Craig M. dePolo

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.

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.

Contemporary tectonics, seismicity, and potential earthquake sources in the white mountains seismic gap, west-central Nevada and east-central California, USA

Abstract The White Mountains seismic gap (WMSG) is a broad area between the 1872 Owens Valley earthquake and the 1932 Cedar Mountain earthquake that has a complicated tectonic setting and seismicity patterns, and is considered to have the potential for a strong or larger-magnitude earthquake in the near future. We take the Sierra Nevada block as the western boundary of the WMSG, the Pancake Range lineament and the southernmost 1932 earthquake ruptures as its approximate northern boundary, a change in structure, tectonic rates and seismicity level as the eastern boundary, and the northernmost 1872 earthquake ruptures as the southern boundary. Seismicity within the WMSG, especially in the southern part, has been at a very high level since 1978. This activity has included six events of magnitude ⩾ 6 and their associated aftershocks. In addition many earthquake swarms have occurred throughout the WMSG, some of which are distinctly located at the ends of fault zones or near changes in structural orientation. Focal mechanisms show a predominance of strike-slip solutions for both small and large earthquakes, with NW-trending right-lateral and NE-trending left-lateral solutions for over half of the mechanisms. These are similar to the sense of displacements and orientations of the larger faults in the WMSG. Thus seismic strain is consistent with the faulting pattern in the WMSG and is accommodating contemporary deformation through a conjugate set of strike-slip and normal-oblique-slip faults. Examination and analysis of eighteen of the larger faults in the WMSG leads to the estimation of characteristic earthquakes ranging from magnitude 6.8 to 7.4. Given the size of the seismic gap (100 km) and the pronounced change in structure near the middle of the seismic gap, it seems likely that it will require two or more magnitude ~ M 7 + events to “fill” the gap.

Earthquake rate analysis

Abstract Earthquake rate analysis explores the quantitative relationship between earthquake occurrence rates and geological deformation rates. One application is the estimation of tectonic deformation rates from seismicity, for comparison with tectonic models. Another application is the use of geological models to help quantify seismic hazards. Seismicity is described by a catalog of the largest earthquakes in the region. The contribution of small earthquakes towards causing geological deformation is insignificant, although the occurrence rate of small events may be predictable from the occurrence rate of large events by a b value relationship. The critical information about each earthquake is its seismic moment, or, better still, its seismic moment tensor. When moment is not available directly, it can be estimated from the earthquake magnitude, although this increases uncertainties considerably. The average annual moment from all the earthquakes in the catalog is the seismic moment rate. Uncertainties in seismic estimates of moment rate arise from the short time period of observations, from the magnitude-moment relationship, from the shape of the distribution function giving number of earthquakes as a function of magnitude and from uncertainties in the physical properties of the Earth near the source. Geological models for deformation rates are described by slip rates on individual faults or by regional strain rates. A regional strain rate can be derived as a limiting case where there are numerous individual faults. Such a strain rate is a particularly useful concept where information about each of the faults is limited. In the case of either faulting or regional strain, the average seismic moment rate is proportional to the deformation rate. Geological uncertainties arise from difficulties in estimating slip rates or strain rates, from fault creep and other aseismic deformation, from uncertainty in the part of the fault that fails during earthquakes and boundaries of the deforming region and from uncertainty about the Earths physical properties. The seismic moment rate is the critical factor for relating the earthquake occurrence rate and the geological deformation rate. Using this intermediate parameter, one can infer earthquake occurrence rates from deformation rates, or infer deformation rates from earthquake occurrence rates. Generally, it is found that deformation rates and earthquake occurrence rates are consistent within uncertainties. Where both are well constrained, they are generally consistent within a factor of two or three.

Latest Quaternary Fault Movement along the Las Vegas Valley Fault System, Clark County, Nevada

Several faults exposed in a 4- to 8-m-deep excavation in North Las Vegas exhibit evidence for two surface-faulting earthquakes that offset latest Pleistocene deposits and paleo-land surfaces about 2 to 3 m per event. These faults are secondary to a major trace of the Las Vegas Valley fault system. The rupture events, defined here as the most recent and penultimate events in the excavation, are expressed as fault displacements within, and buried by, a latest Pleistocene and Holocene stack of intercalated silt-rich and clay-rich deposits. Rapid, brittle faulting offset soft sediments that had the potential to deform plastically and formed a fault scarp 2 to 3 m high in weakly to moderately consolidated materials. A radiocarbon date from organic-rich soil at the surface offset by the most recent event indicates it occurred after about 14,500 14C years before present. The penultimate event likely was a few hundred to a few thousand years before this date, based on our estimate of the time required for intervening sedimentation and lack of soils. Local earthquakes along the Las Vegas Valley fault system likely caused the sudden fault offsets because these are typical features created by earthquakes (e.g., brittle faulting, fault scarps, tectonic colluvium), and the faults are in line with and adjacent to a distinct main fault. The faults are encountered at 2- to 3-m depths, and the events are evident below this. Therefore, exploratory trenches in young sediments within Las Vegas Valley should be deeper than 3 m if the existence of latest Quaternary faulting is to be detected.

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.