Near-source Ground Motions Research Articles

Stress Patterns in Single Deck Floating Roofs Subjected to Ground Motion Accelerations

This paper investigates the induced stresses in circular single deck roofs floating on seismically excited storage tanks. Equations of motion are derived using variational principle. Response of deck floating roofs is evaluated for two different classes of ground motions; near-source and long-period far-field records. Besides time histories and frequency contents for a specific tank, peak value diagrams of stress for tanks with different radii are illustrated. Results indicate two critical locations in the deck roofs: one near the center of the roof and the other along the deck-pontoon interface. It is shown that near-source ground motions produce larger stresses at the inner critical radius of the deck but far-field ground motions lead to larger stresses in deck-pontoon interface. The results could have practical implications in the design process of floating roofed cylindrical tanks.

Empirical correlations between cumulative absolute velocity and spectral accelerations from NGA ground motion database

Considering multiple ground motion intensity measures is important in seismic hazard analysis and ground motion selection process. Using the NGA strong motion database and recently developed ground-motion prediction models, empirical correlations are developed between cumulative absolute velocity (CAV) and spectral accelerations (Sa) at periods from 0.01 to 10s. The CAV–Sa correlations at long periods are significantly influenced by rupture distance due to modification of the frequency content and duration of the acceleration time history through travel path. Similarly, the presence of strong velocity pulses in near-source ground motions also affects the correlations at moderate to long periods. On the other hand, the correlations are not particularly sensitive to the earthquake magnitude, orientation of the ground-motion recordings, selection of ground-motion prediction models and local site conditions. Piecewise linear fitting equations are provided to quantify the correlations for various cases. The application of the CAV–Sa correlations in ground motion selection process is also discussed.

Strong ground motion characteristics observed in the 4 September 2010 Darfield, New Zealand earthquake

This manuscript provides a physically-orientated and engineering-focused assessment of the near-source ground motions from the Darfield earthquake that were recorded by 29 strong motion stations in Christchurch city and the surrounding Canterbury Plains. In discussion of the observed ground motions particular attention is given to: (i) source features such as the complexity of the rupture on multiple fault planes and forward directivity effects; (ii) the effects of the Canterbury Plains sedimentary basin on waveguide effects through the region, and basin-edge effects observed around the Port Hills; and (iii) the importance of local site response as evidenced by observations of large high frequency amplification and liquefaction. Additional context is provided by comparison of ground motion amplitudes with empirical prediction models and reconciling ‘outliers’ based on inferred physical mechanisms. The severity of the horizontal and vertical components of ground motion are also emphasised via comparisons with response spectra prescribed for routine seismic design.

Strong ground motion simulation of the 2003 Bam, Iran, earthquake using the empirical Green’s function method

The 2003 Bam, Iran, earthquake caused catastrophic damage to the city of Bam and neighboring villages. Given its magnitude (Mw) of 6.5, the damage was remarkably large. Large-amplitude ground motions were recorded at the Bam accelerograph station in the center of Bam city by the Building and Housing Research Center (BHRC) of Iran. We simulated the Bam earthquake acceleration records at three BHRC strong-motion stations—Bam, Abaraq, and Mohammad-Abad—by the empirical Green’s function method. Three aftershocks were used as empirical Green’s functions. The frequency range of the empirical Green’s function simulations was 0.5–10 Hz. The size of the strong motion generation area of the mainshock was estimated to be 11 km in length by 7 km in width. To estimate the parameters of the strong motion generation area, we used 1D and 2D velocity structures across the fault and a combined source model. The empirical Green’s function method using a combination of aftershocks produced a source model that reproduced ground motions with the best fit to the observed waveforms. This may be attributed to the existence of two distinct rupture mechanisms in the strong motion generation area. We found that the rupture starting point for which the simulated waveforms best fit the observed ones was near the center of the strong motion generation area, which reproduced near-source ground motions in a broadband frequency range. The estimated strong motion generation area could explain the observed damaging ground motion at the Bam station. This suggests that estimating the source characteristics of the Bam earthquake is very important in understanding the causes of the earthquake damage.

