Skull Mountain Earthquake Research Articles

Validating Nevada ShakeZoning Predictions of Las Vegas Basin Response against 1992 Little Skull Mountain Earthquake Records

Research Article| January 21, 2014 Validating Nevada ShakeZoning Predictions of Las Vegas Basin Response against 1992 Little Skull Mountain Earthquake Records Brady A. Flinchum; Brady A. Flinchum aNevada Seismological Laboratory, University of Nevada, Reno, Reno, Nevada 89557 *Now at the University of Wyoming, Department of Geology and Geophysics, Dept. 3006, 1000 E. University Avenue, Laramie, Wyoming 82071‐2000. Search for other works by this author on: GSW Google Scholar John N. Louie; John N. Louie aNevada Seismological Laboratory, University of Nevada, Reno, Reno, Nevada 89557 Search for other works by this author on: GSW Google Scholar Kenneth D. Smith; Kenneth D. Smith aNevada Seismological Laboratory, University of Nevada, Reno, Reno, Nevada 89557 Search for other works by this author on: GSW Google Scholar William H. Savran; William H. Savran aNevada Seismological Laboratory, University of Nevada, Reno, Reno, Nevada 89557 Search for other works by this author on: GSW Google Scholar Satish K. Pullammanappallil; Satish K. Pullammanappallil bOptim, Inc., 200 South Virginia St., Suite 560, Reno, Nevada 89501 Search for other works by this author on: GSW Google Scholar Aasha Pancha Aasha Pancha bOptim, Inc., 200 South Virginia St., Suite 560, Reno, Nevada 89501 Search for other works by this author on: GSW Google Scholar Author and Article Information Brady A. Flinchum *Now at the University of Wyoming, Department of Geology and Geophysics, Dept. 3006, 1000 E. University Avenue, Laramie, Wyoming 82071‐2000. aNevada Seismological Laboratory, University of Nevada, Reno, Reno, Nevada 89557 John N. Louie aNevada Seismological Laboratory, University of Nevada, Reno, Reno, Nevada 89557 Kenneth D. Smith aNevada Seismological Laboratory, University of Nevada, Reno, Reno, Nevada 89557 William H. Savran aNevada Seismological Laboratory, University of Nevada, Reno, Reno, Nevada 89557 Satish K. Pullammanappallil bOptim, Inc., 200 South Virginia St., Suite 560, Reno, Nevada 89501 Aasha Pancha bOptim, Inc., 200 South Virginia St., Suite 560, Reno, Nevada 89501 Publisher: Seismological Society of America First Online: 14 Jul 2017 Online ISSN: 1943-3573 Print ISSN: 0037-1106 Bulletin of the Seismological Society of America (2014) 104 (1): 439–450. https://doi.org/10.1785/0120130059 Article history First Online: 14 Jul 2017 Cite View This Citation Add to Citation Manager Share Icon Share Facebook Twitter LinkedIn MailTo Tools Icon Tools Get Permissions Search Site Citation Brady A. Flinchum, John N. Louie, Kenneth D. Smith, William H. Savran, Satish K. Pullammanappallil, Aasha Pancha; Validating Nevada ShakeZoning Predictions of Las Vegas Basin Response against 1992 Little Skull Mountain Earthquake Records. Bulletin of the Seismological Society of America 2014;; 104 (1): 439–450. doi: https://doi.org/10.1785/0120130059 Download citation file: Ris (Zotero) Refmanager EasyBib Bookends Mendeley Papers EndNote RefWorks BibTex toolbar search Search Dropdown Menu toolbar search search input Search input auto suggest filter your search All ContentBy SocietyBulletin of the Seismological Society of America Search Advanced Search Abstract Over the last two years, the Nevada Seismological Laboratory has developed and refined Nevada ShakeZoning (NSZ) procedures to characterize earthquake hazards in the Intermountain West. Simulating the ML 5.6–5.8 Little Skull Mountain (LSM) earthquake validates the results of the NSZ process and the ground shaking it predicts for Las Vegas Valley (LVV). The NSZ process employs a physics‐based finite‐difference code from Lawrence Livermore Laboratory to compute wave propagation through complex 3D earth models. Computing limitations restrict the results to low frequencies of shaking. For this LSM regional model the limitation is to frequencies of 0.12 Hz, and below. The Clark County Parcel Map, completed in 2011, is a critical and unique geotechnical data set included in NSZ predictions for LVV. Replacing default geotechnical velocities with the Parcel Map velocities in a sensitivity test produced peak ground velocity amplifications of 5%–11% in places, even at low frequencies of 0.1 Hz. A detailed model of LVV basin‐floor depth and regional basin‐thickness models derived from gravity surveys by the U.S. Geological Survey are also important components of NSZ velocity‐model building. In the NSZ‐predicted seismograms at 0.1 Hz, Rayleigh‐wave minus P‐wave (R−P) differential arrival times and the pulse shapes of Rayleigh waves correlate well with the low‐pass filtered LSM recordings. Importantly, peak ground velocities predicted by NSZ matched what was recorded, to be closer than a factor of two. Observed seismograms within LVV show longer durations of shaking than the synthetics, appearing as horizontally reverberating, 0.2 Hz longitudinal waves beyond 60 s after Rayleigh‐wave arrival. Within the basins, the current velocity models are laterally homogeneous below 300 m depth, leading the 0.1 Hz NSZ synthetics to show insufficient shaking durations of only 30–40 s. You do not have access to this content, please speak to your institutional administrator if you feel you should have access.

