Three-component Accelerograms Research Articles

Ground motion prediction equations based on shallow crustal earthquakes in Georgia and the surrounding Caucasus

Strong ground motions caused by earthquakes with magnitudes ranging from 3.5 to 6.9 and hypocentral distances of up to 300 km were recorded by local broadband stations and three-component accelerograms within Georgia’s enhanced digital seismic network. Such data mixing is particularly effective in areas where strong ground motion data are lacking. The data were used to produce models based on ground-motion prediction equations (GMPEs), one benefit of which is that they take into consideration information from waveforms across a wide range of frequencies. In this study, models were developed to predict ground motions for peak ground acceleration and 5%-damped pseudo-absolute-acceleration spectra for periods between 0.01 and 10 s. Short-period ground motions decayed faster than long-period motions, though decay was still in the order of approximately 1/r. Faulting mechanisms and local soil conditions greatly influence GMPEs. The spectral acceleration (SA) of thrust faults was higher than that for either strike-slip or normal faults but the influence of strike-slip faulting on SA was slightly greater than that for normal faults. Soft soils also caused significantly more amplification than rocky sites.

Data-Driven Synthesis of Broadband Earthquake Ground Motions Using Artificial Intelligence

ABSTRACTRobust estimation of ground motions generated by scenario earthquakes is critical for many engineering applications. We leverage recent advances in generative adversarial networks (GANs) to develop a new framework for synthesizing earthquake acceleration time histories. Our approach extends the Wasserstein GAN formulation to allow for the generation of ground motions conditioned on a set of continuous physical variables. Our model is trained to approximate the intrinsic probability distribution of a massive set of strong-motion recordings from Japan. We show that the trained generator model can synthesize realistic three-component accelerograms conditioned on magnitude, distance, and VS30. Our model captures most of the relevant statistical features of the acceleration spectra and waveform envelopes. The output seismograms display clear P- and S-wave arrivals with the appropriate energy content and relative onset timing. The synthesized peak ground acceleration estimates are also consistent with observations. We develop a set of metrics that allow us to assess the training process’s stability and to tune model hyperparameters. We further show that the trained generator network can interpolate to conditions in which no earthquake ground-motion recordings exist. Our approach allows for the on-demand synthesis of accelerograms for engineering purposes.

Wavelet-Based Stochastic Model for Jointly Simulating Three-Component Ground Motions

ABSTRACTA large number of ground motions are needed in performance-based earthquake engineering. To date, most existing stochastic models are limited to simulating a single horizontal component of earthquake motions. This study proposes a wavelet-based stochastic method for jointly simulating three-component accelerograms using earthquake magnitudes, source-to-site distances, and site conditions as input. Based on the Pacific Earthquake Engineering Research Center Next Generation Attenuation (NGA)-West2 database, prediction equations are developed for wavelet-packet parameters of two horizontal and one vertical components. Correlations among these components are obtained to jointly simulate three-component motions based on earthquake scenarios. The simulated ground motions are systematically compared with existing ground-motion attenuation equations. The model has the capability to simulate the different frequency content of horizontal and vertical motions. The simulated vertical-to-horizontal spectral ratios are found to be consistent with observed strong-motion data. The model is applicable to shallow crustal earthquakes in an active tectonic region with a moment magnitude between 5 and 8, source-to-site distance from 0 to 100 km, and shear-wave velocity in the top 30 m (VS30) in the range of 150–1000 m/s. It can find important applications in 3D time-history analyses in performance-based earthquake engineering.

Influence of earthquake vertical excitations on Sloshing-Created P-Δ effect in elevated water Tanks: Experimental Validation, numerical simulation and proposing a modification for Housner model

