Central Nevada Seismic Belt Research Articles

Inference of the viscosity structure and mantle conditions beneath the Central Nevada Seismic Belt from combined postseismic and lake unloading studies

Inference of the viscosity structure and mantle conditions beneath the Central Nevada Seismic Belt from combined postseismic and lake unloading studies

Identifying block structure in the Pacific Northwest, USA

AbstractWe have identified block structure in the Pacific Northwest (west of 116°W between 38°N and 49°N) by clustering GPS stations so that the same Euler vector approximates the velocity of each station in a cluster. Given the total number k of clusters desired, the clustering procedure finds the best assignment of stations to clusters. Clustering is calculated for k = 2–14. In geographic space, cluster boundaries that remain relatively stable as k is increased are tentatively identified as block boundaries. That identification is reinforced if the cluster boundary coincides with a geologic feature. Boundaries identified in Northern California and Nevada are the Central Nevada Seismic Belt, the west side of the Northern Walker Lane Belt, and the Bartlett Springs Fault. Three blocks cover all of Oregon and Washington. The principal block boundary there extends west‐northwest along the Brothers Fault Zone, then north and northwest along the eastern boundary of Siletzia, the accreted oceanic basement of the forearc. East of this boundary is the Intermountain block; its eastern boundary undefined. A cluster boundary at Cape Blanco subdivides the forearc along the faulted southern margin of Siletzia. South of Cape Blanco, the Klamath Mountains‐Basin and Range block, extends east to the Central Nevada Seismic Belt and south to the Sierra Nevada‐Great Valley block. The Siletzia block, north of Cape Blanco, coincides almost exactly with the accreted Siletz terrane. The cluster boundary in the eastern Olympic Peninsula may mark permanent shortening of Siletzia against the Intermountain block.

Clustering of velocities in a GPS network spanning the Sierra Nevada Block, the Northern Walker Lane Belt, and the Central Nevada Seismic Belt, California‐Nevada

AbstractThe deformation across the Sierra Nevada Block, the Walker Lane Belt, and the Central Nevada Seismic Belt (CNSB) between 38.5°N and 40.5°N has been analyzed by clustering GPS velocities to identify coherent blocks. Cluster analysis determines the number of clusters required and assigns the GPS stations to the proper clusters. The clusters are shown on a fault map by symbols located at the positions of the GPS stations, each symbol representing the cluster to which the velocity of that GPS station belongs. Fault systems that separate the clusters are readily identified on such a map. Four significant clusters are identified. Those clusters are strips separated by (from west to east) the Mohawk Valley‐Genoa fault system, the Pyramid Lake‐Wassuk fault system, and the Central Nevada Seismic Belt. The strain rates within the westernmost three clusters approximate simple right‐lateral shear (~13 nstrain/a) across vertical planes roughly parallel to the cluster boundaries. Clustering does not recognize the longitudinal segmentation of the Walker Lane Belt into domains dominated by either northwesterly trending, right‐lateral faults or northeasterly trending, left‐lateral faults.

Late Pleistocene regional extension rate derived from earthquake geology of late Quaternary faults across the Great Basin, Nevada, between 38.5 N and 40 N latitude

Maps showing Quaternary deposits and active fault traces, paleoseismic trenches, scarp diffusion analyses, and soil characteristics in displaced alluvial surfaces are combined with previous paleoseismic studies to examine the character of late Pleistocene earthquake recurrence and estimate the net extension rate across the interior of the Great Basin of the western United States at the latitude of ~39°N. The study area includes faults bounding the Desatoya, Toiyabe, Monitor, Simpson Park, Toquima, Antelope, Fish Creek, Butte, Egan, and Schell Creek Ranges. The rate of earthquake recurrence is documented to be significantly less than observed within the Walker Lane–Central Nevada seismic belt and along the Wasatch, which respectively define the western and eastern boundaries of the interior of the Great Basin. Late Pleistocene extension across the interior of the Great Basin is calculated to equal ~1 mm/yr across the 450 km transect and is consistent with rates defined by recent geodetic studies. The agreement in extension rate estimates over different time scales indicates that tectonic deformation in the Great Basin has been characterized by relatively slow and consistent extension through the late Pleistocene to the present. The internal deformation of the Great Basin and the pattern of strain release may reflect a broad transition zone from northwest-directed shear in the west to extension along the eastern edge of the Pacific–North American plate boundary.