Analysis of strain‐induced ground‐water fluctuations at Devils Hole, Nevada

AbstractThe pool in Devils Hole is a sensitive indicator of crustal strain and fluctuates in response to changes in atmospheric pressure, earth tides, earthquakes, large‐scale tectonic activity and ground‐water development. Short‐term and cyclic water‐level fluctuations caused by atmospheric pressure and earth tides were found to be on the order of millimeters to centimeters. The 1992 Landers/Little Skull Mountain earthquake sequence and the 1999 Hector Mine earthquake induced water‐level offsets of greater than −12 and −3.6 cm, respectively. The results of a dislocation model used to compute volumetric strain for each earthquake indicates that the coseismic water‐level offsets are consistent in magnitude and sense with poroelastic responses to earthquake‐induced strain. Theoretical postseismic fluid‐flow modeling indicates that the diffusivity of the system is on the order of 0.03 m2 sec−1, and identified areas of anomalous water‐level fluctuations. Interpretation of model results suggests that while the persistent post‐Landers rise in water‐level can be attributed to deformation‐induced channeling of fluid to the Devils Hole fault zone, the cause of the pre‐Hector Mine water‐level rise may be related to postseismic excess fluid pressures or preseismic strain accumulation.

Precarious Rock Evidence for Seismic Shaking during and Prior to the 1992 ML 5.6 Little Skull Mountain, Nevada, Earthquake

The 29 June 1992 Richter local magnitude ( ML) 5.6 Little Skull Mountain earthquake, 20 km southeast of Yucca Mountain, caused rockfalls on steep slopes of Little Skull Mountain, while some semi-precarious rocks remained in place. The observation that ground motion was sufficient to cause rockfalls and yet leave semi-precarious rocks untoppled offers both upper- and lower-bound estimates of the history of ground motion at Little Skull Mountain. A ground-motion synthesis code, used in Yucca Mountain probabilistic seismic hazard analysis (PSHA), was applied to assess the spatial distribution of peak ground acceleration around Little Skull Mountain with respect to the surface attenuation and evidence for ground shaking during the earthquake. Instrumental records for the earthquake, rockfalls, remaining precarious rocks, intensity values estimated from damage to local facilities, and scaling of ground motions based on a recent ML 4.4 Little Skull Mountain aftershock were compared with model predictions. Predicted peak accelerations based on a kappa value of ∼20 ms (used in the Yucca Mountain PSHA) are too high to be consistent with this evidence of ground shaking. This suggests some ground-motion predictions conducted for the Yucca Mountain PSHA may need to be re-examined, and may be too conservative.

