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The Southern San Andreas Fault (SSAF) in California is one of the most thoroughly studied faults in the world, but its configuration at seismogenic depths remains enigmatic in the Coachella Valley. We use a combination of space geodetic and seismic observations to demonstrate that the relatively straight southernmost section of the SSAF, between Thousand Palms and Bombay Beach, is dipping to the northeast at 60–80° throughout the upper crust (<10 km), including the shallow aseismic layer. We constrain the fault attitude in the top 2–3 km using inversions of surface displacements associated with shallow creep, and seismic data from a dense nodal array crossing the fault trace near Thousand Palms. The data inversions show that the shallow dipping structure connects with clusters of seismicity at depth, indicating a continuous throughgoing fault surface. The dipping fault geometry has important implications for the long‐term fault slip rate, the intensity of ground shaking during future large earthquakes, and the effective strength of the southern SAF. Plain Language Summary: The San Andreas Fault (SAF) is capable of large, destructive earthquakes and poses significant seismic hazard in California. In the Coachella Valley, the geometry of the SAF beneath the Earth's surface has been the subject of much debate. Here, we present new constraints on the fault's structure in the uppermost 2–3 km of the crust from several new sets of observations. We measure slow movement of the fault at the surface using satellite‐based radar and perform simulations of what fault geometry is most likely to produce the observed motion. We also installed an array of sensors across the SAF to measure seismic waves that travel through or reflect off the fault. Analysis of both data sets indicates that the shallow portion of the SAF dips to the northeast in the Coachella Valley. Precise locations of small earthquakes at greater depths exhibit a similar trend—we thus suggest the fault dips throughout the Earth's crust. Better understanding the geometry of this section of the SAF has implications for earthquake hazards in Southern California, as well as the fault's geologic history. Key Points: InSAR measurements and models of shallow creep on the southernmost San Andreas Fault indicate a moderate‐to‐steep northeast fault dipSeismic imaging using data from a dense array deployed near Thousand Palms reveal northeast dipping fault damage zonesA joint interpretation of available data suggests that the northeast dip persists throughout the upper 10 km of the crust [ABSTRACT FROM AUTHOR]

Earthquake-induced strong near-fault ground motion is typically accompanied by large-velocity pulse-like component, which causes serious damage to slopes and buildings. Although not all near-fault ground motions contain a pulse-like component, it is important to consider this factor in regional earthquake-induced landslide susceptibility assessment. In the present study, we considered the probability of the observed pulse-like ground motion at each site (PP) in the region of an earthquake as one of the conditioning factors for landslide susceptibility assessment. A subset of the area affected by the 1994 Mw6.7 Northridge earthquake in California was examined. To explore and verify the effects of PP on landslide susceptibility assessment, seven models were established, consisting of six identical influencing factors (elevation, slope gradient, aspect, distance to drainage, distance to roads, and geology) and one or two factors characterizing the intensity of the earthquake (distance to fault, peak ground acceleration, peak ground velocity, and PP) in logistic regression analysis. The results showed that the model considering PP performed better in susceptibility assessment, with an area under the receiver operating characteristic curve value of 0.956. Based on the results of relative importance analysis, the contribution of the PP value to earthquake-induced landslide susceptibility was ranked fourth after the slope gradient, elevation, and lithology. The prediction performance of the model considering the pulse-like effect was better than that reported previously. A logistic regression model that considers the pulse-like effect can be applied in disaster prevention, mitigation, and construction planning in near-fault areas. [ABSTRACT FROM AUTHOR]

Transformer bushing systems have exhibited vulnerability during past earthquakes. Based on past qualification tests, it was found that the seismic performance of transformer bushing systems is improved when the bushings are mounted on a rigid base, as opposed to when mounted on more flexible cover plates of transformers. The addition of flexural stiffeners on the transformer cover plates has been proven to stiffen the base of the bushings and mitigate their seismic vulnerability. In this study, the Performance-Based Earthquake Engineering framework, introduced by the Pacific Earthquake Engineering Research (PEER) center, is used to evaluate in a probabilistic sense the earthquake-induced economic losses of transformer bushing systems both for as-installed and retrofitted mounting conditions. The results are presented in the form of expected losses conditioned on seismic intensity as well as expected annual losses for locations in Southern and Northern California. The results demonstrate the ability of the retrofit approach of adding flexural stiffeners on the transformer cover plates to reduce earthquake-induced economic losses compared to the as-installed conditions/design and further enhance community recovery in the occurrence of a strong seismic event. [ABSTRACT FROM AUTHOR]

