Alan R. Ramelli

Historical surface faulting in the Basin and Range province, western North America: implications for fault segmentation

The distribution of surface ruptures caused by 11 historical earthquakes in the Basin and Range province of western North America provides a basis for evaluating earthquake segmentation behavior of faults in extensional tectonic settings. Two of the three moderate magnitude (5.5 < M < 7) events appear to be confined to individual geometric or structural segments. The remaining nine events, eight of which had large magnitudes (M ≥ 7), ruptured multiple geometric or structural segments. Several of these events had widely distributed surface-rupture patterns, ruptured in complex manners, and extended beyond distinct fault-zone discontinuities. Some of the surface ruptures associated with these events may have resulted from sympathetic or secondary surface faulting. Approximately one-half of the surface rupture end points coincided with distinct fault-zone discontinuities. This study indicates that earthquake ruptures in extensional tectonic settings may not be confined to individual geometric or structural segments. Some rupture-controlling discontinuities may be difficult to identify and significant faulting may occur beyond postulated rupture end points. Rupture of multiple geometric or structural segments should be considered in the evaluation of large earthquakes. Several lines of evidence, particularly timing information, are needed to delineate potential earthquake segments in the Basin and Range province.

Land Subsidence in Las Vegas, Nevada, 1935–2000: New Geodetic Data Show Evolution, Revised Spatial Patterns, and Reduced Rates

Subsidence in Las Vegas Valley has been geodetically monitored since 1935, and several generations of maps have depicted more than 1.5 m of total subsidence. This study presents new geodetic data that reveal insights into the spatial distribution and magnitude of subsidence through the year 2000. In particular, synthetic aperture radar interferometry (InSAR) and global positioning system (GPS) studies demonstrate that subsidence is localized within four bowls, each bounded by Quaternary faults. Conventional level line surveys across the faults further indicate that these spatial patterns have been present since at least 1978, and based on the new geodetic data a revised map showing subsidence between 1963 and 2000 has been developed. A comparison of the location of the subsidence bowls with the distribution of pumping in the valley indicates that subsidence is offset from the principal zones of pumping. Although the reasons for this offset are not well understood, it is likely the result of heavy pumping up-gradient from compressible deposits in the subsidence zones. A compilation of subsidence rates based on conventional, InSAR, and GPS data indicates that rates have significantly declined since 1991 because of an artificial recharge program. The rates in the northwest part of the valley have declined from more than 5–6 cm/year to about 2.5–3 cm/year, a reduction of 50 percent; in the central and southern parts of the valley, rates have declined from about 2.5 cm/year to only a few millimeters per year, a reduction of more than 80 percent.

Pattern and Rates of Faulting in the Central Nevada Seismic Belt, and Paleoseismic Evidence for Prior Beltlike Behavior

The central Nevada seismic belt (CNSB) is a concentration of historical (1915-1932-1954) surface faulting in the western Basin and Range province, forming a linear, nearly continuous 300-km-long rupture zone. Previous results are integrated in this study with new paleoseismic and exploratory trenching data from the historical zones to look for evidence of older, similar beltlike patterns or elevated slip rates that could indicate whether the CNSB is a zone of focused, long-term crustal strain. The data show that the continuous rupture belt produced by the seven earthquakes occurring between 1915 and 1954 is unique in the available paleoseismic record. At the 1954 Fairview Peak fault, the lack of prehistorical faulting in deposits containing the Wilson Creek bed 19 tephra eliminates the possibility of an identical seismic belt in the past 35.4 ka. Our studies also show that the faults have net slip rates ranging from a low of 0.09 mm/yr on the Fairview Peak fault to a high of 0.7 mm/yr on the 1932 Cedar Mountain fault. These are considered moderate to low rates that are similar to most late Quaternary faults in the western Basin and Range province. A space-time comparison shows that the paleoseismic histories for these multiple rupture zones are diverse, and the number and timing of events in each of the zones indicate that there is little evidence for older contemporaneous ruptures of these same faults. Based on these results we reach several conclusions regarding the longer term (≈Holocene) behavior of the CNSB. Although paleoseismic data preclude an older identical rupture belt among the historical zones, consideration of associated Holocene faults within the greater CNSB region indicates that several similar, but not identical, beltlike rupture patterns are plausible during the past 13 ka, although each requires seismic gaps in the along-strike pattern. Although long-term strain (represented by density of young faults) does appear to increase from east to west into the CNSB, the slip-rate data demonstrate that the CNSB is not a belt of concentrated or elevated crustal strain compared with areas that extend west to the Sierra Nevada. The increase in the distribution of Holocene fault activity from east to west into the CNSB is consistent with a marked increase in the 1992-2002 GPS velocity field at the latitude of the 1954 rupture sequence. However, a comparison of the geologic rates across the belt at this same latitude indicates that the extension rates (0.59-1.37 mm/yr) are systematically lower than both the campaign and continuous GPS rates (2.20-3.13 mm/yr) by factors of 2-5. These discrepancies may be due to postseismic strain, or to some form of off-fault deformation. We conclude that the results of our study of fault behavior in the CNSB best support the belt migration model proposed by Wallace (1987) for the western Basin and Range province in which temporal tectonic pulses are believed to migrate regionally, activating different beltlike combinations of late Quaternary faults in an as yet unknown pattern of migration.

