Frequency Ground Motion Research Articles

Influence of Hysteretic Behavior on Seismic Strength Demands: An Analysis for Romanian Vrancea Earthquakes

The paper presents the results of a study performed on a large ground motion database, containing records obtained during the three strongest earthquakes that occurred during the past four decades in the Vrancea seismogenic zone. In order to express strength demands imposed by these earthquakes, constant-ductility nonlinear acceleration spectra were computed for two sets of seismic records, selected as representative for narrow frequency band and broad frequency band ground motions, respectively. The spectra, determined for various types of bilinear hysteretic models, were normalized with respect to peak ground acceleration and mean values, as well as coefficients of variation, were computed for each analysis case. The sensitivity of spectral values to the variation of strength hardening and stiffness degradation parameters was determined, with reference to the elastic-perfectly plastic model. Conclusions were drawn, separately for the two distinct types of ground motion frequency content, on the significance of the considered hysteretic model parameters for the assessment of seismic strength demands.

Site classification of Pondicherry using shear-wave velocity and horizontal-to-vertical spectral ratio

Site classification studies play a vital role in earthquake hazard assessment since in situ ground conditions substantially affect the characteristics of incoming seismic waves during earthquakes. Flat areas along the coast and rivers generally consist of thick layers of soft clay and sand. Such deposits amplify certain frequencies of ground motion, thereby attributing to an increase in the damage due to an earthquake. Hence, site classification studies have been carried out using shear-wave velocity, ground response, and corresponding amplification at 83 locations in Pondicherry, a coastal city in India. The present study is aimed at estimating the shear-wave velocity through multichannel analysis of surface waves and to compute the average shear-wave velocity (V 30 ), stiffness, and N values using empirical relations. Further, site-response studies (horizontal-to-vertical spectral ratio) were conducted to estimate the ground-response frequencies and corresponding amplifications through Nakamura technique. From the results, the study area was classified into three types, i.e., C-class: with V 30 in the range of 360–760 m/s, D-class: with V 30 in the range of 180–360 m/s, and E-class: with V 30 < 180 m/s following the National Earthquake Hazard Reduction Programme norms (BSSC in NEHRP recommended provisions for seismic regulations for new buildings and other structures (FEMA 450), part 1: provisions. Building Seismic Safety Council for the Federal Emergency Management Agency, Washington, 2003). Finally, a site classification map for Pondicherry region has been prepared, which can be used in urban planning and strengthening of existing structures against future earthquakes.

Dynamic mechanisms of earthquake-triggered landslides

Earthquake-triggered landslides usually cause great disasters, and yet their dynamic mechanisms remain poorly understood. This paper will derive a general conceptual landslide model from the geometric and kinematic features of the most landslide masses triggered by the 2008 Wenchuan earthquake. Kinematic characteristics and dynamic processes are simulated here by means of finite element method (FEM) based on the dynamic process of the discontinuous deformable body. The calculated results presented the whole course of landslide motion, and displayed some typical kinematic characteristics such as initiation, sliding, ejection, collision, flying in the air, and climbing of landslides. The simulation result also shows that, under combined seismic inertial forces and gravity, landslides will start to slip once it overcomes the friction between the sliding mass and slip-bed, then it will move from slow to fast along the slippery bed until it ejects from the slip-bed. Moreover, the high frequencies and serious damages by landslides in the Wenchuan earthquake are caused by the strong ground motion on the mountain slopes in and around the epicenter that was dramatically amplified owing to both resonances produced by the seismic event and topographical amplification by seismic motion. In addition, the modeling results suggest that the direction, amplitude, frequency, and duration of strong ground motion have a great influence on the stability of landslide mass. Therefore, the study helps us better understand dynamic mechanism of landslides, seismic hazard assessment, and dynamic earthquake triggering.