Social media is changing the narrative during crisis events. It has facilitated citizen-led emergency dispatch and rescue, information sharing and communication between loved ones in the moments before, during and after a disaster. Researchers of social capital have found that bonding, bridging, and linking social capital can lead to resilient outcomes. With increased use of social media on a day-to-day basis and during a crisis, we still know little about the association between social capital and online social media use. Controlling for demographic characteristics and earthquake intensity, I investigate the association bonding, bridging and linking social capital and hyperlocal social media use following the earthquake and its aftershocks. Using a quantitative cross-sectional longitudinal study across 121 Nextdoor online neighborhoods in California's Napa Valley region across 42 days in August and September of 2014 (N = 3570), I find that bridging and linking social capital led to more online communication via Nextdoor. This finding comes with an important implication, namely, that social media can serve as a primary source of recourse for individuals and communities following a disaster. Social media platforms provide a conduit for accessing and activating bridging and linking ties that can expedite collective action in a time of need. Communities should consider policies that increase levels of social capital, well as social media platforms that can activate social ties when they are needed most. [ABSTRACT FROM AUTHOR]

The longitude, latitude and depth of the hypocenter in 3-D space and the date and time of rupture makes an earthquake a "point" in a spatio-temporal point pattern, observed over a region and months, years or decades. The magnitude of earthquakes marks the point pattern, as would hypocenter depth do if only the longitude and latitude of epicenters were used for location in 2-D space. Stochastic declustering, based on a space–time ETAS model (ETAS: epidemic-type aftershock sequence), is a procedure that can be applied in the preliminary stage of an earthquake catalog data analysis. Stochastic declustering procedures have underlying assumptions, such as the time independence of the background intensity function whether the spatial framework is 2-D or 3-D, and a separate treatment of hypocenter depth from longitude and latitude when the spatial framework is 3-D. Cyclical processes in the Earth, including tides and seasonal surface water loads, can introduce periodic behavior in earthquake occurrence and related variables. The effects of ETAS-based 2-D and 3-D declustering on the outcome of periodicity analyses performed from the resulting earthquake data catalogs are studied. The research objectives and statistical challenges include the detection of periodicities for hypocenter depth in addition to monthly earthquake number, and the risk of missing observations for hypocenter depth when the monthly earthquake number after declustering is zero. A version of the method of multi-frequential periodogram analysis (MFPA) that allows for missing observations in the input temporal series is presented in detail, and applied to hypocenter depth (monthly mean and median) for central and northern California from January 2006 to December 2014. The results obtained for the 2-D and 3-D declustered catalogs are compared with those for the original catalog for this region. A semiannual periodicity in hypocenter depth is detected for the original and 2-D declustered catalogs, and fitted with the goal of relating it to periodicities found in the time series of monthly earthquake numbers. Using these results for central and northern California earthquakes, some of the assumptions on the intensity function of the spatio-temporal point process in stochastic declustering are discussed and future research perspectives are proposed. [ABSTRACT FROM AUTHOR]

Considering spatial correlation of multiple earthquake intensity measures (IMs) is of particular importance in loss assessment of spatially distributed assets. This subject has been investigated in previous studies under the assumption of isotropy. Considering the fact that the assumption of isotropy is not valid in general, the present study employs a non-separable covariance model based on latent dimensions method to investigate anisotropic properties of spatial correlations and cross-correlations of intra-event residuals of multiple earthquake IMs. This method leads to the generation of valid covariance matrix in order to model anisotropic spatially distributed multivariate random fields. Two sets of IMs are considered in this study; the first set consists of peak ground intensity values (acceleration, velocity, and displacement), and the second set consists of spectral accelerations at three different periods. Data of 10 earthquake events in California and Japan are utilized in this study to estimate parameters of marginal and cross-covariance models. Moreover, parameters of covariance model of regional site condition, which is considered as average shear wave velocity of top 30 m of soil profile (Vs30), are obtained in order to investigate the effect of local sited conditions on spatial correlations of IMs. It is shown that maximum range and anisotropy ratio of covariance models of intra-event residuals of IMs are correlated with those of Vs30 values. Also, it is observed that the anisotropy direction of residuals of IMs is consistent with anisotropy direction of Vs30 values. Finally, predictive models are proposed to obtain marginal and cross-covariance functions for different earthquake IMs considering anisotropy. [ABSTRACT FROM AUTHOR]