Seismic sloshing in elevated water tanks creates some additional base moment, which simultaneous with the effect of strong vertical acceleration of near-source earthquakes may increase the P-Δ effect, as an important factor in seismic design of these structures. This study was performed to investigate the extent of this combined effect. First, an experiment was conducted to provide a set of results for validating the finite element analysis (FEA) modeling, conducted as the second part of the study. The test tank was cylindrical with a diameter of 60 cm, installed on an innovative simple shaking table, capable of creating simultaneous horizontal and vertical harmonic excitations. Different frequencies and water depths were used in the tests. For verification, the maximum sloshing height was measured and compared with FEA results. The verified FEA process was used to simulate elevated tanks of actual sizes for various water height to tank diameter and also tank height to tank diameter ratios, using three-component accelerograms of seven selected earthquakes, mostly having strong vertical components, creating totally 126 dynamic FEA cases. In the last stage, a modification was done on the Housner spring-mass model for using it in 3D state with consideration of vertical excitations. Numerical results indicate that the sloshing height and the base moment could increase in average about 20% and 10%, respectively, due to the effect of earthquake vertical excitations. Also, comparison of FEA results with those obtained by using the proposed 3D modified Housner model shows a good agreement of around 90% on average.

A Strong-Motion Database of Costa Rica: 20 Yr of Digital Records

AbstractIn this article, we present a strong-motion database from earthquakes recorded by the Earthquake Engineering Laboratory at the University of Costa Rica. The database consists of 2471 three-component accelerograms from 155 digitally recorded events. It covers the last 20 yr of measurements, including records from the 5 September 2012 Mw 7.6 Nicoya earthquake. The engineering and seismological communities can use this data either to conduct new research or to improve seismic hazard studies in the region. A catalog is also available with metadata of each record containing several intensity measures from the ground-motion time histories.

Seismic Design of the Box for the Gas Flow-Rate Metering Unit

To study seismic resistance of the box for the metering unit of gas flow-rate required to ignite the flare system we used the calculation methods developed on the basis of the method of finite elements and the software designed to define the mode of deformation and the dynamics of structures for the three-dimensional statement. To construct the simulation model, we took into account all the elements that affect the strength and the efficiency of the real structure during possible earthquakes. Structure elements were simulated using the sets of arbitrary oriented rods and lumped masses with inertia moments and those rods were connected absolutely rigidly with each other and using also elastically damped elements. The rod elements with distributed parameters took into consideration all the types of deformations that occur during the vibrations. The plate components of the structure were simulated using the grids made of the rods taking into consideration the ductility and inertia properties of its elements. Lumped masses in the rod connections were used to take into account the masses of installed equipment, pipelines, etc. The seismic loading was simulated using alternating inertia forces transferred from the ground. The soil ductility was taken into account by the Winkler model. Computations were done for the sand and rock stone ground and for the fine-grained sand. Consideration was given to the seismic forces of a different intensity that are represented by three-component accelerograms. Synthesized and real accelerograms were analyzed. The obtained computation data showed that the structure of the box of the metering unit of gas flow rate is quakeproof and it won’t be destroyed during its operation.

Bi-normalized Newmark–Hall spectra for seismic design and assessment

A bi-normalized Newmark–Hall response spectrum is presented for seismic design and assessment of structures. The period axis is normalized with respect to a characteristic period relating only to peak ground-motion parameters. The amplification factors are determined through a statistical analysis by normalizing spectral pseudo-acceleration, spectral pseudo-velocity, and spectral displacement, relative to peak ground acceleration, peak ground velocity, and peak ground displacement, respectively, in the low-, intermediate-, and high-period ranges, which do not need to be prescribed prior to normalization. A total of 671 three-component accelerograms are selected from the New Zealand Strong Motion Database for the statistical analysis. The relationship between peak ground-motion parameters is studied, enabling the construction of a design spectrum from either a given design scenario earthquake, or a code-specified peak ground acceleration value.

A prediction model for vertical-to-horizontal ratios of PGA, PGV, and 5%-damped response spectra (0.01–10 s) for Iran

