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Colorado Earthquake Hazard Colorado Earthquake Hazard Colorado has a significant earthquake history. Over 400 earthquakes have occurred since 1870. The largo est known earthquake, the 6.5 magnitude event on November 7, 1882 caused damage on the eastern and western slopes and was felt throughout the state. In the past few decades, scientists have learned more about the location and size of Colorado earthquakes. Geological and geophysical investiga- tions have successfully identified active faults that are capable of generating future damaging earth- quakes. Many of these faults are visible on the land surface. However, other faults are hidden under rock and soils. In order to locate these hidden faults, net- works of seismic instruments set up in different ar- eas of the state have recorded earthquakes that indicate additional seismic source zones. These in. vestigations suggest Colorado is a moderately active earthquake area with potential for significant earth- quakes capable of causing deaths and damage to buildings, roads, and utilities. The most recent damaging earthquakes in Colorado occurred in 1967 along the Front Range. The August 9, 1967, 5.3 magnitude earthquake caused approxi- mately one million dollars in damage in northern Denver suburbs. The 5.2 magnitude earthquake on November 27, 1967 occurred shortly thereafter, causing additional damage. Minor damage occurred in April, 1982 when a tremor that registered 4.5 on the Richter scale affected the Denver area. The earthquake was felt in downtown office buildings and slight damage occurred in Commerce City and Thornton. Although no complete collapse of substantial struc- tures or deaths have yet resulted from a Colorado earthquake, numerous instances of damage to homes and businesses have resulted. Colorado earthquakes have disrupted power service, affected transportation systems, and caused structural dam- age in buildings. Earthquakes in Colorado have also cracked plaster walls and foundations, toppled chim- neys and un-reinforced masonry walls, and damaged roof tiles. Although no deaths have occurred, people have been injured from these non-structural falling hazards. In addition, earthquakes in Colorado have broken windows, dishes, and other household items. Non-structural damage in businesses has been caused when earthquake ground shaking emptied store shelves of food and other items. Seismic Risk Colorado seismicity rates are consistent with a mag- nitude 6 or larger earthquake occurring within our lifetimes. The geologic record suggests that parts of Colorado have repeatedly experienced earthquakes of up to magnitude 6.5 to 7.5 during the past million years. There are more than 200 known, potentially active faults within Colorado's complex geological struc. tures. Of special concern are faults associated with the Rio Grande Rift zone in south central Colorado. In addition to the known faults, seismograph instru- ments have recorded seismic activity in other areas of Colorado where earthquakes have been felt, but where faults were not visible to geologists. A com-. plete review of existing infonnation on Colorado Date Location Mag/Int 1870, Dee 4 Pueblo Ft Reynolds 5+NI 1871, Oct Lily Psrk Moffat 5+NI 1880, Sept 17 Aspen 5+NI 1882, Nov 7 N. Central CO 6.5+Nll 1891,Dee Axial Basin 5+NI 1901, Nov 15 Buena Vista 5+NI 1944, Sept 9 Montrose-Basalt 5+NI 1955, Feb 10 Steamboat Springs 5+NI 1955, Aug 3 Lake City 5+NI 1980, Oct 11 Montrose~Basalt 5.5NI 1966, Jan 4 N.E. of Denver 5.0N 1966, Jan 23 Dulce, NM . 5.5NII 1967,Aug 9 N.E. of Denver 5.3Nll 1967, Nov 27 N.E. of Denver 5.2IVI 29