We present a synoptic analysis of the ground motions from the 11 March 1933 Mw 6.4 Long Beach, California, earthquake, the largest known earthquake within the central Los Angeles Basin region. Our inferred shaking intensity pattern supports the association of the earthquake with the Newport-Inglewood fault; it further illuminates the concentration of severe damage in the town of Compton, where accounts suggest vertical ground motions exceeding 1 g. We use a broadband simulation approach to develop a rupture scenario for this earthquake, informed by the damage distribution. The predicted shaking for a 25-km-long fault matches the intensity distribution, with an indication that non-linear site response on soft sediments in some near-field regions was stronger than predicted using a simple model to account for non-linearity. Our results suggest that the concentration of damage near Compton can be explained by a combination of local site amplification, source-controlled directivity, and three-dimensional basin effects whereby energy was channeled towards the deepest part of the Los Angeles Basin. [ABSTRACT FROM AUTHOR]

Summary: This paper proposes methodological developments for quantifying the impact of residual axial shortening of first‐story steel columns on earthquake loss estimations in steel moment‐resisting frame (MRF) buildings. A new formulation is proposed that accounts for the likelihood of having to demolish a steel MRF building due to column residual axial deformations in addition to residual story‐drift ratios. The formulation is informed by means of data from a comprehensive survey conducted worldwide to assess the likelihood of steel column repairability due to residual axial shortening. A practical method for quantifying column axial‐shortening in parameterized system‐level numerical simulations is presented. The proposed approach is illustrated by conducting economic seismic loss estimations in two case‐study steel MRF buildings designed in urban California according to the current seismic design practice. It is found that when the ground‐motion duration is appreciable, the examined steel MRFs are more prone to column axial‐shortening than residual story‐drifts at moderate to high seismic intensities. The results suggest that economic losses due to demolition may be underestimated if column residual axial‐shortening is neglected from loss estimations. Limitations as well as directions for future research are discussed. [ABSTRACT FROM AUTHOR]

Seismic risk assessment of lifeline networks or building portfolios requires the quantification of spatial correlation of ground motion intensity over a region during an earthquake event. In this paper, the spatial correlation of vertical spectral accelerations (SAs) at single and multiple periods is studied. In the first part, the spatial correlations of vertical SAs at eight periods in the range of 0.0–3.0 s are investigated. The geostatistical analysis is performed to compute the spatial correlations of SAs using records compiled from ten earthquake events occurred in California, Japan, Taiwan, and Mexico. The results show that the spatial correlations of the vertical SAs at short periods are dependent on the regional site conditions indicated with the spatial correlation of shear wave velocity in the top of 30 m (Vs30). However, the effect of the correlation of Vs30 on the correlation of the vertical SAs decreases as the spectral period increases. A simple predictive model is proposed to quantify the correlation ranges of the vertical SAs considering regional site conditions. The proposed model is compared with the models presented for horizontal SAs. The comparison shows that the correlation ranges of horizontal SAs are generally larger than those of the vertical ones. As an illustrative example, seismic performance of a hypothetical bridge network is studied with different ranges of spatial correlations. The results indicate that ignoring spatial correlation may significantly affect the exceedance probability for spatially distributed engineering demand parameters of bridges in the network. In the second part, the multivariate spatial correlation is investigated using linear model of coregionalization (LMC) approach. Based on the correlation estimates obtained from geostatistical analysis, a predictive model is proposed to simulate vertical SAs at multiple periods and different sites. [ABSTRACT FROM AUTHOR]

Highlights • 1,100+ reports of shaking yield intensity magnitude estimate of M I 7.2 ± 0.2. • Earthquake generated potentially damaging ground motions (MMI 6+) up to 300 km away. • Stronger shaking revealed on the hanging wall of the White Wolf faul. Abstract Seismic intensity data based on first-hand accounts of shaking give valuable insight into historical and early instrumental earthquakes. Comparing an observed intensity distribution to intensity-prediction models based on modern calibration events allows the magnitude to be estimated for many historic earthquakes. Magnitude estimates can also potentially be refined for earthquakes for which limited instrumental data are available. However, the complicated nature of macroseismic data and the methods used to collect and interpret the data introduce significant uncertainties. In this paper, we illustrate these challenges and possible solutions using the 1952 Kern County, California, earthquake as a case study. Published estimates of its magnitude vary from M W 7.2–7.5, making it possibly the second largest in California during the 20th century. We considered over 1100 first-hand reports of shaking, supplemented with other data, and inferred the magnitude in several ways using intensity prediction equations, yielding a preferred intensity magnitude M I 7.2 ± 0.2, where the uncertainty reflects our judgement. The revised intensity distribution reveals stronger shaking on the hanging wall, south of the surface expression of the White Wolf fault, than on the footwall. Characterizing the magnitude and shaking distribution of this early instrumental earthquake can help improve estimation of the seismic hazard of the region. Such reinterpreted intensities for historic earthquakes, combined with USGS Did You Feel It? data for more recent events, can be used to produce a uniform shaking dataset with which earthquake hazard map performance can be assessed. [ABSTRACT FROM AUTHOR]