Strong Motion Simulation of the M8.0 August 15, 2007, Pisco Earthquake; Effect of a Multi-Frequency Rupture Process

We investigated the broadband frequency (0.05-30 Hz) radiation characteristics of the August 15, 2007, Mw8.0 Pisco, Peru, earthquake by simulating the near-source strong ground motion recordings in Parcona city (PCN) and Lima city (NNA). A source model of this earthquake obtained from long-period teleseismic waveforms and InSar data shows two separate asperities, which is consistent with the observation of two distinct episodes of strong shaking in strong motion recordings. We constructed a source model that reproduces near-source records at low frequency (0.05-0.8 Hz) as well as high frequency (0.8-30 Hz) bands. Our results show that the aforementioned teleseismic source model is appropriate for simulating near-source low frequency ground motion. Our modeling of the PCN record in the broad-frequency band indicates that a very strong high frequency radiation event likely occurred near the hypocenter, which generated a large acceleration peak within the first episode of strong shaking at PCN. Using this “broadband frequency” source model we simulated the strong ground motion at Pisco city and obtained accelerations as large as 700 cm/s2and velocities as high as 90 cm/s, respectively, which may explain the heavy damage occurring in the city.

Vertical Motion Inputs for Seismic Analysis of Large Span Bridge

In seismic analysis of large span bridge, inconsistent ground motions in three directions, lengthwise, lateral and vertical are required to input at the base of each of the two main girder piers. In order to adopt synthesized motion field for the inputs, a simple way to prepare the vertical motion is introduced for improvisation at this moment in this paper, since the synthesis in general consists of two parts, the low frequency ground motion calculated by a numerical method, like FEM, and the high frequency motion synthesized by random approach, and the result of the former is in three dimensional, while that of the latter has just horizontal component. The vertical acceleration time histories proposed in the paper show the way is available.

Dynamic Response of Rigid Blocks to Simultaneous Horizontal and Vertical Ground Shock

Surface and underground explosions generate ground shock that might result in overturning of equipment housed in nearby structures owing to rocking responses. Comprehensive studies of rigid structure responses to seismic ground excitations have been reported. It was found that the rocking and sliding response of a rigid structure is highly nonlinear. The structure stability depends on the structure slenderness, as well as the ground motion amplitude, frequency and duration. Compared to an earthquake ground motion, a ground shock has a very large amplitude, high frequency contents and short duration. Moreover, vertical component of a ground shock is often higher than the gravitational acceleration g. This will cause the unanchored equipment fly into air. Therefore, the responses and stability regions of a rigid structure to blast induced ground shock will be very different from those under seismic ground motions. No study of rocking response and overturning of rigid structure to ground shock of amplitude more than 1.0 g can be found in the literature. As there might be some important equipment in buildings near a mining or a quarry site, or a military command center subjected to ground shock, understanding rigid structure response to ground shock is essential for protection of such equipment. In this study, theoretical derivation and numerical prediction of rocking and flying responses and overturning of rigid structures to simultaneous horizontal and vertical ground shock are carried out. Numerical results of stability regions of rigid structures to ground shock are derived. Particular attentions are paid to the case when the vertical ground shock is more than 1.0 g and the rigid structure flies into the air. Results are compared to those obtained with earthquake ground motions. Discussions of the rigid structure stability to ground shock are made.

A study of local amplification effect of soil layers on ground motion in the Kathmandu Valley using microtremor analysis

Past researchers have anticipated the occurrence of a great earthquake in the central Himalayas in the near future. This may cause serious damage in the Kathmandu Valley, which sits on an ancient lake bed zone, with lacustrine sediments of more than 500 m depth. In this study, the predominant frequency of ground motion is evaluated using the Horizontal-to-Vertical (H/V) spectral ratio technique and recordings of ambient noise. The results of the H/V ratio show two peaks in about 20 percent of the locations, which are distributed mainly in and around the center and northern part of the Kathmandu Valley. The predominant frequencies vary from 0.5 Hz to 8.9 Hz in the study area, whereas the second resonance frequency varies from 4 Hz to 6 Hz in the center and northern part of the valley. This indicates that the center and northern part of the valley have a wide range of resonance frequency due to two levels of impedance contrast — one may be from the surface layer and the other may be from the layer underneath. These two levels of resonance indicate the importance of considering the effects of surface and lower layers during the planning and designing of infrastructures in the Kathmandu Valley.

