Earthquake Spectra

The Central Italy earthquake sequence nominally began on 24 August 2016 with a M6.1 event on a normal fault that produced devastating effects in the town of Amatrice and several nearby villages and hamlets. A major international response was undertaken to record the effects of this disaster, including surface faulting, ground motions, landslides, and damage patterns to structures. This work targeted the development of high-value case histories useful to future research. Subsequent events in October 2016 exacerbated the damage in previously affected areas and caused damage to new areas in the north, particularly the relatively large town of Norcia. Additional reconnaissance after a M6.5 event on 30 October 2016 documented and mapped several large landslide features and increased damage states for structures in villages and hamlets throughout the region. This paper provides an overview of the reconnaissance activities undertaken to document and map these and other effects, and highlights valuable lessons learned regarding faulting and ground motions, engineering effects, and emergency response to this disaster.

The Central Italy earthquake sequence produced three main shocks: M6.1 24 August, M5.9 26 October, and M6.5 30 October 2016. Additional M5–5.5 events struck this territory on 18 January 2017 in the Campotosto area. Fault plane solutions for the main shocks exhibit normal faulting (characteristic of crustal extension occurring in the inner central Apennines). Significant evidence, including hypocenter locations, strike and dip angles of the moment tensors, inverted finite fault models (using GPS, interferometric aperture radar, and ground motion data), and surface rupture patterns, all point to the earthquakes having been generated on the Mt. Vettore–Mt. Bove fault system (all three main shocks) and on the Amatrice fault, in the northern sector of the Laga Mountains (portion of 24 August event). The earthquake sequence provides examples of both synthetic and antithetic ruptures on a single fault system (30 October event) and rupture between two faults (24 August event). We describe active faults in the region and their segmentation and present understanding of the potential for linkages between segments (or faults) in the generation of large earthquakes.

The three mainshock events (M6.1 24 August, M5.9 26 October, and M6.5 30 October 2016) in the Central Italy earthquake sequence produced surface ruptures on known segments of the Mt. Vettore–Mt. Bove normal fault system. As a result, teams from Italian national research institutions and universities, working collaboratively with the U.S. Geotechnical Extreme Events Reconnaissance Association (GEER), were mobilized to collect perishable data. Our reconnaissance approach included field mapping and advanced imaging techniques, both directed towards documenting the location and extent of surface rupture on the main fault exposure and secondary features. Mapping activity occurred after each mainshock (with different levels of detail at different times), which provides data on the progression of locations and amounts of slip between events. Along the full length of the Mt. Vettore–Mt. Bove fault system, vertical offsets ranged from 0–35 cm and 70–200 cm for the 24 August and 30 October events, respectively. Comparisons between observed surface rupture displacements and available empirical models show that the three events fit within expected ranges.

The Central Italy earthquake sequence has, to date, generated three mainshocks: M6.1 24 August, M5.9 26 October, and M6.5 30 October 2016. These events, along with aftershocks, were well recorded by Italian networks, and are among the normal fault earthquakes with the highest number of recordings globally. We process records for six events using procedures developed during the latest Next Generation Attenuation (NGA-West2) project, coordinated by the Pacific Earthquake Engineering Research Center (PEER). Many recording sites lacked V_S30 assignments, which we provide using measured shear wave velocity profiles where available and a local geology proxy otherwise. Stations at close distance, including near the hanging wall, exhibit fling step in some cases but no obvious rupture directivity. The data exhibit fast anelastic attenuation at large distances (>100 km), as predicted by recent Italy-adjusted global models, but not by Italy-specific models. We partition residuals from Italy-adjusted global models, finding negative event terms at short periods (weaker than average shaking). We apply Kriging of within-event peak acceleration and velocity residuals using a global semi-variogram model to estimate the spatial distribution of peak accelerations and velocities, which are generally most intense southwest of Mt. Vettore.

The Central Italy earthquake sequence initiated on 24 August 2016 with a moment magnitude M6.1 event, followed by two earthquakes (M5.9 and M6.5) on 26 and 30 October, caused significant damage and loss of life in the town of Amatrice and other nearby villages and hamlets. The significance of this sequence led to a major international reconnaissance effort to thoroughly examine the effects of this disaster. Specifically, this paper presents evidences of strong local site effects (i.e., amplification of seismic waves because of stratigraphic and topographic effects that leads to damage concentration in certain areas). It also examines the damage patterns observed along the entire sequence of events in association with the spatial distribution of ground motion intensity with emphasis on the clearly distinct performance of reinforced concrete and masonry structures under multiple excitations. The paper concludes with a critical assessment of past retrofit measures efficiency and a series of lessons learned as per the behavior of structures to a sequence of strong earthquake events.

