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Earthquake nowcasting has been proposed as a means of tracking the change in large earthquake potential in a seismically active area. The method was developed using observable seismic data, in which probabilities of future large earthquakes can be computed using Receiver Operating Characteristic methods. Furthermore, analysis of the Shannon information content of the earthquake catalogs has been used to show that there is information contained in the catalogs, and that it can vary in time. So an important question remains, where does the information originate? In this paper, we examine this question using stochastic simulations of earthquake catalogs. Our catalog simulations are computed using an Earthquake Rescaled Aftershock Seismicity ("ERAS") stochastic model. This model is similar in many ways to other stochastic seismicity simulations, but has the advantage that the model has only 2 free parameters to be set, one for the aftershock (Omori‐Utsu) time decay, and one for the aftershock spatial migration away from the epicenter. Generating a simulation catalog and fitting the two parameters to the observed catalog such as California takes only a few minutes of wall clock time. While clustering can arise from random, Poisson statistics, we show that significant information in the simulation catalogs arises from the "non‐Poisson" power‐law aftershock clustering, implying that the practice of de‐clustering observed catalogs may remove information that would otherwise be useful in forecasting and nowcasting. We also show that the nowcasting method provides similar results with the ERAS model as it does with observed seismicity. Plain Language Summary: Earthquake nowcasting was proposed as a means of tracking the change in the potential for large earthquakes in a seismically active area, using the record of small earthquakes. The method was developed using observed seismic data, in which probabilities of future large earthquakes can be computed using machine learning methods that were originally developed with the advent of radar in the 1940s. These methods are now being used in the development of machine learning and artificial intelligence models in a variety of applications. In recent times, methods to simulate earthquakes using the observed statistical laws of earthquake seismicity have been developed. One of the advantages of these stochastic models is that it can be used to analyze the various assumptions that are inherent in the analysis of seismic catalogs of earthquakes. In this paper, we analyze the importance of the space‐time clustering that is often observed in earthquake seismicity. We find that the clustering is the origin of information that makes the earthquake nowcasting methods possible. We also find that a common practice of "aftershock de‐clustering", often used in the analysis of these catalogs, removes information about future large earthquakes. Key Points: Earthquake nowcasting tracks the change in the potential for large earthquakes, using information contained in seismic catalogsWe analyze the information contained in the space‐time clustering that is observed in earthquake seismicityWe find that "aftershock de‐clustering" of catalogs removes information about future large earthquakes that the nowcasting method uses [ABSTRACT FROM AUTHOR]

Base isolators and fluid viscous dampers are viable protective devices that have been commonly considered in the seismic protection of civil engineering structures. However, the optimal design of these devices remains a tedious and iterative undertaking due to the uncertainty of ground motions, the nonlinear behavior of the structure, and its change of dynamic characteristics (i.e., effective stiffness and damping ratio) under each new design. The optimal design problem becomes more challenging concerning a multiresponse bridge system where conflicting damage potential is often expected among multiple bridge components (e.g., column, bearing, shear key, deck unseating, foundation). In this respect, this study develops a risk-based optimization strategy that directly links the expected annual repair cost ratio (ARCR) of the bridge to the design parameters of base isolators and fluid dampers. This strategy is achieved by devising a multistep workflow that integrates a seismic hazard model, a design of experiment for bearings and dampers, a logistic regression towards parameterized component-level fragility models, and a bridge system-level seismic loss assessment. The developed ARCR is parameterized as a convex function of the influential parameters of seismic protective devices. As such, optimal bearing and damper designs can be pinpointed by directly visualizing the global minimum of the parameterized ARCR surface. The optimal design is carried out against a typical reinforced concrete highway bridge in California that is installed with the fluid dampers and three types of widely-used isolation bearings—the elastomeric bearing, lead-rubber bearing, and friction pendulum system. It is shown that optimal design parameters can be obtained to significantly reduce the expected ARCR of the bridge, whereas combining optimally designed bearings and dampers can provide the minimum seismic risk. [ABSTRACT FROM AUTHOR]