Micro-seismic measurements of cliff motion under wave impact and implications for the development of near-horizontal shore platforms

Few high-resolution measurements of process-form interactions have been taken on rock coasts, but recent studies in California have shown that portable seismometers enable useful proxy measurements of wave-energy delivery to cliffs. Here we describe measurements over 20days of high frequency ground motion of cliffs formed in sedimentary (flysch) rocks at Okakari Point, north of Auckland, New Zealand. Three sensors were located in a shore-normal array inland from the cliff top and a fourth sensor was bolted to a ledge 2m above the cliff toe. The nearshore wave field in front of the cliff and shore platform was monitored using a shore-normal array of 5 wave gauges. The instrumentation provided measurements of wave-energy delivery and consequent ground motion, including the first observations of motion at the top and bottom of cliffs. Results showed that horizontal ground motion is dominant at the cliff top, whereas vertical motion is dominant at the cliff toe. Power spectra show that several high frequency peaks occur in data from the cliff toe, whereas a single, broader peak frequency occurs at the cliff top resulting from signal modification as seismic waves pass through tens of metres of cliff rock. A 100m wide shore platform at the cliff toe fundamentally controls the patterns of observed energy delivery. The shore platform is nearly horizontal, elevated close to high water level, and abruptly plunges into water >10m deep at its seaward edge. As expected, the magnitude of ground motion at all sensors is greatest during larger waves. Measurements further show that ground motion, both at the bottom and top of the cliff, is strongest at low tide and weakest at high tide. This observation is opposite to that noted at Santa Cruz, where ground motion was greatest at high tide. At Okakari Point the most significant high frequency ground motions occur at low tide when waves are forced to break (sometimes violently) against the seaward edge of the shore platform. Four distinctive frequency peaks between 1 and 50Hz increase in magnitude as tidal stage drops, implying that wave breaking against the outside edge of the shore platform represents an important source of vibration. A detailed understanding of the energy source (e.g. short duration shock pressures) and rock resonance is not provided by this study. However, quantifying the spatial and temporal patterns of energy delivery places strong emphasis on the important role of shore platform geometry in filtering wave-energy delivery to the cliff. During the 20-day experiment most wave energy was delivered to the outside edge of the shore platform, not the cliff toe. The geomorphic role of high-frequency shaking from wave impacts remains to be clearly demonstrated, but if wave impacts are capable of eroding rock then the data from this study imply that under present conditions the outside edge of the shore platform may be subject to higher erosion rates than the cliff toe. It is possible that the shore platform is currently being destroyed rather than created, but a longer programme of measurements is required to test this notion.

Site- and ground motion-dependent nonlinear effects in seismological model predictions

We investigate the empirical relationship between site response nonlinearity, soil properties, and ground motion characteristics, as a first step to enable efficient integration of nonlinear analyses in broadband ground motion simulations. For this purpose, we use 24 downhole array sites with detailed geotechnical information and subject the profiles to broadband ground motion synthetics. We quantify the extent of soil nonlinearity in site-specific response analyses by estimating the divergence between linear and nonlinear ground surface predictions. We show that the parameters controlling the nonlinear response are V S 30 (weighted averaged shear wave velocity in the top 30 m of the soil profile), the site amplification at the fundamental frequency (Amp), the peak ground acceleration (PGA), and the frequency index (FI), a quantitative measure we define to characterize how well the incident motion can “see” the near-surface soil layers. Using the synthetic results, we quantitatively describe the error introduced in ground motion predictions when nonlinear effects are not accounted for, and show that the error is both site and ground motion dependent. Our study indicates that to characterize the susceptibility of a soil profile to nonlinear effects, V S 30 should be complemented with measures of the soil–rock impedance contrast, as well as measures of the ground motion intensity and frequency content.

