This study investigates the directions of structural failures and toppling near Point Reyes Station during the 1906 San Francisco earthquake (M-w 7.9). We examined archives of the Jack Mason Museum of West Marin History and other historical sources for photographs and other evidence relevant to the dynamics of the 1906 rupture in this area. Using historical maps, site investigations, and previously unpublished photographs, we determined the precise locations and orientations of several structures, including a correction to the orientation of the train that was the subject of previous studies. Based on the photographic evidence and written accounts, we estimate the direction of toppling or collapse of each structure. Nearly all objects found were thrown in a direction approximately parallel to the right-lateral San Andreas fault, and in the same direction as the static ground displacement. This suggests that fault-parallel accelerations may have been stronger than fault-normal accelerations, and that the slip on the fault may have begun slowly and stopped more suddenly.

In the absence of long-term instrumental data, the presence of fragile geologic features near active faults can provide physical limits on the level of ground shaking that could potentially have significant implications for seismic hazards. This paper introduces a multidisciplinary investigation that uses unfractured hoodoos in seismically active regions to constrain the level of ground accelerations at those locations. Although there is a large uncertainty associated with the age of the hoodoos because of their rapidly eroding nature, they can still be useful in providing physical limits on ground motions associated with recent large events. Here, we consider the fragilities of two hoodoos in the Red Rock Canyon region within a few kilometers of the Garlock fault, which is an active strike-slip fault in a transtensional region with at least a few large earthquakes in the Holocene. The hoodoos at these sites could be evidence of median or relatively low ground motions associated with large transtensional strike-slip earthquakes. Results of our field and laboratory tests on two hoodoos provide constraints on peak ground accelerations (PGAs) of 0.36 +/- 0.06g and 0.59 +/- 0.14g. Using the U. S. Geological Survey's (USGS) probabilistic seismic hazard (PSH) deaggregation, the dominant earthquake contributing to the hazard at the site of the hoodoos for the recurrence intervals of 475, 975, and 2475 years is located at a distance of 4.8 km and has a magnitude of 7.63, consistent with the observed paleoearthquake evidence on the Garlock fault. The PGAs corresponding to these three return periods are 0.26, 0.40, and 0.61g, respectively. Therefore, the survival of the more fragile hoodoo during a presumably large event on the Garlock fault in the past 550 years would be consistent with the 2008 seismic hazard level if the ground motions during that event were below the median value.

Nevada is a large western state in theUnited States with a seismic hazard that ranges from moderate to high, depending on location. This article identifies priorities to improve estimates of the seismic hazard in the most urbanized parts of the state, specifically the Reno-Carson City urban area of western Nevada and the Las Vegas urban region of southern Nevada. Collaborative task forces are needed to efficiently realize these priorities. For the Reno-Carson City region in western Nevada, the seismic hazard is high because of strain distributed across several active faults, including normal faults that dip beneath parts of the urban areas. The subsurface geometry and possible connections of these faults remain to be determined. The present large uncertainty in estimates of the slip rates can be reduced by future geological and geodetic studies, including trenching at more than one site per fault and increasing the density of geodetic stations to include multiple stations in the mountain ranges between faults to detect rotations. Adjustments to the ground-motion models for the regional properties of western and southern Nevada could reduce ground-motion uncertainties. Ground-motion simulation research needs an improved 3D velocity model. The seismic hazard in Las Vegas is lower than in Reno. An expanded geodetic network and continued geological studies of the active faults are needed. Uncertainties in the geometry and activity of the Frenchman Mountain and Eglington faults particularly introduce significant uncertainties into the seismic hazard in the Las Vegas basin. The more distant Garlock and Death Valley faults in eastern California impact the hazard in Las Vegas because the Las Vegas basin amplifies long-period ground motion and prolongs its duration, so reliable simulations from these sources are needed.