New models are developed for the prediction of vertical-to-horizontal (V/H) spectral ratios of peak ground acceleration (PGA), peak ground velocity (PGV), and 5%-damped elastic response spectra for periods ranging from 0.01 to 10 s. The models use moment magnitude, source-to-site distance, style-of-faulting (SoF), and site classification terms as independent variables. The study is based on a well-compiled Iranian ground-motion databank, consisting of 1350 (*3) three-component accelerograms generated by 370 earthquakes. Only records with known measured time-averaged shear-wave velocity in the uppermost 30 m (VS30) are used for regression analyses. The presented models are valid for moment magnitude ranging from 4.5 to 7.4, distance up to 200 km, and for earthquakes with reverse and strike–slip fault mechanisms. Four alternative mathematical forms are evaluated in terms of magnitude and distance in order to find the optimal functional form for reliable prediction of V/H ratios. Out of four proposed forms, two were chosen for further investigation. The point source (hypocentral and epicentral) as well as extended source (rupture and Joyner-Boore) distance metrics are employed to derive four models. The overall validity of the proposed models is examined by detailed residual analyses as well as comparison with some of the local and regional predictive V/H models. Our proposed models showed significant reduction in the total standard deviation across all periods. The proposed V/H models facilitate assessment of vertical components of response spectrum where horizontal component of response spectrum is known, using V/H spectral ratios as scaling factors.

Relationship between pressure variations at the ocean bottom and the acceleration of its motion during a submarine earthquake

The data provided by ten DONET deep-sea observatories, that on March 11, 2011, registered the Great East Japan Earthquake and tsunami, were used for investigation of the relationship between variations of the ocean bottom pressure and three-component accelerograms. Methods of cross-spectral analysis revealed the existence of a frequency range of “forced oscillations,” within which pressure variations are proportional to the vertical component of the acceleration. This proportionality is manifested by the magnitude-squared coherence (MSC) being close to unity and a phase lag (PL) practically equal to zero. The spectral analysis method showed the proportionality coefficient to be equal the mass of a water column of unit area at the installation point of the observatory or, approximately, to the product of the water density and the ocean depth. The observed boundaries of the frequency range of “forced oscillations” are revealed to correspond to the theoretical frequency values confining the manifestation of surface gravity and acoustic waves in pressure variations near the ocean bottom. The hypothesis is put forward that the small deviations of MSC from unity and of PL from zero observed by a number of stations within the range of “forced oscillations” are due to the contribution of horizontal movements of nearby submarine slopes. A theoretical analysis has been performed of the problem of forced oscillations of a water layer in a basin of varying depth. A formula is obtained that relates pressure variations at the ocean bottom to acceleration components of the bottom motion and the bottom slope. The pressure in the region of forced oscillations is shown to decrease exponentially with the distance from the moving segment of the ocean bed, so pressure variations, originating from movements of the bottom, are registered effectively by a gauge at the ocean bottom only within a radius less than 1–2 ocean depths. A cross-spectral analysis of pressure variations and of three-component accelerograms confirmed the hypothesis concerning the contribution of horizontal movements of nearby submarine slopes to pressure variations.

Velocity spectrum for the Iranian plateau

Peak ground acceleration (PGA) and spectral acceleration values have been proposed in most building codes/guidelines, unlike spectral velocity (SV) and peak ground velocity (PGV). Recent studies have demonstrated the importance of spectral velocity and peak ground velocity in the design of long period structures (e.g., pipelines, tunnels, tanks, and high-rise buildings) and evaluation of seismic vulnerability in underground structures. The current study was undertaken to develop a velocity spectrum and for estimation of PGV. In order to determine these parameters, 398 three-component accelerograms recorded by the Building and Housing Research Center (BHRC) were used. The moment magnitude (Mw) in the selected database was 4.1 to 7.3, and the events occurred after 1977. In the database, the average shear-wave velocity at 0 to 30 m in depth (Vs30) was available for only 217 records; thus, the site class for the remaining was estimated using empirical methods. Because of the importance of the velocity spectrum at low frequencies, the signal-to-noise ratio of 2 was chosen for determination of the low and high frequency to include a wider range of frequency content. This value can produce conservative results. After estimation of the shape of the velocity design spectrum, the PGV was also estimated for the region under study by finding the correlation between PGV and spectral acceleration at the period of 1 s.