Strong ground motion simulation by the Empirical Green’s Function method for Bursa-Yalova Region, Turkey

In this study near field strong ground motion generation of Mw 6.9 scenario events on Gemlik Bay was presented at broadband frequency (0.5–10 Hz) ground motion at 9 stations. In the first stage of the study, focal mechanism of a small earthquake, which was used as the Empirical Green’s Function (EGF) throughout the scenario simulation, was decided by simulating it with a smaller magnitude event. The best waveform fitting was judged with the smallest misfit value. In the second stage, near field ground motion simulation of scenario events was performed. Calculations were achieved by considering three different rupture processes which have the same magnitude but different asperity locations. Fault and asperity parameters for each scenario were determined from empirical scaling laws. It has been found that the peak ground acceleration and peak ground velocities reach maximum values of 1,440 cm/s2 and 125 cm/s, respectively for the worst case scenario. Rupture directivity effect is observed with clear peaks at a forward station. The design spectra for Turkish seismic design code (TSDC 2007) were either nearly or actually exceeded by the scenario earthquakes at periods lower than 0.6 s at all near field stations. Majority of structures in the area were built to lower design spectra before the 1998 code was implemented. The strength of many structures would have been insufficient to resist the forces that may be generated by an earthquake that is similar to Scenario I and Scenario II in this study.

On the unsteady motion and stability of a heaving airfoil in ground effect

This study explores the fluid mechanics and force generation capabilities of an inverted heaving airfoil placed close to a moving ground using a URANS solver with the Spalart-Allmaras turbulence model. By varying the mean ground clearance and motion frequency of the airfoil, it was possible to construct a frequency-height diagram of the various forces acting on the airfoil. The ground was found to enhance the downforce and reduce the drag with respect to freestream. The unsteady motion induces hysteresis in the forces’ behaviour. At moderate ground clearance, the hysteresis increases with frequency and the airfoil loses energy to the flow, resulting in a stabilizingmotion. By analogy with a pitching motion, the airfoil stalls in close proximity to the ground. At low frequencies, the motion is unstable and could lead to stall flutter. A stall flutter analysis was undertaken. At higher frequencies, inviscid effects overcome the large separation and the motion becomes stable. Forced trailing edge vortex shedding appears at high frequencies. The shedding mechanism seems to be independent of ground proximity. However, the wake is altered at low heights as a result of an interaction between the vortices and the ground.

Rigid Structure Response Analysis to Seismic and Blast Induced Ground Motions

Comprehensive studies of rigid structure responses to seismic ground excitations have been reported. It was found that the rocking and sliding response of a rigid structure is highly nonlinear. The structure stability depends on the structure slenderness, as well as the ground motion amplitude, frequency and duration. Compared to an earthquake ground motion, ground shock induced by underground or surface explosion has very large amplitude, high frequency and short duration. Moreover, vertical component of a ground shock may be substantially larger than the gravitational acceleration g. This will cause the unanchored rigid structure jump or fly into air. Therefore, the responses and stability regions of a rigid structure to blast induced ground shock will be very different from those under seismic ground motions. No study of rigid structure response to ground shock of amplitude more than 1.0g can be found in the literature. As there might be many rigid structures such as computers, document shelfs, and other important equipments in a building or a military command center close to an explosion center, understanding rigid structure response to ground shock is essential for protection of such equipments. In this study, theoretical derivation and numerical prediction of rigid structure response to ground shock are carried out. Numerical results of stability regions of rigid structures to ground shock are derived. Particular attentions are paid to the case when the vertical ground shock is more than 1.0g and the rigid structure flies into the air. Results are compared to those obtained with earthquake ground motions. Discussions on the rigid structure stability to earthquake motion and ground shock are made.

