Precarious Rocks Research Articles

Uncertainty in overturning of precariously balanced rocks due to basal contact

AbstractPrecariously balanced rocks (PBRs) are a type of naturally occurring freestanding structure that provides valuable information to constrain seismic hazard at return periods which are important for critical facilities such as nuclear power plants and nuclear repositories. Exposure ages have been established to be in excess of 10,000–30,000 years, which is why precarious rocks are one of the only available means to validate seismic hazard associated with long return periods. One critical component for constraining seismic hazard in this way is the overturning estimate of a given precarious rock as a function of earthquake intensity. However, current state‐of‐the‐art methods for modeling the seismic response of precarious rocks involve significant sources of uncertainty. One of the main sources of uncertainty stems from the interface of the rock, which is usually occluded during surveying and assumed during modeling. Through extensive shake table testing, this study analyzes the uncertainty in the overturning response of a granite precarious rock specimen incorporating various degrees of interface contact. The results indicate that a small variation in the contact geometry could result in a substantial increase in the stability of the specimen, which is significant given the difficulty of surveying the interface of PBRs in the field. Repeatability tests indicate that the overturning demand can vary up to nearly ±50%. The probabilistic overturning responses are compared across the interface changes to bound uncertainty; and, the effect of modeling parameters, namely the contact normal stiffness, is evaluated through a parametric study and comparison with experimental results.

The Rocks That Did Not Fall: A Multidisciplinary Analysis of Near‐Source Ground Motions From an Active Normal Fault

AbstractOn 8 July 2021 a M6.0 normal faulting earthquake rocked the community of Walker and the surrounding region near the California‐Nevada border. In the 1990s, field surveys of nearby Meadowcliff Canyon identified numerous precarious rocks deemed likely to topple in the event of strong shaking. Despite their proximity (∼6 km) to the 2021 earthquake, the precarious rocks still remain standing. In this work, we combine advanced source and ground motion characterization techniques to help unravel this mystery. High‐precision hypocentral locations reveal a clear north/south‐striking, east‐dipping rupture plane along the southern extension of the Slinkard Valley fault. The mainshock nucleated near the base of the fault, triggering thousands of aftershocks. Bayesian source spectral analyses indicate that the mainshock had a moderately‐high stress drop (∼17 MPa), and that aftershocks with deeper hypocenters have higher stress drops. Peak Ground Acceleration (PGA) recordings at regional stations agree well with existing ground motion models, predicting PGA of ∼0.3 g in Meadowcliff Canyon, a level sufficient to topple precarious rocks based on PGA‐derived stability criteria. We demonstrate that despite these large ground accelerations, the pulse duration in Meadowcliff Canyon is too short to supply the impulse necessary to damage these features, observations which support the application of dynamic toppling models that account for the joint effects of pulse amplitude and duration when assessing rock fragility. This study provides a unique vantage point from which to interpret rarely‐observed strong‐motion recordings from close to an active normal fault, one of many that dominate hazard along the eastern Sierra.

Precarious rocks help refine earthquake hazard in California

Precarious rocks help refine earthquake hazard in California

Investigation of the Ground‐Motion Variability Associated with Site Response for Sites withVS30over 500 m/s

Abstract Understanding and reducing uncertainties in ground‐motion prediction are high priorities for seismic‐hazard analysis. This article examines , the variability in synthetic ground motions at rock sites caused by the variability in randomly generated velocity profiles of the geological column from 5 km depth to the surface. Only sites with V S 30 of 500 m/s or higher are considered, and linearity is assumed. These synthetic estimates of the mean value of are a complicated but understandable function of magnitude, period, and V S 30 . The distribution of modeled residual response spectral amplitudes at several oscillator periods is not lognormal, but the deviations are in the central part of the distribution, in which the effect on probabilistic seismic‐hazard analysis may not be very large. Adding another constraint to the velocity profile, namely that the shear‐wave velocity of the uppermost layer should be at least 70% of V S 30 , greatly reduces the uncertainty at high frequencies. We tentatively identify sites with this property as nonresonant rock, because it excludes sites with a strong resonance in a thin shallow layer. The reduction in uncertainty that this allows might reduce or eliminate the contradiction between the U.S. National Seismic Hazard Map and precarious rocks in southern California. Furthermore, for nonresonant rock sites, the residual impact of the ergodic assumption might be reduced to more tolerable levels. Online Material: Tables of variability values.

