High-frequency Ground Motions Research Articles

Seismogram Synthesis of Multi-Scale Layered Transversely Isotropic Saturated Half-Space Using a Revised Stiffness Matrix Method

Seismogram synthesis for a multi-scale transversely isotropic (TI) saturated layered half-space subjected to seismic dislocation source is studied by a revised stiffness matrix method in this paper. First, based on Biot’s theory of wave propagation in a fluid-saturated porous solid, the governing equations of motion in TI saturated medium are transformed by Fourier–Hankel integral transform. Next, the contribution of the seismic dislocation source can be imposed in the form of body forces. Then, the global stiffness matrix with well-conditioned properties is obtained by assembling each layer’s exact stiffness matrix, which can avoid accumulating errors in the matrix transmission. By extracting the divergence exponent terms of the stiffness matrix concerning thickness and wavenumber, the bottleneck of simulating the wave propagation process in multi-scale half-space, where the layer thickness and velocity vary sharply from the near-surface region to the crustal zone, could be overcome, and a fast and accurate seismogram synthesis could be achieved. The accuracy of the proposed method is verified by comparing the calculated results with those of published literature. Finally, the method is applied to the multi-scale system with realistic superficial fine layers. Numerical examples regarding thickness, TI properties and porosity are presented to illustrate the effects of superficial layers on ground motion. The numerical results show that the surface properties affect the wave propagation and energy distribution, leading to variable impacts on the surface response in different directions; this phenomenon is more pronounced in the case of high-frequency ground motions.

Double distance dependence in high-frequency ground motion along the plate boundary in Northern Chile

In subduction zones, the forearc crust transitions from a highly fractured wedge near the trench to a less fractured basement near the volcanic arc. Here we study the role of wedge integrity on the frequency content of strong ground motion produced by subduction earthquakes in Northern Chile. Our data includes aftershocks from the 2014 Mw 8.1 Iquique earthquake and intermediate-depth seismicity, including the 2005 Mw 7.6 Tarapacá earthquake. We focus on the S-wave spectral decay parameter κ obtained from strong ground motion in the frequency band 2−40Hz, paying particular attention to how κ varies with hypocentral distance. We report two distinct trends of κ versus hypocentral distance. One trend applies to events near the trench, where we observe a rapid increase of κ with hypocentral distance. Another trend applies to events near the bottom of the megathrust and greater depths, where we observe a relatively slow increase of κ with hypocentral distance. We interpret this difference as resulting from a heterogeneous tectonic structure with lateral variations in anelastic attenuation (i.e., attenuation seems stronger in the fractured wedge near the trench). Our results improve our understanding of the role of high-frequency ground motion in seismic hazard analysis near the Chilean coastline. In addition, our results stress the need for a better characterization of anelastic attenuation in the Chilean subduction zone, specially close to the highly fractured wedge near the trench.

Investigating Spectral Estimates of Stress Drop for Small to Moderate Earthquakes With Heterogeneous Slip Distribution: Examples From the 2016–2017 Amatrice Earthquake Sequence

AbstractEstimates of spectral stress drop are fundamental to understanding the factors controlling earthquake rupture and high frequency ground motion, but are known to include large, poorly‐constrained uncertainties. We use earthquakes from the 2016–2017 sequence in the Italian Appenines (largest event at Norcia, Mw 6.3) to investigate these uncertainties and their causes. The similarly‐sized events near Amatrice (Mw 6.0) and Visso (Mw 5.9) enable better constrained relative analysis. We calculate S wave source spectra, corner frequencies, and spectral stress drop for 30 of the larger events. We compare both empirical and modeling approaches to isolate the source spectra and calculate source parameters; we also compare our results with those from published studies. Both random and systematic inter‐study variations are larger than the standard errors reported by any individual study. The reported magnitude dependence of stress drop varies between studies, being largest for generalized inversions and smallest for more individual event based approaches. The relative spectral estimates of inter‐event stress drop are more consistent; all approaches estimated higher stress drop in the Amatrice earthquake than the similar‐sized Visso earthquake. In contrast, finite fault inversions of these two earthquakes found that the Visso earthquake had the larger region of concentrated, higher slip, whereas the Amatrice earthquake had multiple, lower slip, subevents. The Amatrice spectra contain more high frequency energy than those of the Visso earthquake. This comparison suggests that consistent measurement of a higher spectral stress drop indicates greater high‐frequency ground motion but may correspond to greater rupture complexity rather than higher stress drop.