Tectonic implications of a dense continuous GPS velocity field at Yucca Mountain, Nevada

A dense, continuous GPS network was established in the Yucca Mountain area in 1999 to provide the most reliable measurements possible of geodetic strain patterns across the nation's only proposed permanent repository for high‐level radioactive waste. The network lies astride a boundary between the geodetically stable central Great Basin and the active western Great Basin, which at the latitude of Yucca Mountain is undergoing distributed right‐lateral shear at a rate of ∼60 nstrain/yr. Monitoring from 1999 to 2003 (3.75 years) yields a velocity field characterized by nearly homogenous N20°W right‐lateral shear of 20 ± 2 nstrain/yr (net velocity contrast of ∼1.2 mm/yr across a 60 km aperture) in the vicinity of the proposed repository site. Comparison of time series of continuous results with earlier campaign surveys indicating ∼50 nstrain/yr of west‐northwest extension from 1991 to 1997 suggests that the more rapid rates were in part transient motions associated with the 1992 Ms 5.4 Little Skull Mountain earthquake. Postseismic motions do not appear to affect the 1999–2003 velocity field in either campaign or continuous data. The magnitude of the velocity contrast across the area, the overall linearity of the gradient, and the large area of undeforming crust to the east of Yucca Mountain are difficult to explain by elastic bending of the crust associated with the Death Valley fault zone, a major right‐lateral strike‐slip fault about 50 km west of the repository site. These observations, along with apparent local variations in the velocity gradient, suggest that significant right‐lateral strain accumulation, with displacement rate in the 1 mm/yr range, may be associated with structures in the Yucca Mountain area. The absence of structures in the area with equivalent late Quaternary displacement rates underscores the problem of reconciling discrepancies between geologic and geodetic estimates of deformation rates.

Linearity of the Earthquake Recurrence Curve to M < -1 from Little Skull Mountain Aftershocks in Southern Nevada

Laboratory and theoretical studies suggest that there is a minimum patch size for rupture on faults, thus limiting observed earthquake sizes to above some moment or magnitude corresponding to this patch dimension. This limit, most likely being at a much lower magnitude than that which can be observed due to background noise of the Earth and to attenuation, is difficult to verify for real earthquakes. We have recorded earthquakes to as small as M -2 (Richter local magnitude) in the aftershock zone of the M 5.6 Little Skull Mountain earthquake of 29 June 1992. The network threshold for location is below M 0. However, by considering all the triggered earthquakes at station LSC, which is just above the aftershock zone, and combining these with the located events, the recurrence curve is extended well below the network threshold. We do this by forming a relation between the trace amplitudes of the LSC recordings and the network magnitudes for larger events and then assigning an estimated M to the smaller triggered events. The recurrence curve plotted for the combined data shows a constant b -value of 0.82 down to roughly M -1.2. We show that this magnitude is somewhat below the detection threshold of LSC. The smallest recorded events are similar in appearance to events larger by as much as 3 magnitude units in the aftershock zone, partly due to the band limitation of the 100-samples/sec data and local attenuation. S -to- P ratios much greater than 1 and broadband signals for these small events are indicative of normal tectonic earthquakes. The seismic moments of the very smallest events near M -2 are approximately 5 × 109 N m, larger than observations in deep mines by at least an order of magnitude. Because the source corner frequency is too high to be seen in the spectrum of the smallest earthquakes, we cannot estimate source radii. Manuscript received 19 February 2003.