Pioneering work by Nicolas Ambraseys and many collaborators demonstrates both the tremendous value of macroseismic data and the perils of its uncritical assessment. In numerous publications he shows that neglect of original sources and/or failure to appreciate the context of historical accounts, as well as use of unreliable indicators such as landsliding to determine intensities, commonly leads to inflated magnitude estimates for historical earthquakes. The U.S. Geological Survey 'Did You Feel It?' (DYFI) system, which now collects and systematically interprets thousands of first-hand reports from felt earthquakes, provides the opportunity to explore further the biases associated with traditional intensity distributions determined from written (media or archival) accounts. I briefly summarize and further develop the results of Hough (), who shows that traditional intensity distributions imply more dramatic damage patterns than are documented by more spatially rich DYFI data, even when intensities are assigned according to the conservative practices established by Ambraseys' work. I further consider the separate intensity-attenuation relations that have been developed to characterize intensities for historical and modern earthquakes in California, using traditionally assigned intensities and DYFI intensities, respectively. The results support the conclusion that traditionally assigned intensity values tend to be inflated by a fundamental bias towards reporting of dramatic rather than representative effects. I introduce an empirical correction-factor approach to correct for these biases. This allows the growing wealth of well-calibrated DYFI data to be used as calibration events in the analysis of historical earthquakes. [ABSTRACT FROM AUTHOR]

Abstract: The temporal evolution of earthquake inter-event time (IET) may provide important clues for the timing of future events and underlying physical mechanisms of earthquake interaction. In this study, we examine ~12yr of local earthquake and low-frequency earthquake (LFE) activity near Parkfield, CA from catalogs of ~50,000 earthquakes and ~730,000 LFEs. We focus on the long-term evolution of IETs after the 2003 Mw6.5 San Simeon and 2004 Mw6.0 Parkfield earthquakes. The IETs of local earthquakes along and to the southwest of the San Andreas fault show clear decreases of several orders of magnitude after the Parkfield and San Simeon earthquakes, followed by recoveries with time scales of ~3yr and >8yr, respectively. We also observe decreases in recurrence times in some of LFE families, followed by long-term recoveries with time scales of ~4 months to several years. The long-term recovery of the earthquake IET is a manifestation of the aftershock decay of the Parkfield and San Simeon earthquakes, and the different recovery time scales likely reflect the different tectonic loading rates in the two regions. The drop in the recurrence times of LFEs after the Parkfield earthquake is likely caused by static and dynamic stresses induced by the Parkfield earthquake, and the long-term recovery in LFE recurrence time could be due to post-seismic relaxation or gradual recovery of fault zone material properties. The recovery time scales for general earthquake IET and LFE recurrence following the Parkfield earthquake are similar to those estimated for repeating earthquake recurrence identified in previous studies, indicating that they could be controlled by similar mechanisms. [Copyright &y& Elsevier]

Analog station LUC near Lucerne, California, recorded strong motions from the nearby (1.25 km) fault rupture during the 28 June 1992 Landers mainshock. The records illustrate general issues that can arise at near-field stations. In the area of the station, weathered granite regolith with 400 m/s S-wave velocity overlies intact granite with 3000 m/s S-wave velocity. A strongly reverberating signal persisted for several seconds after near-field velocity pulse passed. Recovery of the vertical record in digital form provided calibration of resonant site properties and qualitative separation of site and source effects. The dominant signal on the vertical spectrum arises from vertically reverberating P waves in an ∼13.5-m-thick layer. Horizontal spectra marginally resolve the analogous resonance for vertical S waves and coupling of vertical P waves into horizontal motion. Here, resonant amplification of a broad high-frequency band ∼5-40 Hz by a factor of a few over lower frequencies sufficed to dominate acceleration records and to make velocity records jittery. Conversely, the amplitude damping times of these resonances are much less than 1 s, indicating that the time-domain decay of the acceleration signal over ∼8 s is an incident-wave effect. Overall, the raw seismograms represent incident incoming high-frequency 5-40 Hz signals, but poorly resolved directional effects preclude straightforward determination of the incident body waves. [ABSTRACT FROM AUTHOR]

The article presents a study which examines the applicability of a single ground-motion intensity correlation equation (GMICE) to eastern North American (ENA) intraplate and Californian interplate. It determines the peak ground velocity (PGV), peak ground acceleration (PGA), and spectral acceleration (SA) by calculating the median of ground-motion values at modified Mercalli intensity (MMI) level and 95% of confidence limits. It discusses the difference on the GMICEs of surface spectral shapes.

