Large Magnitude Earthquakes Research Articles

Rapid Source Characterization of the Maule Earthquake Using Prompt Elasto‐Gravity Signals

AbstractThe recently identified Prompt Elasto‐Gravity Signals (PEGS), generated by large earthquakes, propagate at the speed of light and are sensitive to the earthquake magnitude and focal mechanism. These characteristics make PEGS potentially very advantageous for tsunami early warning, which relies on fast and accurate estimation of the magnitude of large offshore earthquakes. PEGS‐based early warning does not suffer from the problem of magnitude estimation saturation, that P‐wave based early warning algorithms have, and could be faster than Global Navigation Satellite Systems (GNSS)‐based systems while not requiring a priori assumptions on slip distribution. We use a deep learning model called PEGSNet to evaluate the possibility to estimate in real time the evolution of the magnitude of big earthquakes in the tsunamigenic zone of Chile. The model is a Convolutional Neural Network (CNN) trained on a database of synthetic PEGS—simulated for an exhaustive set of possible earthquakes distributed along the Chilean subduction zone—augmented with empirical noise. The approach is multi‐station and leverages the information recorded by the seismic network to estimate as fast as possible the magnitude and location of an ongoing earthquake. Our results indicate that PEGSNet could have estimated that the magnitude of the 2010 Mw 8.8 Maule earthquake was above 8.7, 90 s after origin time. Our offline simulations using real data and noise recordings further support the instantaneous tracking of the source time function of the earthquake and show that deploying seismic stations in optimal locations could improve the performance of the algorithm.

Velocity Changes Associated to the 2019 Ambon 6.5 Mw Earthquake Using Ambient Seismic Noise

Seismic velocity changes which occurred before, during and after large magnitude earthquake carry information about damage fault surrounding the area. Research focused on Ambon 6.5 Mw earthquake on September 26, 2019. We use ambient noise cross-correlation technique to compute empirical Green’s Function between station pairs from January to December 2019. We use doublet method to determine perturbation calculated in frequency domain by using Moving Windows Cross Spectrum in frequency between 0.1 to 1.0 Hz. Our results show no obvious significant velocity change before the mainshock. Velocity fluctuation average about ±0.1% caused by non-isotropic of noise source around the area. Sudden velocity decrease occurred and reached average value of -0.35% in 7 days after mainshock. After reaching its lowest value, velocity gradually increased about +0.4% in 14 days. Velocity fluctuations about ±0.2% also occurred for three months after the earthquake which may caused by the aftershocks around the earthquake epicentre. Following the large magnitude mainshock, velocity reduction may be induced by cracks opening and fractures due to deformation. While the velocity increase may caused by post-seismic relaxation due to elastic behaviour and crack healing of the fault throughout the surrounding region.

Deaggregation of probabilistic seismic hazard results for some selected cities in Western Mexico

ABSTRACT Here we present a deaggregation appraisal conducted for 15 selected significant cities in Western Mexico, for four oscillation periods (PGA, SA (0.2 s), SA (1.0 s), and SA (2.0 s)), also considering a different input for the soil condition (for B, B/C and C NEHRP site classes), and for two return periods (475 and 975 years). This study is based on a previous complete recently published seismic hazard evaluation for the region. An area source model consisting of thirty-seven seismic sources has been used alternatively in a logic tree with a spatially smoothed seismicity model for the same region. The obtained hazard deaggregation results prove that for most of the studied cities –those located along the Pacific coast–, nearby seismic sources are contributing most to the seismic hazard at the studied location, especially for lower periods (PGA and SA (0.2 s)). However, for a few cities far from the Middle America Trench, distant large-magnitude earthquakes contribute more to the seismic hazard, especially at larger spectral periods (SA (1.0 s) and SA (2.0 s)). Additionally, this study shows that the differences in the soil conditions for the computed return periods, have a little influence on the obtained deaggregation results.