We present a data‐driven approach to clustering or grouping Global Navigation Satellite System (GNSS) stations according to observed velocities, displacements or other selected characteristics. Clustering GNSS stations provides useful scientific information, and is a necessary initial step in other analysis, such as detecting aseismic transient signals (Granat et al., 2013, https://doi.org/10.1785/0220130039). Desired features of the data can be selected for clustering, including some subset of displacement or velocity components, uncertainty estimates, station location, and other relevant information. Based on those selections, the clustering procedure autonomously groups the GNSS stations according to a selected clustering method. We have implemented this approach as a Python application, allowing us to draw upon the full range of open source clustering methods available in Python's scikit‐learn package (Pedregosa et al., 2011, https://doi.org/10.5555/1953048.2078195). The application returns the stations labeled by group as a table and color coded KML file and is designed to work with the GNSS information available from GeoGateway (Donnellan et al., 2021, https://doi.org/10.1007/s12145-020-00561-7; Heflin et al., 2020, https://doi.org/10.1029/2019ea000644) but is easily extensible. We demonstrate the methodology on California and western Nevada. The results show partitions that follow faults or geologic boundaries, including for recent large earthquakes and post‐seismic motion. The San Andreas fault system is most prominent, reflecting Pacific‐North American plate boundary motion. Deformation reflected as class boundaries is distributed north and south of the central California creeping section. For most models a cluster boundary connects the southernmost San Andreas fault with the Eastern California Shear Zone (ECSZ) rather than continuing through the San Gorgonio Pass. Key Points: Unsupervised clustering methods provide a data‐driven way of analyzing and partitioning Global Navigation Satellite System observations of crustal deformationDeformation is distributed across the San Andreas fault system but is localized at the creeping section in central CaliforniaThe Southern San Andreas fault connects with the Eastern California Shear Zone rather than the rest of the San Andreas fault system [ABSTRACT FROM AUTHOR]

Multiframe PC box-girder bridge with intermediate hinges is a kind of bridge having complex structural characters, which is very quintessential in California. In this study, a typical bridge was adopted to establish a nonlinear dynamic model through OpenSees platform. Intermediate hinge and inhibiting devices in it were elaborately simulated. Meanwhile, pushover analysis was used to reinstate a specimen of column test, which has the similar ratio of reinforcement to the typical bridge, and the hysteretic model parameters of the longitudinal steels inside columns were obtained. The damage indexes of column and hinge, which are primary components, under different limit states were acquired by moment-curvature analysis. Taking into account the uncertainty, nonlinear time-history analysis of the bridge was carried out through a suite of synthetic ground motions. Subsequently, a probabilistic seismic demand model was developed, and fragility curves were further focused on. According to fragility assessment, the conclusion shows that columns and hinge restrainers exhibit high fragility, and bridge system fragility is gradually determined by column fragility along with aggravating of the damage state. Unseating of girder can hardly occur at abutments and intermediate hinges. Moderate limit state could be exceeded in the positions of plug-type concrete structures in intermediate hinges, which tends to create transverse and vertical cracks, furthermore causing reinforcements yield. It indicates that it would severely underestimate the seismic fragility of intermediate hinges without considering the elaborate simulation of hinges. [ABSTRACT FROM AUTHOR]

Enhanced flood hazard due to global warming and climate change imposes an additional threat to safety and serviceability of river crossing bridges. This study proposes an integrated approach to assess how climate change might affect bridge performance under multihazard conditions involving floods and earthquakes. The approach is used for an existing bridge spanning over the San Joaquin River, California. The multihazard impact considering climate change is evaluated in terms of risk and resilience of the bridge. Future flood projections under climate change are obtained from general circulation model simulations in conjunction with a macroscale hydrological model. Enhanced intensities of future design floods are observed to cause higher expected scour at around bridge piers. This resulted in significant rise in risk (−21%) and drop in resilience (−14%) of the bridge, when compared to no climate change scenario, at a specific seismic hazard level. To reduce possible consequences, ripraps are applied around piers as a climate change adaptation measure. Size of the riprap is determined based on the maximum expected design flood flow at the bridge site over the projection period. The applied adaptive measure is observed to be cost-effective through a cost-benefit analysis performed over the remaining bridge service life. [ABSTRACT FROM AUTHOR]

This paper analyzed damping ratios inferred from 1,335 seismic responses recorded in 154 instrumented buildings in California. These values were inferred using a parametric system identification technique in the time domain, and subjected to a series of reliability screening tests to retain only high-quality data. The resulting damping ratios conformed a data set of 1,037 high-quality values inferred exclusively from the seismic response of buildings, a database several times larger than previous studies of damping inferred from seismic response. The data set was analyzed using a linear mixed-effects statistical model to account for the fact that many of the data points were clustered, because they came from damping ratios in the same building shaken by various earthquakes. It was shown that damping decreases with increasing building height, which is the factor that best explained the relatively large variance observed in the data. Contrary to some previous recommendations, it was found that once the variation with height is taken into account, the primary structural building material is not statistically significant in the damping ratio of buildings subjected to earthquakes. However, when including the combined material and lateral resistant system as a factor in the statistical model, an additional 6% of the variance was explained. Results showed that steel buildings with moment-resistant frames have, on average, a slightly higher damping ratio than those with steel braced frames. The amplitude dependency of damping showed that there was no significant correlation between damping ratio and the overall lateral deformation demand in the building as measured by the peak roof drift ratio for amplitudes typically observed during moderate earthquake motions once a minimum level of amplitude is exceeded. [ABSTRACT FROM AUTHOR]

