Earthquake Summary Poster

The August 10, 2005, earthquake occurred in the southern Rocky Mountains near the Colorado-New Mexico border, about 40 km (20-30 mi) southwest of Trinidad. The earthquake occurred within a broad region of extensional tectonic stress that is associated with the highlands of the intermountain states and that extends west from westernmost Great Plains to the Pacific coastal states. Earthquakes within the broad zone typically occur as the result of normal faulting or strike-slip faulting. The August 10, 2005, earthquake occurred as the result of slip on a north-striking normal fault.

The August 10 mainshock was preceded by approximately 20 small, instrumentally recorded, earthquakes that occurred beginning in 2001, and whose calculated locations are close enough to the August 10 epicenter that they may have been on the same fault. Few of the earlier shocks were reported felt, and none are known to have caused damage. In addition to previous earthquakes that may have occurred on the same fault, a swarm of small earthquakes occurred in September 2001 several tens of kilometers northeast of the August 10, 2005, shock. The locations and focal mechanism of the September 2001 earthquake swarm imply that those shocks also occurred on a normal fault, but the fault that produced the September 2001 earthquake swarm had a northeasterly rather than northerly strike. The largest earthquake in the 2001 swarm had M 4.6 and produced minor damage. The southern Rocky Mountains and adjacent Great Plains in general have been characterized by a moderately low level of earthquake activity, substantially lower than in some parts of the intermountain states to the west, but higher than in many sections of the Great Plains farther to the east.

Throughout the highlands of the western United States, the largest earthquakes have tended to occur on major faults that outcrop on the earths surface, but small and moderate shocks commonly occur on faults that are too small or situated too deep in the earths crust to be mapped even retrospectively, after the earthquakes have occurred. A major, active, geologic fault has not been mapped in the immediate vicinity of the epicenter of the August 10 shock. The shock was situated 50 km east of the Rio Grande rift, a 1000 km long north- south zone of major normal faults where east- west tectonic extension has been concentrated within the past few millions of years. Geologic evidence indicates that large prehistoric earthquakes have produced surface ruptures on many of the Rio Grande rift faults in the late Pleistocene (10,000 to 130,000 years ago) or Holocene (within the last 10,000 years). The August 2005 shock likely reflects the same regional extensional stress field that has produced, and will continue to produce, occasional larger shocks on faults of the Rio Grande rift.