Jul 032017
 

Nearly 100 potentially hazardous faults have been identified in Colorado. Generally, these are faults thought to have had movement within about the past 2 million years. There are other faults in the state that may have potential for producing future earthquakes. Because the occurrence of earthquakes is relatively infrequent in Colorado and the historical earthquake record is relatively short (only about 130 years), it is not possible to accurately estimate the timing or location of future dangerous earthquakes in Colorado. Nevertheless, the available seismic hazard information can provide a basis for a reasoned and prudent approach to seismic safety.

Faulting

Sudden movement on long faults is responsible for large earthquakes. By studying the geologic characteristics of faults, geoscientists can often determine when the fault last moved and estimate the magnitude of the earthquake that produced the last movement. In some cases it is possible to evaluate how frequently large earthquakes occurred on a specific fault during the recent geological past.

Geological studies in Colorado have discovered about 100 faults that moved during the Quaternary Period (past 2 million years) and could be considered potentially active. The Sangre de Cristo fault, which lies at the base of the Sangre de Cristo Mountains along the eastern edge of the San Luis Valley, and the Sawatch fault, which runs along the eastern margin of the Sawatch Range, are two prominent and potentially active faults in Colorado. However, not all of Colorado’s potentially active faults are in the mountains. For example, the Cheraw fault, which is in the Great Plains Physiographic Province in southeast Colorado, appears to have had multiple movements during the recent geologic past. Some potentially active faults cannot be seen at the earth’s surface. The Derby fault near Commerce City lies thousands of feet below the earth’s surface. It has not been recognized at ground level, and for that reason it is not included on the CGS Earthquake and Late Cenezoic Fault and Fold Map Server [1].

Screen-shot from the Earthquake and Late Cenezoic Fault and Fold Map Server.

Screen-shot from the Earthquake and Late Cenezoic Fault and Fold Map Server.

Several potentially active faults in Colorado are thought to be capable of causing earthquakes as large as magnitude 7.2 based on recent detailed studies. In comparison, California has hundreds of hazardous faults, one or two of which can cause earthquakes of magnitude 8.0 or larger. The time interval between large earthquakes on faults in Colorado is generally much longer than on faults in California.

Past and Possible Future Earthquakes

About 400 earthquake tremors of magnitude 2.5 or higher have been reported in Colorado since 1867. More earthquakes of magnitude 2.5 to 3.0 probably occurred during that time, but were not recorded because of the sparse distribution of population and limited instrumental coverage in much of the state. The largest known historical earthquake in Colorado occurred on November 7, 1882 and had an estimated magnitude of 6.6. The location of this earthquake probably was in the northern Front Range.

Although many of Colorado’s earthquakes occurred in mountainous regions of the state, some have been located in the western valley and plateau region or east of the mountains. The best known Colorado earthquakes were a series of events in the 1960s that were later shown to be triggered by the injection of liquid waste into a deep borehole at the Rocky Mountain Arsenal. Twelve of the so-called “Arsenal” earthquakes caused damage, including a magnitude 5.3 earthquake on August 9, 1967 that resulted in more than a million dollars in damage in Denver and the northern suburbs. This series of earthquakes continued for about ten years and was followed by about six years of quiescence. Earthquake activity resumed in the northeast Denver area in 1978, including a magnitude 4.3 event on April 2, 1981.

Colorado’s earthquake hazard is similar to other states in the intermountain west region. It is less than in states like California, Nevada, Washington, and Oregon, but greater than many states in the central and eastern United States. It is prudent to expect future earthquakes as large as magnitude 6.6, the largest historical event in Colorado.

Conclusions and Recommendations

Based on Colorado’s historical earthquake record and geologic studies, an event as large as magnitude 6.5 to 7.2 could occur somewhere in the state. Scientists are unable to accurately predict when the next major earthquake will take place in Colorado; only that one will occur. The major factors that prevent the prediction of the timing and location of future damaging earthquakes are the limited knowledge of potentially active faults and short historical record of earthquakes. Given Colorado’s continuing active economic growth and the accompanying expansion of population and infrastructure, it is prudent to continue the study and analysis of earthquake hazards. Existing knowledge should be used to incorporate appropriate levels of seismic safety into building codes and practices. Seismic safety of critical facilities and vulnerable structures is especially important. Emergency response and recovery planning should consider earthquake hazards and risk. Concurrently, we should expand earthquake monitoring, geological and geophysical research, and mitigation planning and activities.