Regional seismic risk assessment involves many infrastructure systems, and it is computationally intensive to conduct an individual simulation of each system. This paper suggests an approach using active learning to select informative samples that help build machine learning models with fewer samples for regional damage assessment. The potential of the approach is demonstrated with (1) failure mode prediction of bridge columns, and (2) regional damage assessment of the California two-span bridge inventory with seat abutments. The active learning approach involves the selection of column attributes or bridge models that are more informative to the creation of machine learning-based decision boundaries. The results reveal that an active learning target model based on 100 bridge samples can achieve a level of accuracy of 80%, which is equivalent to a machine learning model based on 480 bridge samples in the case of damage prediction following an earthquake. With the proposed approach, the computational complexity associated with regional risk assessment of bridge systems with specific attributes can be drastically reduced. The proposed approach also will help plan experimental studies that are more informative for damage assessment. [ABSTRACT FROM AUTHOR]

The strainmeter record observed at Isabella (ISA), California, for the 1960 Chilean earthquake (M w = 9.5) is one of the most important historical records in seismology because it was one of the three records that provided the opportunity for the first definitive observations of free oscillations of the Earth. Because of the orientation of the strainmeter rod with respect to the back azimuth to Chile, the ISA strainmeter is relatively insensitive to G (Love) waves and higher order (order ≥ 6) toroidal modes, yet long-period G waves and toroidal modes were recorded with large amplitude on this record. This observation cannot be explained with the conventional low-angle thrust mechanism typical of great subduction-zone earthquakes and requires an oblique mechanism with half strike-slip and half thrust. The strain record at Ogdenburg, New Jersey, the Press–Ewing seismograms at Berkeley, California, and the ultra-long period displacement record at Pasadena, California, also support the oblique mechanism. We tested the performance of the ISA strainmeter using other events including the 1964 Alaskan earthquake and found no instrumental problems. Thus, the ISA observation of large G/R and toroidal/spheroidal ratios most likely reflects the real characteristics of the 1960 Chilean earthquake, rather than an observational artefact. The interpretation of the large strike-slip component is not unique, but it may represent release of the strike-slip strain that has accumulated along the plate boundary as a result of oblique convergence at the Nazca–South American plate boundary. The slip direction of the 2010 Chilean (Maule) earthquake (M w = 8.8) is rotated by about 10° clockwise from the plate convergence direction suggesting that right-lateral strain comparable to that of an M w = 8.3 earthquake remained unreleased and accumulates near the plate boundary. One possible scenario is that the strike-slip strain accumulated over several great earthquakes like the 2010 Maule earthquake was released during the 1960 Chilean earthquake. If this is the case, we cannot always expect a similar behaviour for all the great earthquakes occurring in the same subduction zone and such variability needs to be considered in long-term hazard assessment of subduction-zone earthquakes. [ABSTRACT FROM AUTHOR]

The San Andreas and San Jacinto faults are the primary plate-boundary structures in southern California and present a large earthquake hazard for the region. They approach each other in the Cajon Pass area between the San Gabriel and San Bernardino Mountains, where the northern end of the San Jacinto fault forms a 2-km-wide releasing step with the San Andreas fault. In this study, we used paleoseismic data from sites on the San Jacinto and San Andreas faults near their juncture to evaluate spatial and temporal patterns of surface rupture between these major structures of the North American-Pacific transform plate boundary. We present a new 3700 yr paleoseismic record from the northern San Jacinto fault at Mystic Lake, where trench excavations exposed evidence for at least 16 surface ruptures. A sedimentary gap in our trench exposures separates three ruptures in the oldest part of the record from 13 ruptures during the past 2000 yr. For the past 2000 yr, the mean recurrence times varied from 86 to 312 yr, with a mean recurrence interval of 160 yr. This rate of surface rupture is roughly equal to that of the southern San Andreas fault south of the Cajon Pass juncture, but it is half that of the San Andreas fault north of the juncture, indicating that coseismic strain on the San Andreas fault is split between the southern San Andreas and San Jacinto faults south of Cajon Pass. Comparison of the past 2000 yr of the Mystic Lake record to similar paleoseismic records from nearby sections of the San Andreas fault suggests that: (1) the current open interval on these two faults in the study area is longer than their average recurrence intervals, but that similar intervals of quiescence have occurred in the past 2000 yr; (2) the San Andreas and San Jacinto faults have probably ruptured together multiple times in the past 2000 yr; and (3) a joint rupture of the San Jacinto fault with the Mojave section of the San Andreas fault may be a more likely source of a major earthquake in southern California than a rupture on the San Andreas fault from the Mojave segment to the southern end of the fault. [ABSTRACT FROM AUTHOR]

