Prominent Fault Scarps in Western Nevada

Spring 1992

Although there have been no strong earthquakes in western Nevada in almost 30 years, several have occurred since the region was settled about 150 years ago and many older ones are recognized from geologic evidence. The effects of large earthquakes can be seen throughout the region. This article briefly describes some of the better locations for viewing the effects of past earthquakes in western Nevada.

Earthquakes are caused by the sudden movement along faults in the earth's crust (fractures along which rocks have moved, or slipped, relative to each other). The size of an earthquake is related to the size of the fault surface that slips and to the amount of slip. At the beginning of earthquakes, fault slip starts miles below the earth's surface and then extends outward and upward. Most earthquakes (small to moderate earthquakes, in particular) stop before the fault slip reaches the ground surface, but during large earthquakes the fault slip usually creates a break (or fault scarp) at the ground surface. Fault scarps and other geomorphic features caused by recent earthquakes can be seen in many places throughout western Nevada.

What to look for -- Fault scarps appear as a step in the ground, and can be a few inches to several tens of feet high, but they are extremely difficult to recognize unless they are at least a few feet high. Scarps that were formed very recently (the 1954 earthquake scarps, for example) have a "scarp face" (steep part of the step) that is often quite steep and bare of vegetation. With time, the scarp face becomes worn down by erosion and covered by vegetation, making the scarp much less obvious. Fault scarps are often most obvious when the sun is low on the horizon, creating a shadow on the scarp face while the surrounding surface is sunlit. Early morning and late afternoon hours are thus the best times for fault scarp hunting. Fault scarps are commonly located at or near the base of steep mountain ranges, where the slope abruptly changes. A map of recent earthquake breaks (for example; Quaternary fault map of Nevada - Reno sheet, NBMG Map 79) can be an invaluable guide to locating fault scarps.

1954 earthquake area -- In 1954, four large earthquakes shook western and central Nevada; the largest of these had a magnitude of 7.2. These four earthquakes were centered in the Stillwater Range/Dixie Valley area east of Fallon; this is perhaps the best location in western Nevada for viewing earthquake scarps. One excellent site for visiting the 1954 scarps is located near U.S. Highway 50 about 40 miles east of Fallon. To get there, turn south from U.S. 50 onto a dirt road 6 miles west of the Middlegate turnoff (State Route 361). This dirt road is marked by an "Earthquake Scarps" sign and continues south through Stingaree Valley on the east side of Fairview Peak. The 1954 scarps at this location lie at or near the base of Fairview Peak and are up to 20 feet high. Two locations, one 5.5 miles south of U.S. 50 and the other 9-10 miles south of U.S. 50 (marked by signs), are good sites to view the 1954 earthquake scarps.

Genoa fault -- The Genoa fault (labelled GFZ on the map on page 2) is responsible for the steep mountain front on the west side of Carson Valley (in the Minden/Gardnerville area) and is one of the most active faults in western Nevada. The most recent earthquake on this fault is believed to have occurred within the past few hundred years and probably had a magnitude of about 7.5. During mid- to late afternoon hours, the fault scarp at the base of the mountain range is shaded and quite prominent. About 1.2 miles south of the town of Genoa, near Walley's Hot Springs Resort, the Genoa fault is exposed in a gravel quarry. A polished and grooved bedrock fault surface (slickensides) forms the western wall of the gravel pit. The grooves on this surface were formed as the rocks on either side of the fault moved past one another. Immediately south of the gravel quarry, the fault has uplifted outwash terraces (meltwater deposits from ancient glaciers located higher in the Carson Range) at the mouth of a moderate-size drainage. The very large scarp (more than 50 feet high) at this location is the combined effects of several earthquakes.

Peavine Mountain -- During late afternoon hours, a fault scarp can be seen at the base of the northwest side of Peavine Mountain (adjacent to the Stead/Silver Lake area). The most recent earthquake on this fault (NPMF on the map) occurred sometime within the past several thousand years.

Mount Rose -- Several fault scarps along the Mount Rose fault zone (MRFZ on the map) lie between Steamboat Hills and the base of Mount Rose in the Callahan Ranch area. Some of these cross the Mt. Rose Highway (State Route 431). At mile marker 20.00, the highway lies within a low area created by movement on faults to the east and west. Such downfaulted areas, or grabens, are common in actively extending areas such as Nevada.

---Alan Ramelli, Research Geologist