Subduction as a Smoothing Machine: How Multiscale Dissipation Relates Precursor Signals to Fault Geometry

Understanding the process of earthquake preparation is of utmost importance in mitigating the potential damage caused by seismic events. That is why the study of seismic precursors is fundamental. However, the community studying non-seismic precursors relies on measurements, methods, and theories that lack a causal relationship with the earthquakes they claim to predict, generating skepticism among classical seismologists. Nonetheless, in recent years, a group has emerged that seeks to bridge the gap between these communities by applying fundamental laws of physics, such as the application of the second law of thermodynamics in multiscale systems. These systems, characterized by describing irreversible processes, are described by a global parameter called thermodynamic fractal dimension, denoted as D. A decrease in D indicates that the system starts seeking to release excess energy on a macroscopic scale, increasing entropy. It has been found that the decrease in D prior to major earthquakes is related to the increase in the size of microcracks and the emission of electromagnetic signals in localized zones, as well as the decrease in the ratio of large to small earthquakes known as the b-value. However, it is still necessary to elucidate how D, which is also associated with the roughness of surfaces, relates to other rupture parameters such as residual energy, magnitude, or fracture energy. Hence, this work establishes analytical relationships among them. Particularly, it is found that larger magnitude earthquakes with higher residual energy are associated with smoother faults. This indicates that the pre-seismic processes, which give rise to both seismic and non-seismic precursor signals, must also be accompanied by changes in the geometric properties of faults. Therefore, it can be concluded that all types of precursors (seismic or non-seismic), changes in fault smoothness, and the occurrence of earthquakes are different manifestations of the same multiscale dissipative system.

Evaluation of the Effect of Change in Support Conditions on the Seismic Behavior of a Historical Masonry Bridge

It is believed that the Yılanlı (or Leylekli) Bridge (bridge with stork or snake figures) in Tokat, a city with high seismicity, was built during the Roman or Byzantine period. Although no structural damage was found on the bridge, which is still open to pedestrian traffic, it is very important to determine the seismic behaviour of the bridge considering the possible earthquakes to which the structure will be exposed. This article presents the seismic response of the historical Yılanlı (Leylekli) Bridge, which has been subjected to unfavourable human-made changes in geometric shape over time. The original and modified shapes of the bridge were modelled using the macro modelling approach in ANSYS finite element software. The structural performance of the models was evaluated in terms of stresses, deformations, and especially cracks on the arch and spandrel walls using nonlinear static and time history analyses. The results of the analysis show that the change in shape, which is not desirable from an architectural and aesthetic point of view, adversely affects the seismic performance of the historical bridge and makes it more vulnerable to possible earthquakes. In addition, the analyses clearly showed that both models are subject to serious structural damage that could lead to a collapse in the event of a potential large magnitude earthquake.

Stochastic strong-ground motion simulation in the Santiago metropolitan region considering an Mw 7.8 intraplate intermediate-depth earthquake

Large intraplate intermediate-depth (IID) events are the most destructive to Chilean structures and they occurred throughout the country. Cities located above the hypocenters of these large earthquakes have been completely destroyed. In recent years, only small- and moderate-magnitude IID events have been reported below Santiago, the most populated Chilean city; however, the occurrence of large-magnitude IID events cannot be ruled out. In this study, we investigated the strong-ground motion generated by large-magnitude earthquakes occurring below the Santiago metropolitan region. We used a stochastic methodology to simulate synthetic records, considering IID events of magnitude Mw 7.8. To validate this method, we simulated intermediate-depth events of magnitudes Mw ∼5-6 that occurred near Santiago. We further calibrate our results by reproducing the strong-ground motion data recorded during the 2005 Tarapacá Mw 7.8 Northern Chile IID earthquake. We observe that in some areas of the Santiago Basin, high PGA values can reach values close to 1 g, in which, in addition to directivity effects and takeoff angles, the rupture distances (∼100 km) and soil type (C) play a key role in amplifying strong-ground motion. Finally, our findings highlight the importance of reproducing high-frequency time histories as a proxy to re-evaluate the seismic hazard due to large IID earthquakes.