The 2016–2017 Central Italy earthquake sequence consisted of several moderately high-magnitude earthquakes, between M5.5 and M6.5, each centered in a different location and with its own sequences of aftershocks spanning several months. To study the effects of this earthquake sequence on the built environment and the impact on the communities, a collaborative reconnaissance effort was organized by the Earthquake Engineering Research Institute (EERI), the Eucentre Foundation, the European Centre for Training and Research in Earthquake Engineering (EUCentre), and the Rete dei Laboratori Universitari di Ingegneria Sismica (ReLuis). The effort consisted of two reconnaissance missions: one following the Amatrice Earthquake of 24 August 2016 and one after the end of the earthquake sequence, in May 2017. One objective of the reconnaissance effort was to evaluate existing strengthening methodologies and assess their effectiveness in mitigating the damaging effects of ground shaking. Parallel studies by the Geotechnical Extreme Events Reconnaissance (GEER) Association, presented in a companion paper, demonstrate that variations in-ground motions due to topographic site effects had a significant impact on damage distribution in the affected area. This paper presents that, in addition to these ground motion variations, variations in the vulnerability of residential and critical facilities were observed to have a significant impact on the level of damage in the region. The damage to the historical centers of Amatrice and Norcia will be used in this evaluation: the historical center of Amatrice was devastated by the sequence of earthquakes; the significant damage in Norcia was localized to individual buildings. Amatrice has not experienced the same number of devastating earthquakes as Norcia in the last 150 years. As a result, its building stock is much older than that of Norcia and there appeared to be little visual evidence of strengthening of the buildings. The distribution of damage observed throughout the region was found to be indicative of the effectiveness of strengthening and of the need for a comprehensive implementation of retrofit policies.

The 2016 Central Italy earthquake sequence caused numerous landslides over a large area in the Central Apennines. As a result, the Geotechnical Extreme Events Reconnaissance Association (GEER) organized post-earthquake reconnaissance missions to collect perishable data. Given the challenging conditions following the earthquakes, the GEER team implemented a phased reconnaissance approach. This paper illustrates this approach and how it was used to document the largest and most impactful seismically induced landslides. This phased approach relied upon satellite-based interferometric damage proxy maps, preliminary published reports of observed landslides, digital imaging from small unmanned aerial vehicles (UAVs), traditional manual observations, and terrestrial laser scanning. Data collected from the reconnoitered sites were used to develop orthophotos and meshed three-dimensional digital surface models. These products can provide valuable information such as accurate measurements of landslide ground movements in complex topographic geometries or boulder runout distances from rock falls. The paper describes three significant landslide case histories developed and documented with the phased approach: Nera Valley, Village of Pescara del Tronto, and near the villages of Crognaleto and Cervaro.

The region of the central Apennines affected by the 2016 earthquake sequence has numerous towns, villages, and isolated dwellings connected by local secondary roads and a few state highways. The roadway network includes several bridges that are important to the economy of the region and play an important role in the post-earthquake resilience of local communities. Within this network, 12 bridges and a rockfall protection tunnel were inspected in coordination with local officials, with relatively cursory reconnaissance of most of the remainder of the network. All inspected reinforced concrete and steel–concrete composite bridges performed adequately. Two historic masonry bridges near Amatrice and Tufo suffered significant damage after the 24 August 2016 main shock, and collapsed after the 30 October 2016 event. Recovery strategies related to the bridge collapse near Amatrice, where two temporary bridges were built within 10 days from the first main shock in August, are discussed. An inspected rockfall protection tunnel experienced earthquake pounding effects.

This paper presents the application of a rigorous probabilistic framework that estimates the number, severity, and distribution of casualties over a region. A brief summary of the model is included in this paper. The application is for casualties resulting from a Mw 8.8 earthquake scenario occurring on the sub-duction fault along the coastline of Lima, Peru. The case study demonstrates an application of the casualty model, including the procedures for acquiring the required information, the selection of model parameters, and a step-by-step explanation of the model-solving algorithms. The model provides an estimate of the joint probability distribution of multiseverity casualties, including spatial and across-severity correlations. This paper also shows how the model can be useful for (1) estimating 90th-percentile casualties, (2) identifying unsafe communities and structural typologies, and (3) providing evidence to support hospital collaboration policies across different districts to increase the patient treatment reliability. Additionally, the results demonstrate that empirical fatality prediction methodologies can underestimate fatality rates in countries with scarce and outdated fatality data.

