Extreme Ground Motions Research Articles

Ambient vibrations of a deep maar resonator

We introduce non-invasive seismic methods for identifying and characterizing maar sedimentary infills. We conducted a dense ambient vibration survey and employed state-of-the-art 3D resonance analysis techniques to map the lateral extent and depths of the sedimentary layers, a challenge with traditional geophysical methods due to unfavorable aspect ratios (depth > lateral dimension). The ambient vibrations of maars are predominantly driven by normal mode motions due to 3D resonance of crater infills. Dense station coverage enabled detailed images of the normal mode shapes revealing infill symmetries. The resonance results in extreme ground motion amplification, with factors reaching up to 30 on the vertical component, challenging conventional beliefs about site effects and methodologies based on widely used horizontal-to-vertical spectral ratios. These results are supported by numerical simulations of the maar's seismic response. The observed response is so specific that it can be used to identify partly eroded maar structures in the field.

Seismic response characteristics of seismically isolated buildings with dead zone mechanism connected to oil damper

AbstractIn recent years, there has been concern about the occurrence of extreme ground motions that greatly exceed the amplitude levels and durations expected at the conventional design level. Therefore, seismically isolated buildings are required to ensure seismic isolation performance for small earthquakes and suppress the excessive displacement of the seismic isolation layer for extreme ground motions. In this paper, we analyze the seismic response of seismically isolated buildings with the dead zone mechanism connected to oil dampers to understand the response characteristics.

Shaking table test of a timber building equipped with a novel cost-effective, impact-resilient seismic isolation system

In most cases, construction of Light Frame Timber Buildings (LFTBs) in areas of high seismic hazard requires strong wood frame shear walls and continuous rod systems, which increases the cost of LFTBs. Frictional seismic isolation might be applied to protect LFTBs against extreme ground motions and mitigate the cost of continuous rod systems. However, there are no experimental studies on the response of LFTBs equipped with frictional isolation and subjected to extreme seismic ground motions that might cause impacts between the slider and the perimetral ring or between the isolated base and the perimetral moat wall.This study explores the potential of using impact-resilient frictional isolators as a feasible solution to alleviate stiffening and overturning costs of LFTBs while making them resilient to impacts in case of extreme events. This issue has been researched by evaluating the response of a 1:2 scaled 3-story LFTB isolated with a novel Impact Resilient Double Concave Frictional Pendulum recently developed by the authors. The specimen was subjected to a suite of shaking table tests (white noise, harmonics signals, and seismic records), including strong ground motions such as the Concepcion record (2010 Maule, Chile earthquake, Mw = 8.8) scaled to 130 %. Results indicate that despite being subjected to extreme excitations, peak acceleration ratios (i.e., the ratio of peak floor acceleration to peak ground acceleration) did not exceed 0.75, and story drift ratios were smaller than 0.52 % in most cases. Thus, the superstructure remained in the elastic range without damage. The study demonstrates the potential of achieving effective seismic protection of LFTBs using Impact Resilient devices. In addition, this paper presents a numerical model developed with experimental data, which provides insight into modeling issues such as, for instance, damping properties.

Shaking Table Testing of a Scaled Nuclear Power Plant Structure with Base Isolation

To investigate the seismic performance and isolation effect of a high-temperature gas-cooled reactor, a 1/20 scale model including a reactor, a spent-fuel plant, and a nuclear auxiliary plant was fabricated. In addition, 220 mm lead-rubber bearings were designed and produced for use in the shaking table test, which included both isolated and nonisolated conditions. Two historical earthquake records and three artificial earthquake motions were used to input the ground motion in the tests. The results demonstrated that the seismic performance of the plant was better and that the structure was in an elastic state, under a safe shutdown earthquake event. Isolation bearings were found to effectively reduce the dominate frequency of the structure. The acceleration amplification factor of the superstructure was found to be less than 1. The isolation test results showed that the peak of the floor response spectrum at the pressure vessel support was less than 0.1 g. In the nonisolation test, the peak of the floor response spectrum was greater than 1 g. In the isolation test, the relative displacement of the structure was less than 1.1 mm, which was relatively small. The structure maintained a good isolation performance and exhibited improved safety under extreme ground motion.

