Paleoseismology

Earthquakes are caused by movement on faults. It is important to understand the past behavior of faults in Colorado in order to understand what future hazard. Paleoseismicity is the discipline of trying to determine the past movement behavior of faults.

Intro

To understand the earthquake hazard in an area, one must know what the earthquake history has been. Because the interval between large earthquakes on any given fault is often in the neighborhood of hundreds or even thousands of years, it is necessary to study when large earthquakes have occurred on the faults themselves. These studies of faults fall under the category of paleoseismicity, or the study of ancient 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.

Trenching

Trenching is a common technique for investigating a known active fault with a surface expression or rupture. By carefully excavating a series of trenches across the fault, the structure and magnitude of the fault displacement may be determined.

Paleoseismic studies

Paleoseismic studies are most commonly performed when a critical facility is in the planning stages, such as a dam or a power plant. For example:

• The Ken Caryl fault and Kennedy fault trenches were excavated to study earthquake hazards that might affect the construction of the Two Forks Dam.
• The Golden fault was extensively studied as part of the due diligence for building the Rocky Flats Technology Site (the official name for the cleanup operation).
• The Rampart Range fault trench was excavated to determine risks associated with construction of the Air Force Academy water treatment plant.

Other studies are to determine more general paleoseismic histories, albeit ones that can inform current construction codes, emergency management plans, and land use issues:

Ostenaa, Dean A., and Mark S. Zellman. “Paleoseismic Investigation of the Cheraw Fault at Haswell, Colorado.” Paleoseismic. Miscellaneous Investigations. Golden, CO: Colorado Geological Survey, November 2018.

Kirkham, Robert M., David C. Noe, Lauren Heerschap, James P. McCalpin, Shannon Mahan, and Matthew L. Morgan. “MI-100 Summary Report on the McQueary Gulch Trench, Williams Fork Mountains Fault, Grand County, Colorado.” Paleoseismic. Miscellaneous Investigation. Golden, CO: Colorado Geological Survey, June 2020.

Mapping

ON-006-15M — Colorado Earthquake and Fault Map — shows a variety of information including Cenozoic faults. (supersedes publications B-46; B-52; B-52B, and IS-60).

Sackungen

Sackungen are a special category of faults that disrupt the ground surface and are typically found in glaciated, alpine regions near the crest of a mountain range. They are not interpreted to extend very deep into the crust and are probably not capable of generating earthquakes. However, they can potentially yield information on past earthquakes on nearby faults, and so, are the object of research here in Colorado.

A sackung structure can be a trench (small-scale graben) or an upslope-facing scarp. Their origin is interpreted to be a result of post-glacial, gravitational spreading of the ridge crest and over-steepened ridge flanks. The plural of this German word is sackungen.