Large Ground Motions Research Articles

Deterministic seismic hazard assessment for Sultanate of Oman

The Sultanate of Oman forms the southeastern part of the Arabian plate, which is surrounded by relatively high active tectonic zones. Studies of seismic risk assessment in Oman have been an important on-going socioeconomic concern. Using the results of the seismic hazard assessment to improve building design and construction is an effective way to reduce the seismic risk. In the current study, seismic hazard assessment for the Sultanate of Oman is performed through the deterministic approach with particular attention on the uncertainty analysis applying a recently developed method. The input data set contains a defined seismotectonic model consisting of 26 seismic zones, maximum magnitudes, and 6 alternative ground motion prediction equations that were used in four different tectonic environments: obduction zone earthquake (Zagros fold thrust belt), subduction zone earthquakes (Makran subduction zones), normal and strike-slip transform earthquakes (Owen and Gulf of Aden zones), and stable craton seismicity (Arabian stable craton). This input data set yielded a total of 76 scenarios at each point of interest. A 10 % probability that any of the 76 scenarios may exceed the largest median ground acceleration is selected. The deterministic seismic hazards in terms of PGA, 5 % damped spectral acceleration at 0.1, 0.2, 1.0 and 2.0 s are performed at 254 selected points. The ground motion was calculated at the 50th and 84th percentile levels for selected probability of exceeding the median value. The largest ground motion in the Sultanate of Oman is observed in the northeastern part of the country.

The seismic retrofit of UC Berkeley's California Memorial Stadium

SUMMARYThis paper addresses the seismic retrofit of the University of California at Berkeley's California Memorial Stadium (CMS) to accommodate surface fault rupture movements as well as intense seismic shaking from the Hayward fault. For the surface fault rupture aspect of the project, the project team took a multidisciplinary approach that involved structural engineers, geotechnical engineers, seismologists and geologists collaborating to establish design criteria and possible surface rupture scenarios. The mechanics of surface fault rupture are discussed relative to the structural design approach that mitigates the effects of surface deformations caused by the fault during an earthquake. The effects of the surface fault rupture were studied using small scale physical models and nonlinear finite element models of the building and rupturing ground surface. The retrofit scheme accommodates the concentrated ground deformations of an estimated 6 ft of horizontal movement and 2 ft of vertical offset caused by a surface fault rupture. Segments of the CMS that overlay the regions of possible surface fault rupture were decoupled from the main structure via movement gaps to accommodate relative displacements of the building segments, including tilting in the event of earthquake‐produced fault rupture. The results of the analyses are discussed along with the design decisions and challenges of retrofitting a historic structure that resides on an active fault. While a large portion of the engineering effort was devoted to solving the issue of accommodating surface rupture, another large engineering challenge also dominated the design of the CMS. One of the signature architectural features of the stadium is a two‐story, 375‐ft long press box that hovers above the new west side of the seating bowl. Although designing the press box on limited supports to give the appearance of ‘hovering’ was a challenge, the real challenge came in safely bracing this structure for large potential ground motions at this site. The interface between the flexible support of the press box and the rigid seating bowl became a challenge, solved by providing a separation between the bowl and press box structures allowing them to move independently while linking them only with fluid viscous dampers. Copyright © 2012 John Wiley & Sons, Ltd.

Effects of rotation motions on strong-motion data

Rotation motion and its effects on strong-motion data, in most cases, are much smaller than that of translational motion and have been ignored in most analyses of strong-motion data. However, recent observations from near-fault and/or extreme large ground motions suggest that these effects might be underestimated and quantitative analyses seem to be necessary for improving our understating of these effects. Rotation motion-related effects include centrifugal acceleration, the effects of gravity and effects of the rotation frame. Detailed analyses of these effects based on the observed data are unavailable in the literature. In this study, we develop a numerical algorithm for estimating the effects of rotational motion on the strong-motion data using a set of six-component ground motions and apply it to a set of rotation rate-strong motion velocity data. The data were recorded during a magnitude 6.9 earthquake. The peak value of the derived acceleration and rotation rate of this dataset are about 186 cm/s/s and 0.0026 rad/s. Numerical analyses of data gives time histories of these rotational motion-related effects. Our results show that all the rotation angles are less than 0.01°. The maximum centrifugal acceleration, effect from gravity and effect of the rotation frame are about 0.03 and 0.14 cm/s/s, respectively. Both these two effects are much smaller than the peak acceleration 186 cm/s/s. This result might have been expected because our data are not near-field and wave motions are expected to be nearly plane waves. However, it is worth noticing that the centrifugal acceleration is underestimated and a small rotational effect can cause large waveform difference in acceleration data. The waveform difference before and after the correction for rotational motion can reach 16 cm/s/s (about 10 %).