Use of Fragile Geologic Structures as Indicators of Unexceeded Ground Motions and Direct Constraints on Probabilistic Seismic Hazard Analysis

Abstract We present a quantitative procedure for constraining probabilistic seismic hazard analysis results at a given site, based on the existence of fragile geologic structures at that site. We illustrate this procedure by analyzing precarious rocks and undamaged lithophysae at Yucca Mountain, Nevada. The key metric is the probability that the feature would have survived to the present day, assuming that the hazard results are correct. If the fragile geologic structure has an extremely low probability of having survived (which would be inconsistent with the observed survival of the structure), then the calculations illustrate how much the hazard would have to be reduced to result in a nonnegligible survival probability. The calculations are able to consider structures the predicted failure probabilities of which are a function of one or more ground‐motion parameters, as well as structures that either rapidly or slowly evolved to their current state over time. These calculations are the only way to validate seismic hazard curves over long periods of time.

Capacity Assessment of the Titus Tunnel Bridge Using Analytical and Numerical Techniques

A great number of masonry arch bridges are still in service in Europe, the Middle East, and other parts of the world. Assessing the capacity of these historical structures is an important problem both for safety and preservation purposes. This paper addresses the assessment of the Titus Tunnel Bridge, a single-span, Roman stone arch pedestrian bridge (Common Era 70) located in Antakya, Turkey, using analytical and numerical techniques. The bridge is located in an active seismic zone and has survived several major earthquakes. Therefore, it is important to understand the Bridge’s subtle features that have helped the structure survive for the last two millennia. Furthermore, being a single-span semicircular stone arch, the bridge exemplifies the main construction blocks of several masonry bridges around the world. First, the analytical techniques, i.e., Heyman’s geometrical factor of safety and the mechanism method, are used to assess the capacity of the bridge under simple loading conditions and to validate the finite-element (FE) models. Then, discrete FE modeling with explicit time integration is used to investigate the arch behavior under both gravity and earthquake loading conditions. It is observed that the Titus Tunnel Bridge has a significant margin of safety against collapse, which also explains the approximately 2,000-year lifetime of the structure. The results are also used to provide estimates on the maximum expected ground motions in the region using a precarious rocks analogy.

Workshop Report: Applications of Precarious Rocks and Related Fragile Geological Features to U.S. National Hazard Maps

The Workshop on the Applications of Precarious Rocks and Related Fragile Geological Features to U.S. National Hazard Maps met on the University of Nevada campus in Reno (UNR), Nevada, on 4–5 October 2010. The goal of this workshop was to develop recommendations to the U.S. Geological Survey (USGS) on the use of precariously balanced rocks and related fragile geological features to improve the national seismic hazard maps (NSHM). The context of the workshop is the growing realization that it is essential to regard hazard maps as the calculated output of hazard models that should be tested, and that fragile geological features provide the only data to validate the predictions of these models at low probabilities. A fragile geological feature (FGF) was defined as a feature that might be easily destroyed by strong earthquake ground motions and is mechanically simple enough to allow for analysis of the ground motions that might cause it to be destroyed. For testing the prediction of a seismic hazard model, an FGF must also have been in essentially the present geometry for enough time to constrain past ground motions. A precariously balanced rock (PBR) is the type example of an FGF. A PBR is a rock that is balanced on, but not mechanically attached to, its pedestal, and it has a high ratio of height to width of the contact with the pedestal, so that it is easily toppled. Brune and Whitney (1992) and Brune (1996) suggested that PBRs might serve as low-resolution seismoscopes to put upper bounds on ground motions over the lifetime of the rock. FGFs were recognized by Hanks et al. (2006) for their critical contribution to evaluation of extreme ground motions at Yucca Mountain. Because PBRs have been studied more than other FGFs, most attention at the workshop focused on their distribution …

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.