Seismic site effects at near-fault strong-motion stations along the Aterno River Valley during the Mw=6.3 2009 L'Aquila earthquake

An Mw=6.3 earthquake originating at a normal fault in proximity of the city of L'Aquila produced a significant amount of near-fault strong-motion data recorded by an array deployed in the upper Aterno River Valley, NW of L'Aquila. This set of data deserves attention since it is the first well documented earthquake occurring in a near-fault area which was recorded from a moderate magnitude earthquake on a normal fault. For most of the stations geotechnical conditions are ascertained by means of drilling and geophysical investigation. To provide insights on the relevance of site conditions on measured near-source ground motions, peak and spectral values of acceleration and velocity recordings at stiff soil sites are compared with those at “rock” reference sites. Events of magnitude ranging between 3.0 and 6.3 (main event) were used. The results show that peak and response spectra values of ground motion, for the horizontal component and to a lesser extent for the vertical one, are affected by soil profile characteristics, especially for the station located on alluvium at the centre of the valley. For this station, observed amplification ratios were compared with numerical ones computed from 1D site response analyses. Evidence is presented that 1D numerical modelling is not able to completely explain the recorded motions.

Performance-based semi-active control algorithm for protecting base isolated buildings from near-fault earthquakes

Base isolated structures have been found to be at risk in near-fault regions as a result of long period pulses that may exist in near-source ground motions. Various control strategies, including passive, active and semi-active control systems, have been investigated to overcome this problem. This study focuses on the development of a semi-active control algorithm based on several performance levels anticipated from an isolated building during different levels of ground shaking corresponding to various earthquake hazard levels. The proposed performance-based algorithm is based on a modified version of the well-known semi-active skyhook control algorithm. The proposed control algorithm changes the control gain depending on the level of shaking imposed on the structure. The proposed control system has been evaluated using a series of analyses performed on a base isolated benchmark building subjected to seven pairs of scaled ground motion records. Simulation results show that the newly proposed algorithm is effective in improving the structural and nonstructural performance of the building for selected earthquakes.

Analysis of Seismic Responses During Near-Source Ground Motions

Seismic responses can strongly determine damages of bridge structures, e.g. due to pounding and unseating of girders as observed in many major earthquakes in the past. In the investigations often uniform ground excitation and pounding at only one location are considered. Studies on the influence of non-uniform ground excitation on bridge responses are still very limited, and they are restricted to flat ground-surface condition. Influence of multi-sided pounding is also not much investigated. If the impediment effect of abutments is considered at all. The study reveals that commonly assumed girder stiffness for the end restraint overestimates the actual effective end-restraint stiffness. Non-linear elastomeric bearings can significantly reduce the adjacent girder relative displacements. A consideration of non-uniform excitation is essential for a realistic estimation of bridge response and damage potential.

Common Observations for Near-Source Ground Motions and Seismo-Traveling Ionosphere Disturbances Following the 2011 off the Pacific Coast of Tohoku Earthquake, Japan

The time history and spatial dependence of seismic-wave propagation on the ground surface and through the ionosphere following the 2011 off the Pacific coast of Tohoku Earthquake were reconstructed from dense seismic networks and from Global Positioning System (GPS) array observations, respectively. Using total electron content (TEC) data recorded by a dense GPS receiver network, the near-source ionosphere perturbations induced by this giant earthquake were analyzed and high-resolution images of seismic-wave propagation in the ionosphere are presented. Similar spatial images of ground motions were reconstructed from observations by a dense seismic array. Observations of this event provide, for the first time, the opportunity to compare near-source ground motions with the near-field seismo-traveling ionosphere disturbance (STID) excited by the ground motions. Based on the results, the nature of the source rupture and seismic-wave propagation are discussed. Both seismic and ionosphere observations indicate that seismic energy propagated radially outward initially from the hypocenter, but that the circular shape of the propagation front became gradually distorted as the source rupture became extended. Coherent wavefronts from the two analyses show contrasting patterns during the later stage of propagation, possibly due to different patterns of spatial variations in the physical properties of the solid earth and of the ionosphere.

Reduction Measure for 3-D structure During Near-Source Ground Motions

In this study an approach for reducing the influence of the strong long-period pulses as well as the effect of the simultaneous ground excitation on a three-dimensional (3D) frame structure is presented. The considered ground excitation is the 1995 Kobe earthquake and the 1999 Turkey earthquake. The investigation reveals that the common base isolators are not reducing the response of the structure in the case of the near-source earthquakes with long period impulses. Even they can amplify the response of the structure. By modifying the fundamental natural mode of the structure, we can reduce the effect of the strong pulses of the horizontal ground motion. The improvement of the dynamics of the structure is achieved with the combination of the lateral elastic spring and the viscous damper at the first story level of the structure and the base isolation. The proposed approach reduces not only the displacement but also the activated forces in the structural members.