1D Seismic Velocity Models for West-Central and Western Montana

Abstract In seismically active areas with infrequent large-magnitude earthquakes, high-quality seismic data are critical for determining accurate seismic velocity models. Here, we present a new local-scale crustal seismic velocity model in west-central Montana as well as a new regional-scale crustal and upper mantle seismic velocity model for broader western Montana. These models are constrained by phase arrivals from several seismic networks, including the University of Montana Seismic Network, the Montana Regional Seismic Network, the Advanced National Seismic System, temporary deployments by the U.S. Geological Survey, and the USArray Transportable Array. We jointly invert for hypocenters and velocity structure using the VELEST software. The local seismic velocity model is the first model specific to west-central Montana, using P-wave arrivals from aftershocks of the 2017 M 5.8 Lincoln, Montana, earthquake. The local model consists of eight layers down to 30 km depth below the mean sea level and spans a region of approximately 40,​000 km2 (200 km × 200 km). The velocity of the uppermost layer in the local model is 4.80 ± 0.11 km/s, and the velocity of the lowermost layer resolved from the data is 7.00 ± 0.05 km/s. Using an expanded dataset across a broader geographical area, we develop a “regional” seismic velocity model that represents spatially averaged velocity structure across western Montana. The regional model consists of thirteen layers down to 45 km depth below the sea level and is appropriate to an area of 160,​000 km2 (400 km × 400 km). The velocity of the uppermost layer is 4.30 ± 0.15 km/s, and the velocity of the lowermost layer is 8.00 ± 0.04 km/s. The new models are similar to prior velocity models for western Montana, but feature improved vertical resolution of layer velocities.

Uncertainties in Intensity-Based Earthquake Magnitude Estimates

Abstract Estimating the magnitude of historical earthquakes is crucial for assessing seismic hazard. Magnitudes of early-instrumental earthquakes can be inferred using a combination of instrumental records, field observations, and the observed distribution of shaking intensity determined from macroseismic observations. For earthquakes before 1900, shaking intensity distributions often provide the only information to constrain earthquake magnitude. Considerable effort has been made to develop methods to estimate the magnitude of moderate-to-large historical earthquakes using shaking intensities derived from macroseismic data. In this study, we consider earthquakes in California with known instrumental magnitudes to explore uncertainties in estimating the magnitude of historical earthquakes from intensity information alone. We use three California-specific intensity prediction equations (IPEs) and an IPE based on a global ground-motion model (GMM) to determine optimum intensity-based magnitudes for 33 moderate-to-large California earthquakes between 1979 and 2021. Intensity-based magnitudes are close to instrumental magnitudes on average. However, intensity-based magnitudes for individual events differ by as much as 2.2 magnitude units from instrumental magnitudes. This result reflects the weak dependence of ground motions and shaking intensities on moment magnitude and their strong dependence on stress drop. Considering the intensity distributions of the 1906 San Francisco and 1989 Loma Prieta earthquakes, we show that information that could constrain rupture length is discarded when considering only the 2D decay of intensity with distance. We also show that ground-motion intensity conversion equations used in a GMM-based approach may cause a systematic overestimation of large historical earthquake magnitudes. This study underscores both the reducible and potentially irreducible uncertainties associated with using intensity data to estimate magnitudes of historical earthquakes using IPEs and highlights the value of using additional information to constrain rupture dimensions. Using intensity observations alone, moment magnitude uncertainties are typically on the order of a full unit.

Estimating seismic vulnerability in West Bengal by AHP-WSM and AHP-VIKOR

West Bengal is situated primarily in the Surma Valley at the foothills of the Himalayas and near the western foreland of the Assam-Arakan Orogenic Belt. Several low to moderate-magnitude earthquakes are felt in the region frequently. In this study, we use integrated multi-criteria decision-making (MCDM) models to assess the seismic vulnerability in West Bengal. Twenty-four parameters that were susceptible to seismicity in the region have been used to evaluate geotechnical, structural, social, and physical vulnerability. The analytical hierarchy process (AHP) model was used to estimate the priorities of the parameters, which was then used to estimate seismic vulnerability using the VIseKriterijumska Optimizacija I Kompromisno Resenje (VIKOR) and the weighted sum method (WSM). The results reveal that approximately ∼17.81% of the total area and ∼65.36% population may fall under a high to very high-vulnerable zone, causing concerns for planning and disaster mitigation. The receiver operating characteristic curve estimated to validate the results, indicate that the AHP-VIKOR performs better for seismic vulnerability estimation. The results of this study may help various mitigation and planning agencies in identifying earthquake-vulnerable zones and preparing in advance for any potential large magnitude earthquakes that may occur in the region.