Surface faulting and paleoseismic history of the 1932 Cedar Mountain earthquake area, west-central Nevada, and implications for modern tectonics of the Walker Lane

The 1932 Cedar Mountain earthquake (M s 7.2) was one of the largest historical events in the Walker Lane region of western Nevada, and it produced a complicated strike-slip rupture pattern on multiple Quaternary faults distributed through three valleys. Primary, right-lateral surface ruptures occurred on north-striking faults in Monte Cristo Valley; small-scale lateral and normal offsets occurred in Stewart Valley; and secondary, normal faulting occurred on north-northeast–striking faults in the Gabbs Valley epicentral region. A reexamination of the surface ruptures provides new displacement and fault-zone data: maximum cumulative offset is estimated to be 2.7 m, and newly recognized faults extend the maximum width and end-to-end length of the rupture zone to 17 and 75 km, respectively. A detailed Quaternary allostratigraphic chronology based on regional alluvial-geomorphic relationships, tephrochronology, and radiocarbon dating provides a framework for interpreting the paleoseismic history of the fault zone. A late Wisconsinan alluvial-fan and piedmont unit containing a 32–36 ka tephra layer is a key stratigraphic datum for paleoseismic measurements. Exploratory trenching and radiocarbon dating of tectonic stratigraphy provide the first estimates for timing of late Quaternary faulting along the Cedar Mountain fault zone. Three trenches display evidence for six faulting events, including that in 1932, during the past 32–36 ka. Radiocarbon dating of organic soils interstratified with tectonically ponded silts establishes best-fit ages of the pre-1932 events at 4, 5, 12, 15, and 18 ka, each with ±2 ka uncertainties. On the basis of an estimated cumulative net slip of 6–12 m for the six faulting events, minimum and maximum late Quaternary slip rates are 0.2 and 0.7 mm/yr, respectively, and the preferred rate is 0.4–0.5 mm/yr. The average recurrence (interseismic) interval is 3600 yr. The relatively uniform thickness of the ponded deposits suggests that similar-size, characteristic rupture events may characterize late Quaternary slip on the zone. A comparison of event timing with the average late Quaternary recurrence interval indicates that slip has been largely regular (periodic) rather than temporally clustered. To account for the spatial separation of the primary surface faulting in Monte Cristo Valley from the epicenter and for a factor-of-two-to-three disparity between the instrumentally and geologically determined seismic moments associated with the earthquake, we hypothesize two alternative tectonic models containing undetected subevents. Either model would adequately account for the observed faulting on the basis of wrench-fault kinematics that may be associated with the Walker Lane. The 1932 Cedar Mountain earthquake is considered an important modern analogue for seismotectonic modeling and estimating seismic hazard in the Walker Lane region. In contrast to most other historical events in the Basin and Range province, the 1932 event did not occur along a major range-bounding fault, and no single, throughgoing basement structure can account for the observed rupture pattern. The 1932 faulting supports the concept that major earthquakes in the Basin and Range province can exhibit complicated distributive rupture patterns and that slip rate may not be a reliable criterion for modeling seismic hazard.

A methodology for probabilistic fault displacement hazard analysis (PFDHA)

We present a methodology for conducting a site-specific probabilistic analysis of fault displacement hazard. Two approaches are outlined. The first relates the occurrence of fault displacement at or near the ground surface to the occurrence of earthquakes in the same manner as is done in a standard probabilistic seismic hazard analysis (PSHA) for ground shaking. The methodology for this approach is taken directly from PSHA methodology with the ground-motion attenuation function replaced by a fault displacement attenuation function. In the second approach, the rate of displacement events and the distribution for fault displacement are derived directly from the characteristics of the faults or geologic features at the site of interest. The methodology for probabilistic fault displacement hazard analysis (PFDHA) was developed for a normal faulting environment and the probability distributions we present may have general application in similar tectonic regions. In addition, the general methodology is applicable to any region and we indicate the type of data needed to apply the methodology elsewhere.