Assessment of seismic hazards in southern California may be improved with more accurate characterization of the active San Andreas fault strands within the San Gorgonio Pass region. Ongoing debate centers on the activity and geometry of the Mill Creek and Mission Creek strands. Here, we investigate crustal deformation models with six geologically plausible geometries of the Mill Creek and Mission Creek strands. Model results suggest that differences in active fault geometry along the San Andreas fault impact slip rates along the San Jacinto fault by up to 3 mm/yr. Each model fits many but none fits all of the available geologic strike-slip rates. The calculated misfits to the geologic strike-slip rates reveal two best-fitting models: the Inactive Mill Creek model and the West Mill Creek model, which incorporate active portions of the Mill Creek, Mission Creek, and Galena Peak strands, consistent with recent studies. The cumulative strike-slip rates across faults of the two best-fitting models differ from each other by ~5 mm/yr, suggesting that fault slip rates do not always sum to the plate rate. Consequently, kinematic slip budgets should consider off-fault deformation. The two best-fitting models produce uplift patterns with significant differences in the hanging walls of dipping faults. New uplift rate data in these regions and additional geologic slip rates along the northern fault strands could further support plausible interpretations of active fault configuration. An assessment of the seismic hazard of the region indicates the potential for a rupture through the San Gorgonio Pass region with Mw ~7.8. [ABSTRACT FROM AUTHOR]

Recent researches are directed towards the regional seismic risk assessment of structures based on a bridge inventory analysis. The framework for traditional regional risk assessments consists of grouping the bridge classes and generating fragility relationships for each bridge class. However, identifying the bridge attributes that dictate the statistically different performances of bridges is often challenging. These attributes also vary depending on the demand parameter under consideration. This paper suggests a multi-parameter fragility methodology using artificial neural network to generate bridge-specific fragility curves without grouping the bridge classes. The proposed methodology helps identify the relative importance of each uncertain parameter on the fragility curves. Results from the case study of skewed box-girder bridges reveal that the ground motion intensity measure, span length, and column longitudinal reinforcement ratio have a significant influence on the seismic fragility of this bridge class. [ABSTRACT FROM AUTHOR]

Slip rates represent the average displacement across a fault over time and are essential to estimating earthquake recurrence for probabilistic seismic hazard assessments. We demonstrate that the slip rate on the western segment of the Puente Hills blind thrust fault system, which is beneath downtown Los Angeles, California (USA), has accelerated from ~0.22 mm/yr in the late Pleistocene to ~1.33 mm/yr in the Holocene. Our analysis is based on syntectonic strata derived from the Los Angeles River, which has continuously buried a fold scarp above the blind thrust. Slip on the fault beneath our field site began during the late-middle Pleistocene and progressively increased into the Holocene. This increase in rate implies that the magnitudes and/or the frequency of earthquakes on this fault segment have increased over time. This challenges the characteristic earthquake model and presents an evolving and potentially increasing seismic hazard to metropolitan Los Angeles. [ABSTRACT FROM AUTHOR]

Fragility analyses is a powerful tool for the seismic risk assessment of highway bridges. Multispan continuous concrete box-girder bridges account for the bulk of the California bridge inventory. Although several studies have looked at the seismic response and fragility of individual box-girder bridges, very little research has explored the fragility of the box-girder bridge class holistically. Exacerbating this situation is the lack of considerations of design details unique to the era in which the bridges were designed and constructed, in their fragility development. This paper addresses this gap by investigating the influence of evolution of seismic design principles and details on the seismic performance of multispan continuous concrete box-girder bridge class, as well as the failure probability through the development of fragility curves across three design eras demarcated by the 1971 San Fernando and 1989 Loma Prieta earthquakes. Detailed nonlinear analytical models capturing the bridge characteristics within a design era are developed and nonlinear time history analysis is employed to develop analytical fragility curves. Significant improvement in performance is seen in modern bridges designed according to capacity design principles. [ABSTRACT FROM AUTHOR]

