Investigation of an earthquake swarm near Trinidad, Colorado, August-October 2001

Abstract

A swarm of 12 widely felt earthquakes occurred between August 28 and September 21, 2001, in the area west of the town of Trinidad, Colorado. The earthquakes ranged in magnitude between 2.8 and 4.6, and the largest event occurred on September 5, eight days after the initial M 3.4 event. The nearest permanent seismograph station to the swarm is about 290 km away, resulting in large uncertainties in the location and depth of these events. To better locate and characterize the earthquakes in this swarm, we deployed a total of 12 portable seismographs in the area of the swarm starting on September 6. Here we report on data from this portable network that was recorded between September 7 and October 15. During this time period, we have high-quality data from 39 earthquakes. The hypocenters of these earthquakes cluster to define a 6 km long northeast-trending fault plane that dips steeply (70-80?) to the southeast. The upper bound of well-constrained hypocenters is near 3 km depth and lower bound is near 6 km depth. Preliminary fault mechanisms suggest normal faulting with movement down to the southeast. Significant historical earthquakes have occurred in the Trinidad region in 1966 and 1973. Reexamination of felt reports from these earthquakes suggest that the 1973 events may have occurred in the same area, and possibly on the same fault, as the 2001 swarm. In recent years, a large volume of excess water that is produced in conjunction with coal-bed methane gas production has been returned to the subsurface in fluid disposal wells in the area of the earthquake swarm. Because of the proximity of these disposal wells to the earthquakes, local residents and officials are concerned that the fluid disposal might have triggered the earthquakes. We have evaluated the characteristics of the seismicity using criteria proposed by Davis and Frohlich (1993) as diagnostic of seismicity induced by fluid injection. We conclude that the characteristics of the seismicity and the fluid disposal process do not constitute strong evidence that the seismicity is induced by the fluid disposal, though they do not rule out this possibility.