Large Earthquake Triggering, Clustering, and the Synchronization of Faults

Large earthquakes are sometimes observed to trigger other large earthquakes on nearby faults. The magnitudes of the calculated Coulomb stress transfers presumed to cause the triggering are 10-2–10-3 of the earthquake stress drops. The earthquake stress drops and the triggering delay times are similarly small with respect to the natural recurrence time of the earthquakes. This requires that both faults be simultaneously very close to the ends of their seismic cycles. Paleoseismological data show that for the same regions prior earthquakes have occurred in clusters of ruptures of several faults separated by long quiescent periods. Both observations suggest that synchronization is occurring between faults. Theory and observations indicate that synchronization can occur between nearby faults with positive stress coupling and intrinsic slip velocities within an entrainment threshold. In the south Iceland seismic zone, the central Nevada seismic belt, and the eastern California shear zone, several synchronous clusters that apparently act independently can be recognized. This behavior is the 3D equivalent of the phase locking that results in the seismic cycle of individual faults being dominated by large characteristic earthquakes, and for synchronization of fault segments along a given fault. Rupture patterns of repeated individual earthquakes or earthquake clusters are not identical in either the 2D or 3D cases. The state of this system, which exhibits strong indications of synchrony without exact repetition, may be called fuzzy synchrony.

Crustal deformation across the Sierra Nevada, northern Walker Lane, Basin and Range transition, western United States measured with GPS, 2000–2004

Global Positioning System (GPS) data collected in campaigns in 2000 and 2004 were processed and interpreted with other GPS data in the western Basin and Range province to provide new constraints on the rate, style, and pattern of deformation of the central and northern Walker Lane (WL), which lies near the western boundary of the Basin and Range. Across the central WL, near 38°N latitude, the velocities with respect to North America increase westward by ∼10 mm/yr inducing dextral shear. Farther north between 40° and 41°N latitude, a western zone of ∼7 mm/yr relative motion undergoes dextral shear, and an eastern zone of ≤3 mm/yr relative motion undergoes extension and shear. These data show that the northern WL is essentially a dextral shear zone experiencing minor net dilatation (ɛΔ = 2.6 ± 0.8 nstrain/yr). Near most Holocene normal faults, dilatation inferred from the velocity field is not greater than the uncertainties. However, near the central Nevada seismic belt we detect significant dilatation expressed as extension in a direction approximately normal to the range fronts (ɛΔ = 23.0 ± 3.9 nstrain/yr), some of which is attributable to transient postseismic deformation following large historic earthquakes. A block model constrained by velocities corrected for transient effects shows that the sum of dextral slip rates across the Honey Lake, Warm Springs, east Pyramid fault system, and Mohawk Valley faults is ∼7 mm/yr. The WL is a zone whose width and dilatation rate increase northwestward, consistent with counterclockwise rotation of the Sierra Nevada microplate and transfer of deformation into the Pacific Northwest.

Postseismic Mantle Relaxation in the Central Nevada Seismic Belt

Holocene acceleration of deformation and postseismic relaxation are two hypotheses to explain the present-day deformation in the Central Nevada Seismic Belt (CNSB). Discriminating between these two mechanisms is critical for understanding the dynamics and seismic potential of the Basin and Range province. Interferometric synthetic aperture radar detected a broad area of uplift (2 to 3 millimeters per year) that can be explained by postseismic mantle relaxation after a sequence of large crustal earthquakes from 1915 to 1954. The results lead to a broad agreement between geologic and geodetic strain indicators and support a model of a rigid Basin and Range between the CNSB and the Wasatch fault.