The mechanism of REE-Y impregnation on active carbonate normal fault scarps

The fluctuations of the Rare Earth Elements and Yttrium (REE-Y) concentrations on exhumed carbonate normal fault scarps may reveal the number and size of paleoearthquakes that exposed the scarp subaerially. This is because, prior to each large-magnitude earthquake, narrow (<50 cm) sections of the fault plane which are in direct contact with the soil become enriched in REE-Y before they are exhumed co-seismically, together with deeper, non-enriched, scarp sections. Following exhumation, depletion in REE-Y commences on both the enriched (i.e. ‘soil rupture zone’) and non-enriched (i.e. ‘rock rupture zone’) scarp sections. Although these processes are commonly described to occur on carbonate scarps, the mechanisms through which they operate remains poorly understood. Here, we present a series of laboratory tests that mimic the natural process of REE-Y enrichment/depletion to elucidate the mechanism of REE-Y impregnation. Our results indicate a fast uptake of REE-Y by the carbonate plane, when in contact with soil, either as (REE, Y)2(CO3)3 precipitate or by adsorption on calcite surfaces. The source of REE-Y in soil solution is released in a “pulses” due to alternations of dry and wet periods, characteristic of Mediterranean climatic conditions. Organic matter oxidation during the first rain events, triggers the Mn reductive dissolution and the release of REE-Y into the soil solution. The pH decrease due to organic matter dissolution is buffered by calcite, especially in the vicinity of the scarp, where calcite dissolution and re-precipitation occurs with a marked pH oscillation between 9.3 and 7.7. Further, comparison of these results with empirical data from three co-seismically exhumed fault scarps in Greece and Italy places quantitative constraints on the timing of these processes: the REE-Y enrichment within the ‘soil rupture zone’ may reach a maximum of ∼50% in about 500 years (+0.53 μg/kg/year), while the REE-Y depletion from the scarp is slow (−0.021 μg/kg/year), with a maximum recorded retention time of ∼16 ka. These enrichment and depletion characteristics work together to preserve paleoearthquake signal on carbonate scarps. Thus, this methodology is a valuable tool for quantifying the number of past earthquakes on carbonate fault scarps and allows more targeted use of expensive dating techniques (i.e. with cosmogenic nuclides) in order to derive the precise timing of these paleoearthquakes.

Seismic Risk Regularization for Urban Changes Due to Earthquakes: A Case of Study of the 2023 Turkey Earthquake Sequence

Damage identification soon after a large-magnitude earthquake is a major problem for early disaster response activities. The faster the damaged areas are identified, the higher the survival chances of inhabitants. Current methods for damage identification are based on the application of artificial intelligence techniques using remote sensing data. Such methods require a large amount of high-quality labeled data for calibration and/or fine-tuning processes, which are expensive in the aftermath of large-scale disasters. In this paper, we propose a novel semi-supervised classification approach for identifying urban changes induced by an earthquake between images recorded at different times. We integrate information from a small set of labeled data with information from ground motion and fragility functions computed on large unlabeled data. A relevant consideration is that ground motion and fragility functions can be computed in real time. The urban changes induced by the 2023 Turkey earthquake sequence are reported as an evaluation of the proposed method. The method was applied to the interferometric coherence computed from C-band synthetic aperture radar images from Sentinel-1. We use only 39 samples labeled as changed and 9000 unlabeled samples. The results show that our method is able to identify changes between images associated with the effects of an earthquake with an accuracy of about 81%. We conclude that the proposed method can rapidly identify affected areas in the aftermath of a large-magnitude earthquake.