Evaluation of Attenuation Models for the Northeastern United States/Southeastern Canada

This study uses a database of 800 km to evaluate published attenuation models for the region. The models include functional forms that are linear (Atkinson 1989; Atkinson and Mereu 1992), bilinear (Street and Turcotte 1975; Atkinson and Mereu 1992; Boatwright and Seekins 2011) and trilinear (Atkinson and Boore 1995; Atkinson 2004) in shape. Each model is evaluated by examining residual distance trends in ground-motion data after correction of the data for the attenuation effects according to that model. None of the models provide a satisfactory description of attenuation over all distance ranges. A simple 1/ R geometric spreading model at all distances and frequencies (with an associated Q ∼ 2,000) does as well as any model in providing (on average) low bias at near-source distances while being pegged by the amplitude levels observed at regional distances—even though more complex models provide a statistically better fit to the attenuation shape overall. We need to rethink the concepts used in deriving attenuation models for the development of ground-motion prediction equations such that the model shapes are not driven so strongly by regional observations. Any attenuation model that is not significantly constrained by data at hypocentral distances from 10 to 50 km is unlikely to provide reliable estimates of near-source ground motions or source parameters.

Seismic Isolation with No Strain Energy – Research on New Seismic Isolation System with No Resonance Characteristics –

Since the safety of seismically-isolated buildings during earthquakes depends mainly on the largedeformation stability of the isolation devices, it is necessary to either provide large isolator deformation capacity or to reduce seismic response deformation to ensure enhanced building safety. From the viewpoint of earthquake demand, it must be recognized that, the isolator response deformation may exceed the allowable capacity when the isolation system resonates with strong, near-source earthquake ground motions. This paper first establishes the problem of the damping capacity of conventional isolation systems that a system with strong restoration spring causes large elastic strain energy to accumulate in largely deformed isolators. Then, a new hysteresis behavior is proposed to reduce the strain energy developed in isolators. Based on studies of the fundamental characteristics of the proposed hysteresis behavior, this paper proposes a new isolation system concept called “Seismic Isolation with No Strain Energy (NSE)” which does not result in resonance because it eliminates the strain energy stored in deformed isolators, even if the period of the isolation system coincides with predominant period of the input ground motions. The superior performance of NSE Seismic Isolation is confirmed by the results of dynamic response analyses for strong, near-source earthquake ground motions.

Development of the EDR-Spring Element Using Foamed Polymer Materials and the Design of NSE-Isolated Buildings Using EDR-Spring Elements

Seismic Isolation can provide superior building safety during strong earthquakes, but it is known that some near-source ground motions produce excessive response deformations, and these may be larger than the allowable capacity of the isolation devices. “Seismic Isolation with No Strain Energy” (NSE), a new isolation concept, has been proposed as a solution to the problem. By eliminating the strain energy stored in deformed isolators, the resulting resonance problem can be completely resolved. The goal of the paper is to successfully produce an NSE spring element which has the restoring characteristics of the EDR-spring model for NSE-isolated buildings. The paper tested 15 foamed polymer materials and selected the most suitable one for the EDR-spring element. It has an elimination ratio of stored strain energy of ß≥0.7. The design loop for the spring using the selected material was established, and two model buildings of 5 stories and 15 stories were designed using the EDR-spring and sliders supporting the buildings’ weight. The designed NSE-isolated buildings’ superior dynamic performance against very strong earthquakes, even in cases in which input ground motions have the same predominant period as the isolated structures, was confirmed by dynamic response analyses.