The efficacy of various types of intervention measures intended to facilitate post-earthquake housing recovery can be evaluated ahead of time by using simulation models to quantify their benefits and tradeoffs. Towards this end, this paper presents a conceptual framework comprised of three components for modeling post-earthquake housing recovery. The modeling framework starts with a probabilistic assessment of building-level damage using recovery-based limit states that characterize post-earthquake functionality, inhabitability, and repairability. These limit states are the basis for the second component, which includes two different utility-based models for representing post-earthquake household decision making. Stochastic models to probabilistically quantify building-level recovery trajectories comprise the third and final component of the framework. Collectively, these alternative models can integrate the effect of building states, available resources, household decisions, and endogenous factors such as lifeline restoration. The modeling framework can be scaled to model spatiotemporal scenarios of housing recovery to inform jurisdictional-level policies, plans, and interventions to increase residential community resilience.

Post earthquake decisions on whether to repair or to demolish and rebuild a damaged commercial building can be influenced by factors other than repair costs. These factors include the property's ability to generate income and the conditions of the real estate market—factors not currently considered in seismic performance estimation models. This paper introduces a framework that unifies performance-based earthquake engineering and real estate investment analysis to model cases in which repair of damaged buildings is feasible, but redevelopment or leaving the building unrepaired and vacant might offer greater economic value. A three-stage approach for quantifying the likelihood of repair, redevelopment, or leaving the property vacant is proposed. First, building seismic performance analysis is conducted using FEMA P-58 and Resilience-based Earthquake Design Initiative (REDi) methodologies; then, given repair and redevelopment costs and times, the net present value decision rule is used to evaluate alternative outcomes; and finally, the results from the two stages are integrated to quantify the probability of different decisions. An illustrative case study of four reinforced concrete buildings highlights the insights provided by the proposed framework.

The seismological community acknowledges the essential contribution of macroseismic assessment to the compilation of the seismic catalogs used for seismic hazard assessment. Furthermore, macroseismic observations are routinely employed by civil protection authorities in the aftermath of damaging events to improve their decision making. In this paper, we describe a novel methodology for the rapid, probabilistic estimation of macroseismic intensity in the epicentral area of a major event according to the European Macroseismic scale. The methodology includes mobile mapping and a collaborative online platform for rapid post earthquake reconnaissance. A Bayesian scheme is proposed to integrate direct damage observations and prior information, allowing consideration of ancillary data and expert judgment. According to a feasibility study carried out in the area affected by the 2016 Amatrice (Central Italy) earthquake, the proposed methodology should provide a reliable estimation of intensity, efficiently integrating further post earthquake building damage surveys.

This paper describes an experimental investigation of the low-cycle fatigue (LCF) behavior of welded flange-bolted web (WFBW) connections, which are commonly employed in high-rise steel moment-resisting frames (MRFs) in Japan. The main parameters investigated in this study were (1) bolt configuration of the web connection and (2) steel grade. According to test results, LCF capacity depends on the slip behavior of different bolt configurations, even at relatively minor inelastic rotations. Slip behavior effects can be evaluated by the yield strength of the shear plate or the slip-critical strength; for specimens whose shear plate yield strength was designed to be higher than the slip-critical strength, LCF capacity showed that the upper limit can neglect slip behavior. Furthermore, it was shown that LCF behavior can be evaluated by the same fatigue curve in the relationship of fatigue capacity and beam rotation amplitude, regardless of steel yield stress.

Viscoelastic coupling dampers (VCDs) are installed in lieu of traditional reinforced concrete (RC) coupling beams in high-rise buildings to provide distributed supplemental damping for all dynamic loading conditions without affecting the architectural layout. When distributed effectively over the height of the building, VCDs provide viscous damping in all lateral modes of vibration and an elastic restoring force that enhances the lateral stiffness of the coupled system. In this paper, a first extensive numerical case study is carried out to compare the seismic performance of a conventional coupled shear wall high-rise building to a high damping alternate of the same design in which VCDs replace all diagonal RC beams in the core to enhance its seismic resilience. The added damping from VCDs is intended to reduce the peak responses under low amplitude earthquakes, but for larger amplitude maximum credible earthquakes, the peak responses are similar; however, structural damage is greatly reduced. Three seismic hazard levels were investigated, and the results indicate that the use of VCDs reduces peak floor accelerations, story drifts, and story shears over all seismic intensities. Nonlinear time-history analysis results also highlighted the improved resilience of the VCD structure at the maximum credible seismic hazard level where the use of VCDs eliminated all damage to coupling beams that would otherwise require repair over most of the height of the building.