Machine-learning has recently gained considerable attention in the earthquake engineering community, as it can map the complex relationship between the expected damage and the input parameters. It is often necessary to understand the reasons for the behavior and predictions of the machine-learning model. This paper addresses this issue through interpretable machine-learning approaches such as partial dependence plots, accumulated local effects, and Shapely additive explanations. The evaluation of these approaches is carried out (1) at a component level by analyzing the shear strength predictions by a machine-learning model and (2) at a regional level through the machine-learning model for the regional damage assessment of bridges in California. The comparison helps to identify (1) the proper implementation of these approaches for the efficient use of machine-learning models and (2) key influential variables and thresholds that govern the prediction of the machine-learning models. [ABSTRACT FROM AUTHOR]

SUMMARY We explore a recently developed procedure for kinematic inversion based on an elliptical subfault approximation. In this method, the slip is modelled by a small set of elliptical patches, each ellipse having a Gaussian distribution of slip. We invert near-field strong ground motion for the 2004 September 28 Mw 6.0 Parkfield, California, earthquake. The data set consists of 10 digital three-component 18-s long displacement seismograms. The best model gives a moment of 1.21 × 1018 N m, with slip on two distinct ellipses, one with a high-slip amplitude of 0.91 m located 20 km northwest of the hypocentre. The average rupture speed of the rupture process is ∼2.7 km s−1. We find no slip in the top 5 km. At this depth, a lineation of small aftershocks marks the transition from creeping above to locked below, in the interseismic period. The high-slip patch coincides spatially with the hypocentre of the 1966 Mw6.0 Parkfield, California, earthquake. The larger earthquakes prior to the 2004 Parkfield earthquake and the aftershocks of the 2004 earthquake ( Mw > 3) also lie around this high-slip patch, where our model images a sharp slip gradient. This observation suggests the presence of a permanent asperity that breaks during large earthquakes, and has important implications for the slip deficit observed on the Parkfield segment, which is necessary for reliable seismic hazard assessment. [ABSTRACT FROM AUTHOR]

Industry seismic reflection data, oil test well data, interpretation of gravity and magnetic data, and seismic refraction deepcrustal profiles provide new perspectives on the subsurface geology of San Fernando Valley , home of two of the most recent damaging earthquakes in southern California. Seismic reflection data provide depths to Miocene-Quaternary horizons; beneath the base of the Late Miocene Modelo Formation are largely nonreflective rocks of the Middle Miocene Topanga and older formations. Gravity and seismic reflection data reveal the North Leadwell fault zone, a set of down-tothe- north faults that does not offset the top of the Modelo Formation; the zone strikes northwest across the valley, and may be part of the Oak Ridge fault system to the west. In the southeast part of the valley, the fault zone bounds a concealed basement high that infl uenced deposition of the Late Miocene Tarzana fan and may have localized damage from the 1994 Northridge earthquake. Gravity and seismic refraction data indicate that the basin underlying San Fernando Valley is asymmetric, the north part of the basin (Sylmar subbasin) reaching depths of 5-8 km. Magnetic data suggest a major boundary at or near the Verdugo fault, which likely started as a Miocene transtensional fault, and show a change in the dip sense of the fault along strike. The northwest projection of the Verdugo fault separates the Sylmar subbasin from the main San Fernando Valley and coincides with the abrupt change in structural style from the Santa Susana fault to the Sierra Madre fault. The Simi Hills bound the basin on the west and, as defined by gravity data, the boundary is linear and strikes ∼N45 E. That northeasttrending gravity gradient follows both the part of the 1971 San Fernando aftershock distribution called the Chatsworth trend and the aftershock trends of the 1994 Northridge earthquake. These data suggest that the 1971 San Fernando and 1994 Northridge earthquakes reactivated portions of Miocene normal faults. [ABSTRACT FROM AUTHOR]

Probabilistic assessments of the seismic ductility demand for hysteretic bilinear single-degree-of-freedom systems have been reported in the literature. However, a systematic assessment of possible differences in the estimated ductility demand for different earthquake types using recorded ground motions is not available, although ground motion prediction equations for different earthquake types are developed. The assessment of the differences can be important for estimating structural reliability and expected damage cost under seismic excitations since partial damage and collapse could be related to the ductility demand. Therefore, if the differences are significant one must use consistent sets of ground motion prediction equation and ductility demand relation for each earthquake type affecting a site of interest to evaluate the seismic hazard and risk. To assess the differences of the ductility demand, 413 records for Mexican interplate earthquakes, 275 records for Mexican inslab earthquakes, and 592 records for California earthquakes are employed. The evaluation considers ranges of values of natural vibration periods and ratios of initial to postyield stiffness. The obtained results indicate that the statistics of displacement ductility demand differs for different earthquake types. The results are used to develop empirical relations for predicting the expected displacement ductility demand. [ABSTRACT FROM AUTHOR]