Strong Motion Characteristics of the 2011 Tohoku-Oki Earthquake

The 2011 M 9.0 Tohoku-Oki earthquake is the largest earthquake that occurred in and around Japan since the beginning of the recorded history. This megathrust event initiated approximately 100km off-shore Miyagi prefecture, in northeast Japan, and its rupture extended 400-500km along the subducting Pacific plate. This is the first M 9-class earthquake that has been closely recorded by a dense seismograph network. Strong motions of this earthquake are characterized by large seismic intensities and peak ground accelerations (PGA), long durations, and wideness of the area that experienced intense shaking. The ground motions were recorded by 1223 K-NET and KiK-net stations. The PGA exceeded gravity at 20 sites; the largest PGA, of 2933 gals, was observed at the K-NET Tsukidate station (MYG004). The attenuation of the recorded peak values shows a possibility of saturating strong ground motion amplitude with the magnitude. The complex features of the accelerograms and velocity waveforms are discussed in connection with the source processes estimated using long and short period waveform data. Due to the large ground motions and tsunamis associated with this event, more than 16 thousand people were killed and more than 360 thousand houses and buildings were totally or partially destroyed. Although the tsunamis were the primary cause of damage, the strong shaking, liquefaction and landslides also brought serious destruction. However, it was reported that the damage ratios of houses and buildings directly due to shaking were not as high as for the former earthquakes having comparable seismic intensities and PGA. The recorded ground motions at most stations where the seismic intensities and PGA were large had dominant periods shorter than 0.5s and relatively poor power in the 1-2s period range which has strong influence on the damage of few-stories wooden houses. The main reason for the short-period predominance is the amplification due to the low-velocity superficial layer and can be roughly explained by empirical amplification factors for 0.1-0.5s periods. Long-period ground motions were also observed. The velocity response spectra (5% dumping) for periods of 4-20s reached around 100cm/s at many stations, mainly in the Tohoku and Kanto regions. This level may be considered not very large taking into account that the Tohoku-Oki earthquake was an M 9-class event. In the Kanto district, at epicentral distances of 300-400km, liquefaction widely occurred at the artificially reclaimed land in Tokyo and Chiba bay areas and the basin of major rivers, such as the Tone and Ara. The damage (e.g., cutoff of lifelines and differential settlement of house-basements) due to liquefaction was very severe. In this paper we summarize the strong motion characteristics associated with the M 9.0 Tohoku-Oki earthquake and review the latest results from the viewpoint of strong motions.

Large Apparent Stresses from the Canterbury Earthquakes of 2010 and 2011

An earthquake of Mw 6.1–6.31 (Beavan et al. 2011, page 789 of this issue) that struck Christchurch, New Zealand, on 22 February (21 February, UTC) produced recorded ground motion acceleration over 2 g. The event caused widespread damage with dense recordings of non-linear site behavior. Globally, dense near-field recordings of shallow intraplate earthquakes are rare. It is possible that extreme ground motions are common with this type of earthquake and that their rarity is merely a function of inadequate seismic sampling in the near field of such low-probability, high-potency events. To better define the nature of these events, we calculate apparent stress ( τa ) of the three largest earthquakes in the Canterbury sequence and compare them to global and regional data. We then place recorded PGA and spectral accelerations into the context of regional and global ground motion prediction equations and discuss the implications of high-stress events for future seismic hazard estimates for the region. For the February event, we also briefly explore the implications of directivity on measured ground motions in central Christchurch. The earthquakes that occurred in the Canterbury region of the South Island, New Zealand, from September 2010 to the present have disproportionately large energy magnitudes ( Me ) to their moment magnitudes ( Mw ). They have produced the largest ground motions ever measured in New Zealand. The sequence began with the Mw 7.1 earthquake that occurred about 40 km west of the city of Christchurch on 4 September 2010. The maximum recorded ground acceleration recorded during the event was over 1.25 g, which was experienced near the intersection of the triggering thrust on which the rupture began and the strike-slip Greendale fault that carried most of the moment in the earthquake (Gledhill et al. 2010). Peak ground accelerations (PGA) in the central business district of Christchurch averaged …

