Paleoseismic Studies in the Basin and Range Province, Northern Nevada

Figure 1
Map of Quaternary faults in a part of the northern Basin and Range province
Map of Quaternary faults in a part of the northern Basin and Range province
The Basin and Range province is a large region of extensional tectonics in the Intermountain western United States that is marked by numerous north-trending mountain ranges and adjacent basins, which formed primarily by normal faulting in late Tertiary and Quaternary time. Our current understanding of the region seismic hazard is based in large part on the rates and locations of both historical and prehistoric earthquakes. To increase our knowledge of the locations, size and frequency of prehistoric earthquakes, we are conducting paleoseismic investigations to extend the short historical record of seismicity in this sparsely populated region. Our studies focus on gathering information on fault slip rates and the time intervals between past earthquakes, which are used to quantify earthquake activity on a fault. We can then compare long-term fault slip rates obtained from geological studies with rates of modern extension obtained from geodetic networks in the region. In northwestern Nevada and adjacent southeastern Oregon, we have studied two major faults that are thought to have played a significant role in the recent tectonic history of the region.

Santa Rosa Range fault zone

Figure 2
Photograph showing the grass-covered fault scarp at the Orovada trench site, Santa Rosa Range fault zone
Photograph showing the grass-covered fault scarp at the Orovada trench site, Santa Rosa Range fault zone. Note the steep front of the Santa Rosa Range in the distance
The Santa Rosa Range is a prominent mountain range in northwestern Nevada and southeastern Oregon. The Santa Rosa Range fault zone, which bounds the western side of the range, has been reported to have a relatively high vertical slip rate. To obtain information about prehistoric earthquakes on the fault, we excavated a trench on the southern part of the fault at a site near Orovada, Nevada. The deposits that were exposed in the trench indicated that the last four paleoearthquakes were accompanied by vertical displacements of the ground surface of 1 to 2.8 m.
Figure 3
Example of a detailed trench wall map of the Orovada trench site, Santa Rosa Range fault zone
Example of a detailed trench wall map of the Orovada trench site, Santa Rosa Range fault zone
Based on the size of these vertical displacements, these paleoearthquakes had estimated magnitudes of M 6.8-7.4. The most recent earthquake occurred between 11,000 and 16,000 years ago. The two previous earthquakes occurred between 90,000 and 108,000 years ago, and between 125,000 and 155,000 years ago. Overall, these studies indicate that the slip rates on this fault are low (0.01-0.16 mm/yr), which is surprising considering the prominent topography of the Santa Rosa Range. Significantly, these slip rates are much lower than earlier estimates, which suggests that the seismic hazard posed by this fault is less than inferred from previous studies.

Santa Rosa Range fault zone Publications

Personius, S.F., Anderson, R.E., Okumura, K., Mahan, S.A., and Hancock, D.A., 2004, Logs and data from an investigation of the Orovada trench site, Santa Rosa Range fault zone, Humboldt County, Nevada: U.S. Geological Survey Scientific Investigations Map 2815.
This publication is available on-line at: http://pubs.usgs.gov/sim/2004/2815/

Personius, S.F., and Mahan, S.A., 2005, Unusually low rates of slip on the Santa Rosa Range fault zone, northern Nevada: Bulletin of the Seismological Society of America, v. 95, no. 1, p. 319-333.

Steens fault zone

Figure 4
View of the Bog Hot Valley trench, Steens fault zone
View of the Bog Hot Valley trench, Steens fault zone. The main fault zone is exposed in the area above the head of the person in the bottom of the trench
Recent geodetic data from the northern part of the Basin and Range province in northwestern Nevada, northeastern California, and southern Oregon define a narrow zone of relatively high extension that coincides with the Steens fault zone, which bounds the eastern side of Steens Mountain and the Pueblo Mountains. These are the most prominent mountain ranges in this part of the province. To better determine the recent history of movements on this fault system, we excavated a trench across a 2- to 5-m-high scarp in the Bog Hot Valley in northernmost Nevada, where a series of right-stepping fault scarps extend southward from the southern end of the Pueblo Mountains and cross the floor of the valley. At this site, well-preserved fault scarps are present in lake and alluvial sediments.
Figure 5
Schematic diagram showing the reconstruction of geological events that resulted in the sediments and faults exposed in the Bog Hot Valley trench
Schematic diagram showing the reconstruction of geological events that resulted in the sediments and faults exposed in the Bog Hot Valley trench
In addition, two nearby geodetic stations will allow comparison of the geodetically determined, modern slip rate with our geologically determined, long-term slip rate for the fault zone. Our results from this trench study are preliminary, but we found evidence for three large paleoearthquakes that had estimated magnitudes of M 6.8-7.2, all of which occurred since the valley was last flooded by a large lake about 15,000-20,000 years ago. Each paleoearthquake vertically displaced the ground surface about 1.1-2.1 m. Based on this information, we calculate preliminary slip rates of 0.2-0.6 mm/yr, which are lower than the rates on high-slip faults on the margins of the Basin and Range province, but are higher than the rates for most faults in the interior of the province. The high topography of Steens Mountain suggests that the long-term slip rate on the fault zone is even higher on the central part of the fault system in the Alvord basin of southern Oregon; preliminary geophysical and geologic data suggest vertical slip rates of 1-2.5 mm/yr in this area. These relations suggest that the deformation rate along the Steens fault zone decreases southward, which is consistent with the decreasing topographic relief of the Pueblo Mountains.

Steens fault zone Publications

Personius, S.F., Crone, A.J., Machette, M.N., Kyung, J.B., Cisneros, H., and Lidke, D.J., 2004, Preliminary paleoseismology of the southern Steens fault zone, Bog Hot Valley, Nevada: Western States Seismic Policy Council, Basin and Range Province Seismic Hazards Summit II, Program and Abstracts, p. 124-125.

Personius, S.F., Crone, A.J., Machette, M.N., Kyung, J.B., Cisneros, H., Lidke, D.J., and Mahan, S.A., 2004, Preliminary paleoseismology of the Steens fault zone, Bog Hot Valley, Nevada: Geological Society of America, Abstracts with Programs. v. 36, no. 5, p. 137.