Intermountain West Region

Figure 1
Aerial view of Holocene fault scarp on the Wasatch fault, near American Fork Canyon, Utah

Aerial view of Holocene fault scarp on the Wasatch fault, near American Fork Canyon, Utah


The differential motion between the North American, Pacific, and Juan de Fuca plates is causing deformation throughout most of western North America. The majority of this deformation is occurring close to the plate boundaries in California and off the Oregon and Washington coasts, but a significant part of this deformation extends eastward across the Basin-and-Range Province of Nevada and Utah and throughout the Intermountain West, all the way to the eastern front of the Rocky Mountains. Even though the occurrence of earthquakes in the Intermountain West region is modest in comparison to the West Coast, it contains abundant evidence of geologically young (Holocene and Pleistocene in age) large earthquakes that have ruptured the ground surface and produced fault scarps (Fig. 1). The widespread distribution of young fault scarps in the region demonstrates the existence of a significant, but in general, poorly quantified seismic hazard. Furthermore, the large earthquakes that created these fault scarps generated strong shaking, yet we know few details about the strength and characteristics of this strong shaking in most of the region's urban centers.

Primary Objectives

  1. Collect data that directly contributes to improving the National Seismic Hazard maps with specific focus on faults that have slip rates sufficiently high to directly impact the maps.
  2. Obtain data and information that better quantify the frequency and characteristics of earthquake-generated strong ground motion in seismically hazardous parts of the Intermountain West, particularly in the more heavily populated urban corridors of the region.
Figure 2
Map showing historical seismicity in the Intermountain West
Map showing historical seismicity in the Intermountain West
To help define the seismic hazard in the Intermountain West and to better understand the nature of ground shaking, we are undertaking geological of specific faults and geophysical studies in urban areas. For the purposes of the USGS Earthquake Hazards Program, we define the Intermountain West as that portion of the Western U.S. extending from roughly the eastern side of the Sierra Nevada Range and Mojave Desert in eastern California, and the eastern side of the Cascade Range in Oregon and Washington, to the eastern margin of the Rocky Mountains in central Montana, Wyoming, Colorado, and southward through New Mexico and west Texas. It includes the extended terrain of the Basin-and-Range Province, the Colorado Plateaus, the Rocky Mountains, and the Rio Grande rift of west Texas, New Mexico and Colorado (Fig. 2). Within this vast region, historical and instrumental earthquakes are largely concentrated in the Intermountain Seismic Belt and in the Central Nevada Seismic Belt, but hundreds of Pleistocene and Holocene faults are mapped throughout the region, and few of these faults have been studied in detail. Although substantial parts of this area are sparsely settled, the region does contain many of the fastest growing urban areas in the country, including Las Vegas and Reno-Sparks in Nevada, the Wasatch Front urban corridor in Utah, the Front Range urban corridor in Colorado, Santa Fe-Albuquerque in northern New Mexico, and Las Cruces-El Paso-Juarez in southern New Mexico, Texas, and Mexico.
Figure 3
Map showing historical seismicity in the Intermountain West
Geologist studying and mapping deposits that have been displaced by movement on the Surprise Valley fault in northeastern California.

With few exceptions, our knowledge of the history of the Quaternary faults throughout the region is limited, and the characteristics of earthquake-induced ground motions are very poorly quantified. We seek to address these important issues by using geological and geophysical techniques to characterize the basin geometry and levels of shaking in the basin-fill deposits beneath selected urbanized areas, and by detailed and reconnaissance studies of selected faults that directly influence the National Seismic Hazard maps. We are attacking problems at a variety of scales ranging from detailed studies of individual faults (Fig. 3), to regional studies that clarify critical details of the tectonic processes operating in the region, to coordinating with colleagues who are conducting basin-wide studies of velocity structure and basin geometry.