Verifying a Computational Method for Predicting Extreme Ground Motion

Large earthquakes strike infrequently and close-in recordings are uncommon. This situation makes it difficult to predict the ground motion very close to earthquake-generating faults, if the prediction is to be based on readily available observations. A solution might be to cover the Earth with seismic instruments so that one could rely on the data from previous events to predict future shaking. However, even in the case of complete seismic data coverage for hundreds of years, there would still be one type of earthquake that would be difficult to predict: those very rare earthquakes that produce very large ground motion.

Factors Influencing Maximum and Residual Deformations of SDOF Systems Subjected to Large Ground Motions

The maximum and residual deformations of structures subjected to strong ground motions are the most importance indexes, particularly under the performance-based design framework, thus understanding the influencing factors is of great importance to seismic design. In this study, single degree of freedom (SDOF) systems with varying structural properties are analyzed using a series of strong ground motions from FEM/SAC project. The influences of three structural parameters, i.e., yield force, second stiffness after yielding, and stiffness degradation, on the maximum and residual deformations are investigated based on the statistics of the analysis results. The analysis results suggest the follows: (1) larger yield forces lead to smaller residual and maximum deformations for short period structures, and they lead to smaller residual deformations but no necessarily smaller maximum deformation for intermediate and long period structures; (2) larger second stiffness lead to smaller residual and maximum deformations for short period structures, and they lead to smaller residual deformations but no necessarily smaller maximum deformation for intermediate and long period structures; (3) smaller stiffness degradation index leads to smaller maximum deformations but larger residual deformations.

Analysis of Vibration Control due to Strong Earthquakes

The paper proposes an aseismic hybrid control system to control the response of structures subjected to large ground motions caused by large magnitude earthquakes. The proposed hybrid control system consists of a base isolation system (laminated rubber bearings) connected to an active control system (a tuned mass damper and an actuator). The base isolation system is used to decouple the horizontal ground motions from the structure, whereas the active control system is used to protect the safety and integrity of the base isolation system. A 5-story benchmark building model is developed to study the effectiveness of the hybrid control system against different ground motions. It was found from the numerical results that rubber bearing system alone shows good performance and resists ground motion due to Hachinohe 1968, Kobe 1995, and Northridge 1994 earthquakes, but is somewhat unable to protect the model against El-Centro 1940 earthquake. After the installation of an active control system onto the rubber-isolated model, further improvements to earthquake resistance against these four earthquakes were observed, especially against the El-Centro earthquake. The merit of the hybrid control system lies in its capability to protect against different ground motions, with varying intensity and frequency content.

Considering Spatial Correlation in Mixed-Effects Regression and the Impact on Ground-Motion Models

Abstract Ground-motion models are commonly used in earthquake engineering to predict the probability distribution of the ground-motion intensity at a given site due to a particular earthquake event. These models are often built using regression on observed ground-motion intensities and are fitted using either the one-stage mixed-effects regression algorithm proposed by Abrahamson and Youngs (1992) or the two-stage algorithm of Joyner and Boore (1993). In their current forms, these algorithms ignore the spatial correlation between intraevent residuals. This paper emphasizes the theoretical importance of considering spatial correlation while fitting ground-motion models and proposes an extension to the Abrahamson and Youngs (1992) algorithm that allows the consideration of spatial correlation. By refitting the Campbell and Bozorgnia (2008) ground-motion model using the mixed-effects regression algorithm considering spatial correlation, it is apparent that the variance of the total residuals and the ground-motion model coefficients used for predicting the median ground-motion intensity are not significantly different from the published values even after the incorporation of spatial correlation. However, there is an increase in the variance of the intraevent residual and a significant decrease in the variance of the interevent residual. These changes have implications for risk assessments of spatially-distributed systems because a smaller interevent residual variance implies lesser likelihood of observing large ground-motion intensities at all sites in a region.