Regional distance seismic moment tensors of nuclear explosions

Nuclear explosions, because of their known location, depth and theoretical source mechanism, provide a means to explore the resolution of non-double-couple and isotropic seismic moment tensors. We perform seismic moment tensor inversions on long-period (50–20 s), three-component velocity seismograms from the Caltech TERRAscope network and the Berkeley Digital Seismic Network (BDSN) to determine best-fitting double-couple, isotropic plus double-couple, deviatoric and full-moment tensor source mechanisms for the Little Skull Mountain Earthquake and three large ( M L≥5.5, M W≥4.5) Nevada Test Site (NTS) nuclear explosions (JUNCTION, MONTELLO and BEXAR). The significance of solutions with higher degrees of freedom is evaluated using the F-test. The stability of the moment tensor solutions for variations in station configuration is investigated using a cross-validation method. Our results show that strongly non-double-couple seismic moment tensors and shallow source depth characterize the nuclear explosions. The full-moment tensor inversions recover a volume increase. However, our analysis indicates that the improvement in fit afforded by the extra degree of freedom is not statistically significant due to the similarity of the vertical compensated linear vector dipole (CLVD) and isotropic surface wave Green's functions at these periods. An isotropic plus double-couple source model was found to provide the same level of fit as the deviatoric moment tensor inversions. Determination of the nonisotropic source mechanism is not unique and we discuss our results with respect to the proposed source models for NTS. While the results of this study indicate that regional distance seismic moment tensor analysis is not suitable for directly discriminating nuclear explosions from earthquakes, the shallow source depth and non-double-couple seismic moment tensors obtained for these events suggest that it may be useful for identifying suspect events for further screening.

Location and mechanism of the Little Skull Mountain earthquake as constrained by satellite radar interferometry and seismic waveform modeling

We use interferometric synthetic aperture radar (InSAR) and broadband seismic waveform data to estimate source parameters of the 29 June 1992, Ms 5.4 Little Skull Mountain (LSM) earthquake. This event occurred within a geodetic network designed to measure the strain rate across the region around Yucca Mountain. The LSM earthquake complicates interpretation of the existing GPS and trilateration data, as the earthquake magnitude is sufficiently small that seismic data do not tightly constrain the epicenter but large enough to potentially affect the geodetic observations. We model the InSAR data using a finite dislocation in a layered elastic space. We also invert regional seismic waveforms both alone and jointly with the InSAR data. Because of limitations in the existing data set, InSAR data alone cannot determine the area of the fault plane independent of magnitude of slip nor the location of the fault plane independent of the earthquake mechanism. Our seismic waveform data tightly constrain the mechanism of the earthquake but not the location. Together, the two complementary data types can be used to determine the mechanism and location but cannot distinguish between the two potential conjugate fault planes. Our preferred model has a moment of ∼3.2 × 1017 N m (Mw 5.6) and predicts a line length change between the Wahomie and Mile geodetic benchmarks of ∼5 mm.

The 1992 Little Skull Mountain Earthquake Sequence, Southern Nevada Test Site

The ML 5.6-5.8 Little Skull Mountain, Nevada, 29 June 1992 earth- quake occurred in the southwest portion of Nevada Test Site (NTS) approximately 20 km from Yucca Mountain, a potential site for a high-level radioactive waste repository. The earthquake involved predominantly down-to-the-southeast dip-slip motion with a small left-slip component on a steeply dipping, 70, northeast-striking fault. The mainshock nucleated near the base of the aftershock zone and ruptured up and to the northeast, and the mainshock rupture and the majority of the aftershock sequence were confined to depths between 6 and 12 km. All three ML 4 (largest ML 4.5) aftershocks occurred off the mainshock fault plane on secondary structures within the aftershock zone. The nearest strong-motion instrument located 11 km southwest of the epicenter recorded a peak acceleration of 0.206g. The earthquake occurred adjacent to the Rock Valley fault zone within an area of prior concentrated background seismicity and near the intersection of several northeast-striking faults in the southern NTS that have experienced Quaternary motion.