A novel physical device to realize rate-independent linear damping for performance enhancement of seismically isolated structures

It is common concerned that seismically isolated structures may suffer excessive responses when subjected to extreme ground motions containing dominated long-period components. Supplementing damping via traditional damping devices is indeed beneficial in reducing the isolator displacement but perhaps at the expense of floor response accelerations. In the previous study, a conceptual rate-independent linear damping model (known as MNS model), which consists of linear Maxwell and negative stiffness elements in parallel, was found to be capable of simultaneously controlling response displacements and floor accelerations of low-frequency structures under strong ground motions. In the present study, a novel damping device, referred to as Maxwell-negative-stiffness damper (MNSD), is proposed to physically realize the conceptual rate-independent MNS model, and an efficient method of designing the properties of the MNSD with its nonlinearity embedded is presented. The performance of the proposed MNSD incorporated into a seismically isolated building structure is systematically studied by using both short- and long-period ground motions, and parametric studies are conducted to investigate the influences of key characteristic parameters on the control capability of the MNSD. Numerical examples illustrate the advantages of the proposed MNSD over the commonly used fluid viscous and hysteretic dampers, as well as the conventional negative stiffness dampers, in improving the performance of seismically isolated building structures under both sets of short- and long-period ground motions.

Elastic and viscous bumpers for seismic mitigation of base-isolated liquid storage tanks in near-fault zones

The use of friction pendulum system (FPS) as a base isolation system is one of the efficient and reliable techniques for seismic response reduction of liquid storage tanks. However, recent studies have shown that the FPS may experience substantial displacements under pulse-like near-fault earthquakes, which could lead to the collision of its articulated slider against the retainer ring. This internal impact in the FPS can make the isolation system less effective or even damaged. The best solution to this problem is to provide an appropriate displacement capacity of the FPS bearing to accommodate the resulting movements. However, due to any practical limitation or under extreme ground motions, the base displacement may exceed the displacement capacity provided and an internal impact may occur. This paper proposes and investigates the use of elastic and viscous bumpers in FPS-isolated tanks subjected to near-fault earthquakes to limit excessive bearing displacements and to prevent direct impact with the retainer. The material properties of the bumpers and their initial gap distance are used as variable parameters in the nonlinear time-history analyses of selected isolated tanks with different aspect ratios. The results, while showing the advantages of viscous bumpers over elastic ones, indicate the effectiveness of both shock absorbers in isolated tanks that are subject to large displacement demands. Based on the results of the parametric study, design recommendations are given for the use of such bumpers in FPS-isolated tanks.

Time-dependent seismic reliability of isolated bridges considering ageing deterioration of lead rubber bearings

Seismic isolation is an effective technology for mitigating the seismic damage to bridges. However, rupture and large cracks of laminated rubber bearings, including lead rubber bearings, have occurred at several bridges during recent earthquakes. Preventing the rupture of seismic isolators by considering uncertainties and variations in the mechanical properties due to ageing deterioration can improve the ductility capacity of the bridge systems when subjected to extreme ground motions. This study presents a time-dependent seismic reliability assessment methodology for isolated bridges that considers long-term ageing deterioration of lead rubber bearings. The effects of capacity hierarchy between the isolator and the column, as well as the ageing deterioration of isolators on the seismic reliability and the predominant ultimate failure mode were clarified. It was found that the effect of ageing deterioration of lead rubber bearings on seismic reliability is relatively smaller than that of the difference in seismic hazards at different sites. Furthermore, the seismic safety of isolated bridges can be significantly improved by setting an adequate capacity hierarchy between the isolator and the column, as it can prevent the rupture of the isolator under extreme ground motions considering the ageing deterioration of the isolator.