Record-based simulation of three-component long-period ground motions: Hybrid of surface wave separation and multivariate empirical mode decomposition

Far-field long-period ground motion (LPGM) records are critical for the performance evaluation of high-rise buildings in the LPGM potential area. However, the typical LPGM records are extremely scarce because of their harsh formation condition (e.g., large magnitude earthquake and basin site). To this end, this study develops a novel record-based approach for simulating three-component LPGMs based on the combination of Hilbert spectra theory, surface wave separation method, and multivariate empirical mode decomposition (MEMD). To be specific, the recorded three-component LPGMs are first divided into surface waves and body waves via the particle polarization-based separation method. Then, MEMD is introduced to decompose these body waves into a set of intrinsic mode functions (IMFs) with a well mode-alignment attribute. Finally, the Hilbert spectra theory is employed to explain the characteristics of the recorded three-component LPGMs, thereby realizing the corresponding simulation. This developed method not only embodies the surface wave polarization feature effectively, but also can well reflect body wave time-frequency characteristics. Numerical examples based on the measured three-component LPGM records demonstrate the effectiveness of the proposed method. Therefore, this method may be an effective way to provide adequate LPGM inputs in the structural performance evaluation.

Large earthquakes along slow converging plate margins: Calabrian Arc paleoseismicity based on the submarine turbidite record

The Calabrian Arc subduction-rollback system hosts seismogenic faults capable of generating earthquakes exceeding magnitude 7. Since earthquakes are the result of long-term geodynamic processes, documenting seismic activity during a sufficiently long time interval is of fundamental importance for hazard scenarios. Instrumental and historical data provide critical information on seismogenesis, but they cover time periods shorter than the recurrence times of large earthquakes, especially in areas with low deformation rates such as Calabria. If onshore paleoseismological studies are fundamental to compile earthquake catalogs, they are sometime affected by the relatively poor continuity of sedimentation in the subaerial environment.In this study we applied the paleoseismological approach to the submarine environment to reconstruct the record of high-energy sedimentary events triggered by seismic activity. We analyzed three gravity cores collected in disconnected sedimentary basins to reconstruct resedimentation processes during the Holocene, integrating inland information for a better assessment of tectonic activity and seismogenesis. Multiproxy analyses of the sedimentary record constrained by radiometric dating allowed reconstructing event stratigraphy and linking resedimented deposits to specific earthquakes.Onshore and offshore data allow to identify large-magnitude earthquakes in the central Calabrian Arc subduction system during the Holocene, with inferred epicenters located either along normal faults onshore and/or related to the slab dynamics. The turbidite record reveals 20 major events during the last 10 ka, with sources including crustal faults in Calabria (i.e. Lakes, Rossano and Cittanova faults). Analyses of sediment samples and high-resolution seismic reflection images allowed identification of different types of resedimented deposits during the last 30–50 ka. The basin-wide occurrence of three megaturbidites/homogenites suggests they are related to megatsunamis sourced by far field earthquakes along the Hellenic Arc. Megaturbidites with a more limited spatial extent are interpreted as subduction-type events in the Calabrian Arc, while thinner seismo-turbidites record the activity of crustal structures including faults onshore. Results suggest a recurrence time of 2–3 ka for major Calabrian Arc events that needs to be considered for a reliable hazard assessment in the Mediterranean region.

Magnetotellurics (MT) and Gravity Study of a Possible Active Fault in Southern Garut, West Java, Indonesia

In recent years, dozen low-intensity earthquakes occurred in southern Garut, West Java Indonesia; two of them were reported destructive. However, those shallow earthquake clusters are hardly associated with well-known active faults in the area. Hence, we conducted 3D gravity combined with 2D magnetotellurics (MT) inversions to study the subsurface. Gravity and MT modeling confirm a basin with around 5 km depth consisting of two subbasins separated by a NE-SW trending local-high ridge. The local high coincides with the magmatic intrusion in geothermal fields and aligns with a series of volcanic bodies’ lineament observed on the surface. We interpret this structural high as a preexisting fault that serves as a magma pathway in the tectonomagmatic interaction. Shallow low-magnitude seismicity in the southern Garut area tends to occur in the resistive bodies. We interpret that heat from the cooling magmatic intrusion may decrease the effective fault-normal stress of the rocks, leading to a decrease in fault failure resistance and may initiate rupture. The resistivity structure around the initial rupture may affect whether or not the nucleation will end up as a large-magnitude earthquake. Furthermore, the unconsolidated young volcanic cover in this area could amplify ground shaking when earthquake occurs that might lead to more extensive damage.