Studies on ‘precarious rocks’ in the epicentral area of the AD 1356 Basle earthquake, Switzerland

SUMMARY For the first time precarious rocks have been analysed in the epicentral area of the AD 1356 Basle earthquake in northern Switzerland. Several cliff sites in flat-lying, thickly bedded Upper Jurassic coral limestones in the Jura Mountains were investigated. Seven blocks are regarded as precarious with respect to earthquake strong ground motions. The age of these precarious rocks could not be determined directly as for instance by radiometric dating methods; however, based on slope degradation processes it can be concluded that the formation of these blocks predates the AD 1356 Basle earthquake. The acceleration required to topple a precarious rock from its pedestal is estimated using geometrical data for individual block sections and earthquake strong-motion records from stations on rock sites in the European Strong-Motion Database as input data for the computer program ROCKING V1.0 from the Seismological Laboratory, University of Nevada, Reno. The calculations indicate that toppling of a precarious rock largely depends on earthquake strength but also on the frequency spectrum of the signal. Although most investigated precarious rocks are surprisingly stable for ground motions similar to those expected to have occurred during the AD 1356 Basle earthquake, at least two blocks are clearly precariously balanced, with peak toppling accelerations lower than 0.3 g. Possible reasons why these blocks did not topple during the AD 1356 Basle earthquake include incomplete separation from their base, sliding of precarious rocks, their size, lower than assumed ground accelerations and/or duration of shaking.

Methodology for Obtaining Constraints on Ground Motion from Precariously Balanced Rocks

Precariously balanced rocks provide constraints on the level of ground motion that could have occurred during the time the rocks have been in their current positions. Field measurements of the quasi-static toppling accelerations of precariously balanced rocks provide information for computer modeling of the response of the rocks to given ground-motion histories. The quasi-static toppling acceleration is determined by the ratio of the quasi-static force through the center of mass whose moment about the rocking point counterbalances that of gravity and the mass of the rock. For an estimate of dynamic toppling peak ground accelerations, we use a finite-difference numerical code to model the dynamic response of rocks to arbitrarily complex acceleration time histories. We test our computational methodology and results using the University of Nevada shake table. The shake-table tests of both idealized rectangular shapes and actual rocks confirm the accuracy of our methodology for estimating dynamic toppling accelerations of precarious rocks for different waveforms representing earthquake ground-motion time histories. A study of statistical variation of the dynamic toppling acceleration for a suite of seismograms provides constraints on the peak ground acceleration on hard-rock sites in Mojave Desert, about 15 km from the San Andreas fault, of about 0.5 g for synthetic seismograms with relatively high frequency content, or about 0.4 g for seismograms with spectra similar to the recent Izmit, Turkey, earthquake. A major source of uncertainty is the uncertainty in the spectrum of ground motion for large earthquakes. Waveforms with relatively more low frequencies can topple the rocks with lower peak ground accelerations. We describe methods for reducing the uncertainties with further study and more data.

Precarious Rock and Overturned Transformer Evidence for Ground Shaking in the Ms 7.7 Kern County Earthquake: An Analog for Disastrous Shaking from a Major Thrust Fault in the Los Angeles Basin

Precariously balanced rocks and overturned transformers in the vicinity of the White Wolf fault provide constraints on ground motion during the 1952 M s 7.7 Kern County earthquake, a possible analog for an anticipated large earthquake in the Los Angeles basin (Shaw et al. , 2002; Dolan et al. , 2003). On the northeast part of the fault preliminary estimates of ground motion on the footwall give peak accelerations considerably lower than predicted by standard regression curves. On the other hand, on the hanging-wall, there is evidence of intense ground shattering and lack of precarious rocks, consistent with the intense hanging-wall accelerations suggested by foam-rubber modeling, numerical modeling, and observations from previous thrust fault earthquakes. There is clear evidence of the effects of rupture directivity in ground motions on the hanging-wall side of the fault (from both precarious rocks and numerical simulations). On the southwest part of the fault, which is covered by sediments, the thrust fault did not reach the surface (“blind” thrust). Overturned and damaged transformers indicate significant transfer of energy from the hanging wall to the footwall, an effect that may not be as effective when the rupture reaches the surface (is not “blind”). Transformers near the up-dip projection of the fault tip have been damaged or overturned on both the hanging-wall and footwall sides of the fault. The transfer of energy is confirmed in a numerical lattice model and could play an important role in a similar situation in Los Angeles. We suggest that the results of this study can provide important information for estimating the effects of a large thrust fault rupture in the Los Angeles basin, specially given the fact that there is so little instrumental data from large thrust fault earthquakes.