Hybrid broadband ground motion simulations in the Indo-Gangetic basin for great Himalayan earthquake scenarios

This study presents broadband ground motions for the Indo-Gangetic basin, a large sedimentary basin in India, for potential future great (Mw 8.5) Himalayan earthquakes. We use a recently developed 3D earth structure model of the basin as an input to simulate low-frequency ground motion (0–0.5 Hz). These ground motions are further combined with high-frequency scattering waveforms by using a hybrid approach, thus yielding broadband ground motions (0–10 Hz). We calibrate the 3D model and scattering parameters by comparing the simulated ground motions against available recorded data for two past earthquakes in Himalaya. Our approach accounts for the physics of interaction between the scattered seismic waves with deep basin sediments. Our results indicate that the ground motion intensities exhibit frequency-dependent amplification at various basin depths. We also observe that in the event of a great earthquake, the ground motion intensities are larger at deep basin sites near the source and exhibit an attenuating trend over distance similar to the ground motion models. The extreme ground motion simulations performed in our study reveal that the national building codes may not provide safe recommendations at deep basin sites, especially in the near field region. The period-dependent vertical-to-horizontal spectral ratio deviates from the code-recommended constant 2/3 at least up to 6 s at these sites.

New Metallic Damper with Multiphase Behavior for Seismic Protection of Structures

This paper proposes a new metallic damper based on the plastic deformation of mild steel. It is intended to function as an energy dissipation device in structures subjected to severe or extreme earthquakes. The damper possesses a gap mechanism that prevents high-cycle fatigue damage under wind loads. Furthermore, subjected to large deformations, the damper presents a reserve of strength and energy dissipation capacity that can be mobilized in the event of extreme ground motions. An extensive experimental investigation was conducted, including static cyclic tests of the damper isolated from the structure, and dynamic shake-table tests of the dampers installed in a reinforced concrete structure. Four phases are distinguished in the response. Based on the results of the tests, a hysteretic model for predicting the force-displacement curve of the damper under arbitrary cyclic loadings is presented. The model accurately captures the increment of stiffness and strength under very large deformations. The ultimate energy dissipation capacity of the damper is found to differ depending on the phase in which it fails, and new equations are proposed for its prediction. It is concluded that the damper has a stable hysteretic response, and that the cyclic behavior, the ultimate energy dissipation capacity and failure are highly predictable with a relatively simple numerical model.

Topology optimization of buildings subjected to stochastic base excitation

In seismically active regions, buildings are inevitably exposed to extreme ground motions. Traditionally, the main structural system is designed iteratively to resist these loads, which provides safe systems, but is usually suboptimal. Topology optimization provides an approach to obtain optimal material layout; however, most approaches only accommodate deterministic loads. Moreover, typical structural design goals require minimization of the maximum of some set of responses; such a goal is typically non-smooth, which impairs the use of efficient gradient-based optimizers. This study models the stochastic ground excitation as a zero-mean filtered white noise and combined with the model of the structure to form an augmented system. The structural response stationary covariances are obtained by solving a corresponding Lyapunov equation. The optimization problem is formulated to minimize the maximum structural response covariances, employing equivalent smooth formulations. Dynamic condensation is also employed to increase the efficiency. Sensitivities are computed by solving an adjoint Lyapunov equation, allowing for a gradient-based solver to be used. This study implements the following building features: additional discrete floor masses, boundary elements, and floor diaphragms. The proposed strategy is illustrated for seismically excited buildings with different properties. The results presented herein demonstrate the efficacy of this approach for efficient topology optimization of buildings subjected to stochastic ground motion.