Earthquake recurrence histories may provide clues to the timing of future events, but long intervals between large events obscure full recurrence variability. In contrast, small earthquakes occur frequently, and recurrence intervals are quantifiable on a much shorter time scale. In this work, I examine an 8.5-year sequence of more than 900 recurring low-frequency earthquake bursts composing tremor beneath the San Andreas fault near Parkfield, California. These events exhibit tightly clustered recurrence intervals that, at times, oscillate between ∼3 and ∼6 days, but the patterns sometimes change abruptly. Although the environments of large and low-frequency earthquakes are different, these observations suggest that similar complexity might underlie sequences of large earthquakes. [ABSTRACT FROM AUTHOR]

Abstract: We employ our semi-empirical kinematic model for shear body waves (KF) [Sirovich L. A simple algorithm for tracing out synthetic isoseismals. Bull Seism Soc Am 1996;86(4):1019–27; Sirovich L. Synthetic isoseismals of three earthquakes in California-Nevada. Soil Dyn Earthquake Eng 1997;16:353–62] to back-predict and then validate the regional intensity scenario of a destructive earthquake (Loma Prieta, California, M s 7.1, October 17, 1989). Only the pre-1988 geological and seismotectonic knowledge was used to set the 11 source parameters of KF (in this sense, our procedure was deterministic). Then, the ranges of the pre-1988 uncertainties were explored by grid search and the parametric combination produced 59,049 sources. The quality of our prediction was measured using the field intensities of 1989 by the US Geological Survey (in the MMI scale). The squared scenario residuals are: 73 for the mean KF scenario and 123 for the best available empirical attenuation of MMI intensity. We stress that, before using KF in the forward mode, its automatic inverse application has already been validated by refinding a series of earthquake sources [Gentile F, Pettenati F, Sirovich L. Validation of the automatic nonlinear source inversion of the US geological survey intensities of the Whittier Narrows, 1987 Earthquake. Bull Seism Soc Am 2004;94(5):1737–47; Pettenati F, Sirovich L. Intensity-based source inversion of three destructive California earthquakes. Bull Seism Soc Am 2007;97(5):1587–606; Sirovich L, Pettenati F. Source inversion of intensity patterns of earthquakes: a destructive shock in 1936 in northeast Italy. J Geophys Res 2004;109:B10309, doi:10.1029/2003JB002919:1–16]. If our technique had been available at the time, the 1989 pattern of damage south of San Francisco would have been conservatively foreseen even from 1983 on. [Copyright &y& Elsevier]

High-resolution seismic compressed high intensity Radar pulse (CHIRP) data and piston cores acquired in Fallen Leaf Lake (FLL) and Lake Tahoe provide new paleoseismic constraints on the West Tahoe-Dollar Point fault (WTDPF), the westernmost normal fault in the Lake Tahoe Basin, California. Paleoearthquake records along three sections of the WTDPF are investigated to determine the magnitude and recency of coseismic slip. CHIRP profiles image vertically offset and folded strata along the southern and central sections that record deformation associated with the most recent event (MRE) on the WTDPE Three faults are imaged beneath FLL, and the maximum vertical offset observed across the primary trace of the WTDPF is ~3.7 m. Coregistered piston cores in FLL recovered sediment and organic material above and below the MRE horizon. Radiocarbon dating of organic material constrained the age of the MRE to be between 3.6 and 4.9 k.y.B.P., with a preferred age of 4.1-4.5 k.y.B.P. In Lake Tahoe near Rubicon Point, approximately 2.0 m of vertical offset is observed across the WTDPF. Based on nearby core data, the timing of this offset occurred between ~3-10 k.y.B.P., which is consistent with the MRE age in FLL. Offset of Tioga-aged glacial deposits provides a long-term record of vertical deformation on the WTDPF since ~13-14 k.y.B.P., yielding a slip rate of 0.4-0.8 mm/yr. In summary, the slip rate and earthquake potential along the WTDPF is comparable to the nearby Genoa fault, making it the most active and potentially hazardous fault in the Lake Tahoe Basin. [ABSTRACT FROM AUTHOR]