Evaluation of Evidence for Inhibition of Very Strong Ground Motions in the Abrahamson and Silva Next Generation Attenuation Ground-Motion Model

Abstract A modification to the Rhoades et al. (2008) procedure for evaluating inhibition of very strong ground motions in strong-motion models is developed that accounts for the correlation in the total residuals. The modified method treats the intraevent and interevent residuals separately. The alternative method is applied to the Abrahamson and Silva (2008) Next Generation Attenuation (NGA) ground-motion model and strong-motion data set. We find that there is no statistical basis to indicate that the Abrahamson and Silva (2008) NGA model overpredicts the occurrence of very large ground motions.

Simulating soil stiffness degradation in transient site response predictions

A series of nonlinear seismic ground response analyses are conducted to investigate the effect of soil degradation on the seismic response of typical sedimentary deposits in Southern California. For this purpose, three downhole array station sites at the Los Angeles basin are subjected to broadband synthetic ground motion excitations. A characteristic strain measure, defined as the averaged maximum shear strain in the top 30 m of the soil profile ( γ 30 ), is proposed to enable the correlation between the ground shaking intensity level (here measured by means of the peak ground acceleration on the surface of rock-outcrop, PGA RO ) and the average deviation in ground motion response spectral predictions (denoted as e SA ) attributed to the effect of modulus degradation due to cyclic loading. It is shown that strong linear correlations exist among log ( PGA RO ) , log ( γ 30 ) and log ( e SA ) for the sites investigated. On the other hand, the number of loading cycles typically employed to indicate the importance of soil degradation is shown to play a much less important role. This becomes more apparent for near-field large amplitude ground motions characterized by a dominant pulse and overall low number of cycles, which nonetheless impose strains large enough to trigger modulus degradation effects. Based on this observation, it is suggested that the soil degradation effect in seismic ground motion is primarily controlled by the ground shaking intensity. The level of PGA RO beyond which soil degradation should be accounted for and incorporated in seismic ground response analysis is also depicted for the sites investigated.

Microstructure and Mechanical Properties of Ultra-High Strength Steel Plates with High Deformability

High deformability has been considered as a critical factor of ultra-high strength steel plates subjected to compressive, tensile, and bending deformation induced by large ground movements. In this paper, various dual phase microstructures consisting of soft ferrite and strong low-temperature transformation phases without deformation in the (austenite + ferrite) two-phase temperature region after controlled rolling were introduced and then the mechanical properties were discussed with emphasis on deformability such as yield ratio and uniform elongation. Ultra-high strength steel plates fabricated by a modified thermo-mechanical control process showed lower yield ratio of under 0.75 and higher uniform elongation of 5% as a minimum, as compared to commercial API X100 and X120 grade pipeline steels, without much sacrifice of Charpy impact properties because of an appropriate formation of soft ferrite and strong low-temperature transformation phases.

Tactile Pressure Sensors for Soil-Structure Interaction Assessment

This paper provides an assessment of tactile pressure sensors for geotechnical applications. A tactile pressure sensor is an array of small sensing units, called sensels, embedded in a polymeric sheet or pad that measures the magnitude and distribution of stresses normal to the sheet surface. Methods for minimizing the effects of shear on sensor measurements are discussed and the efficacy of these methods are demonstrated by laboratory experiments. The time-dependent characteristics of the sensors are evaluated and recommendations are provided for measurements that account for time-dependent effects. Tactile pressure sensor measurements in response to vertical loading and unloading and to lateral loads on full-scale pipelines affected by large horizontal ground movements are compared with independent measurements of the loads. Sensor measurements are used to show the distribution of normal stress on pipelines subject to large lateral soil movement.