It has been 153 yr since the last large earthquake on the southern San Andreas fault (California, United States), but the average interseismic interval is only 100 yr. If the recurrence of large earthquakes is periodic, rather than random or clustered, the length of this period is notable and would generally increase the risk estimated in probabilistic seismic hazard analyses. Unfortunately, robust characterization of a distribution describing earthquake recurrence on a single fault is limited by the brevity of most earthquake records. Here we use statistical tests on a 3000 yr combined record of 29 ground-rupturing earthquakes from Wrightwood, California. We show that earthquake recurrence there is more regular than expected from a Poisson distribution and is not clustered, leading us to conclude that recurrence is quasi-periodic. The observation of unimodal time dependence is persistent across an observationally based sensitivity analysis that critically examines alternative interpretations of the geologic record. The results support formal forecast efforts that use renewal models to estimate probabilities of future earthquakes on the southern San Andreas fault. Only four intervals (15%) from the record are longer than the present open interval, highlighting the current hazard posed by this fault. [ABSTRACT FROM AUTHOR]

Using an inversion strategy based on adjoint methods, we developed a three-dimensional seismological model of the southern California crust. The resulting model involved 16 tomograpic iterations, which required 6800 wavefield simulations and a total of 0.8 million central process unit hours. The new crustal model reveals strong heterogeneity, including Cecal changes of ±30% with respect to the initial three-dimensional model provided by the Southern California Earthquake Center. The model illuminates shallow features such as sedimentary basins and compositional contrasts across faults. It also reveals crustal features at depth that aid in the tectonic reconstruction of southern California, such as subduction-captured oceanic crustal fragments. The new model enables more realistic and accurate assessments of seismic hazard. [ABSTRACT FROM AUTHOR]

• New self-centering precast concrete frames with increasing post-decompression stiffness. • Seismic design method considers the changing stiffness of variable friction dampers. • A method to mitigate the concentration of story drifts was proposed. • A method to mitigate detrimental higher mode effects was proposed. This paper presents the numerical investigation of the seismic performance of post-tensioned self-centering precast concrete (SCPC) frames with variable friction dampers (VFDs) and hidden corbels (HCs). The VFDs are characterized by the combination of flat and grooved plates, where the friction forces change as the connection deformation increases, which enables designs to meet target seismic performance objectives at different seismic hazard levels. Theoretical and numerical models for the connection are proposed based on connection geometry, and a performance-based seismic design approach for the SCPC frames is outlined to meet target performance objectives at two common seismic hazard levels, the design basis earthquake (DBE) and the maximum considered earthquake (MCE). SCPC frames for a six-story prototype building located in a high seismic zone in California were designed using this procedure. Three frames with HC-VFD-SPSC connections were designed using various methods to mitigate concentration of drifts in the first story, and one frame was designed with conventional constant friction devices (CFDs) for comparison. Nonlinear time-history analyses were performed for these different designs using two sets of ground motions representing the DBE and MCE seismic hazard levels. The analysis results indicate that the frames with the HC-VFD-SCPC connections achieved the target performance objectives. The proposed approaches of modifying post-tensioning and VFD design parameters or column stiffness in the first story were effective in mitigating the concentration of drifts. Additionally, through comparison with the results of the frame with CFDs, it was found that the VFDs are effective in decreasing overall drift demands at the MCE level, as well as mitigating detrimental higher mode effects observed in SCPC frames with CFDs. [ABSTRACT FROM AUTHOR]

Abstract: Determining late Quaternary offset rates on specific faults within active mountain belts is not only a key component of seismic hazard analysis, but sheds light on regional tectonic development over geologic timescales. Here we report an estimate of dip–slip rate on the eastern Big Pine oblique-reverse fault in the upper Cuyama Valley within the western Transverse Ranges of southern California, and its relation to local landscape development. Optically stimulated luminescence (OSL) dating of sandy beds within coarse-grained alluvial deposits indicates that deposition of alluvium shed from the Pine Mountain massif occurred near the southern margin of the Cuyama structural basin at the elevation of the Cuyama River between 25 and 14 ka. This alluvial deposit has been offset ∼10 m vertically by the eastern Big Pine fault, providing a latest Quaternary dip–slip rate estimate of ∼0.9 m/ky based on a 50° fault dip. Incision of the adjacent Cuyama River has exposed a section of older Cuyama River sediments beneath the Pine Mountain alluvium that accumulated between 45 and 30 ka on the down-thrown footwall block of the eastern Big Pine fault. Corroborative evidence for Holocene reverse-slip on the eastern Big Pine fault is ∼1 m of incised bedrock that is characteristically exposed beneath 2–3.5 ka fill terraces in tributaries south of the fault. The eastern Big Pine fault in the Cuyama Valley area has no confirmed record of historic rupture; however, based on our results, we suggest the likelihood of multiple reverse-slip rupture events since 14 ka. [Copyright &y& Elsevier]