Seismic Design Evaluation of Reinforced Concrete Buildings for Near-Source Earthquakes by Using Nonlinear Time History Analyses

Seismic design codes mostly claim that their requirements lead to Life Safety (LS) Performance Level (PL) for buildings. This is while many buildings, designed based on the current codes have shown unacceptable performance, and even have collapsed in some recent earthquakes, particularly near-source events. On this basis, it seems that the code provisions still need further improvement to create sufficient confidence in the engineering community. This study has been conducted to find out how IBC 2009 And ACI 318-2014 codes are effective in providing the LS PL in reinforced concrete multi-story regular buildings with special moment frame lateral load bearing system. For this purpose, a set of multi-story buildings up to 16 stories were considered in the highest seismic hazard zone of Tehran, and were designed based on the codes. Then, a set of near-source three-component accelerograms were employed and scaled according to the code, and a series of nonlinear time history analyses were conducted for all buildings. Roof displacement and acceleration, and base shear forces were calculated, and also the formation trend of plastic hinges and their distribution in the structures were investigated for evaluating the seismic performances. Results show that for some earthquakes the buildings performance exceed LS PL, and even in some cases they reach collapse level.

Rotation-invariant mean duration of strong ground motion

Strong-motion duration is usually computed separately for three components of recorded ground-motion time series. This results in different values of duration for the three components. Furthermore, the computed duration values are dependent on the sensor orientation. Physically, such dependence is not desirable. In this work, computing duration based on resultant recorded motion instead of individual components is proposed. Such a measure of duration is shown to be rotation-invariant and hence independent of the sensor axes. Furthermore, it is demonstrated that the duration of resultant motion represents the mean duration for all possible arbitrary sensor orientations in three-dimensional space. The results indicate that the apparent difference between duration of horizontal and vertical motion reported in the literature is not universal to all definitions of duration. A set of 462 three-component accelerograms from Europe and the Middle East is used to demonstrate and support the presented findings and arguments.

Seismic treatment for a maximal credible earthquake in Guwahati city area of northeast India region

Strong ground motion parameters for the Guwahati city area, the capital city of the state of Assam in northeast India, are examined with the help of data accrued from local as well as worldwide network. Empirical relations are proposed for the ground motion parameters as a function of earthquake magnitude, distance, fault type, source depth and velocity characteristics of medium. Seismotectonics of the study region is examined, and a maximum credible earthquake M S ~ 8.0 is presumed from the Brahmaputra fault, the nearest source zone in the city area. Such great/major event may cause intensity of the of 9.3 with a probability of 0,95 in the Guwahati city during time interval of 500 years. Further, the design spectrum with 67 % confidence level and the synthetic three-component accelerograms are constructed. These results are much relevant and useful for structural engineering to mitigate seismic hazards in the region.

Implications of the Mw9.0 Tohoku-Oki Earthquake for Ground Motion Scaling with Source, Path, and Site Parameters

The Mw9.0 Tohoku-oki Japan earthquake produced approximately 2,000 ground motion recordings. We consider 1,238 three-component accelerograms corrected with component-specific low-cut filters. The recordings have rupture distances between 44 km and 1,000 km, time-averaged shear wave velocities of VS30 = 90 m/s to 1,900 m/s, and usable response spectral periods of 0.01 sec to >10 sec. The data support the notion that the increase of ground motions with magnitude saturates at large magnitudes. High-frequency ground motions demonstrate faster attenuation with distance in backarc than in forearc regions, which is only captured by one of the four considered ground motion prediction equations for subduction earthquakes. Recordings within 100 km of the fault are used to estimate event terms, which are generally positive (indicating model underprediction) at short periods and zero or negative (overprediction) at long periods. We find site amplification to scale minimally with VS30 at high frequencies, in contrast with other active tectonic regions, but to scale strongly with VS30 at low frequencies.

Stochastic Finite Fault Modeling of Subduction Zone Earthquakes in Northeastern India

In this article, a stochastic finite fault source model is calibrated to estimate ground motion in northeastern India for intermediate depth events originating in the Indo-Burmese tectonic domain. A total of 47 three-component accelerograms from eight events with magnitudes ranging from M w 4.8–6.4 are used to estimate the input source and site parameters of the finite fault source model. Key seismic parameters such as stress drop (Δσ) and site amplification function are determined from the recorded strong motion data. The obtained stress drop of the eight recorded events lies in between 105 and 165 bars.