Strain accumulation at Yucca Mountain, Nevada, 1983–1998

A 14‐station, 50‐km aperture geodetic array centered on the proposed radioactive waste disposal site at Yucca Mountain, Nevada, was surveyed in 1983, 1984, 1993, and 1998 to determine the rate of strain accumulation there. The coseismic effects of the 1992 (MS=5.4) Little Skull Mountain earthquake, which occurred within the array, were calculated from a dislocation model and removed from the data. The measured principal strain accumulation rates determined over the 1983–1998 interval are ε1 = 2±12 nanostrain/yr N87°W±12° and ε2 = −22±12 nanostrain/yr N03°E±12° (extension reckoned positive and quoted uncertainties are standard deviations). The N65°W extension rate is −2±12 nanostrain/yr, significantly less than the 1991–1997 N65°W rate of 50±9 nanostrain/yr reported by Wernicke et al. [1998]. The implied maximum right‐lateral engineering‐shear, strain accumulation rate is γ=ε1−ε2 = 23±10 nanostrain/yr, a marginally significant rate. Almost half (ε1 = 6 nanostrain/yr N90°W, ε2 = −6 nanostrain/yr N00°E, and γ = 12 nanostrain/yr ) of the measured strain rate can be attributed to strain accumulation on the Death Valley‐Furnace Creek (50 km distant) and Hunter Mountain‐Panamint Valley (90 km distant) faults. The residual strain rate after the removal of those fault contributions is not significant at the 95% confidence level.

Detecting Strain in the Yucca Mountain Area, Nevada

From repeated surveys of the relative motions between geodetic stations Claim, Black, Mile, 67TJS, and Wahomie (Fig. 1) from 1991 through 1997, Wernicke et al. (1) deduce a N65°W strain accumulation rate of 50 ± 9 nanostrains per year (2) across the proposed high-level radioactive waste disposal repository at Yucca Mountain, Nevada. That strain rate is sufficient to indicate a higher-than-expected earthquake hazard at the repository. An earlier (1983–1993) U.S. Geological Survey (USGS) measurement (Table 1) of the strain accumulation in the network (black triangles) in Fig. 1 found a N65°W strain rate of 8 ± 20 nanostrains per year, a rate not significantly different from zero (3). Although the two measurements are marginally consistent at the 95% confidence level [that is, the difference 42 ± 22 nanostrains per year is less than 2 standard deviations, (SD)], the earthquake hazard implications are somewhat different. The USGS strain determination is the better of the two measurements. Wernicke et al. did not include the effects of monument instability in their error budget and, as a consequence, have significantly underestimated the uncertainties in their measurements. Moreover, in their interpretation of the data, they did not give proper weight to the coseismic and postseismic effects of the Little Skull Mountain earthquake (June 29, 1992, M = 5.4).

Effect of Site Amplification and Basin Response on Strong Motion in Las Vegas, Nevada

The June 29, 1992, Little Skull Mountain earthquake (M=5.6) was recorded at nine strong-motion stations in Las Vegas, Nevada. Ground motion on alluvial sites is much greater than on rock sites, especially at low frequencies, and had a longer duration than expected. Five of the seven stations on sediment have Fourier spectral amplifications that average at least a factor of five greater than the rock reference sites from 0.5 to 2 Hz. Thus, large earthquakes at moderate distances, that would excite strong long-period waves, are a concern. We generated synthetic seismograms, including the site amplification, for magnitude 7.4 earthquakes on the Death Valley fault system 150km from Las Vegas. The synthetics have average peak accelerations ranging from 0.058 to 0.13g at the rock and from 0.051 to 0.22g at the sediment sites. The average peak velocity and displacement is 49 cm/sec and 33 cm, respectively, on the larger component of a sediment site.

Anomalous strain accumulation in the yucca mountain area, nevada

Global Positioning System (GPS) surveys from 1991 to 1997 near Yucca Mountain, Nevada, indicate west-northwest crustal elongation at a rate of 1.7 +/- 0.3 millimeters per year (1final sigma) over 34 kilometers, or 50 +/- 9 nanostrain per year. Global Positioning System and trilateration surveys from 1983 to 1997 on a 14-kilometer baseline across the proposed repository site for high-level radioactive waste indicate that the crust extended by 0.7 to 0.9 +/- 0.2 millimeter per year (50 to 64 +/- 14 nanostrain per year), depending on the coseismic effect of the Ms 5.4 1992 Little Skull Mountain earthquake. These strain rates are at least an order of magnitude higher than would be predicted from the Quaternary volcanic and tectonic history of the area.