The impact of an earthquake event on the performance of a highway transportation network depends on the extent of damage sustained by its individual components, particularly bridges. Seismic damageability of bridges expressed in the form of fragility curves can easily be incorporated into the scheme of risk analysis of a highway network under the seismic hazard. In this context, this article focuses on a nonlinear static method of developing fragility curves for a typical type of concrete bridge in California. The method makes use of the capacity spectrum method (CSM) for identification of spectral displacement, which is converted to rotations at bridge column ends. To check the reliability of this current analytical procedure, developed fragility curves are compared with those obtained by nonlinear time history analysis. Results indicate that analytically developed fragility curves obtained from nonlinear static and time history analyses are consistent. [ABSTRACT FROM AUTHOR]

A spectacular band of precariously and semiprecariously balanced rocks extends from Riverside to near Borrego Valley, California, about halfway between the Elsinore and San Jacinto fault zones. The rocks are distributed in a band a few kilometers wide midway between the San Jacinto and Elsinore fault zones, an indication that the distribution is caused by attenuation of strong ground motion from numerous large events along these two fault zones. These rocks have apparently been in place for thousands of years, and thus place important constraints on ground motions from earthquakes. On the basis of field tests and photographic analysis, the estimated quasi-static toppling accelerations for the precariously balanced rocks are ∼0.32 ± 0.10 g (g = gravitational acceleration). The dynamic toppling accelerations are within this range for earthquakes with waveforms similar to those recorded during the Izmit and Denali earthquakes. These constraints are roughly consistent with the median predicted value of ground motion for M7 earthquakes, but somewhat lower than +σ ground motion curves, and much lower than the values from the 2% in 50 yr hazard maps. The evidence from these rocks has importance for some of the assumptions that go into calculating probabilistic seismic hazard assessment, including median and standard deviation of ground motion attenuation curves (especially for hard rock, for which few instrumental data are available), the possible existence of random background earthquakes, and the smoothing distance for historical seismicity. [ABSTRACT FROM AUTHOR]

Abstract: California is in a highly seismically active region, and structures must be designed and constructed to withstand earthquakes. Seismic hazard analysis to estimate realistic earthquake ground motions and surface fault rupture offsets is done for various mitigation measures. The best policy is to avoid constructing structures crossing seismogenic faults. Because earthquake timings are unpredictable within our current understanding, the best method is time-invariant deterministic seismic hazard analysis (DHSA) to assess effects from the largest single earthquake called Maximum Credible Earthquake (MCEs) expected from seismogenic faults. Time-dependent hazard estimates such as those arrived at through probabilistic seismic hazard analysis (PSHA) are inherently unreliable. Hazard analyses based on MCEs have been in continuous use for the design and construction of highways and bridges in California for over 30 years. This paper presents an alternative to other methods of analysis, e.g., Abrahamson (2000) [Abrahamson, N.A., 2000. State of the practice of seismic hazard evaluation. Melbourne: proceedings of GeoEng, 2000]. [Copyright &y& Elsevier]

Despite a lack of reliable deterministic earthquake precursors, seismologists have significant predictive information about earthquake activity from an increasingly accurate understanding of the clustering properties of earthquakes. In the past 15?years, time-dependent earthquake probabilities based on a generic short-term clustering model have been made publicly available in near-real time during major earthquake sequences. These forecasts describe the probability and number of events that are, on average, likely to occur following a mainshock of a given magnitude, but are not tailored to the particular sequence at hand and contain no information about the likely locations of the aftershocks. Our model builds upon the basic principles of this generic forecast model in two ways: it recasts the forecast in terms of the probability of strong ground shaking, and it combines an existing time-independent earthquake occurrence model based on fault data and historical earthquakes with increasingly complex models describing the local time-dependent earthquake clustering. The result is a time-dependent map showing the probability of strong shaking anywhere in California within the next 24?hours. The seismic hazard modelling approach we describe provides a better understanding of time-dependent earthquake hazard, and increases its usefulness for the public, emergency planners and the media. [ABSTRACT FROM AUTHOR]