We analyze Boatwright and Bundock's (2005) modified Mercalli intensity (MMI) map for the 18 April 1906 San Francisco earthquake, reviewing their interpretation of the MMI scale and testing their correlation of 1906 cemetery damage with MMI intensity. We consider in detail four areas of the intensity map where Boatwright and Bundock (2005) added significantly to the intensity descriptions compiled by Lawson (1908). We show that the distribution of off-fault damage in Sonoma County suggests that the rupture velocity approached the P-wave velocity along Tomales Bay. In contrast, the falloff of intensity with distance from the fault appears approximately constant throughout Mendocino County. The intensity in Humboldt County appears somewhat higher than the intensity in Mendocino County, suggesting that the rupture process at the northern end of the rupture was relatively energetic and that there was directivity consistent with a subsonic rupture velocity on the section of the fault south of Shelter Cove. Finally, we show that the intensity sites added in Santa Cruz County change the intensity distribution so that it decreases gradually along the southeastern section of rupture from Corralitos to San Juan Bautista and implies that the stress release on this section of rupture was relatively low. [ABSTRACT FROM AUTHOR]

This article reports a study of modeled, long-period building responses to ground-motion simulations of earthquakes in the San Francisco Bay Area. The earthquakes include the 1989 magnitude 6.9 Loma Prieta earthquake, a magnitude 7.8 simulation of the 1906 San Francisco earthquake, and two hypothetical magnitude 7.8 northern San Andreas fault earthquakes with hypocenters north and south of San Francisco. We use the simulated ground motions to excite nonlinear models of 20-story, steel, welded moment-resisting frame (MRF) buildings. We consider MRF buildings designed with two different strengths and modeled with either ductile or brittle welds. Using peak interstory drift ratio (IDR) as a performance measure, the stiffer, higher strength building models outperform the equivalent more flexible, lower strength designs. The hypothetical magnitude 7.8 earthquake with hypocenter north of San Francisco produces the most severe ground motions. In this simulation, the responses of the more flexible, lower strength building model with brittle welds exceed an IDR of 2.5% (that is, threaten life safety) on 54% of the urban area, compared to 4.6% of the urban area for the stiffer, higher strength building with ductile welds. We also use the simulated ground motions to predict the maximum isolator displacement of base-isolated buildings with linear, single-degree-of-freedom (SDOF) models. For two existing 3-sec isolator systems near San Francisco, the design maximum displacement is 0.5 m, and our simulations predict isolator displacements for this type of system in excess of 0.5 m in many urban areas. This article demonstrates that a large, 1906-like earthquake could cause significant damage to long-period buildings in the San Francisco Bay Area. [ABSTRACT FROM AUTHOR]

We estimate the ground motions produced by the 1906 San Francisco earthquake making use of the recently developed Song et al. (2008) source model that combines the available geodetic and seismic observations and recently constructed 3D geologic and seismic velocity models. Our estimates of the ground motions for the 1906 earthquake are consistent across five ground-motion modeling groups employing different wave propagation codes and simulation domains. The simulations successfully reproduce the main features of the Boatwright and Bundock (2005) ShakeMap, but tend to over predict the intensity of shaking by 0.1-0.5 modified Mercalli intensity (MMI) units. Velocity waveforms at sites throughout the San Francisco Bay Area exhibit characteristics consistent with rupture directivity, local geologic conditions (e.g., sedimentary basins), and the large size of the event (e.g., durations of strong shaking lasting tens of seconds). We also compute ground motions for seven hypothetical scenarios rupturing the same extent of the northern San Andreas fault, considering three additional hypocenters and an additional, random distribution of slip. Rupture directivity exerts the strongest influence on the variations in shaking, although sedimentary basins do consistently contribute to the response in some locations, such as Santa Rosa, Livermore, and San Jose. These scenarios suggest that future large earthquakes on the northern San Andreas fault may subject the current San Francisco Bay urban area to stronger shaking than a repeat of the 1906 earthquake. Ruptures propagating southward towards San Francisco appear to expose more of the urban area to a given intensity level than do ruptures propagating northward. [ABSTRACT FROM AUTHOR]