Toward earthquake early warning: A convolutional neural network for rapid earthquake magnitude estimation

Earthquake early warning (EEW) is one of the important tools to reduce the hazard of earthquakes. In contemporary seismology, EEW is typically transformed into a fast classification of earthquake magnitude, i.e., large magnitude earthquakes that require warning are in the positive category and vice versa in the negative category. However, the current standard information signal processing routines for magnitude fast classification are time-consuming and vulnerable to data imbalance. Therefore, in this study, Deep Learning (DL) algorithms are introduced to assist with EEW. For the three-component seismic waveform record of 7 s obtained from the China Earthquake Network Center (CENC), this paper proposes a DL model (EEWMagNet), which accomplishes the extraction of spatial and temporal features through DenseBlock with Bottleneck and Multi-Head Attention. Extensive experiments on Chinese field data demonstrate that the proposed model performs well in the fast classification of magnitude. Moreover, the comparison experiments demonstrate that the epicenter distance information is indispensable, and the normalization has a negative effect on the model to capture accurate amplitude information.

Capturing seismic velocity changes in receiver functions with optimal transport

SUMMARY Temporal changes in seismic velocities are an important tool for tracking structural changes within the crust during transient deformation. Although many geophysical processes span the crust, including volcanic unrest and large-magnitude earthquakes, existing methods for seismic monitoring are limited to the shallow subsurface. We present an approach for deep seismic monitoring based on teleseismic receiver functions, which illuminate the crustal velocity structure from the bottom-up. Using synthetic waveform modelling, we show that receiver functions are uniformly sensitive to velocity changes throughout the crust and can locate the depth of the perturbation. We introduce a novel method based on optimal transport for measuring the non-linear time–amplitude signal variations characteristic of receiver function monitoring. We show that optimal transport enables comparison of full waveform distributions rather than relying on representative stacked waveforms. We further study a linearized version of optimal transport that renders time-warping signal variations into simple Euclidean perturbations, and use this capability to perform blind source separation in the space of waveform variations. This disentangles the effects of changes in the source–receiver path from changes in subsurface velocities. Collectively, these methods extend the reach of seismic monitoring to deep geophysical processes, and provide a tool that can be used to study heterogeneous velocity changes with different spatial extents and temporal dynamics.

2019 M7.1 Ridgecrest earthquake slip distribution controlled by fault geometry inherited from Independence dike swarm

Faults often form through reactivation of pre-existing structures, developing geometries and mechanical properties specific to the system’s geologic inheritance. Competition between fault geometry and other factors (e.g., lithology) to control slip at Earth’s surface is an open question that is central to our knowledge of fault processes and seismic hazards. Here we use remote sensing data and field observations to investigate the origin of the 2019 M7.1 Ridgecrest, California, earthquake rupture geometry and test its impact on the slip distribution observed at Earth’s surface. Common geometries suggest the fault system evolved through reactivation of structures within the surrounding Independence dike swarm (IDS). Mechanical models testing a range of fault geometries and stress fields indicate that the inherited rupture geometry strongly controlled the M7.1 earthquake slip distribution. These results motivate revisiting the development of other large-magnitude earthquake ruptures (1992 M7.3 Landers, 1999 M7.1 Hector Mine) and tectonic provinces within the IDS.

