Summary Of the Late Quaternary Tectonics of the Basin and Range Province in Nevada, Eastern California, and Utah

(DERIVED FROM USGS DATABASE ON QUATERNARY FAULTS AND FOLDS IN THE UNITED STATES)
Michael Machette, Kathleen Haller, Richard Dart, and Susan Rhea
1996-2004

Most Quaternary faults in the Great Basin portion of the Basin and Range Province (Fig. 1) trend north-south, have normal slip displacement, and bound uplifted or tilted mountain ranges. Although the uplifted ranges are spectacular geomorphic features, the associated Quaternary faults whose relatively low slip rates result in relatively long recurrence intervals between Magnitude 6.5+ or larger earthquakes. A small percentage of the faults are quite active, especially those at the eastern and western margins of the province, including the Genoa (2-3 mm/yr), Death Valley (4-5 mm/yr), and Wasatch (1-1.5 mm/yr) faults. Hundreds of more typical Basin and Range faults are clearly less active, but their long-term behavior is not well known. Recent paleoseismic studies show that some of these faults have average slip rates of 0.05-0.15 mm/yr and recurrence intervals of tens to hundreds of thousands of years for earthquakes that rupture the ground surface. Although individual faults pose relatively low hazard, the cumulative hazard is significantly greater because hundreds of Quaternary faults riddle the province and, therefore, increase the average rate of earthquake recurrence in any particular region.

The USGS's new compilation of Quaternary faults (http://earthquake.usgs.gov/regional/qfaults/) contains about 750 structures that are reported to have evidence of Quaternary movement in the Great Basin (Fig. 2). In past 150 years, there have been 14 earthquakes large enough to rupture the ground surface. This historic earthquakes are concentrated mainly on the margins of the Great Basin (Fig. 3). Roughly 150 of these faults (20 percent) have evidence of surface rupture in the past 15,000 years, whereas 320 (43 percent) have similar evidence of rupture in the past 130,000 years. One result of recent paleoseismic investigations is that, in many cases, dating of the faulted deposits shows that the most recent movement is younger than the age that would be inferred on the basis of expression of the fault scarp on the ground surface (fault-scarp morphology), or from detailed mapping of geologically young deposits on the surface. Many geological processes can make a fault-scarp appear to be older than its true age, such as by burial by eolian, colluvial, or alluvial deposits.

In contrast, there are only few a ways to make a scarp look morphologically younger. Thus, many estimates of the time of most recent movement shown in the fault database probably err on the old side. In addition, the database groups the time of fault activity into inclusive time categories, such as less than 130,000 years, to bracket the times of faulting; thus, each category includes some younger faults. We suspect that the above cited number and percentages of faults with less than 15,000 years and less than 130,000 years since the time of the last movement are minimum values that will increase as more faults are studied in detail. One result of our analysis of the time/space patterns for faulting in the province is that the less than 130,000-year time window captures almost one-half of the Quatenary faults and realistically reflects their true distribution. This time window is long enough to span at least one typical earthquake cycle on most faults (two successive earthquakes on the same part of a fault, which defines one recurrence interval), whereas the less than 15,000-year time window is geologically too short to adequately sample all of the potential earthquake-generating faults. In 1988, de Polo and Slemmons pointed out that only about half of the historical ruptures in the Basin and Range Province occurred on mapped Holocene faults, thus showing that 10,000 years to too short to sample historic and future surface-rupturing faults.

Except for aftershock activity associated with some historical ruptures in the province, it is difficult to associate recorded seismicity with specific faults and there are virtually no examples of foreshock activity preceding large earthquakes. For example, the Wasatch fault zone, the longest and most prominent fault in the province is poorly defined by earthquakes on Utah seismicity maps, and the Thousand Springs segment of the Lost River fault in Idaho was virtually aseismic at M>3.5 for at least two decades before it ruptured during the 1983 Borah Peak earthquake. Similar examples are common in the Great Basin, especially in its southern half. For the most part, normal faults of the Great Basin seem to be aseismic and locked, but some may be close to the point of failure as was the case with the 1954 Fairview Peak and Dixie Valley earthquakes.