A comparison of direct S-wave and coda-wave site amplification determined from aftershocks of the Little Skull Mountain earthquake

Abstract Site amplifications of direct S waves and coda waves are studied and compared using high-quality, three-component digital data from aftershocks of the Little Skull Mountain, Neveda, earthquake. We use data from 12 stations installed on a variety of geological and topographic site conditions, distributed widely in space with different azimuths and epicentral distances. S-wave site amplifications are obtained in the frequency range from about 0.5 to over 30 Hz, while coda-wave site amplifications are obtained in a frequency range from about 1.5 to over 30 Hz. A thorough statistical analysis of these results was performed. We find that (1) for S waves, all three components at a station follow a similar frequency-dependent trend. The amplitudes of the two horizontal components match more closely, while the vertical component shows consistently lower amplification than the horizontal components at low frequencies. (2) For coda waves, all three components share both similar frequency-dependent trends and amplification level. (3) S-wave and coda-wave site amplification are consistent for stations with epicentral distance greater than about 10 km (which is about the average focal depth of the earthquakes we used). Within the epicentral distance of 10 km, however, some stations show discordant S-wave and coda-wave site amplifications. Possible factors are that the direct S waves are affected by particular wave propagation paths and that at short distance SV is partitioned with more energy on the horizontal components and less on the vertical components than at larger distances.

Constraints on the 29 June 1992 Little Skull Mountain, Nevada, earthquake sequence provided by robust hypocenter estimates

Abstract The M s 5.4 Little Skull Mountain earthquake of 29 June 1992 occurred within 21 km of Yucca Mountain, Nevada, the site of a proposed high-level nuclear-waste repository. Data from a dense portable seismic network have enabled us to examine the faulting process in detail. Standard linearized least-squares procedures were used to determine hypocenter estimates for all the events. A grid-search technique was applied to test the robustness of estimates of the hypocenters of the mainshock and six of the largest aftershocks. Projection of the mainshock fault plane to the surface, delineated by its focal mechanism and the distribution of aftershocks, defines a strike subparallel to a north-northeast structural trend within Jackass Flats and collinear with the Mine Mountain fault system. This result implies reactivation of mapped faults with no known Quaternary movement, and growth or reactivation of buried faults having no clear surface manifestation. The aftershock distribution suggests that postseismic strain release occurred up-dip from the mainshock on the mainshock fault plane and on several secondary faults. The mainshock focal mechanism and the aftershock distribution indicate a normal-slip rupture on a plane with a strike of N55°E, a dip of 56°, and a rake of −72°. Results of the grid-search analysis show that the mainshock and three of the largest aftershocks probably occurred on the same fault plane and that the other three occurred off the mainshock fault plane; two on a steeply dipping, N200°E-striking plane (with dextral strike-slip displacements), and the other possibly on a plane conjugate to the mainshock rupture plane (focal mechanism unknown).

The Little Skull Mountain, Nevada, earthquake of 29 June 1992: Aftershock focal mechanisms and tectonic stress field implications

