Nevada's Quaternary Fault Hazard

Spring 1993

A statewide seismic hazard study commissioned by the Nevada Department of Transportation included the identification and characterization of major Quaternary faults in and adjacent to Nevada. The first-cut data set produced by this NBMG study consists of the 304 faults and seismogenic sources shown on the map. Only the largest faults in each area were considered because it was impractical to analyze all of the thousands of individual Quaternary faults in Nevada.

The data set includes fault systems, fault zones, faults, and seismogenic lineaments. Faults are generally singular, continuous features. Fault zones are more distributed in nature and involve a collection several faults. Fault systems are collections of faults or fault zones that may have different characteristics, but overall are part of the same structural package or bound the same mountain range. For example, the Carson Range fault system bounds the eastern side of Carson Range, but consists of two fault zones, the Genoa fault zone in Carson Valley and the Mt. Rose fault zone to the north. Seismogenic lineaments are collections of discontinuous fault scarps and/or other tectonogeomorphic features, and/or lineaments of seismicity that are illuminating an active structure at depth. Seismogenic lineaments are essentially reactivated older fault zones or zones of weakness in the crust. They are treated differently than the faults in that earthquakes are thought to occur over smaller portions of these features and to be more limited in potential size.

Most of the faults on this map are range-bounding faults along most of their extent. There are several cases, however, where faults are located within the basins, and there are numerous smaller Quaternary faults not considered in this study that are likewise intrabasinal. In a few cases, large range-front warps or folds that have evidence of Quaternary activity and appear to overlie faults are included. However, these features are harder to identify and characterize, especially with respect to their activity, and many more probably exist in Nevada.

The majority of the faults considered in this study appear to have dominantly a normal sense of displacement, although several significant strike-slip faults exist as well, especially in western Nevada.

Geomorphic features associated with Nevada Quaternary faults are commonly spectacular, primarily due to the and climate, and some of these are shown in the accompanying block diagram. This diagram separates normal- and strike-slip faults for clarity, but a combination of the two along the same fault is not unusual. The A and T on the side of the block diagram represent away and toward and indicate the strike-slip movement along the fault in the reference frame of the viewer.

These faults have been, and continue to be, analyzed for their seismic potential. First-cut estimates of the potential earthquake magnitudes associated with these faults, how often earthquakes occur, and how fast the faults are moving have been made for most of the faults. In most cases, however, these values need to be supported or adjusted with field measurements. The most uncertain parameters are how often earthquakes occur along a fault and, especially, when the next large earthquake will occur along a particular fault or in an area. We need to know better not only the paleofrequency of earthquakes along individual faults, but also the earthquake behavior of the faults and what affects that behavior. The few data presently available on Nevada faults suggests that earthquakes do not occur at regular or periodic intervals, but they seem to cluster in time or show some sort of "chaotic" behavior. Stress and strain interactions between adjacent faults influence earthquake activity as well, and may also help us understand the likelihood of earthquakes.

The results of this study underscore that earthquake faults are a statewide hazard. Large earthquakes can occur nearly everywhere in Nevada. The main variation across the State is the frequency of these earthquakes. This is consistent with the historical earthquake pattern in which most events have occurred in western Nevada, fewer in eastern Nevada, and fewer still in southern Nevada. The fault data are inconsistent with the historical earthquake record in that no large earthquakes have occurred during the historical record in eastern and southern Nevada. However, the fault data assure that at some time in the future, they will.

---Craig dePolo, Research Geologist