Data from the global positioning system (GPS) of satellites shows some close associations with the fault data in the Great Basin. The entire province is between the Sierra Nevada Mountains in eastern California and the Wasatch Mountains in central Utah is expanding in a roughly east-southeast to west-northwesterly direction at a rate of about 13 mm/yr. Recent analyses of GPS data show a simple pattern in which the extension is concentrated in three belts: 1) along the Wasatch Front Utah, in the Intermountain seismic belt (ISB), 2) in the Central Nevada seismic belt (CNSB) in west-central Nevada, and 3) along the Eastern California seismic belt (ECSB) in extreme eastern California (Fig. 4).

Generally, the central part of the Great Basin (eastern Nevada and western Utah) show little evidence for contemporary extension and the geological evidence indicates that the timing most of the surface rupturing on faults in this area is late Quaternary (<130,000 years) or older. One conclusion from the pattern of fault slip rates is that most of the gross topography of the interior Great Basin is probably a relict of the late Miocene and Pliocene normal faulting, with minor rejuvenation during the Pleistocene.

The CNSB and ECSB have been the preferred areas where historical earthquakes larger than M 6.5 have occurred in the Basin and Range Province (Fig. 3). Between 1872 and 1954, seven large earthquakes caused surface ruptures along this NNE-trending belt yielding an incredibly high average of one rupture every 14 years. Recent summaries of paleoseismic investigations of the CNSB have shown that this rate of earthquakes that rupture the surface and the spatial pattern of activity is anomalous. There is no compelling evidence that a similar pattern of concentrated activity has occurred in this belt in the past 50,000 years, and there has been almost 50 years of quiescence since the last large earthquake. So, two of the most pertinent but unanswered questions about the CNSB are ?why here and where next?? Ultimately, the broader scientific challenge in the Basin and Range Province is to compare geologically determined rates and styles of deformation to contemporary strain fields determined by GPS to see if regions of accelerated extension are relicts of geologically recent activity or precursors of future activity. Hopefully, the new compilation of faults in the Basin and Range will provide an ever-growing archive of paleoseismic information that will encourage such comparisons.

References

Machette, M.N., Haller, K.M., Dart, R.L.; Thatcher, W.A., and Wernicke, B., 2002, Quaternary tectonics of the Basin and Range province of Nevada and Utah Faults, folds, and GPS: Geological Society of America Abstracts with Program, v. 34, no. 4, p. A-12.

Machette, M.N, 2003, Quaternary extensional tectonics of the Basin and Range province: Eos [Transactions of the American Geophysical Union], v. 84, no. 46, Fall meeting, Abstract S12E-01.

Machette, M.N, Haller, K.M., and Dart, R.L 2003, Quaternary fault database for the Basin and Range province of Nevada and Utah: Eos [Transactions of the American Geophysical Union], v. 84, no. 46, Fall meeting, Abstract G31B-0711.

Machette, M.N, Haller, K.M., Rhea, B.S., and Dart, R.L., 2004, Quaternary fault database for the Basin and Range province of Nevada and Utah: Proceedings of the Basin and Range Province Seismic Hazards Summit II, Western States Seismic Policy Council meeting, Reno, Nevada, May 16-91, 2004.

Machette, M.N, 2005, Summary of the late Quaternary tectonics of the Basin and Range province in Nevada, Eastern California, and Utah, in Lund, W.R., ed., Proceedings of the Basin and Range Province Seismic Hazards Summit II, Western States Seismic Policy Council meeting, Reno, Nevada, May 16-91, 2004: Utah Geological Survey Special Publication 05-2 (CD-ROM), 18 p., 12 full-color figures, 2 tables