Earthquakes in central Nevada (1932-1959) were used to develop a modified Mercalli intensity (MMI) attenuation model for estimating moment magnitude M for earthquakes in the Basin and Range province of interior western North America. M is 7.4-7.5 for the 26 March 1872 Owens Valley, California, earthquake, in agreement with Beanland and Clark's (1994) M 7.6 that was estimated from geologic field observations. M is 7.5 for the 3 May 1887 Sonora, Mexico, earthquake, in agreement with Natali and Sbar's (1982) M 7.4 and Suter's (2006) M 7.5, both estimated from geologic field observations. MMI at sites in California for earthquakes in the Nevada Basin and Range apparently are not much affected by the Sierra Nevada except at sites near the Sierra Nevada where MMI is reduced. This reduction in MMI is consistent with a shadow zone produced by the root of the Sierra Nevada. In contrast, MMI assignments for earthquakes located in the eastern Sierra Nevada near the west margin of the Basin and Range are greater than predicted at sites in California. These higher MMI values may result from critical reflections due to layering near the base of the Sierra Nevada. [ABSTRACT FROM AUTHOR]

Fourway is a simple graphical tool for estimating the conditional dependence of decision variables involved in the seismic design process on the earthquake intensity. The tool was formulated for use with the Pacific Earthquake Engineering Research Center’s performance-based earthquake engineering framework. The tool simplifies the development of decision fragilities by exact determination of first moments and approximate determination of second central moments (variance) of the corresponding probability distributions, without the need for numerical integration of intermediate probabilistic models. For commonly assumed probability distributions of the interim variables, the Fourway method was compared to closed-form and numerical solutions, including measures for evaluating the variance approximations. It is shown that Fourway is a useful first-pass assessment tool for approximating the parameters of the decision variable distributions. Furthermore, Fourway is a tool for evaluating the effects of varying the interim model parameters on the final decision variable fragilities. Examples are provided for the case of a reinforced concrete highway overpass bridge in California. [ABSTRACT FROM AUTHOR]

Finite-fault source inversions reveal the spatial complexity of earthquake slip or pre-stress distribution over the fault surface. The basic assumption of this study is that a stochastic model can reproduce the variability in amplitude and the long-range correlation of the spatial slip distribution. In this paper, we compute the stochastic model for the source models of four earthquakes: the 1979 Imperial Valley, the 1989 Loma Prieta, the 1994 Northridge and 1995 Hyogo-ken Nanbu (Kobe). For each earthquake (except Imperial Valley), we consider both the dip and strike slip distributions. In each case, we use a 1-D stochastic model. For the four earthquakes, we show that the average power spectra of the raw, that is, non-interpolated, data follow a power-law behaviour with scaling exponents that range from 0.78 to 1.71. For the four earthquakes, we have found that a non-Gaussian probability law, that is, the Lévy law, is better suited to reproduce the main features of the spatial variability embedded in the slip amplitude distribution, including the presence and frequency of large fluctuations. Since asperities are usually defined as regions with large slip values on the fault, the stochastic model will allow predicting and modelling the spatial distribution of the asperities over the fault surface. The values of the Lévy parameters differ from one earthquake to the other. Assuming an isotropic spatial distribution of heterogeneity for the dip and the strike slip of he Northridge earthquake, we also compute a 2-D stochastic model. The main conclusions reached in the 1-D analysis remain appropriate for the 2-D model. The results obtained for the four earthquakes suggest that some features of the slip spatial complexity are universal and can be modelled accordingly. If this is proven correct, this will imply that the spatial variability and the long-range correlation of the slip or pre-stress spatial distribution can be described with the help of five parameters: a scaling exponent controlling the spatial correlation and the four parameters of the Lévy distribution constraining the spatial variability. [ABSTRACT FROM AUTHOR]

The Landers earthquake in June 1992 redistributed stress in southern California, shutting off the production of small earthquakes in some regions while increasing the seismicity in neighboring regions, up to the present. This earthquake also changed the ratio of small to large events in favor of more small earthquakes within about 100 kilometers of the epicenter. This implies that the probabilistic estimate for future earthquakes in southern California changed because of the Landers earthquake. The location of the strongest increase in probability for large earthquakes in southern California was the volume that subsequently produced the largest slip in the magnitude 7.1 Hector Mine earthquake of October 1999. [ABSTRACT FROM AUTHOR]