Historic Surface Faulting and Paleoseismicity in the Area of the 1954 Rainbow Mountain-Stillwater Earthquake Sequence, Central Nevada

The Rainbow Mountain area was the site of three surface-rupturing earthquakes on 6 July and 23 August 1954. More than 50 field measurements of surface offsets constrain the distribution of slip along the discontinuous and distrib- uted rupture zone that formed during the earthquake sequence. Vertical offsets reach a maximum of 0.8 m with the average vertical offset being 0.2 m. In contrast to original reports, we see evidence for a right-lateral component of slip along portions of the rupture zone, including offset stream channels (0.5-1.0 m), left-stepping en echelon scarps, and a well-preserved, 100-m-long mole track. The right-slip com- ponent is consistent with focal plane solutions for the events and recent geodetic results. Previously unmapped surface ruptures now extend the known rupture length of the sequence by 25 km to a total of 70 km. Surface ruptures along the previously unmapped Fourmile Flat fault are subparallel to and form a 10-km left step to the southeast of the Rainbow Mountain fault. Event locations and anecdotal information indicate that the Fourmile Flat ruptures represent minor, primary surface rupture associated with the large 6 July aftershock, triggered 11 hr after the initial 6 July Rainbow Mountain event. The paleoseismic histories of the Rainbow Mountain and Fourmile Flat faults, as recorded in natural and trench exposures, are different although both faults experi- enced three post 15-ka surface rupturing events, including 1954. Bracketing ages for triultimate events on both faults do not overlap. However, constraints on the penultimate event for the Rainbow Mountain and triultimate event for the Fourmile Flat fault do overlap slightly, allowing the possibility that they may have ruptured close in time as in 1954. The Holocene slip rate for the Fourmile Flat fault (0.40 mm/yr) is similar to the post-latest Pleistocene rate for the Rainbow Mountain fault (0.20-0.46 mm/yr) even though the total length of the Fourmile flat (10 km) is much shorter than the overall length of the Rainbow Mountain rupture zone (60 km), indicating that even minor faults can be important for assessing regional strain rates and patterns.

Late Quaternary geomorphology and soils in Crater Flat, Yucca Mountain area, southern Nevada

Crater Flat is an alluvium-filled structural basin on the west side of Yucca Mountain, Nevada, which is under consideration for a high-level nuclear waste repository. North-trending, late Quaternary faults offset alluvium in Crater Flat both along the canyons of the western flanks of Yucca Mountain and out on the piedmont slope. We believe the initial lack of young offsets at Yucca Mountain was in part due to unrecognized late Quaternary stratigraphy. We hypothesize that alluviation in the Yucca Mountain region was more active during the late Quaternary than previously thought. Several techniques were tried to test this hypothesis. Results are compared with previous soils and surface-exposure dating studies, and correlated to stratigraphy of other late Quaternary units in the southern Nevada, Death Valley, and Mojave Desert areas, and provide new stratigraphic data relevant to understanding climatic-alluvial processes in the Basin and Range Province during the late Quaternary. 76 refs., 7 figs., 6 tabs.

Ground Cracks Associated with the 1994 Double Spring Flat Earthquake, West-Central Nevada

The 1994 Double Spring Flat earthquake ( M W 5.8) occurred within a densely faulted step-over between the Genoa and Antelope Valley faults, two principal normal faults of the transition zone between the Basin and Range Province and the northern Sierra Nevada. The earthquake created zones of ground cracks from 0.1 to 2.8 km long along at least five northwest- to north-northwest-striking faults in the epicentral area. Individual cracks had extensional openings generally from 1 to 10 mm wide. No cracks displayed obvious vertical separation, and only one zone showed permissive evidence of right-lateral separation. Over the 8 days following the mainshock (the period over which the cracks were found), aftershocks formed a dominant northeast trend suggesting the earthquake occurred along a northeast-striking structure. However, no ground breakage was found along faults striking parallel to this northeast aftershock alignment, and subsequent aftershocks formed a conjugate northwest trend. Based on the location and character of the five zones, the observed cracks are attributed to secondary fault slip and shaking effects. The earthquake also created ground cracks along at least two faults 15-25 km from the epicenter. In both of these cases, the faults had documented histories of prior ground cracking, indicating that they are particularly susceptible to such triggered deformation. Manuscript received 21 August 2002.