Comparative Analysis of Two Methods for Instrumental Intensity Estimations using the Database Accumulated during Recent Large Earthquakes in Japan

A database containing records from nine large earthquakes in Japan, obtained by K-NET and KIK-net strong motion stations, was used for the analysis of two techniques for the estimation of instrumental seismic intensity from accelerograms. The first technique is the standard method for JMA intensity evaluation from filtered three-component accelerograms. The second technique is the so-called FAS-intensity, which was developed for MM and MSK scales and which is based on the correlation between levels of the Fourier Amplitude spectrum (FAS) and observed intensity. The relation between these two types of instrumental intensities ( JMAI and spectral MMI) may be described by linear function for intensities larger than JMAI 3.5–4 and MMI 5.0–5.5, but large discrepancy arises at small intensities. The variation is most probably caused by differences in the spectral content of the ground motions, since the JMAI calculation is sensitive to the spectral amplitude within a narrow frequency band around 0.5 Hz.

Sediment Thicknesses and Qs vs. Qp Relations in the Kachchh Rift Basin, Gujarat, India Using Sp Converted Phases

Delineation of the top sedimentary structure and its Qs vs. Qp relationship using the travel-time difference of direct S and converted Sp phase is key to understanding the seismic hazard of any sedimentary basin area. We constructed filtered displacement waveforms from local ETNA Episensor acceleration recordings as well as local velocity recordings of aftershocks of the 2001 Bhuj earthquake recorded by the Kachchh seismological network of the National Geophysical Research Institute (NGRI), Hyderabad, India during 2001–2004. Stations are within 15–70km of epicenters, and the resulting displacement waveforms are generally simple, displaying prominent P, Sp, and S wave pulses. Particle motion of P and S waves suggest near-vertical raypaths consistent with preliminary depth estimates. The direct S wave on the horizontal component is characterized by lower frequency content than the converted Sp phase on the vertical component. This difference in frequency content between S and Sp phases can be explained in terms of different attenuation effects for P and S waves in the unconsolidated sediments. The Sp phase is generated by S-to-P phase conversion at the base of Mesozoic sediments of the Kachchh basin. Travel-time inversion (VELEST) of 2565 P and 2380 S arrivals from 658 well located aftershocks recorded at 8–14 three-component local seismic stations led to 1 D velocity models indicated very slow sediments in the upper 0–2 km depth range (Vp: 2.92 km/s and Vs: 0.90 km/s) and an increasing trend of velocities with depth at 2–40 km depth. The estimated sediment thicknesses beneath 12 accelerograph and 6 seismograph sites from the estimated velocity model and the travel-time difference between S and converted Sp phases reaches a maximum of (1.534 ± 0.117) km beneath Bandri (near the location of 2001 Bhuj mainshock) and attains a minimum sediment thickness of (0.858 ± 0.104) km beneath Ramvav and Burudia. The spectral ratios between Sp and S from 159 three-component accelerograms have been used to study seismic wave attenuation in the Kachchh rift basin. The estimated Qs vs. Qp relations for 12 accelerograph sites vary from Qs = 0.184 Qp (at Chobari) to Qs = 0.505 Qp (at Dudhai). For stations Chobari, Chopdwa, Jahawarnagar, Vondh and Tapar, the spectral ratio slopes and hence the calculated Qs vs. Qp relations are effectively the same, and the correlation coefficients are quite high (0.91–0.93). Stations Adhoi, Manfara, New Dudhai, Dudhai and Sikara have similar Qs vs. Qp relationships to each other and also have high correlation coefficients (0.78–0.87). The spectral ratios for stations Anjar and Ramvav are small and poorly constrained, resulting in less reliable Qs vs. Qp relations. This could be due to noisy data, fewer available waveforms, or scattering due to velocity heterogeneities and/or interface irregularities.

Attenuation Relationships of Peak Ground Acceleration and Velocity for Crustal Earthquakes in Taiwan

