THE 1986 CRESTED-BUTTE EARTHQUAKE SWARM AND ITS IMPLICATIONS FOR SEISMOGENESIS IN COLORADO

In August and September 1986, an earthquake swarm of possibly several hundred earthquakes occurred near Crested Butte, Colorado. The epicent ral area is located within the Ruby Range in a region of extensive mid dle Tertiary volcanic and intrusive activity. The recording of this se quence has provided the best data to date to evaluate the source chara cteristics of an earthquake sequence in Colorado and the associated te ctonic stresses. At least 200 events were recorded at regional distanc es; 30 events were Richter magnitude (M(L)) 1.6 and greater, and 16 we re reported felt. The largest event, M(L) 3.5, occurred on 3 September . In addition to the regional recordings, a portable seismographic net work was deployed from 19 to 26 August. Based on these data, 78 events were relocated using a master event technique. The earthquakes define a 6-km-long, northwest-striking planar zone dipping steeply to the no rtheast, between the depths of 2 and II km. Focal mechanisms indicate predominantly normal faulting with a minor left-lateral component on a n approximately northwest-striking, northeast-dipping plane. These obs ervations are all consistent with possible slip on the Treasure Mounta in fault, a late Tertiary structure within the Ruby Range. The northea st-oriented T axes exhibited by the Crested Butte focal mechanisms are consistent with the regional extensional stress direction characteris tic of the southern Rocky Mountains as indicated by other earthquake f ocal mechanisms. Within this extensional stress regime, earthquakes in western Colorado appear to be the result of normal slip on reactivate d preexisting faults that are favorably oriented to the contemporary s tress field.