• The uncertainty in fragility functions depends on the ground motion records. • The strategy used for selecting the records yields different levels of uncertainty. • Two strategies are proposed to improve fragility analysis done by random selection. • The methodology yields more accurate fragility functions than random selection. Fragility functions enable the assessment of a structural system for a given hazard scenario. Specifically, the fragility function provides the probability of an undesirable structural state conditioned on the occurrence of a specific hazard level. Multiple sources of uncertainty are present when estimating fragility functions, e.g., record-to-record variation, uncertain material and geometric properties, model assumptions, and limited data to characterize the hazard. The objective of this study is to develop a methodology that will accelerate the process of fragility function estimation under limitations in computational resources and data. The approach used in the methodology is as follows. The stochastic map between hazard level and structural response is first constructed using Bayesian inference for a finite number of simulations. The Bayesian approach enables the quantification of the epistemic uncertainty due to a limited number of simulations. This epistemic uncertainty is exploited to sequentially select subsequent simulations that accelerate learning based on up to two different earthquake intensity measures, peak ground velocity and spectral velocity. The methodology is applied to a benchmark model of a twenty-story nonlinear building. Simulations are performed using a set of synthetic ground motions obtained from scenario earthquakes in California. Through this case study the methodology developed here is demonstrated. Additionally, the case study highlights the ability of the methodology to achieve lower levels of epistemic uncertainty than traditional techniques using the same number of simulations. This approach is expected to enable more efficient fragility function determination. [ABSTRACT FROM AUTHOR]

Probabilistic hazard curves for earthquake-induced permanent slope displacements incorporate the important variabilities associated with predictions of slope movements, such that a rational assessment of slope performance can be obtained. To date, displacements estimated from sliding block analyses predominantly have been used to compute displacement hazard curves, despite the fact that nonlinear finite element analysis is becoming the preferred method to evaluate the performance of slopes. This paper extends the probabilistic approach for use with displacements from finite element analyses. We develop slope-specific displacement prediction models using displacements computed by nonlinear finite element analyses. Various models are developed that utilize different ground motion intensity measures and each model is used to compute a displacement hazard curve for a site in Northern California. The computed hazard curves reveal that the different models generate a large range of epistemic uncertainty, and the model with the smallest standard deviation does not necessarily predict the smallest displacement hazard due to the influence of the shape of the ground motion hazard curve and of the displacement model. As a result, multiple displacement models that incorporate different intensity measures should be considered in any assessment of the seismic performance of slopes, in the same way that multiple ground motion models are incorporated into ground motion hazard analyses. • Slope-specific models of earthquake-induced displacement were developed from finite element simulations. • Models were developed using single ground motion intensity parameters and sets of two intensity parameters. • The model with the smallest standard deviation does not necessarily predict the smallest displacement hazard. • The different models generate epistemic uncertainty in the computed displacement hazard curves. • Multiple displacement models that incorporate different intensity measures should be considered. [ABSTRACT FROM AUTHOR]

The article presents a response by Susan E. Hough and Kate Hutton to a comment about their article "Revisiting the 1872 Owens Valley, California, Earthquake" which appeared in a previous issue. They note that the commentary stresses that their conclusions over the 1872 earthquake in Owens Valley, California are wrong as the Sierra Nevada attenuation model has not been taken into consideration in their study. They emphasize that inferences made about key historic earthquakes tend to be only as good as the intensity assignments used as basis.

We propose the energy-dependent magnitude-frequency (EDMF) model as an alternative of the classical (tapered or not) Gutenberg-Richter (G-R) law for the magnitude of seismic events, the most widely used in earthquake forecasting and seismic hazard applications. The model we present is a modification of the Kagan's tapered G-R law: the corner moment parameter is assumed to be an energy-varying function, changing with respect to the size of the area under analysis, and to the seismic history occurred in the area itself. The basic rationale behind our model rests in the criticism to the applicability of the G-R law on small space-time windows, many times raised in the literature and already tackled for some operational forecasting projects, as for example in the UCERF3 for California. The EDMF model is built in such a way to decrease the probability of large events in a small area where a strong shock has just occurred and to reobtain the classical G-R when enlarging the analyzed region. We do not account for a parental connection between events, neither for an order relation between their magnitudes. We simply condition to the elastic energy available in the area considered, which depends on the time elapsed since the last strong event that is assumed to have resetted the energy available, as well as on the length of the longest fault included in the area.The applicability of the proposed EDMF model is shown for the Californian seismic catalog; we obtain results that support the validity of the EDMF model and its potential to improve large earthquake forecasting at different time scales. [ABSTRACT FROM AUTHOR]

The author comments on the article "Revisiting the 1872 Owens Valley, California, Earthquake," by Susan E. Hough and Kate Hutton published in a previous issue. Hough and Hutton arrived with the conclusions that the 1872 earthquake in Owens Valley, California had a magnitude of 7.8-7.9 and was larger than the 1906 earthquake in San Francisco. He expresses his disagreement over such conclusions and the interpretation by Hough and Hutton of macroseismic observations. He adds that the authors made a mistake in their analysis of intensity data.