The peak ground acceleration (PGA) and peak ground velocity (PGV) from 5058 ruptures of a foam rubber stick-slip model are not distributed according to a lognormal probability distribution function. PGA and PGV values are decomposed using the method of Anderson and Uchiyama (2011). The statistically significant deviations from the lognormal distribution occur near the peak of the distribution. In some cases, high-amplitude tails differ by a much greater ratio, but the statistical significance of this effect is low. This result is true of both raw data and data adjusted for site and magnitude. Event terms are also not lognormal but can be modeled as a sum of three or four lognormal subdistributions, which possibly represent different preferred rupture initiation points rather than a uniform distribution of initiation points. The event term subdistributions with highest median values have small standard deviations, so if shapes of this nature were used in ground-motion prediction equations (GMPEs) during a probabilistic seismic-hazard analysis, the effect of the long tail of the lognormal distribution in controlling the hazard would be weakened considerably. Static stress drop was recorded for each event, and event terms for PGA and PGV are well correlated with static stress drop. Unlike Next Generation Attenuation-West 2 GMPEs, residual variances for the foam model are dominated by variability in the source slip function, rather than the path and site effects. This difference in the variance budget results from the way in which the source and site residuals are defined in this study; the source uncertainty includes variation in the rupture size (magnitude) and location, along with deviations in distance and path. We do not know if these results apply to earthquakes, but we do think tests of repeating stick-slip events in a physical system are useful to expand the set of credible hypotheses regarding possible behavior modes of earthquake faults.

Field studies of historic rupture traces show that fault stepovers commonly serve as endpoints to earthquake ruptures. This is an effect that is corroborated by past dynamic modeling studies. However, field studies also show a great deal of complexity in fault‐zone structure within a stepover, which is often simplified out of modeling studies. In the present study, we use the 3D finite‐element method to investigate the effect of one type of smaller‐scale complexity on the rupture process: a smaller fault segment positioned between the two primary strands of a strike‐slip fault stepover. We find that such small faults can have a controlling effect on whether or not a rupture is able to jump the stepover and on the resulting ground motions from these ruptures. However, this effect is neither straightforward nor linear: the length of the intermediate segment and its basal depth, as well as whether the stepover is extensional or compressional, all contribute to the rupture behavior and ground‐motion distribution. These results have important implications for assessing the probability of a rupture propagating through small‐ and large‐scale discontinuities in faults, as well as for evaluating ground‐motion intensities near fault stepovers. Because of the sensitivity of results to so many parameters, these results also suggest that modeling studies on idealized fault geometries may not be sufficient to describe the rupture behaviors of specific complex fault systems. Site‐specific modeling studies, where possible, will provide better inputs and constraints for probabilistic rupture length assessments as well as for ground‐motion estimates.

Research Article| August 05, 2015 Reconciling Precariously Balanced Rocks (PBRs) with Large Earthquakes on the San Andreas Fault System Lisa Grant Ludwig; Lisa Grant Ludwig aProgram in Public Health, University of California Irvine, 2085 AIRB, 653 East Peltason Drive, Irvine, California 92697‐3957 U.S.A.lgrant@uci.edu Search for other works by this author on: GSW Google Scholar James N. Brune; James N. Brune bSeismological Laboratory, University of Nevada Reno, Reno, Nevada 89557 U.S.A. Search for other works by this author on: GSW Google Scholar Abdolrasool Anooshehpoor; Abdolrasool Anooshehpoor cU.S. Nuclear Regulatory Commission, MS C5A24M, Washington, D.C. 20555‐0001 U.S.A. Search for other works by this author on: GSW Google Scholar Matthew D. Purvance; Matthew D. Purvance dItasca Consulting Group, 111 Third Avenue South, Suite 450, Minneapolis, Minnesota 55401 U.S.A. Search for other works by this author on: GSW Google Scholar Richard J. Brune; Richard J. Brune e3107 Murray Lane, Costa Mesa, California 92626‐2735 U.S.A. Search for other works by this author on: GSW Google Scholar Julian C. Lozos Julian C. Lozos fDepartment of Geophysics, Stanford University, 397 Panama Mall, Stanford, California 94305 U.S.A. Search for other works by this author on: GSW Google Scholar Author and Article Information Lisa Grant Ludwig aProgram in Public Health, University of California Irvine, 2085 AIRB, 653 East Peltason Drive, Irvine, California 92697‐3957 U.S.A.lgrant@uci.edu James N. Brune bSeismological Laboratory, University of Nevada Reno, Reno, Nevada 89557 U.S.A. Abdolrasool Anooshehpoor cU.S. Nuclear Regulatory Commission, MS C5A24M, Washington, D.C. 20555‐0001 U.S.A. Matthew D. Purvance dItasca Consulting Group, 111 Third Avenue South, Suite 450, Minneapolis, Minnesota 55401 U.S.A. Richard J. Brune e3107 Murray Lane, Costa Mesa, California 92626‐2735 U.S.A. Julian C. Lozos fDepartment of Geophysics, Stanford University, 397 Panama Mall, Stanford, California 94305 U.S.A. Publisher: Seismological Society of America First Online: 14 Jul 2017 Online ISSN: 1938-2057 Print ISSN: 0895-0695 © 2015 by the Seismological Society of America Seismological Research Letters (2015) 86 (5): 1345–1353. https://doi.org/10.1785/0220140239 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 Lisa Grant Ludwig, James N. Brune, Abdolrasool Anooshehpoor, Matthew D. Purvance, Richard J. Brune, Julian C. Lozos; Reconciling Precariously Balanced Rocks (PBRs) with Large Earthquakes on the San Andreas Fault System. Seismological Research Letters 2015;; 86 (5): 1345–1353. doi: https://doi.org/10.1785/0220140239 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 SocietySeismological Research Letters Search Advanced Search A major goal of seismology is to predict earthquake shaking (Frankel, 1999). Earthquakes are generated by rupture of faults, but it is difficult to predict the endpoints of fault rupture (Wesnousky, 2006) and the ground motions that will likely be produced. Precariously balanced rocks (PBRs) are a class of fragile landforms that are susceptible to toppling by earthquakes (Brune, 1996). PBRs have been reported to exist in many areas, including seismically active regions of the United States and New Zealand (Stirling and Anooshehpoor, 2006; Anderson et al., 2011). The population... You do not have access to this content, please speak to your institutional administrator if you feel you should have access.