Earthquakes Effect on the Behavior of Multi-Story RC Building Using Non-Linear Dynamic Analysis

One of the buildings that were designed depending on the old Syrian Code is selected and modeled using SAP2000 by taking its non-linear properties to be checked under dynamical loading. Three earthquake records are taken and applied to the model as Time History loading cases. The dynamical displacement of the top roof of the building and the hysterical diagrams of the relation between base shear and roof displacement is compared and discussed. Asymmetry of shear walls and cores is responsible of the differences in responses of building elements, and insufficient nonlinear modeling of shear walls prevents from finding the real capacity of the system, although comparing pushover curves with hysteric loops from the applied ground motion excitations shows that the building is capable, depending on its old design, to withstand various types of extreme ground motions and earthquakes.

Comparative risk-based seismic assessment of 1970s vs modern tall steel moment frames

This study benchmarks the performance of older existing tall steel moment resisting frame buildings designed following historic code-prescriptive requirements (1973 Uniform Building Code) against modern design standards (2015 International Building Code). The comparison is based on seismic risk assessments of alternative designs of a 50-story archetype office building, located at a site in San Francisco, CA. The mean annual frequency collapse risk of the 1973 building is 28 times greater than the equivalent 2015 building (28 × 10−4 versus 1 × 10−4), or approximately 13% versus 0.5% probability of collapse in 50 years. The average annual economic loss (based on cost of repair) is 65% higher for the 1973 as compared to the 2015 building (0.66% versus 0.40% of building replacement cost). The average annual downtime to re-occupancy for the 1973 building is 72% longer (8.1 vs 4.7 days) and to functional recovery is about 100% longer (10.4 vs 5.0 days). Building performance evaluations at the design basis earthquake (DBE) and the maximum considered earthquake (MCE) shaking intensities further suggest that 1970s tall steel moment frames have much higher risks of collapse under extreme ground motions and risks of damage and building closure in moderate earthquakes. Furthermore, while modern building code requirements provide acceptable seismic collapse safety, they do not necessarily ensure a level of damage control to assure a swift recovery after a damaging earthquake due to extensive downtime. A set of vulnerability functions are proposed for both archetype buildings considered in the assessment.

Peak ground motions and characteristics of nonlinear site response during the 2018 Mw 6.6 Hokkaido eastern Iburi earthquake

The observed peak ground accelerations and peak ground velocities (PGVs) of the 2018 Mw 6.6 Hokkaido eastern Iburi earthquake generally followed the median values from ground motion prediction equations with reasonable errors at fault distances ≥ 50 km. However, at smaller distances, the equations significantly underestimated the peak ground motions, and it was eminent for PGVs. A comparison of surface-to-borehole spectral ratios of S-waves during the mainshock and other events revealed that the sites at smaller distances experienced various degrees of nonlinear site response. The two most widely known characteristics of nonlinear site response are the weakening of higher-frequency components and shifting of predominant frequencies to lower ones in comparison with the linear site response. At one of the sites that recorded the largest intensity of 7 in JMA scale of 0–7, the latter nonlinear effect was so dominant that the ground motions around the new predominant frequency got intensified by one order of magnitude in comparison with that during the weak-motions. Two sites, which were closely located, recorded vertical peak ground accelerations exceeding 1 g for the up-going motions. The recordings showed asymmetric waveforms and amplitudes characteristics of the nonlinear site response in extreme vertical ground motions recorded during a few earthquakes in the past. Few sites having lower vertical peak ground accelerations were also suspected of being experienced nonlinear site response on vertical motions. These findings suggest taking a cautious approach to enumerate the reduction in amplification at higher frequencies using the single-station horizontal-to-vertical (H-to-V) spectral ratio technique. However, we found that the H-to-V technique was still useful to detect nonlinearity. Finally, an ad hoc equation was derived to correct the nonlinear site amplification in predicting horizontal PGVs with respect to one of the most widely used attenuation models in Japan. The results indicated that the effect was much stronger for a larger input motion than that for a proportional change in the Vs30 values.