Effect of gravity columns on mitigation of drift concentration for braced frames

In strong earthquake events, multi-story concentrically braced steel frames (CBFs) are prone to form a story-collapse mechanism after buckling and yielding of the braces in a story. With the addition of continuous gravity columns and the increased lateral stiffness and strength they provide, however, it becomes possible to reduce the concentration of story drift and prevent the story collapse. Nonlinear dynamic analysis is implemented for a three-story CBF, and it is demonstrated that a drift concentration is inevitable, particularly in the first story, when the CBF is subjected to a set of large ground motions with a probability of exceedance of 2% in 50 years. A simplified theoretical formulation is presented to characterize the effect of gravity columns. Two cases are considered: gravity columns perfectly fixed, and gravity columns pinned at the base. It is noted that significant mitigation of drift concentration can be achieved, particularly when gravity columns are fixed at the base. Nonlinear time-history analysis is further performed to validate the theoretical observations and quantify the stiffness/strength demands of gravity columns to avoid drift concentration. The analysis results support the trends and observations obtained from the theoretical formulation. Finally, numerical simulations are implemented with the seismic force reduction factor D s , slenderness ratio of braces λ , and number of stories, as major analysis variables. The results demonstrate that a few (e.g. four) fixed-base gravity columns can lead to collapse prevention for reasonably designed low-rise CFBs under the set of large ground motions.

The Slapdown Phase in High-acceleration Records of Large Earthquakes

The 2008 Iwate-Miyagi Nairiku earthquake (M_w 6.9, M_(jma) 7.2) produced strong shaking throughout northern Honshu, Japan, with severe damage to buildings and extensive landslides. The shallow event occurred in southwestern Iwate Prefecture (39.03°N, 140.88°E, depth 8 km) on 13 June 2008 at 23:43:45 GMT (Japan Meteorological Agency 2008). This earthquake produced relatively high-frequency ground motions, which resulted in large values of peak ground acceleration (PGA). The surface accelerometer of the station IWTH25 of KiK-net, located 3 km southwest of the epicenter, produced one of the largest strong-motion values of PGA (4,278 cm/s^2 for the vector sum of the three components) ever recorded (http://www.kik.bosai.go.jp/kik/index_en.shtml). The new accelerometers installed in KiK-net last year have a recording range up to 4,000 cm/s^2, which made it possible to record such large ground motions near the source (http://www.kik.bosai.go.jp/kik/index_en.shtml). The sampling rate of the record of IWTH25 is 100 Hz (http://www.kik.bosai.go.jp/kik/index_en.shtml). The surface acceleration record at station IWTH25 shows an asymmetric amplification in the vertical components (Aoi et al. 2008). The upward vertical acceleration is much larger than the downward direction, although in the borehole record at a depth of 260 m at the same site, the upward and downward accelerations have symmetric amplitudes (Figure 1). On the other hand, the horizontal components do not show this asymmetric effect. This difference between the surface and borehole recordings for the vertical component implies a strong nonlinear amplification. In this paper, we will analyze these records and propose a mechanism to produce the large vertical accelerations. The predominance of large upward acceleration spikes is not unique to the Iwate-Miyagi Nairiku earthquake, so our proposed mechanism may be applicable to a number of large vertical acceleration records.

Strong ground motions from an Mj 6.1 inland crustal earthquake in Hokkaido, Japan: the 2004 Rumoi earthquake

AbstractAn inland crustal earthquake (Mj6.1) occurred on December 14, 2004 in the northern part of Hokkaido, Japan. A large ground acceleration of 1127 cm/s2and a large pseudo-velocity response of over 100 cm/s were recorded at the nearest strong-motion station, HKD020, about 10 km from the hypocenter. This large ground motion is considered to be attributable to the source effect and the site effect. The site effect is investigated using the traditional spectral ratio technique and the theoretical evaluation based on the subsurface structure model. The results imply that the site effect has an insignificant effect on the large ground motion at HKD020. The source effect is investigated by constructing the source model that explains the broad-band strong-motion records at four stations around the epicenter using the empirical Green’s function method. The estimated source model satisfies the empirical relationship between the strong motion generation areas and the seismic moment for inland crustal earthquakes. The high-frequency level of the acceleration source spectrum is also consistent with the empirical relationship. These results suggest that this earthquake is a normal crustal event and that the large ground motion at HKD020 is mainly attributable to the source effect, short distance from the strong motion generation area and the forward directivity effect. Finally, the temporal change of the site response at HKD020 is examined using long duration records including the main shock and several aftershocks. The site response based on theS-wave horizontal-to-vertical spectral ratio method shows the nonlinearity for the main shock and an aftershock occurring about 20 s after the main shock. However, the site response shows linearity for other later aftershocks. This site response change is attributed to the difference in ground motion amplitude.