Paleoseismic transect across the northern Great Basin

The relationship of strain accumulation to strain release over different timescales provides insight to the dynamics, structural development, and spatial and temporal pattern of earthquake recurrence in regions of active tectonics. The Great Basin physiographic province of the western United States is one of the Earth's broadest regions of ongoing continental extension, encompassing an area reaching ∼800 km in width between the Sierra Nevada to the west and Wasatch mountains to the east. We present observations arising from excavations, scarp profiling, optically stimulated luminescence, and radiocarbon dating to place limits on the late Pleistocene paleoseismic history of faults bounding eight ranges across the interior of the northern Great Basin, specifically, the Shawave, Hot Springs, Humboldt, Sonoma, Shoshone, Tuscarora, Dry Hills, and Pequop ranges. Combining the observations with similar previously published studies within and at the margins of the Great Basin yields a transect that extends eastward across the basin between the 40th and 41st parallels. The sum of observations provides a picture of the patterns and rates of earthquake recurrence over the region during the last ∼20–45 kyr that may be compared to patterns of contemporary seismicity and recently reported measures of strain accumulation across the area using GPS. The recurrence rate of large surface rupture paleoearthquakes along ranges at the margins of the Great Basin is systematically greater than observed along ranges in the interior. The pattern is similar to seismological and geodetic measurements that show levels of background seismicity and strain accumulation are also concentrated along the margins of the Great Basin. An east‐west extension rate across the interior of the Great Basin on the order of 1/2 mm yr−1 (strain rate of ∼1 nstrain yr−1) over the last ∼20–45 kyr is estimated by summing the record of paleoseismic displacements across the 400 km breadth of the transect, as compared to ∼2 mm yr−1 of strain accumulation indicated by a recently reported analysis of a collinear GPS survey. The comparison is hindered by significant uncertainties coupled to the geologic rate estimate. The transect also crosses the northern limit of the central Nevada seismic belt. The central Nevada seismic belt is defined by a north‐northeast trending alignment of historical surface rupture earthquakes, increased levels of background seismicity, and strain accumulation rates greater than observed elsewhere in the interior of the Great Basin. The reported recurrence rate of late Pleistocene surface rupture earthquakes within the central Nevada seismic belt is also generally greater than observed along our transect. The observations when taken together suggest that the characteristics of strain release observed historically within the central Nevada seismic belt have been operative over the latest Pleistocene and that the apparently greater rates of strain accumulation and release in the central Nevada seismic belt are diminished or less localized in regions to the north and east. Thus, while the historical alignment of surface ruptures that defines the central Nevada seismic belt remains a unique clustering of earthquakes in time and space, the likelihood of the cluster at its observed location appears greater than would be expected to the north or eastward in the interior of the Great Basin.

Unusually Low Rates of Slip on the Santa Rosa Range Fault Zone, Northern Nevada

The Santa Rosa Range fault zone (srrfz) is one of the most topographically prominent normal fault systems in the northern Basin and Range province of the western United States. It has been assigned high rates of vertical slip by others and has been identified as a possible site of the future extension of the central Nevada seismic belt (cnsb). We use detailed trench mapping and luminescence dating to estimate displacements and timing of the last several large-magnitude paleoearthquakes on the southern part of the srrfz at a trench site near Orovada, Nevada. Coseismic vertical displacements ranged from 1 to 2.8 m for each of the last four events. Luminescence ages provide time limits for the last three events of 125–155 ka, 90–108 ka, and 11–16 ka. These data yield recurrence intervals of 17–65 k.y. and 74–97 k.y. and an elapsed time of 11–16 k.y. since the youngest event. Slip-rate determinations at the Orovada site are complicated by multiple fault strands, but rates calculated from a variety of data are surprisingly low (0.01–0.16 mm/yr), given the topographic prominence of the Santa Rosa Range. A lack of compelling patterns in a comparison of paleoseismic parameters indicate that the srrfz is no more likely a location for a large-magnitude earthquake than previously identified seismic gaps or along faults that lie directly north of the cnsb.

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.