Fundamental site frequency estimation at New Domiat city, Egypt

The horizontal-to-vertical spectral ratio technique has applied to detect the fundamental frequency at the sites of ambient noise recordings for New Domiat city. Noise measurements are acquired at 90 of sites for 1 h of continuous recording with a sampling rate of 100 Hz. Then, these data are processed following to SESAME project scheme. The presence of deep sedimentary basin in the Nile Delta suggests that the site response should be important. Consequently, the obtained fundamental fre- quency has lower values (0.2-0.6 Hz). However, low- frequency ground motions attenuate more gradually with distance and can excite vibrations in large engineered structures, such as tall buildings and long bridges. There is hazardous threat even from the distant earthquakes originated from Mediterranean convergence zone for the structures in the city. It is recommended that the results of this study must be taken into consideration from civil engineering point of view before construction of civil engineering structures at this part.

Localized damage caused by topographic amplification during the 2010 M 7.0 Haiti earthquake

Microzonation maps use local geological conditions to characterize seismic hazard, but do not generally consider topography. Ground motions during the Haiti earthquake are found to have been significantly amplified along a high topographic ridge, which caused substantial structural damage, indicating that topography can play an important role in seismic hazard. Local geological conditions, including both near-surface sedimentary layers1,2,3,4 and topographic features5,6,7,8,9, are known to significantly influence ground motions caused by earthquakes. Microzonation maps use local geological conditions to characterize seismic hazard, but commonly incorporate the effect of only sedimentary layers10,11,12. Microzonation does not take into account local topography, because significant topographic amplification is assumed to be rare. Here we show that, although the extent of structural damage in the 2010 Haiti earthquake was primarily due to poor construction, topographic amplification contributed significantly to damage in the district of Petionville, south of central Port-au-Prince. A large number of substantial, relatively well-built structures situated along a foothill ridge in this district sustained serious damage or collapse. Using recordings of aftershocks, we calculate the ground motion response at two seismic stations along the topographic ridge and at two stations in the adjacent valley. Ground motions on the ridge are amplified relative to both sites in the valley and a hard-rock reference site, and thus cannot be explained by sediment-induced amplification. Instead, the amplitude and predominant frequencies of ground motion indicate the amplification of seismic waves by a narrow, steep ridge. We suggest that microzonation maps can potentially be significantly improved by incorporation of topographic effects.

Hydrodynamic pressures behind flexible and rigid dams

During earthquakes, hydrodynamic pressures are generated by the impounded reservoir on the dam face. The magnitude and distribution of the hydrodynamic pressures vary with factors such as frequency and intensity of earthquake-induced ground motion, depth of impounded reservoir, stiffness of dam and geological conditions. It is difficult to obtain experimental data on hydrodynamic pressures from the field owing to uncertainties associated with earthquake loading. This paper aims at using dynamic centrifuge modelling to measure hydrodynamic pressures behind both relatively stiff and flexible model dams. Comparisons of the experimental data with theoretical hydrodynamic pressures show that Westergaard's equation gives a conservative estimation of hydrodynamic pressures. Comparison with Chopra's method revealed that it underpredicts hydrodynamic pressures for low reservoir depths but gives reasonably good predictions for higher depths of reservoir. It is concluded that dynamic centrifuge modelling may be an effective experimental method to estimate the hydrodynamic pressures acting on a dam.

Seismic Isolation of Bridges Using Variable Frequency and Variable Friction Pendulum Isolator System

Friction pendulum system (FPS) with spherical sliding surface is an effective isolation system for bridges subjected to earthquake ground motion. However, due to constant curvature, this sliding surface may encounter resonance problem at low frequency. In order to overcome this limitation, an isolation system with variable frequency and variable friction is proposed for bridges in this study. The isolator is similar to typical FPS, but the curvature of the sliding surface and the friction coefficient along the surface are varied and they become a function of sliding displacement. A three-span continuous bridge isolated with variable frequency and variable FPS is analysed to study the effectiveness of the proposed system. It is concluded from the study that the resonance problem due to low frequency ground motion can be prevented using the proposed isolator. The proposed isolator reduces the base shear considerably without increasing the residual displacements. Also, the proposed isolator has the advantages of both the FPS and the pure friction (PF) system.