Earthquake mitigation efforts in the United States currently use long-term probabilistic hazard assessments and rapid post-earthquake notification to reduce the potential damage of earthquakes. Here we present the seismological design for and demonstrate the feasibility of a short-term hazard warning system. Using data from past earthquakes, we show that our Earthquake Alarm System (ElarmS) could, with current TriNet instrumentation, issue a warning a few to tens of seconds ahead of damaging ground motion. The system uses the frequency content of the P-wave arrival to determine earthquake magnitude, an approach that allows magnitude determination before any damaging ground motion occurs. [ABSTRACT FROM AUTHOR]

Presents a study which suggests that aseismic deformation mechanisms attribute to regional shortening strain in southern California. Methodology used to conduct study; Results of study; Discussion on findings.

Presents research which demonstrated that a significant component of seismic moment release and shortening in the Los Angeles, California, region is due to extrusion along known strike-slip and oblique-slip faults. Prevalence of thrust faults in the area; Use of Global Positioning System geodetic measurements.

Presents an application of the stress-based, nonlinear numerical procedure DYSAC2 and the total stress-based, equivalent-linear procedure SHAKE to the analysis of the seismic response of soil deposits in the San Francisco marina district. Indications of soil liquefaction and building damage; Evaluation of three sites' dynamic behavior.

Presents research which studied earthquakes at the Sierra Madre fault. Location of fault; Restoration of slip on the fault; Total cumulative slip; Seismic hazard and risk assessments of earthquakes in the Los Angeles region.

The spatial and temporal distribution of fault slip is a critical parameter in earthquake source models. Previous geomorphic and geologic studies of channel offset along the Carrizo section of the south central San Andreas Fault assumed that channels form more frequently than earthquakes occur and suggested that repeated large-slip earthquakes similar to the 1857 Fort Tejon earthquake illustrate typical fault behavior. We found that offset channels in the Carrizo Plain incised less frequently than they were offset by earthquakes. Channels have been offset by successive earthquakes with variable slip since ∼1400. This nonuniform slip history reveals a more complex rupture history than previously assumed for the structurally simplest section of the San Andreas Fault. [ABSTRACT FROM AUTHOR]

Discusses the four projects in the United States known as EarthScope, and how they will advance understanding of volcanoes, fault systems, and earthquakes. Their impact on the understanding of seismic hazards; Likely cost of the projects, and the potential for cost overruns; Details of the projects, which include the San Andreas Fault Observatory at Depth (SAFOD), The USArray project, The Plate Boundary Observatory (PBO), and a satellite fitted with interferometric synthetic aperture radar (InSAR). INSET: Keeping up with the neighbors.

Discusses the use of balanced cross-section to study the Northridge fault in the Transverse Ranges fold-and-thrust belt in southern California. Occurrence of the Northridge fault as cause of the earthquake in 1994; Features of balanced cross-section; Identification and quantification of the seismic hazard posed by thrust faults; Data gathered from the cross-sectional modelling.

This article discusses how research facilitating the increased understanding of slow earthquakes may provide a more complete understanding of the processes associated with ordinary earthquakes. The term slow earthquake encompasses a number of phenomena, including slow-slip events and very-low-frequency earthquakes. Slow earthquakes are typically accompanied by deep tremor and occur as shear slip events of tectonic plates. These tremors can be sensitive to small variations in geophysical stress, as they have been triggered by lunar tides. Monitoring of tremor phenomena via seismic stations is discussed, and peculiarities associated with the San Andreas Fault are elucidated.

Explores potential implications of the Northridge and Loma Prieta earthquakes in California for hazard-mitigation strategies. Combined financial losses due to the natural disaster; Assessment of earthquake sources; Prediction of earthquake effects; Ground shaking levels recorded in the Northridge epicentral area; Need to increase efforts in identifying and mapping hazard areas.

The article discusses the great chance that a major seismic activity would produce a catastrophic failure of multiple levees around Sacramento and San Joaquin River Delta in California. It is stated that the delta is laced with more than 1700 kilometers of earthen levees wherein many lies in a major flood or quake fault. According to Jeffrey Mount, a geologist at the University of California in Davis, the system of delta levees are in worse shape. In 2005, he denotes that for the next 50 years a combination of seismic action and increased flooding due to climate change would devastate the area.

On 28 September 2004 there was an earthquake of magnitude 6.0 at Parkfield, California. Here we show that the size distribution of the micro-earthquakes recorded in the decades before the main shock occurred allowed an accurate forecast of its eventual rupture area. Applying this approach to other well monitored faults should improve earthquake hazard assessment in future. [ABSTRACT FROM AUTHOR]