Postseismic relaxation across the Central Nevada Seismic Belt

Two GPS geodetic surveys across the Basin and Range (BR) province of western North America have detected an anomalous compression east of the Central Nevada Seismic Belt (CNSB). The first network, installed by the U.S. Geological Survey along an east‐west trending transect across the BR at approximately 40°N, consisted of 63 monuments observed campaign style in 1992, 1996, and 1998. The second network, installed by the California Institute of Technology, consisted of 50 continuously operating GPS receivers, 18 of which were roughly along the same transect as the campaign survey. We have used a viscoelastic postseismic relaxation model of the four largest earthquakes in the CNSB (on the Pleasant Valley fault in 1915, the Cedar Mountain fault in 1932, and on the Fairview Peak and Dixie Valley faults in 1954) to interpret the geodetic velocities observed across the CNSB. A model of postseismic relaxation with a lower crustal Maxwell viscosity of 5–50 × 1018 Pa s can explain the apparent compression regardless of the assumed mantle viscosity. The geodetic velocities that we determined after removing the postseismic contribution indicate that within current GPS detection limits the BR is stable east of the CNSB and is undergoing rapid right‐lateral shear west of the CNSB.

Contemporary strain rates in the northern Basin and Range province from GPS data

We investigate the distribution of active deformation in the northern Basin and Range province using data from continuous GPS (CGPS) networks, supplemented by additional campaign data from the Death Valley, northern Basin and Range, and Sierra Nevada–Great Valley regions. To understand the contemporary strain rate field in the context of the greater Pacific (P)–North America (NA) plate boundary zone, we use GPS velocities to estimate the average relative motions of the Colorado Plateau (CP), the Sierra Nevada–Great Valley (SNGV) microplate, and a narrow north‐south elongate region in the central Great Basin (CGB) occupying the longitude band 114–117°W. We find that the SNGV microplate translates with respect to the CP at a rate of 11.4 ± 0.3 mm yr−1oriented N47 ± 1°W and with respect to NA at a rate of ∼12.4 mm yr−1also oriented N47°W, slower than most previous geodetic estimates of SNGV‐NA relative motion, and nearly 7° counterclockwise from the direction of P‐NA relative plate motion. We estimate CGB‐CP relative motion of 2.8 ± 0.2 mm yr−1oriented N84 ± 5°W, consistent with roughly east‐west extension within the eastern Great Basin (EGB). Velocity estimates from the EGB reveal diffuse extension across this region, with more rapid extension of 20 ± 1 nstr yr−1concentrated in the eastern half of the region, which includes the Wasatch fault zone. We estimate SNGV‐CGB relative motion of 9.3 ± 0.2 mm yr−1oriented N37 ± 2°W, essentially parallel to P‐NA relative plate motion. This rate is significantly slower than most previous geodetic estimates of deformation across the western Great Basin (WGB) but is generally consistent with paleoseismological inferences. The WGB region accommodates N37°W directed right lateral shear at rates of (1) 57 ± 9 nstr yr−1across a zone of width ∼125 km in the south (latitude ∼36°N), (2) 25 ± 5 nstr yr−1in the central region (latitude ∼38°N), and (3) 36 ± 1 nstr yr−1across a zone of width ∼300 km in the north (latitude ∼40°N). By construction there is no net extension or shortening perpendicular to SNGV‐CGB relative motion. However, we observe about 8.6 ± 0.5 nstr yr−1extension on average in the direction of shear from southeast to northwest within the Walker Lane belt, comparable to the average east‐west extension rate of 10 ± 1 nstr yr−1across the northern Basin and Range but implying a distinctly different mechanism of deformation from extension on north trending, range‐bounding normal faults. An alternative model for this shear parallel deformation, in which extension is accommodated across a narrow, more rapidly extending zone that coincides with the central Nevada seismic belt, fits the WGB data slightly better. Local anomalies with respect to this simple kinematic model may reveal second‐order deformation signals related to more local crustal dynamic phenomena, but significant improvements in velocity field resolution will be necessary to reveal this second‐order pattern.