Distribution of precarious rocks at the Nevada Test Site: Comparison with ground motion predictions from nuclear tests

Precarious rocks have been proposed as ground motion seismoscopes that have been in place thousands of years, thereby providing important constraints on seismic hazard estimates. Numerous precarious rocks exist in the region of large nuclear explosions at Nevada Test Site (NTS). Ground motion recordings from nuclear tests have provided regression equations for ground acceleration and ground velocity as a function of distance and explosion yield for comparison with constraints from precarious rocks. Very near large NTS explosions, there are no precarious rocks or semiprecarious rocks. Cliff faces are shattered and numerous recent rockfalls have fresh white surfaces covered by caliche: proof that the rockfalls have been caused by the explosions. As the distance from the explosions increases and the contours for predicted ground motion decrease, fewer and fewer rockfalls are observed, some semiprecarious rocks begin to appear, and finally, numerous precarious rocks are found with no evidence of recent rockfalls. To quantify the calibration of precarious rocks and ground motion we (1) do field tests on the rocks to determine the quasi‐static and dynamic toppling accelerations, (2) carry out computer tests using estimates of waveforms from NTS explosions, and (3) compare the results with ground motions predicted by empirical regressions for nuclear explosions. We conclude that these comparisons provide an important validation of the precarious rock methodology and an approximate validation of preliminary estimates of toppling accelerations.

Precarious rock evidence for low near-source accelerations for trans-tensional strike-slip earthquakes

This paper describes precarious rock evidence for low ground motions associated with extensional sections of strike-slip faults. Recent evidence from physical and numerical models and data regressions has indicated that ground motion for extensional strike-slip regions may be lower than for strike-slip faults with a large fault-normal tectonic stress component, and for thrust faults in general. Data from compressional strike-slip and thrust earthquakes dominates the database used in most regression curves for ground acceleration, and in the calculation of current probabilistic seismic hazard maps. Therefore, estimates of ground accelerations on these seismic hazard maps may be too high for sites near extensional sections of strike-slip faults. This paper discusses precariously balanced rock data from three areas near extensional sections of strike-slip faulting: (1) the region of the Honey fault, California, with an active Holocene fault, (2) the Red Rock Canyon region of the Garlock fault, near a dilatational step-over, and (3) the region just south of Beaumont, California, near the Hemet dilatational step-over in the San Jacinto fault. These are all active strike-slip faults, with at least a few large earthquakes in the Holocene, and, in the case of the San Jacinto example, historic large earthquakes ( M=7). Thus, the precarious rocks at these sites are evidence of relatively low ground motions associated with extensional strike-slip faulting. The results of this study could be very important in developing more detailed seismic hazard maps in the future.

Precarious-Rock Constraints on Ground Motion from Historic and Recent Earthquakes in Southern California