Simulation of Strong Ground Motion During the 1950 Great Assam Earthquake

In this paper, ground motion during the Independence Day earthquake of August 15, 1950 (Mw 8.6, Ben-Menahem et al., 1974) in the northeastern part of India is estimated by seismological approaches. A hybrid simulation technique which combines the low frequency ground motion simulated from an analytical source mechanism model with the stochastically simulated high-frequency components is used for obtaining the acceleration time histories. A series of ground motion simulations are carried out to estimate the peak ground acceleration (PGA) and spectral accelerations at important cities and towns in the epicentral region. One sample PGA distribution in the epicentral region encompassing the epicenter is also obtained. It is found that PGA in the epicentral region has exceeded 1 g during this earthquake. The estimated PGA’s are validated to the extent possible using the MMI values. The simulated acceleration time histories can be used for the assessment of important engineering structures in northeastern India.

Offshore triangular tension leg platform earthquake motion analysis under distinctly high sea waves

TLPs are compliant structures designed to withstand moderate loads without damage and severe loads without seriously endangering the occupants. Dynamic behavior of TLPs under distinctly high sea waves in the presence of both horizontal and vertical seismic excitations is examined and method of analysis is discussed. Seismic forces imposed at tether bottom make tether tension unbalanced when the hull is under offset condition. Tether tension varies nonlinearly under vertical seismic excitations generated using Kanai-Tajimi ground acceleration spectrum. Analytical studies conducted on triangular TLPs show that this tension variation is much higher than the regulation values indicating the necessity for examining them for seismic safety. Clearly, the peaks seen in the response of all active degrees-of-freedom occurring near to the average sum frequencies of waves and input ground motion is a significant influence of seismic excitations on TLP tethers under high sea waves. The numerical results obtained also verify/establish the fact that TLPs built in deeper sea locations show significantly lesser response to the combined wave and earthquake loading.

Seismic record analysis of the Liyutan earth dam

In this paper the authors present an analysis of the seismic records of the Liyutan dam, which is a zoned compacted earth dam. It is 96 m in height, and it suffered some minor cracks and settlement during the 1999 Chi-Chi earthquake. A large amount of seismic motion has been recorded for 42 past earthquake events. These records were used to analyze the vibration characteristics of the dam. The fundamental frequency and nonlinear amplification of the peak ground motion of the dam body have been clearly identified. Based on the fundamental frequency identified from the small earthquake data, the seismic motion of the dam during the Chi-Chi earthquake could be well simulated by an equivalent linear dynamic procedure. An empirical decomposition method was used to decompose the Chi-Chi earthquake motion histories, from the top and the bottom of the dam, into several orthogonal and complete intrinsic mode functions (IMFs). These IMFs show that the low frequency parts of the seismic motion at the top and bottom of the dam are nearly the same, however, the high frequency parts are significantly amplified when the seismic motion propagates from the bottom to the top of the dam.

New method for generation of artificial ground motion by a nonstationary Kanai-Tajimi model and wavelet transform

Considering the vast usage of time-history dynamic analyses to calculate structural responses and lack of sufficient and suitable earthquake records, generation of artificial accelerograms is very necessary. The main target of this paper is to present a novel method based on nonstationary Kanai-Tajimi model and wavelet transform to generate more artificial earthquake records, which are compatible with target spectrum. In this regard, the generalized nonstationary Kanai-Tajimi model to include the nonstationary evaluation of amplitude and dominant frequency of ground motion and properties of wavelet transform is used to generate ground acceleration time history. Application of the method for El Centro 1940 earthquake and two Iranian earthquakes (Tabas 1978 and Manjil 1990) is presented. It is shown that the model and identification algorithms are able to accurately capture the nonstationary features of these earthquake accelerograms. The statistical characteristics of the spectral response of the generated accelerograms are compared with those for the actual records to demonstrate the effectiveness of the method. Also, for comparison of the presented method with other methods, the response spectra of the synthetic accelerograms compared with the models of Fan and Ahmadi (1990) and Rofooei et al. (2001) and it is shown that the response spectra of the synthetic accelerograms with the method of this paper are close to those of actual earthquakes.