Active displacement transfer and differential block motion within the central Walker Lane, western Great Basin

Velocities determined for 50 global positioning system sites within the central Walker Lane indicate differential motion among tectonic blocks forming a boundary zone between the Great Basin extensional province and the Sierra Nevada. The velocity field is related to displacement transfer from the Owens Valley and Furnace Creek fault systems of east- ern California to transtensional structures of the Walker Lane and extensional faults of the central Nevada seismic belt. Block boundaries are sharp and appear to be inherited from pre-Tertiary crustal structure. The block geometries exert strong influence on dif- ferential displacements concentrated along boundaries as belts of divergent, transcurrent, and convergent motion. The aggregate velocity accounts for about 25% of the relative motion between the Pacific and North American plates. About 5 mm/yr of the motion is localized along the eastern margin of the Sierra Nevada, whereas about 10 mm/yr is stepped 100 km east along a belt of east-northeast-trending transtensional faults that merge with northwest-trending transcurrent structures of the Walker Lane. About 6 mm/ yr of the velocity field is transferred to north-northeast-trending extensional faults of the central Nevada seismic belt. The heterogeneous distribution of motions is consistent with partitioning of a regional velocity field formed by westward extension and N408W-directed shear.

Continuous GPS measurements of contemporary deformation across the Northern Basin and Range Province

We have acquired and analyzed data from the northern Basin and Range (NBAR) continuous GPS network since July 1996. The RMS residual with respect to the best fitting lines through the individual station position estimates is 2–3 mm in the horizontal and 6–10 mm in the vertical. After the first 395 days of operation, uncertainties in horizontal velocity estimates are 1–2 mm/yr (1‐σ). Relative motion among NBAR sites located in eastern Nevada and in Utah is small, but east‐west extension is significant assuming uniform strain accumulation across the whole network. The relative motion observed across the Wasatch fault zone is 2 ±2 mm/yr, east‐west. Relative motions among stations in western Nevada and California, in contrast, are dominated by northwest, right‐lateral shear. We infer an integrated deformation across the northern Basin and Range of 11 ± 2 mm/yr, northwest. These rates are consistent with previous geodetic measurements. Our GPS velocity estimates, however, reveal a possibly abrupt transition from east‐west extension in eastern Nevada and Utah to right‐lateral shear in western Nevada. This transition is roughly coincident with the central Nevada seismic belt and is consistent with the right‐oblique focal mechanisms of the 1954 Dixie Valley and Fairview Peak earthquakes. The transition also appears to correlate spatially with a transition in upper mantle structure.

The Sou Hills: A barrier to faulting in the central Nevada Seismic Belt

Bedrock zones transverse to regional structures may indicate barriers at depth that inhibit the propagation of fault ruptures in the Basin and Range Province. The Tertiary bedrock of the Sou Hills separates Dixie and Pleasant valleys and is transverse to the trends of physiography and historic surface faulting in central Nevada. Four lines of evidence indicate that the Sou Hills are a barrier to faulting in the seismic belt. First, total late Cenozoic vertical displacement on range‐bounding faults decreases toward the Sou Hills. Second, analyses of landforms that reflect rates of relative uplift show that Quaternary uplift decreases where range‐bounding faults meet the Sou Hills. Third, the most recent prehistoric faulting south of the transverse zone is several thousand years younger than faulting to the north. Fourth, patterns of late Quaternary fault scarps in the Sou Hills are similar to rupture patterns observed at the termination of faults elsewhere.

Patterns and timing of Late Quaternary faulting in the Great Basin Province and relation to some regional tectonic features

Large‐scale surface faulting events, each tens of kilometers long and involving more than a meter of displacement, of late Quaternary age have not been uniformly distributed but have been concentrated in subprovincial and smaller belts and areas within the Great Basin province. Furthermore, faulting apparently has not been uniformly distributed over time. The average recurrence interval of large‐scale faulting events on individual faults in the province is measured in thousands or tens of thousands of years. In contrast to such long recurrence intervals, in the central Nevada and eastern California seismic belts, large‐scale faulting events have occurred during the last 111 years in an apparently coherent, sequential belt‐filling pulse of activity in which events were separated by only a few years or a few decades. The questions are, where are other pulses of large‐scale faulting likely to occur and what controls the localization of such belts and areas characterized by higher rates of faulting? Two examples of tectonic subprovinces that may play a role in delimiting smaller zones of faulting are the Black Rock‐Carson Sink zone of extension and the central Nevada downwarp. The central Nevada seismic belt lies along the dividing line between these two subprovinces, and the northwest margin of the Black Rock‐Carson Sink zone of extension coincides with the margin of a fingerlike belt of dense faulting in northwestern Nevada.