New examples of precarious rocks are presented. These rocks provide constraints on ground motion for historical and recent large earthquakes. An approximate field calibration of the precarious-rock methodology was provided by the M 7.1 Hector Mine earthquake of 16 October 1999. Previously documented rocks at Granite Pass near Kelso, California (Brune, 1996), were overturned by the earthquake, and a nearby strong-motion station indicated ground motions of about 0.2 g , consistent with the toppling accelerations estimated in the published article. The time of the last earthquake for the section of the fault nearest the precarious rocks has recently been determined to be more than 10 ka, consistent with previously estimated ages for precarious rocks. This first example of an actual earthquake field test lends strong support to the precarious-rock methodology. The other rocks discussed here give constraints on ground motions for historic earthquakes, such as the 1812 and 1857 San Andreas fault earthquakes, the 1899 and 1912 San Jacinto Fault earthquakes, the 1952 Kern County earthquake, and possibly recent but prehistoric earthquakes on the Banning and Garlock faults. The ground-motion constraints for these earthquakes are lower than predicted by some recent ground-motion attenuation curves (which are extrapolations to near-fault distances from a data set dominated by data from larger distances) but are generally consistent with peak ground accelerations observed from the recent large Turkey and Taiwan earthquakes and provide important additional information for seismic-hazard analysis. On the other hand, there are areas where precarious rocks would be expected on the basis of previous studies but are apparently not found. This suggests possible earthquakes on previously unrecognized, or only recently recognized, faults. One such area is in northwestern San Diego and southwestern Orange County between the Elsinore and Newport–Inglewood faults. The lack of precarious rocks in this area might be attributed to recent earthquakes on the blind-thrust faults proposed in the area by Grant et al. (1999, 2002) and Rivero et al. (2000).

Verification of precarious rock methodology using shake table tests of rock models

Precariously balanced rocks in seismically active regions are effectively upper-limit strong motion seismoscopes that have been in place for thousands of years. Thus, estimates of the dynamic toppling acceleration of these rocks (through rigid body rocking) can provide constraints on the peak ground accelerations experienced during past earthquakes. We have developed a methodology that uses a two-dimensional numerical code to calculate the dynamic rocking response of precarious rocks to realistic ground acceleration time histories. Statistical analyses of the dynamic response of these rocks to a range of synthetic seismograms, as well as strong motion records, can provide important information about the ground motion attenuation curves and seismic hazard maps. We use shake table tests to investigate the dynamic rocking response of 13 wooden rectangular blocks of various sizes and aspect ratios subjected to realistic seismograms and compare the results with those of numerical tests. Our results indicate good agreement between the shake table and numerical results.

Assessment of the Site Conditions of Precariously Balanced Rocks in the Mojave Desert, Southern California

We examine whether groups of precariously balanced rocks in the Mo- jave Desert, southern California, may have remained standing for thousands of years because anomalous site conditions prevent the rocks from toppling during large earth- quakes. Measurements of peak ground velocity at bandwidths of 0.5-1, 1-2, 2-4, 4-8, and 8-16 Hz for 56 earthquakes (ML 3.5-4.8) at two sites of precarious rocks are compared with velocities recorded for the same earthquakes by three TRI- NET* stations located on engineering rock (National Earthquake Hazard Reduction Program (NEHRP) site class B). We find that the residuals (the logarithm of the ratio of the amplitudes recorded at the precarious-rock sites to the TRINET amplitudes for the same earthquake and epicentral distance) at frequencies less than about 4 Hz are negative (i.e., deamplification of 50%-250%), whereas the residuals are slightly positive at the higher frequencies (i.e., amplification of up to 25%-50%). High- frequency ground motions (e.g., peak ground acceleration) may therefore be slightly amplified at the precarious-rock sites, which means that site conditions do not appear to explain the existence of the precarious rocks in areas where high peak ground accelerations are predicted in recent probabilistic seismic-hazard (PSH) models. This discrepancy between the precarious rocks and the PSH models should be urgently resolved.

Three-Dimensional Ground Motion Simulations for Large Earthquakes on the San Andreas Fault with Dynamic and Observational Constraints

I have simulated 0–0.5 Hz viscoelastic ground motion in Los Angeles from M 7.5 earthquakes on the San Andreas fault using a fourth-order staggered-grid finite-difference method. Two scenarios are considered: (a) a southeast propagating and (b) a northwest propagating rupture along a 170-km long stretch of the fault near Los Angeles in a 3D velocity model. The scenarios use variable slip and rise time distributions inferred from the kinematic inversion results for the 1992 M 7.3 Landers, California, earthquake. The spatially variable static slip distribution used in this study, unlike that modeled in a recent study,1 is in agreement with constraints provided by rupture dynamics. I find peak ground velocities for (a) and (b) of 49 cm/s and 67 cm/s, respectively, near the fault. The near-fault peak motions for scenario (a) are smaller compared to previous estimates from 3D modeling for both rough and smooth faults.1,2 The lower near-fault peak motions are in closer agreements with constraints from precarious rocks located near the fault. Peak velocities in Los Angeles are about 30% larger for (b) 45 cm/s compared to those for (a) 35 cm/s.