A fused steel diagrid frame (FSDF) is an innovative earthquake resilient structural system. FSDF combines the steel diagrid structural system with shear links (SLs) and moment connections (MCs) to dissipate earthquake energy. SLs are placed between diamond-shaped grid units and serve as the primary seismic force-resisting system (SFRS). MCs provide the additional resistance as the secondary SFRS. To facilitate the design of the proposed FSDF system, a performance-based equivalent energy design procedure (EEDP) is utilized to design a prototype building. EEDP allows designers to select multiple performance objectives under different earthquake hazard intensities. A detailed finite element model is developed to simulate the nonlinear dynamic response of the structure under earthquake shaking intensities. The results of nonlinear dynamic analyses show that the FSDF has superior seismic performance and can be efficiently designed using EEDP. Lastly, detailed collapse risk assessment of the prototype building utilizing the FSDF is conducted using the Federal Emergency Management Agency FEMA P695 (2009) methodology. The result shows that the EEDP-designed FSDF has superior margin against collapse.

Deaggregation is one of the products of probabilistic seismic hazard analysis (PSHA) suitable for identifying the relative contributions of various magnitude-distance bins to a hazard or intensity measure (IM) level. In this paper, we elucidate some interesting features of deaggregations, such as: their monotonically decreasing nature with IM; their invariance to any minimum IM level; and the pertinence of their bins to a complementary cumulative distribution function (CCDF). We use these features of hazard deaggregation along with copula functions in a simplified method for computing vector deaggregation and vector hazard given the scalar counterparts. We validate our simplified procedure at a hypothetical site surrounded by multiple fault sources where seismic hazard is calculated using a logic tree. We also demonstrate the application of our approach to a real site in Los Angeles, CA. Finally, we explore whether the invariance property of deaggregations can be used to compute scalar hazard curves using new ground motion prediction models/IMs, and find that for low to moderate levels of IM, a reasonable approximation is obtained.

The random vibration theory offers a framework for the conversion of response spectra into power spectral densities (PSDs) and vice versa. The PSD is a mathematically more suitable quantity for structural dynamics analysis and can be straightforwardly used to compute structural response in the frequency domain. This allows for the computation of in-structure floor response spectra and peak responses by conducting only one structural analysis. In particular, there is no need to select or generate spectrum-compatible time histories to conduct the analysis. Peak response quantities and confidence intervals can be computed without any further simplifications such as currently used in the response spectrum method, where modal combination rules have to be derived. In contrast to many former studies, the Arias intensity-based definition of strong-motion duration is adopted here. This paper shows that, if the same definitions of strong-motion duration and modeling assumptions are used for time history and RVT computations, then the same result can be expected. This is illustrated by application to a simplified model of a reactor building.

The current practice for selecting bidirectional ground motions (GM pairs) to conduct nonlinear response history analysis (RHA) of multistory buildings is restricted to those with a symmetric plan. To overcome such limitations, we propose selecting GM pairs to be consistent with a pair of target spectra defined along the structural axes, enabling a unique azimuth to be determined for each GM pair. We develop two new target spectra: (1) the s-GCMS for two horizontal components of GM and (2) the CMS-UHS Composite Spectrum. Based on nonlinear RHAs of buildings with both symmetric and unsymmetric plans, the CMS-UHS Composite Spectrum is shown to be the best alternative to the current practice of utilizing multiple CMSs, because it provides accurate demands with minimal computational effort and can be easily constructed using existing PSHA tools.

Using initial P-wave records at 298 seismic stations from the Kiban-Kyoshin network (KiK-net), the P-wave seismograms method is employed to estimate the near-surface shear wave velocity in Japan. The applicability of this method is validated by comparisons between the measured and estimated time-averaged shear wave velocity to depth Z(V_SZ, 5< Z < 300 m). Using a second-order polynomial regression relating log V _SZ and log V_S30, the estimated V_S30 values agree well with the measured values. We also find that V_S30 is directly related with the ratio of radial to vertical components of the initial P-wave velocity time series (R _P ). Compared with the theoretical method, the empirical relationship between R _P and V_S30 has an improvement in the accuracy for V_S30 estimation, is basically region-independent for Japan and Central and Eastern North America (CENA), does not need any other parameters, and is potentially useful for other regions of the world.

Ergodic site response models are generally conditional on the time-averaged shear wave velocity in the upper 30 m (V_S30). Ground motion databases contain many recordings from Taiwan, and because of site characterization efforts, 56% of recording sites have V_S30 derived from measurements. We develop proxy-based V_S30 prediction models, one application of which is for the remaining 44% of Taiwan sites. Our approach, which can be suitable for other regions, differs from previous studies in which proxies are based on detailed geologic categories and possible within-category topographic gradient effects. Instead, we use three broad, age-based geologic categories, and for the youngest category of Holocene and Quaternary undivided sediments, we propose models conditioned on gradient and elevation. We also adapt a geomorphic terrain-based method, thus providing two V_S30-prediction models. We describe a model weighting scheme that combines the models in consideration of their relative dispersions and correlation, producing a weighted mean and standard deviation natural-log V_S30. Included as an electronic supplement is a profile database file and a site database with site parameters for Taiwan ground motion stations.