The ML = 5.6 29 June 1992 Little Skull Mountain earthquake occurred in the southern Nevada Test Site, Nye County, Nevada, within 20 km of a potential national nuclear waste repository at Yucca Mountain, Nevada. While microseismicity in the immediate epicentral vicinity occurred for years prior to the mainshock, foreshock activity ( ML 2) were also sufficiently well recorded to provide local network P -wave focal-mechanism solutions. The mainshock and the majority of the aftershocks through September 1992 display predominantly normal-slip focal mechanisms, but unambiguous strike-slip and oblique-slip focal-mechanism solutions are also present in the aftershock series. The mainshock and immediate foreshock depths were 10 ± 1 km below sea level, and depths of focus of aftershocks range from about 6 to 12 km. A large subset of the aftershock hypocenters define a southeast-dipping structure, probably the mainshock fault plane. No variation of mechanism style with depth is apparent. Two mainshock and 84 aftershock focal-mechanism solutions were input into Gephart's (1990) stressfield inversion program, and principal compressional stress directions, σ j , j = 1, 2, 3, were determined, as well as the stress difference ratio R = (σ1 − σ2)/(σ1 − σ3). The results indicate a uniform stress field in which σ3 (minimum stress) is oriented 116°/5° (azi, plunge) and σ1 (maximum stress) is oriented 219°/69° with R = 0.35. The inversion's selection of preferred nodal planes invokes Bott's (1959) criterion, which states that slip occurs in the direction of resolved shear traction on preexisting fault surfaces. For the predominantly normal-slip Little Skull Mountain earthquake mechanisms having ∼northeast-striking planes, the southeast, moderately to steeply dipping (dip > 55°) nodal planes are selected. However, for the dip-slip earthquake mechanisms exhibiting more northerly trending strikes, shallowly to moderately west-dipping (25° < dip < 65°) nodal planes are selected. When a Coulomb-Mohr analysis was applied to the focal mechanisms using the above principal stress directions and an assumed ratio σ1/σ3, the ratio of resolved shear stress to normal stress, τ/σ n , on the southeast-dipping nodal plane of the mainshock exhibited a near-maximum level, suggesting a high fault strength. In contrast, a wide range of fault strengths, or apparent coefficients of friction, was suggested by the τ/σ n ratios for Little Skull Mountain aftershock preferred nodal planes. Seismicity on misoriented faults, i.e., those on which shear stress was relatively low, may have been the result of a temporarily elevated fluid pore pressure following the mainshock.

Strain accumulation near Yucca Mountain, Nevada, 1983–1993

Over the decade 1983–1993 the U.S. Geological Survey has measured the deformation of a 50‐km‐aperture trilateration network centered on Yucca Mountain, the proposed disposal site for high‐level nuclear waste in the United States. The network was surveyed in 1983, 1984, and 1993. The average annual principal strain rates are 0.010±0.020μstrain/yr N90°W±24° and −0.009±0.021 μstrain/yr N00°E±24°, indicating no significant strain accumulation. The southeast corner of the network was disturbed on June 29, 1992, by the Little Skull Mountain earthquake (Ms = 5.4), the epicenter of which is about 20 km southeast of the Yucca Mountain site. Using the seismically determined fault plane (dip 54°S55°E), we find that 0.580±0.075 m of normal slip on a 5‐km square rupture surface at a depth of about 8 km provides a good fit to the observed deformation in the southeast corner of the network. The inferred seismic moment is (4.4±0.6)×1017 N m, which compares well with the observed seismic moment of 4.1×1017 N m.

Source parameters of the June 29, 1992 Little Skull Mountain Earthquake from complete regional waveforms at a single station

The June 29, 1992, mb=5.5, Little Skull Mountain earthquake occurred 20 km southeast of Yucca Mountain within the boundaries of the Nevada Test Site. This event was recorded at a very broadband station located about 310 km to the east at Kanab, Utah. Both body and surface waves from this single station were used in a grid search technique for the best fitting double couple moment tensor. The technique minimizes the misfit between reflectivity generated complete synthetic seismograms and the three‐component displacement data via a search over the strike, dip and rake of the source. In the passband of 50–15 s period, the grid search results in a well defined, nearly pure normal mechanism with a strike of 35°, a dip of 54°, a rake of −87°, and a seismic moment of 4.1 × 1017 N‐m. Applying the same technique to the large aftershock on July 5, 1992 we obtain a somewhat less well defined minimum with a strike of 358°, a dip of 36°, a rake of −177°, and a moment of 4.3 × 1015 N‐m. The minimum‐compression axes of these mechanisms are consistent with northwest‐southeast extension in the Basin and Range.