THREE-DIMENSIONAL GROUND MOTION SIMULATIONS FOR LARGE EARTHQUAKES ON THE SAN ANDREAS FAULT WITH DYNAMIC AND OBSERVATIONAL CONSTRAINTS

I have simulated 0–0.5 Hz viscoelastic ground motion in Los Angeles from M 7.5 earthquakes on the San Andreas fault using a fourth-order staggered-grid finite-difference method. Two scenarios are considered: (a) a southeast propagating and (b) a northwest propagating rupture along a 170-km long stretch of the fault near Los Angeles in a 3D velocity model. The scenarios use variable slip and rise time distributions inferred from the kinematic inversion results for the 1992 M 7.3 Landers, California, earthquake. The spatially variable static slip distribution used in this study, unlike that modeled in a recent study,1is in agreement with constraints provided by rupture dynamics. I find peak ground velocities for (a) and (b) of 49 cm/s and 67 cm/s, respectively, near the fault. The near-fault peak motions for scenario (a) are smaller compared to previous estimates from 3D modeling for both rough and smooth faults.1,2The lower near-fault peak motions are in closer agreements with constraints from precarious rocks located near the fault. Peak velocities in Los Angeles are about 30% larger for (b) 45 cm/s compared to those for (a) 35 cm/s.

Precarious Rock Evidence for Low Ground Shaking on the Footwall of Major Normal Faults

Preliminary interpretations of precarious rock observations suggest relatively low near-fault (<3 km) footwall ground motions for normal faults relative to strike-slip faults, as reported for a dynamic foam rubber normal fault model by Brune and Anooshehpoor (1999). Since for large normal fault earthquakes there are no instrumental data at such close distances from the fault trace, precarious rocks may provide important data for estimating the ground motion for these earthquakes. Use of empirical curves in the literature, based mainly on strike-slip data, may lead to overestimates of seismic hazard for the near-fault footwall of normal faults. These preliminary results may warrant more comprehensive and more quantitative followup studies.

Methodology for using precarious rocks in Nevada to test seismic hazard models

AbstractFields of precariously balanced rocks indicate that strong earthquake motions have not occurred at that site since the precarious rocks developed. These fields can be characterized with an estimate of the peak acceleration that would be sufficient to topple the rocks and an estimate of how long the rocks have been precarious. This article uses this information to test the input to probabilistic seismic hazard analysis. The fundamental assumption is that the probability of exceeding a ground motion capable of toppling a precarious rock during a time period equal to the age of the rock is equal to the confidence level at which the inputs to the probabilistic seismic hazard analysis can be rejected.We performed a probabilistic seismic hazard analysis for 26 sites of observed precarious rocks in Nevada, using preliminary estimates of the toppling acceleration and the age of the features. Two standard models are rejected with over 95% confidence by most of the precarious rock observations. We consider several possible explanations for the inconsistency. A probabilistic seismic hazard analysis that eliminates the area sources and only includes faults is consistent with the precarious rock observations at most of the sites. Spatial clustering of small-magnitude events could maintain this effect without contradicting the precarious rock observations. However, it may not be necessary to completely reject the area sources from our probabilistic seismic hazard analysis. The physics of rock stability may allow increasing the minimum magnitude to 6.0 in the area sources, because the short duration of high-frequency accelerations in smaller events may not topple all precarious rocks. The precarious rocks could generally have small site effects. Attenuation models may overestimate ground motions on the foot wall of normal faults, and a different attenuation model in general might decrease the ground-motion predictions. The ergodic assumption that is made in estimating the uncertainty in attenuation models may be contributing to overestimate the hazard. Individually, each of these effects might allow more of the precarious rock sites to be consistent with the area source zones.