Near-source Strong Ground Motions Observed in the 22 February 2011 Christchurch Earthquake

On 22 February 2011 at 12:51 p.m. local time, a moment magnitude Mw 6.3 earthquake occurred beneath the city of Christchurch, New Zealand, causing an level of damage and human casualties unparalleled in the country's history. Compared to the preceding 4 September 2010 Mw 7.1 Darfield earthquake, which occurred approximately 30 km to the west of Christchurch, the close proximity of the 22 February event led to ground motions of significantly higher amplitude in the densely populated regions of Christchurch. As a result of these significantly larger ground motions, structures in general, and commercial structures in the central business district in particular, were subjected to severe seismic demands and, combined with the event timing, structural collapses accounted for the majority of the 181 casualties (New Zealand Police 2011). This manuscript provides a preliminary assessment of the near-source ground motions recorded in the Christchurch region. Particular attention is given to the observed spatial distribution of ground motions, which is interpreted based on source, path, and site effects. Comparison is also made of the observed ground motion response spectra with those of the 4 September 2010 Darfield earthquake and those used in seismic design in order to emphasize the amplitude of the ground shaking and also elucidate the importance of local geotechnical and deep geologic structure on surface ground motions. New Zealand resides on the boundary of the Pacific and Australian plates (Figure 1) and its active tectonics are dominated by: 1) oblique subduction of the Pacific plate beneath the Australian plate along the Hikurangi trough in the North Island; 2) oblique subduction of the Australian plate beneath the Pacific plate along the Puysegur trench in the southwest of the South Island; and 3) oblique, right-lateral slip along numerous crustal faults in the axial tectonic belt, of which the …

Assessment of Reduction Measure for Seismicresponse of Bridge Decks during near-Source Earthquakes

During the strong near-source ground motions, adjacent bridge decks often collide each other because of the insufficient separations. Pounding can cause severe bridge damage, or even result in unseating if the seating length is not enough. In this study, the bridge structure is modeled by a finite element method. The subsoil is represented by steel springs and viscous dampers. The considered reduction measure consists of steel springs with additional friction element. The nonlinear analysis is performed in the time domain. The considered earthquakes are the 1994 Northridge earthquake and the 1995 Kobe earthquake. The reduction of the relative displacement and pounding forces of the two bridge decks is investigated. The investigations reveal that the proposed reduction measure can be used to reduce the seismic responses between two bridge decks with their neighboring abutments.

A Study on the Response Instability of Seismically Isolated Structures Affected by Ground Inclination During Earthquakes Part 1 : Estimation of Ground Inclination During Earthquakes and the Influence of Static Ground Inclination

Seismic isolation can provide superior building safety and dynamic performance during strong earthquakes, however, it is known that some near-source ground motions produce excessive response deformations, which may be larger than the allowable capacity of the isolation devices. Isolation systems with longer periods and higher damping are more capable of resisting such deformations, and the author proposed new isolation systems with periods of 10 sec, or even longer. Longer isolation periods, however, mean less resistance and restoring force in the isolation systems, which may cause concerns about unstable response characteristics during strong earthquakes. One such unstable behavior is the possibility of excessive horizontal displacements of the isolation system resulting from ground inclination. Based on numerical analyses using existing vertical ground motion records, this paper estimates the ground inclination during earthquakes and studies the anticipated horizontal deformation of isolated structures that might be induced by such ground inclination.

Empirical equations for the prediction of displacement spectrum intensity and its correlation with other intensity measures

Displacement spectrum intensity ( DSI), defined as the integral of a ground motion's displacement response spectrum from 2.0 to 5.0 s, is proposed as an indicator of the severity of the long period content of a ground motion. It is demonstrated how the distribution of DSI can be predicted using existing ground motion prediction equations for (pseudo) spectral accelerations, which is necessary for it to be a useful intensity measure (IM) in either probabilistic or deterministic seismic hazard analysis. Empirical correlation equations between DSI and other common ground motion IMs are developed for active shallow crustal earthquakes using a dataset of ground motions from active shallow crustal earthquakes. The ability of DSI to account for near-source ground motions exhibiting forward directivity, potentially damaging far-source long-period ground motion, and its use with other spectrum intensity parameters to characterise short, medium, and long period severity of ground motions is discussed. The developed ground motion prediction and correlation equations enable DSI to be utilised in rigorous ground motion selection frameworks such as the generalised conditional intensity measure (GCIM) approach.