References:

[1] Kirkham, R. M., W. P. Rogers, L. Powell, M. L. Morgan, V. Matthews, and G. R. Pattyn. “Bulletin 52B – Earthquake and Late Cenezoic Fault and Fold Map Server.” Earthquake. Bulletin. Denver, CO: Colorado Geological Survey, Department of Natural Resources, 2004.

Jun 272017
 
OF-16-02 Geologic Map of the Watkins Quadrangle, Arapahoe and Adams Counties, Colorado

We’ve just uploaded the next of our free STATEMAP quadrangle map products to our online store: the Geologic Map of the Watkins Quadrangle, Arapahoe and Adams Counties, Colorado. The STATEMAP series in general provides a detailed description of the geology, mineral and ground-water resource potential, and the geologic hazards of an area. Digital PDF/ZIP download.

Location of the Watkins Quadrangle, Arapahoe and Adams Counties, Colorado.

Location of the Watkins Quadrangle, Arapahoe and Adams Counties, Colorado.

Matt Morgan, Senior Research Geologist and CGS Deputy Director, along with Senior Engineering Geologist (Emeritus) Jon White generated this map with special input from Richard Madole (surficial geology) and Shannon Mahan (OSL analysis), both of the USGS. This free release from the CGS includes two PDF plates (with a geologic map, cross-section with correlation, oblique 3D view, and legend) along with the corresponding GIS data package that allows for digital viewing, all in a single ZIP file.

This mapping project was funded jointly by the U.S. Geological Survey through the STATEMAP component of the National Cooperative Geologic Mapping Program, which is authorized by the National Geologic Mapping Act of 1997, and also by the CGS using the Colorado Department of Natural Resources Severance Tax Operational Funds. The CGS matching funds come from the severance paid on the production of natural gas, oil, coal, and metals. Geologic maps produced through the STATEMAP program are multi-purpose information sources useful for land-use planning, geotechnical engineering, geologic-hazard assessment, mineral-resource development, and ground-water exploration.

This particular 7.5-minute, 1:24,000 quadrangle is situated within the Denver Basin, a Laramide-age structural basin that is an important resource for water along with oil & gas. Growth of the Denver Metro area is occurring in the northern half of the quadrangle which is crossed by Interstate 70 and is minutes from Denver International Airport. Dips within the quadrangle typically range from 3° to 7° to the N-NE which reflects the regional structural dip of the basin. Bedrock units consist of the lower part of the Dawson Arkose and the Denver Formation. The widespread Dawson Arkose is white to tan in color and composed of cross-bedded arkoses, pebbly arkoses and arkosic pebble conglomerates with sparse claystone and siltstone beds. The arkoses were shed off the uplifting Front Range into the subsiding Denver Basin during the latter phases of the Laramide Orogeny. Cobble-rich conglomeratic lenses were recognized in the lower part of the Dawson Arkose and represent localized flooding events in a typically quiet fluvial environment. The Denver Formation is finer grained, more clay rich, and yellower in color than the overlying Dawson Arkose and is part of a low-energy alluvial plain environment also related to the Laramide. The units are separated by a basin-wide, yet occasionally discontinuous variegated paleosol that is a regional unconformity and an important time-stratigraphic marker at the Paleocene-Eocene boundary.

Surficial deposits consist of middle Pleistocene to Holocene flood-plain and terrace-forming alluviums and Holocene sand deposits of predominantly eolian origin. The sand deposits are composed of disaggregated sediments derived from the weathering and subsequent mobilization of the underlying Dawson Arkose. New Optically Stimulated Luminescence (OSL) ages, collected during this project, indicate that these eolian deposits were first active during the lowermost Holocene. High-level gravel deposits of Neogene-early Quaternary age cap isolated buttes in the southern half of the quadrangle. These gravels consist of cobbles and boulders of granite, quartzite, sandstone and tuffaceous igneous rocks and were likely derived from the erosion of the late Eocene Castle Rock Conglomerate.

Citation: Morgan, Matthew L., and Jonathan L. White. “OF-16-02 Geologic Map of the Watkins Quadrangle, Arapahoe and Adams Counties, Colorado” Geologic. Open File Reports. Golden, CO: Colorado Geological Survey, 2016.