We use the 3D finite‐element method to conduct dynamic models of rupture and resulting ground motion on the Claremont–Casa Loma stepover of the northern San Jacinto fault. We incorporate complex fault geometry (from the U.S. Geological Survey [USGS] Quaternary Faults Database; see [Data and Resources][1]), a realistic velocity structure (the Southern California Earthquake Center Community Velocity Model‐S), a realistic regional stress field with an orientation taken from seismicity relocation literature, and several stochastic self‐similar shear stress distributions. As we incorporate more types of complexity, the specific effects of any individual factor become less apparent within the overall rupture behavior. We also find that the distribution of high and low shear stress that arises from combining regional and stochastic stress fields has the strongest control over where the rupture terminates. Using a regional stress field alone, as well as with the combined regional and stochastic stress realization, we find that the stepover presents a significant barrier to rupture, regardless of our choice of initial nucleation point and that it is difficult for rupture to propagate the full length of either fault segment. Greater heterogeneity of stresses tends to produce shorter ruptures. Within this result, we find that the Claremont strand is more favorable for long ruptures than the Casa Loma–Clark strand. Low‐frequency ground‐motion intensity and distribution are controlled largely by the velocity structure rather than by stress heterogeneity. The strongest motions produced in these models are in the San Bernardino basin. Although directivity effects do contribute to the low‐frequency ground‐motion distribution, particularly in the near field, they are secondary to the effects of the velocity structure.Online Material: Figures of ground motions from models used to calibrate the stress conditions for dynamic rupture propagation. [1]: #sec-18

The Southern California San Jacinto fault is geometrically complex, consisting of several major strands with smaller scale complexity within each strand. The two northernmost strands, the Claremont and the Casa Loma–Clark, are separated by a 25‐km‐long extensional stepover with an average of 4 km separation between the strands. We use a combined modeling method to assess probable rupture and ground‐motion behaviors for this stepover. First, dynamic rupture modeling on geometrically complex fault strands embedded in a state‐of‐the‐art 3D crustal velocity model is used to generate a series of scenario earthquakes. We then use the resulting near‐fault low‐frequency (<1  Hz) ground‐motion time histories to generate broadband synthetic seismograms with a hybrid approach. These synthetics are then compared with a distribution of precariously balanced rocks (PBRs) near the fault to constrain our results and assess shaking hazard for the region surrounding the fault. Our dynamic models produce sources between M w 5.4 and 6.9, with rupture limits imposed by sharp contrasts in fault stress or by geometrical barriers. The main stepover serves as a primary barrier to rupture in our model, producing event sizes that are consistent with the historical behavior of the San Jacinto fault. The largest broadband synthetics are a good match to leading ground‐motion prediction equations and are generally consistent with the distribution of PBRs, none of which experience accelerations that produce toppling probabilities significantly higher than zero. Thus, although the PBRs do not rule out any of our model scenarios, they confirm that our models produce realistic rupture extents and shaking. Online Material: Figures of total slip for additional rupture models, low‐frequency intensity plots, synthetic seismograms, and comparison with ground‐motion prediction equations.