Comment on "Statistical Features of Short-Period and Long-Period Near-Source Ground Motions" by Masumi Yamada, Anna H. Olsen, and Thomas H. Heaton

Yamada et al. (2009) collected recorded ground motions from the near-source region of large earthquakes that occurred before 2005 and investigated to what extent this historical record could provide a basis for estimating future ground motions at similar sites. Their main conclusion, which they refer to as a paradox, is that the largest peak ground acceleration (PGA) observed in the past can be considered representative of future ground motions with the largest PGA (even though at present earth scientists cannot reliably simulate PGA values by means of numerical models). In contrast to this, the largest peak ground displacement (PGD) observed in the past may not be representative of the future largest PGD (even though this is easier to estimate through numerical simulations). The previous statement, supported by Yamada et al. (2009) with theoretical considerations based on the physics of earthquake sources, stems from the widely recognized weak (if any) dependence of PGA on magnitude and from the still-debated saturation of PGA in the near field of large earthquakes. This is generally mirrored into the functional forms of many state-of-the-art ground-motion-prediction equations (GMPEs), see e.g., Abrahamson et al. (2008) in the special issue of Earthquake Spectra devoted to the Next Generation Attenuation (NGA) Project. The same does not occur for PGD values that, being strongly correlated to magnitude, would tend to follow a uniform probability distribution for ![Graphic][1] , so that, quoting Yamada et al. (2009), the estimation of the largest PGD from future earthquakes is not clear. We first remark here that a statistical analysis of near-source ground motions would have at least required (1) a homogeneous distribution of records, taking into account that about 2/3 of those used in Yamada et al. (2009) come from only two earthquakes and, as acknowledged by the authors themselves, some ranges of magnitudes are not represented at all; (2) a … [1]: /embed/inline-graphic-1.gif

Influence of Super-Shear Earthquake Rupture Models on Simulated Near-Source Ground Motion from the 1999 Izmit, Turkey, Earthquake

We numerically simulate seismic wave propagation from the 1999 Mw 7.4 earthquake in Izmit, Turkey, using a 3D finite difference method based on published finite-source models obtained by waveform inversions. This earthquake has been reported, based on observations at the near-fault station SKR, as an example of supershear rupture propagation toward the east. Although the modeled ground motion does show a characteristic Mach wave from the fault plane, it is difficult to identify any particular effects in terms of peak ground velocity (PGV), an important parameter in earthquake engineering. This is because the fault's spatial heterogeneity is strong enough to mask the properties of supershear rupture, which has been reported through several numerical simulations mostly based on homogeneous fault conditions. This article demonstrates the importance of studying ground motions for known earthquakes through numerical simulations based on finite-fault source models.

Reply to "Comment on 'Statistical Features of Short-Period and Long-Period Near-Source Ground Motions' by Masumi Yamada, Anna H. Olsen, and Thomas H. Heaton" by Roberto Paolucci, Carlo Cauzzi, Ezio Faccioli, Marco Stupazzini, and Manuela Villani

The comment by Paolucci and colleagues (Paolucci et al., 2011) states that a probabilistic seismic hazard analysis (PSHA) can provide reliable prediction of long-period spectral ordinates. The result of such an analysis would be in contrast to the more uncertain prediction suggested by our empirical, and proposed theoretical, distribution of near-source ground displacements in past, large magnitude earthquakes (Yamada et al., 2009). After addressing two specific concerns of Paolucci and colleagues, we use the balance of this reply to discuss the apparent differences between a PSHA and our observations. These two approaches to understanding the seismic hazard of long-period ground motions should be consistent even though they view the problem from different perspectives.

Selection and quantification of near-fault velocity pulses owing to source directivity

Design ground motions are typically governed by large earthquakes at close distances, but the increasing number of near-field recordings manifest the large variability in near-source ground-motion amplitudes which result in significant differences in the building response. In the near-field, this variability arises mainly from source-directivity effects, which generate strong near-fault velocity pulses. We investigate the effect of source complexity on the generation of velocity pulses using a geometrical approach to quantify directivity at near-fault sites based on kinematic rupture models. We propose selection algorithms that can be implemented as search strategies for directivity-related strong ground motion records which may serve as ground motion selection tools in large databases for structural analysis and liquefaction studies. We find that the existence of directivity pulses is strongly related to slip heterogeneity on the fault plane, i.e. that the location and size of asperities (large slip areas) determine directivity pulse generation. In this context we quantify several pulse properties, testing a variety of approaches, and develop predictive relationships between a number of source parameters and pulse properties. We find strong dependence of pulse period on total area of asperities, as well as on a geometrical directivity parameter. The empirical observations on velocity pulse generation determined by the proposed selection procedure are compared with the predictions using the geometrical directivity model. The results are important for determining the probability of observing a pulse at a site as a function of magnitude, distance, and slip heterogeneity on the fault plane.