Strong seismic ground-motion data obtained by the Taiwan Strong Motion Instrumentation Program (tsmip) and Central Mountain Strong Motion Array (cmsma) are used to derive new attenuation relationships for the vertical and horizontal peak ground acceleration (pga) and peak ground velocity (pgv) for crustal earthquakes in Taiwan. More than 7900 three-component accelerograms recorded from 51 crustal earthquakes in Taiwan, with M w magnitudes ranging from 4.0 to 7.1, have been analyzed to study the dependence of peak ground motion parameters on magnitude, distance, regional and local site effects, through attenuation relationships. We first found that, for both pga and pgv, the attenuation relationships decay faster with distance for the vertical component than for the horizontal component. Also, the attenuation relationships decay faster with distance for the vertical pga than for the vertical pgv. We further compared the attenuation relationships for three subregions (chy, iwa, and nto) and the whole Taiwan region (twn). It is found that the chy area has higher ground motion, either in pga or pgv, than the other areas, especially at near-source distances. This is because the chy area is located on a thick, recent alluvial plain. Comparison of our new attenuation relationships with strong-motion data from the 1999 Chi-Chi earthquake ( M w 7.7) and the 2003 Cheng-Kung earthquake ( M w 6.8) shows that the attenuation relationships developed for M w ≤7 can be extrapolated to make reasonable estimates of strong motion from larger M w ≥7 earthquakes. Finally, we analyzed the residuals to investigate variations of pga and pgv with respect to site conditions. The results show that (1) the residual contour maps, especially for the pgv, have high consistency with the regional geology and topography of Taiwan. (2) The pgv residual contours reveal that Taipei Basin, Changhua Plain, Chianan Plain, Pingtung Valley, Ilan Plain, and Taitung Longitudinal Valley have high residual values. Note that most major metropolitan areas all fall in high residual areas. (3) The site classification based on geologic criteria by Lee et al. (2001) can be simplified into three classes, that is, class E for soft soils, a combination of classes C and D for dense and stiff soils, and class B for rocks. These three site classes are mostly distributed, respectively, in the alluvial plains at an elevation less than 50 m, in terraces and hills at an elevation less than 1000 m, and in high mountainous areas.

Sediment non-linearity and attenuation of seismic waves: a study of accelerograms from Lefkas, western Greece

We investigate the dependence of the S-wave high-frequency spectral-decay parameter, κ (“kappa”) — a measure of wave attenuation — on ground-motion amplitude. 21 three-component accelerograms from two adjacent sediment sites in the town of Lefkas, western Greece, are used, representing 17 earthquakes with magnitudes Mw 4.7–7.0 and hypocentral distances 12–93km. Recorded peak horizontal ground accelerations (PGA) and velocities (PGV) are 22–540cm/s2 and 1.3–54.5cm/s.Fourier amplitude spectra are computed for S-wave windows, and the frequency range is visually determined where the high-frequency spectral decay can be approximated by a straight line on the linear-log plot; its slope (and hence κ) is computed by linear regression. κ is found to depend on hypocentral distance as κ=0.108+0.058R (r=0.518).As PGV increases from 1.3 to 54.5cm/s, κ0 (κ at 0km, characterising inelastic attenuation in the site's subsurface geology) varies between 0.060 and 0.160s. κ0 is found to correlate very strongly with logMGA (r=0.645) (MGA — mean horizontal acceleration in the S-wave window) but also with logPGA (r=0.447) and logPGV (r=0.627). We attribute this behaviour to sediment non-linearity (shear-modulus degradation), resulting in the decrease of the site's dominant-resonance frequency (from about 3.5 to 2.4Hz) and leading to the increase of κ0. Our results imply that at sediment sites, an important contribution to κ comes from wave attenuation (damping) in the softest sediments and show that κ0 is amplitude dependent, thus being a measure of sediment non-linearity.

Evidence of nonlinear site response in horizontal-to-vertical spectral ratio from near-field earthquakes

The issue is addressed as to whether the horizontal-to-vertical spectral ratio (HVSR) method is sensitive to the amplitude of ground motion from near-field earthquakes. Twenty-one three-component accelerograms from two closely located similar soil sites in the town of Lefkas are used. The recordings represent 17 earthquakes covering a wide range of magnitudes, epicentral distances and azimuths. Peak horizontal accelerations (PGA) and velocities (PGV) lie in the ranges 20–540cm/s2 and 1.4–55.2cm/s. For each HVS ratio, the site's fundamental-resonance frequency, fres, is determined visually. Linear correlation analysis shows that fres is strongly (negatively) correlated to PGA and PGV (r between −0.7 and −0.8); no correlation is found with resonance amplitude or epicentral distance. We show that the observed correlation is attributable to soil nonlinearity and indicate how weak-motion estimates of fres can be corrected for use in assessing site response during strong shaking.