Precariously balanced rocks (PBRs) are naturally occurring geological features that could be easily toppled by strong earthquake shaking. They bring two forms of information valuable for understanding seismic hazard. In the specific regions where PBRs occur, they provide direct information about strong ground motions not exceeded during their lifetimes. Their more general application is to provide useful limits on ground motion variability. This article explains how precarious rocks may be interpreted, and why they are important for understanding strong ground motions in engineering application.

A crustal normal-faulting earthquake (M-JMA 7.0; M-w 6.7) occurred in eastern Tohoku, Japan, on 11 April 2011. K-NET and KiK-net stations recorded 82 records from within 100 km of fault rupture. These data and data from associated foreshocks and aftershocks will make a critical contribution to future improvements of ground-motion prediction for normal-faulting earthquakes.Peak ground accelerations (PGA) and peak ground velocities (PGV) are compared with four ground-motion prediction equations (GMPEs) that include the style of faulting as a predictor parameter. For distances under 100 km, and using a network average value of V-S30, the average ratio of PGA to the selected GMPEs (the event term) is high by factors of 2.3-3.7. Event terms for PGV are high by factors of 1.4-1.8. Adjusting PGA and PGV with customized site terms (Kawase and Matsuo, 2004a, b), the standard deviations of PGA and PGV residuals are reduced from 0.59 to 0.43, and from 0.53 to 0.35, respectively. The event terms decreased to relatively small factors of 1.1-1.8 for PGA and increased slightly to 1.5-2.0 for PGV. Thus, site terms are very important, but positive event terms remain. The remaining positive event terms are not explained by high stress drop, which was typical of crustal events of all mechanisms globally or in Japan. Two subparallel faults ruptured, but source inversions, which we reviewed, revealed that they ruptured sequentially, so simultaneous contributions from the two faults did not cause high motions. Although these observations may tend to suggest that ground motions in large normal-faulting events are larger than predicted by the tested models, we are not aware of any observations from this event that contradict the precarious rock evidence of Brune (2000) that ground shaking is low on the footwall near the rupture.

The Fort Sage Mountains fault zone is a normal fault in the Walker Lane of the western Basin and Range that produced a small surface rupture (<20 cm) during an M-L 5.6 earthquake in 1950. We investigate the paleoseismic history of the Fort Sage fault and find evidence for two paleoearthquakes with surface displacements much larger than those observed in 1950. Rupture of the Fort Sage fault similar to 5.6 ka resulted in surface displacements of at least 0.8-1.5 m, implying earthquake moment magnitudes (M-w) of 6.7-7.1. An older rupture at similar to 20.5 ka displaced the ground at least 1.5 m, implying an earthquake of M-w 6.8-7.1. A field of precariously balanced rocks (PBRs) is located less than 1 km from the surface-rupture trace of this Holocene-active normal fault. Ground-motion prediction equations (GMPEs) predict peak ground accelerations (PGAs) of 0.2-0.3g for the 1950 rupture and 0.3-0.5g for the similar to 5.6 ka paleoearthquake one kilometer from the fault-surface trace, yet field tests indicate that the Fort Sage PBRs will be toppled by PGAs between 0.1-0.3g. We discuss the paleoseismic history of the Fort Sage fault in the context of the nearby PBRs, GMPEs, and probabilistic seismic hazard maps for extensional regimes. If the Fort Sage PBRs are older than the mid-Holocene rupture on the Fort Sage fault zone, this implies that current GMPEs may overestimate near-fault footwall ground motions at this site.

Yucca Mountain is the designated site of the underground repository for the United States' high-level radioactive waste (HLW), consisting of commercial and military spent nuclear fuel, HLW derived from reprocessing of uranium and plutonium, surplus plutonium, and other nuclear-weapons materials. Yucca Mountain straddles the western boundary of the Nevada Test Site, where the United States has tested nuclear devices since the 1950s, and is situated in an arid, remote, and thinly populated region of Nevada, ~100 miles northwest of Las Vegas. Yucca Mountain was originally considered as a potential underground repository of HLW because of its thick units of unsaturated rocks, with the repository horizon being not only ~300 m above the water table but also ~300 m below the Yucca Mountain crest. The fundamental rationale for a geologic (underground) repository for HLW is to securely isolate these materials from the environment and its inhabitants to the greatest extent possible and for very long periods of time. Given the present climate conditions and what is known about the current hydrologic system and conditions around and in the mountain itself, one would anticipate that the rates of infiltration, corrosion, and transport would be very low—except for the possibility that repository integrity might be compromised by low-probability disruptive events, which include earthquakes, strong ground motion, and (or) a repository-piercing volcanic intrusion/eruption. Extreme ground motions (ExGM), as we use the phrase in this report, refer to the extremely large amplitudes of earthquake ground motion that arise at extremely low probabilities of exceedance (hazard). They first came to our attention when the 1998 probabilistic seismic hazard analysis for Yucca Mountain was extended to a hazard level of 10-8/yr (a 10-4/yr probability for a 104-year repository "lifetime"). The primary purpose of this report is to summarize the principal results of the ExGM research program as they have developed over the past 5 years; what follows will be focused on Yucca Mountain, but not restricted to it.