Fragility Assessments of Multi-Story Piping Systems within a Seismically Isolated Low-Rise Building

A successful, advanced safety design method for building and piping structures is related to its functionality and sustainability in beyond-design-basis events such as extremely strong ground motions. This study develops analytical models of seismically isolated building-piping systems in which multi-story piping systems are installed in non-isolated and base-isolated, low-rise buildings. To achieve the sustainable design of a multi-story piping system subjected to strong ground motions, Triple Friction Pendulum (TFP) elements, specifically TFP bearings, were incorporated into the latter building structure. Then, a seismic fragility analysis was performed in consideration of the uncertainty of the seismic ground motions, and the piping fragilities for the seismically non-isolated and the base-isolated building models were quantified. Here, the failure probability of the piping system in the non-isolated building was greater than that in the seismically isolated building. The seismic isolation design of the building improved the sustainability and functionality of the piping system by significantly reducing the seismic energy of extreme ground motions which was input to the building structure itself.

Applying modified discontinuous deformation analysis to assess the dynamic response of sites containing discontinuities

The accurate consideration of seismic wave propagation through discontinuous media is crucial in rock engineering. Discontinuous deformation analysis (DDA), with the ability to study discontinuity behaviors, is modified by incorporating a seismic input method based on a viscous boundary and the free-field theory. After confirming the accuracy of the modified DDA using several verification examples, two practical applications, (1) the extreme ground motion during the 2008 Iwate–Miyagi earthquake, Japan, and (2) the Donghekou landslide induced by the 2008 Wenchuan earthquake, China, are simulated and reproduced by the modified DDA. The results show that seismic wave propagation through discontinuous media can be accurately simulated by the modified DDA. Further, the simulation results indicate that discontinuities are critical in the dynamic response of structures. Conclusively, the modified DDA provides an alternative approach for analyzing the dynamic response of sites containing discontinuities.

An operational-oriented approach to the assessment of low probability seismic ground motions for critical infrastructures

Extreme natural hazard events have the potential to cause significant disruption to critical infrastructure (CI) networks. Among them, earthquakes represent a major threat as sudden-onset events with limited, if any, capability of forecast, and high damage potential. In recent years, the increased exposure of interdependent systems has heightened concern, motivating the need for a framework for the management of these increased hazards. The seismic performance level and resilience of existing non-nuclear CIs can be analyzed by identifying the ground motion input values leading to failure of selected key elements. Main interest focuses on the ground motions exceeding the original design values, which should correspond to low probability occurrence. A seismic hazard methodology has been specifically developed to consider low-probability ground motions affecting elongated CI networks. The approach is based on Monte Carlo simulation, which allows for building long-duration synthetic earthquake catalogs to derive low-probability amplitudes. This approach does not affect the mean hazard values and allows obtaining a representation of maximum amplitudes that follow a general extreme-value distribution. This facilitates the analysis of the occurrence of extremes, i.e., very low probability of exceedance from unlikely combinations, for the development of, e.g., stress tests, among other applications. Following this methodology, extreme ground-motion scenarios have been developed for selected combinations of modeling inputs including seismic activity models (source model and magnitude-recurrence relationship), ground motion prediction equations (GMPE), hazard levels, and fractiles of extreme ground motion. The different results provide an overview of the effects of different hazard modeling inputs on the generated extreme motion hazard scenarios. This approach to seismic hazard is at the core of the risk analysis procedure developed and applied to European CI transport networks within the framework of the European-funded INFRARISK project. Such an operational seismic hazard framework can be used to provide insight in a timely manner to make informed risk management or regulating further decisions on the required level of detail or on the adoption of measures, the cost of which can be balanced against the benefits of the measures in question.

