ALBERT

Description: 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.

Description: We estimate the variance in ground motions related to repeated large earthquakes occurring on the same fault segment with similar magnitudes. We find eight earthquake pairs for which suitable strong-motion records exist. Two are crustal strike-slip earthquakes from California and six are subduction zone earthquakes from Japan. We consider only large earthquakes and deal with frequencies greater than the earthquake corner frequency, so the variability that is considered here is related to smaller scale differences in the rupture process, particularly on the part of the fault nearest the station. We find that the variance of the 5% damped spectral accelerations of these pairs, termed , averages to about 45% and 80% of 2 for the crustal and subduction zone earthquakes, respectively, in which 2 is the contribution of source variability to the total variability of ground motion estimated by some recent ground-motion prediction equations. We suggest that is lower than 2 , for the frequencies at which is estimated, because it depends primarily on only local physical properties of a fault that are the same in repeated earthquakes. We therefore suggest that at sites where the hazard is controlled by a single rerupturing source, one could potentially use a between-event variance that is smaller than 2 in seismic-hazard calculations. Thus, these results may help to resolve the inconsistencies that are now present between the national hazard maps and some precariously balanced rocks in southern California.

Description: 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.

Description: A crustal normal-faulting earthquake ( ; 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 S 30 , 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.

Description: Earthquakes occur in clusters, which classically are described as foreshock–mainshock–aftershock sequences or swarms. In this paper, every earthquake in a seismicity catalog is assigned to a cluster if it is separated from at least one other event in the cluster by less than t in time and less than r in space. The minimum cluster size is one earthquake. For catalogs that are complete to small magnitudes, this approach is successful in capturing the full spatial extent of an extensive cluster even for r much smaller than the actual cluster dimension. The declustered catalogs are much closer to Poissonian distribution than the originals. This was applied to seismicity catalogs for Japan, Southern California, and Nevada. Cluster sizes measured by the number of earthquakes in the cluster exhibit an approximate power-law frequency distribution. An upper bound to cluster durations is proportional to K 0.5 , where K is the number of earthquakes in the cluster. This paper demonstrates an analytical approach suitable for selecting values of t and r that are appropriate for the earthquake catalog.

Description: The goal of probabilistic seismic-hazard analysis (PSHA) is to summarize the rates of seismic ground-motion hazards at a site. The basic assumption is that true hazard curves exist to express the exceedance rates of any ground-motion amplitude at a site. Procedurally, PSHA depends on a complete and accurate description of seismicity combined with a model for ground motions using standard probabilistic methods to estimate the hazard curve. The hazard curve can be improved by improving inputs and by identifying and then resolving inconsistencies between observations and estimated hazard. However, these inconsistencies do not invalidate the existence of the hazard curve or the probability theory used to estimate it.

Description: INTRODUCTION The objective of our study is to supplement the regional P/S 3-D velocity model in the Reno basin with shear-wave velocity models derived from ambient noise interferometry (Aki 1957; Claerbout 1968; Shapiro et al. 2005; Sabra et al. 2005; Lin et al. 2008; Yang et al. 2008). We use a variety of seismic sensors in the Reno-Carson area (Figure 1). There is a gap for demonstrated extraction of Green's functions (GFs) from ambient noise between short and long interstation distances. The lateral resolution of existing tomographic models exceeds the dimensions of the Reno basin area (~60 km2), and their depth resolution is larger than the Reno basin depth to basement (

Description: We empirically evaluate spectral amplification relative to a nearby rock site at 18 Advanced National Seismic System strong-motion stations within the basin containing the urban areas of Reno and Sparks, Nevada. The near-surface site conditions have a strong effect on ground motion, which is clearly demonstrated through analysis of weak motion. The study uses multiple regional earthquake events of varying azimuth. Averages of these empirical amplifications, grouped by generalized geological formations (volcanic rock, Pliocene and early to mid-Quaternary sediments, and younger Quaternary sedimentary sites), show dependence on the geology. However, when the stations are grouped according to V S 30 values, the mean amplifications are more distinct, indicating that V S 30 is a more useful predictor of amplification for the Reno–Sparks, Nevada, urban area. Spectral amplifications computed from ground motions of the local Mogul, Nevada, 2008 earthquake swarm ( M L  0.6–4.7; Anderson et al. , 2009 ) have similar spectral shapes for 8 of the 12 stations where comparisons could be made, but the absolute amplitudes are slightly inconsistent. Four stations show unexplained peaks in the amplitude spectral ratios from the mainshock ground motions. The comparisons verify that multiple regional ground motions can be used to obtain relative site-response amplification functions for seismic-hazard applications, but the discrepancies are a reminder that directional effects contribute uncertainty to the amplification functions. Online Material: Tables describing earthquakes recorded in the vicinity of Reno, Nevada, and figures of the Fourier spectral ratios (FSRs) and of the amplitude and basin depth relationships.