Damping formulation for nonlinear 1D site response analyses

Measurements and observations of ground shaking during large earthquakes have demonstrated the predominant role of site effects in the response of infrastructure during a seismic event. Despite significant efforts to model the hysteretic response and nonlinearity of soils due to medium and large ground motions, the most widely accepted nonlinear site response methods are not able to represent simultaneously the changes of stiffness and energy dissipation (damping) observed in both laboratory tests and during earthquake events. This paper presents two new soil damping formulations implemented in nonlinear one-dimensional site response analysis for small and large strains. The first formulation introduces an approach to construct a frequency-independent viscous damping matrix which reduces the over-damping at high frequencies, and therefore, the filtering at those frequencies. The second formulation introduces a reduction factor that modifies the extended Masing loading/unloading strain–stress relationship to match measured modulus reduction and damping curves simultaneously over a wide range of shear strains. A set of examples are introduced to illustrate the effect of using the two proposed formulations, separately and simultaneously, in nonlinear site response analyses.

Differences in Attenuation of Ground Motion Perpendicular to the Mexican Subduction Zone between Colima and Guerrero: An Explanation Based on Numerical Modeling

Abstract A recent article (Tejeda-Jacome and Chavez-Garcia, 2007) showed that significant differences in seismic ground-motion attenuation perpendicular to the Pacific coast exist between Guerrero and Colima, in western Mexico. For hypocentral distances larger than 100 km, larger ground motions are predicted using a model derived from local data in the northern section of the subduction zone (Colima) relative to ground-motion models derived using data from the southern section (Guerrero). In this article we consider two possible explanations for the differences. The first possible explanation is differences in the geometry of the subducting slab between Colima and Guerrero. The second is the varying shallow structure of the overriding plate due to the presence of the trans-Mexican volcanic belt (TMVB), oblique to the Middle American trench. We use 2D numerical modeling for P-SV waves to investigate which of the two factors can explain the differences in the observed attenuation. Our results show that it is the presence of the TMVB, closer to the coast in Colima, that better explains the differences.

A STUDY ON SEISMIC ISOLATION SYSTEM WITH ROBUSTNESS FOR INCREASING GROUND MOTION LEVEL

The development of seismology and recent observation records have been evolving the ground motions to be considered in structural engineering. The future research will seems to require quite an amount of revisions in ground motions. Since it is difficult to envisage the goal of seismology, some measures to deal with increasing levels of ground motions must be included in structural engineering. Consideration of excessive strength in structural engineering is not economical. However, making extra margin in sliding bearings for seismic isolation devices require less expenditures in structural design. The authors have conducted analytical study of this kind of system and have confirmed the sliding isolation system has the potential to keep isolation characteristics for large input ground motion level. However, the maximum displacement and residual displacement become quite large, especially when the elements to provide restoring force are not employed, it was over 1.0m. The authors think this level of displacement can be considered in the structural design of foundations to withstand P-Delta effect by building weight. From this study, the authors concluded that if the foundations are well designed to accept P-Delta moment in isolation interface, sliding bearings can be a potential candidate to provide robustness to deal with excessive level of input ground motions.

ELASTO-PLASTIC RESPONSES AND PROCESS LEADING TO A COLLAPSE MECHANISM

Shaking table test of full-scale 4-story building was carried out at E-defense. The objectives of this experiment are to evaluate structural and functional performances of the steel building under design-level ground motions, and to determine the safety margin against collapse under exceedingly large ground motions. This paper presents detailed results concerning the nonlinear responses of the specimen by 0.4 and 0.6 times Takatori records of 1995 Hyogoken-Nanbu earthquake. The cause of difference between the mechanism assumed in design and the mechanism actually generated in experiment is investigated from elasto-plastic behaviors of structural members under three dimensional external forces.