Explaining extreme ground motion in Osaka basin during the 2011 Tohoku earthquake

AbstractDespite being 770 km away from the epicenter, observed ground motions due to the Tohoku earthquake in the Osaka Basin were unexpectedly large, with an amplification of more than a factor of 20 compared to immediately outside the basin, and including 2.7 m peak‐to‐peak roof displacements at one high‐rise building. The local ground motions exceeded expectations based on standard computations of site response by a factor of 3, predicted frequencies of peak acceleration were off by at least 50%, and such discrepancies have not yet been explained quantitatively. Here we show that utilizing semianalytic theory for surface‐wave amplification, we are able to accurately predict both the amplitudes and frequencies of large ground amplification in the Osaka Basin using only knowledge of the local one‐dimensional structure. Comparison between this simple prediction and observed amplification was not expected to be so favorable and suggests that simple one‐dimensional surface‐wave site amplification factors can be useful in the absence of full three‐dimensional wave propagation simulations. Such surface‐wave amplification factors can be included in addition to the standard measures of site‐specific site amplification and should help explain strong ground motion variability in future large earthquakes that shake Osaka Basin and elsewhere in the world.

Simulation of Near-Fault High-Frequency Ground Motions from the Representation Theorem

“What is the maximum possible ground motion near an earthquake fault?” is an outstanding question of practical significance in earthquake seismology. In establishing a possible theoretical cap on extreme ground motions, the representation integral of elasticity, providing an exact, within limits of applicability, solution for fault radiation at any frequency, is an under-utilized tool. The application of a numerical procedure leading to synthetic ground displacement, velocity, and acceleration time histories to modeling of the record at the Lucerne Valley hard-rock station, uniquely located at 1.1 km from the rupture of the M w 7.2 Landers, California event, using a seismologically constrained temporal form of slip on the fault, reveals that the shape of the displacement waveform can be modeled closely, given the simplicity of the theoretical model. High precision in the double integration, as well as carefully designed smoothing and filtering, are necessary to suppress the numerical noise in the high-frequency (velocity and acceleration) synthetic motions. The precision of the integration of at least eight decimal digits ensures the numerical error in the displacement waveforms generally much lower than 0.005% and reduces the error in the peak velocities and accelerations to the levels acceptable to make the representation theorem a reliable tool in the practical evaluation of the magnitude of maximum possible ground motions in a wide-frequency range of engineering interest.

Rupture processes of the 2016 Kumamoto earthquake sequence: Causes for extreme ground motions

AbstractWe performed joint inversions of strong motion, teleseismic, and geodetic data to investigate the rupture processes of three notable (Mw ≥ 6.0) events of the 2016 Kumamoto earthquake sequence. Multisegment fault models for the three events were constructed based on focal mechanisms, hypocenter distributions of this sequence, and active faults, as well as geodetic features. The results reveal the spatial relationships between the slip distributions of the three events over complex fault planes. In the largest event, the rupture primarily propagated to a northeastern region along the Futagawa fault zone. The extreme pulse‐like waveforms observed at the near‐fault stations during the largest event can be attributed to the event's upward rupture directivity, slip rate, and the nearly simultaneous slip of two subparallel fault planes.

Modeling on collapse behaviour of high-rise concrete-filled steel composite frames under over-design seismic excitations

This paper presents a numerical study on seismic collapse capacity of high-rise concrete-filled steel tubular moment resisting frames (CFT-MRFs) under extreme ground motions that are beyond the design levels. The ground motions characterised as flat velocity spectral shape were selected to neglect record-to-record uncertainty induced by the changing of the natural periods of structures. A simple numerical approach was developed using fibre element with stiffness and strength degradation accounted in stress–strain models. Then, incremental dynamic analyses with various parameters were performed to evaluate the P-Delta and degradation effects on local and global collapse mechanisms. Results have shown that a drift concentration at the lower stories triggered the side-sway collapse mode that was dominated by the post-buckling strength degradation of CFT columns. The height of drift concentration indicates the range where the collapse mechanism was formed, and a drift concentration factor was proposed to estimate the extent of drift concentration. Last, a factor indicating the safety margin under collapse was computed based on the probability of earthquake occurrence.