The history of the study of surface-wave dispersion in the Pacific Ocean beginning about 1950 is reviewed. During this period the quality of dispersion data has improved and powerful theoretical methods have been developed. Several of the most difficult problems have been solved, such as: (1) the effect of water and sedimentary layers on short-period surface waves, (2) the relation of the G-wave to Love-wave dispersion, (3) the effect of curvature and gravity on surface waves, and (4) determination of the dispersion of mantle Rayleigh and Love waves. Measurements of phase and group velocities in a wide period range provide the principal data in these studies and are summarized in graphical form. A generalized structure of the Pacific Basin proper based on observed surface wave dispersion curves and theoretical studies is presented. The important structural features of the Pacific are a water layer 5 km thick, a sedimentary layer about 1 km thick with very low shear velocity, the crustal layer 5 km thick, and a low velocity channel in the upper mantle extending from a depth of about 60 to 150 km. For the lower mantle the Gutenberg model is consistent with the Pacific dispersion data.

Using 3D dynamic models, we investigate the effect of fault stepovers on near-source ground motion. We use the finite-element method to model the rupture, slip, and ground motion of two parallel strike-slip faults with an unlinked overlapping stepover of variable width. We model this system as both an extensional and a compressional stepover and compare the results to those of single planar faults. We find that, overall, the presence of a stepover along the fault trace reduces the maximum ground motion when compared to the long planar fault. Whether the compressional or extensional stepover exhibits higher ground motion overall depends on the width of the separation between the faults. There is a region of reduced ground motion at the end of the first fault segment, when the faults are embedded in a homogeneous material. We also experiment with stress fields leading to supershear and subshear rupture velocities, and with different stress drops within those conditions. We find that subshear rupture produces stronger motions than supershear rupture, but supershear ruptures produce that maximum over a larger area than subshear areas, even though the overall area that experiences any shaking at all is not drastically different between the two cases. Lastly, we experiment with placing realistic materials along and around the faults, such as a sedimentary basin in an extensional stepover, a damage zone around the fault, and a soft rock layer on top of bedrock through the entire model area. These configurations alter the pattern of ground motion from the homogeneous case; the peaks in ground motion for the bimaterial cases depend on the materials in question. The results may have implications for ground-motion prediction in future earthquakes on geometrically complex faults.

Kappa is a one-parameter estimator of the spectral amplitude decay with frequency of a seismogram. Low values (similar to 5 ms) indicate limited attenuation of high-frequency energy whereas higher values (similar to 40 ms) indicate high-frequency energy has been removed. Kappa is often assumed to be a site term and used in seismic designs. We address two key questions about kappa: (1) how to identify source, path, and site contributions to kappa; and (2) can kappa estimates from smaller earthquakes, and more readily accessible weak- motion recordings, be reasonably extrapolated to estimate kappa of larger earthquakes? The use of small earthquakes (M-L < 1) presents many challenges and requires new approaches. We develop estimates of kappa for seismograms from 1137 small earthquakes recorded by the ANZA seismic network in southern California, and compare these to results from the stronger recorded shaking generated by 43 M-L > 3.5 earthquakes inside the network. We find kappa from small earthquakes predicts the relative values of kappa for larger earthquakes (e.g., measurements at stations PFO and KNW are small compared with those at stations TRO and SND). For the SND and TRO data, however, kappa values from small earthquakes overpredict those from moderate and large earthquakes. Site effects are the most important contributor to kappa estimates, but the scatter within kappa measurements at a given station is likely caused by a significant contribution from near the source, perhaps related to near-source scattering. Because of this source-side variability, care is recommended in using individual small events as Green's functions to study source-time effects of moderate and large events.