The 1986 Crested Butte earthquake swarm: A model for contemporary seismogenesis in Colorado

Historically, western and central Colorado have been characterized by a low to moderate level of seismicity; one event, however, has exceeded M[sub L]6. Contemporary seismicity appears to occur as earthquake swarms of which only one other (1984 Carbondale swarm) has been studied in detail. The Crested Butte sequence of Aug. and Sept. 1986 provides the best data at present to evaluate the source characteristics of an earthquake swarm in Colorado and the associated tectonic stresses. This swarm is one of the largest known tectonic releases of seismic energy in Colorado in recent times and provides a model for contemporary seismogenic faulting in the Rocky Mountains. It occurred in the Ruby Range, a region of extensive middle Tertiary volcanic and intrusive activity. NEIC recorded 30 events of M[sub L] 1.6 and greater, 16 of which were reported felt; the largest event was a M[sub L] 3.5. In addition to the regional recordings, the USGS and Microgeophysics Inc. deployed a portable network from 19--26 Aug. to monitor the swarm. Based on these data the authors relocated 29 regionally recorded events and 49 microearthquakes recorded by the portable network using a master event technique. The events define a 6 km long, northwest-trending zonemore » dipping steeply to the northeast, between the depths of 2--14 km. Focal mechanisms for the six master events and the largest earthquake indicate predominantly normal faulting with minor right-lateral strike-slip components on approximately NW-striking, NE-dipping planes. These planes and the hypocentral distribution are consistent with the location and orientation of a Late Tertiary/Quaternary fault recognized by Kirkham and Rogers (1981) which they informally name of the Treasure Mountain fault. Earthquakes in Colorado appear to be the result of normal slip on reactivated faults that are favorably oriented to the contemporary stress field.« less