Climate-controlled sensitivity of lake sediments to record earthquake-related mass wasting in tropical Lake Towuti during the past 40 kyr

Located at the triple junction of the Pacific, Eurasian and Sunda plates, the Island of Sulawesi in Indonesia is one of the most tectonically active places on Earth. This is highlighted by the recurrence of devastating earthquakes such as the 2018 Mw 7.5 earthquake that damaged the city of Palu and caused several thousand fatalities in central Sulawesi. The majority of large-magnitude earthquakes on Sulawesi are related to stress release along major strike-slip faults such as the Palu-Koro Fault and its southern extensions, the Matano and Lawanopo Faults. To date, information on the frequency and magnitude of past major events on these faults is limited to instrumental records and historical sources restricted to the last century, whereas information from natural archives is completely lacking. Lake-sediment records can fill this gap, but a detailed assessment of the various factors that influence the sensitivity of sediment successions to past earthquakes is required to evaluate their suitability. Lake Towuti, situated in Eastern Sulawesi, is known for its paleoclimate record and also promises to be a key site to generate a paleoseismology record for Sulawesi. The lake lies close to the highly active Matano and Lawanopo strike-slip faults and thereby is an ideal archive for past earthquakes that have occurred in the surrounding area. Here we combine high-resolution chirp seismic data with lithostratigraphic and petrophysical data of sediment piston cores to assess the recurrence of seismically generated mass-transport and turbidite deposits. Three major seismic-stratigraphic units are distinguished in the upper ∼10 m of the sediment succession and linked to differences in the frequency of mass-wasting during the past 60 kyrs. The evidence of a more turbidite-prone period between 12 and 40 ka is roughly coincident with a dry phase and associated lake-level lowstand during the last glacial period at Lake Towuti. Hence, we suggest that climate strongly influences the sensitivity of slopes to fail during seismic shaking in this tropical setting as a consequence of lowstand-forced sediment redeposition from the shelves onto the slopes and into the basins. As climate significantly impacts the sensitivity of the lacustrine sediments to record earthquake-related mass wasting deposits, we suggest that the frequency of mass-transport deposits can additionally be employed as a quantitative indicator for past changes in hydroclimate in these tropical settings.

A fast nonlinear dynamic analysis automated approach to produce fragility curves for 3D RC frames

This study describes an automated procedure to assess seismic fragility of 3D reinforced concrete frame structures. A number of 9600 3D nonlinear dynamic analyses are performed using the recently developed 3D Force Analogy Method, implemented in an automated Matlad-based non-commercial code. The numerical efficiency of 3D Force Analogy is first illustrated for a broad range of structures and groud motion recordings and scaling factors. The ground motion dataset consisting of 20 horizontal recordings used in the study is characteristic for soft deep soil sites (e.g Bucharest) which are affected by large magnitude earthquakes originating in the Vrancea intermediate-depth seismic source. A total of 24 RC frame structures, both regular and irregular are employed in the study. The Park-Ang damage index is employed in order to evaluate the damage states of the structures. Fragility functions corresponding to life-safety and collapse damage states are obtained for both principal directions of the structures. The results show similar median fragility values on both principal directions, and a larger variability on the transversal direction. The analyses have also shown that the standard deviations evaluated for the mean annual failure rates corresponding to the two considered damage states and on both principal directions is almost equal to the mean value. The mean value of the collapse probability evaluated for the peak ground acceleration with a mean return period of 2475 years and based on the results of all 24 models is 6.1 % for the longitudinal direction and 6.5 % for the transversal direction.

Evaluation of random-vibration procedures to estimate response spectral ordinates on soft soil sites from fourier amplitude spectra

This study evaluates Random-Vibration Procedures (RVP) to estimate 5% damped response spectra of ground motions recorded on soft soil sites from Fourier Amplitude Spectra (FAS). The study makes use of 136 ground motions recorded in the soft soil Mexico City region during two recent large magnitude earthquakes. Seven different RVP are considered in this investigation consisting of four peak factor models, three strong ground motion duration models, and four non-stationary empirical correction factors. Spectral ordinates computed through conventional time integration were compared to approximate spectral ordinates computed using the FAS of the recorded acceleration time series in combination with RVP. A statistical error analysis was then conducted to determine how effective the various RVP were at estimating the exact pseudo-acceleration response spectra computed with response history analyses. It is found that most RVP commonly used today produce significant systematic overestimations of spectral ordinates at the fundamental mode of the soil deposit. Modifications to durations are proposed to be used in conjunction with various peak factors to reduce overestimations at the site period.