakes having S-P interval times less than 5 s (distance less than about 40 km) were detected. Six of these earthquakes were located within the field in a single swarm on February 18 and 19, at a depth of 2–3 km. Radiation patterns are consistent with right lateral strike-slip faulting on a north-west — south-east trending fault. P- and S-wave times for the events within the field reveal a high Poisson's ratio of 0.4 in the upper 2 km of sediments at the field. The field appears to be characterized by high attenuation near stations 5 and 6. There is some indication of slightly higher P-wave velocity throughout the field area, with a locally anomalous low velocity near the center of the production zone around station 6. Although data obtained thus far are limited, attenuation and velocity studies at Cerro Prieto show anomalies related to the production area. This suggests that these data might be useful for delineating the field boundaries. A five-station semi-permanent network of 150 m deep, three-component digitally recorded seismographs will be installed over the field in the fall of 1978 for more continuous monitoring of the field characteristics.