Investigating the effect of seismicity on spatial sediment sources and loads using the fingerprinting approach

Elevated soil erosion and suspended sediment loss are some of the most severe environmental problems in the river catchments of Iran. Seismic activity is known to elevate sediment loss and this study investigated sediment sources and loads in the Talar Drainage Basin in Iran, in the context of earthquake frequency and magnitude. A catalogue of earthquakes in the study region was assembled to estimate the expected high-frequency ground motions (i.e., Peak Ground Acceleration (PGA)). The horizontal ground acceleration was estimated using five global and reginal attenuation relationships. To fingerprint sub-basin spatial sediment sources in the study area, two size fractions (<63 μm and 63–125 μm) and 49 geochemical elements were analysed on 30 sediment samples collected from three potential tributary sub-basin spatial sediment sources and 10 target sediment samples collected at the overall basin outlet. Source contributions were estimated using a Bayesian un-mixing model. The estimated relative contributions of the individual spatial sediment sources for both the <63 μm fraction (51.9% (48.6–55.3), 48 % (44.6–51.3), and 0. 1% (0–0.2)) and 63–125-μm fraction (68.2% (66.4–69.8), 31.7% (30.1–33.6), and 0.1 % (0–0.2)) correlated with recorded seismic events. The correlations between sub-basin specific sediment loads and PGA were r = 0.68 for the <63 μm fraction and r = 0.99 for the 63–125 μm fraction. The results demonstrate that seismic activity and ground acceleration can elevate erosivity and erodibility factors. This study supports environmental planners for targeting management to reduce elevated suspended sediment loads and preserve fluvial habitats.

Estimation of broadband ground motion characteristics considering source parameter uncertainty and undetermined site condition in densely populated areas of Pingwu

Three destructive earthquakes occurred in Pingwu and Songpan, Sichuan Province, China, between August 16 and 23, 1976. Due to the seismic monitoring capability at that time, the ground motion characteristics of these earthquakes are very vague. Realistc and reliable strong ground motion input plays an important role in seismic building design and urban-scale earthquake damage simulation. This study reproduces the main broadband ground motion characteristics of the 1976 Ms7.2 Songpan earthquake in densely populated areas of Pingwu. The empirical Green’s function method and finite difference method are used to simulate high-frequency and low-frequency ground motion, respectively, and the broadband ground motion is obtained by superposition within the frequency range. In addition, in combination with the “Recipe” source parameter scheme, various uncertainties in the source parameters are considered, including the source mechanism, source depth, asperity parameters, etc. We obtain 36 kinds of broadband ground motion at six typical locations in the Pingwu area. Moreover, we test the rationality of the obtained broadband ground motion by ground motion prediction equations(GMPEs), and the broadband ground motions are consistent with the local ground motion characteristics. The results show broadband ground motions obtained from the scenario earthquake in this paper can meet the destructive capacity of earthquakes of this magnitude. The hybrid method can effectively compensate for the lack of long-period components of the original empirical Green function method. This research also proves that the peak ground acceleration (PGA) of ground motion is mainly contributed by high-frequency ground motion components. Long-period ground motion contributes most to the peak ground velocity (PGV), and about twice the contribution of high-frequency ground motion. Concerning the Chinese seismic intensity scale (GB/T 17742-2020) and China Seismic Ground Motion Parameter Zoning Map (GB18306-2015), the basic fortification intensity in the Pingwu area is VIII. In this paper, the seismic intensity of PWN is VI-VII, indicating that the buildings at this location are less likely to be damaged after the earthquake. The seismic intensity of other regions is VII-IX and buildings are more likely to be damaged during the earthquake at these locations. There are many mountains and valleys in the Pingwu area, and the probability of landslides, debris flows, and other disasters after an earthquake is very high, and we should give special attention to the impact of secondary disasters caused by earthquakes. It is necessary to prevent dammed lakes and other disasters caused by landslides and debris flows.

Estimates of κ0 and Effects on Ground Motions in the San Francisco Bay Area

ABSTRACTGround-motion studies are a key component of seismic hazard analyses and often rely on information of the source, path, and site. Extensive research has been done on each of these parameters; however, site-specific studies are of particular interest to seismic hazard studies, especially in the field of earthquake engineering, as near-site conditions can have a significant impact on the resulting ground motion at a site. There has been much focus on the constraint of site parameters and their application to seismic hazard studies, especially in the development of ground-motion models (GMMs). Kappa is an observational parameter describing the high-frequency attenuation of spectra, and its site contribution (κ0) has shown to be a good predictor of high-frequency ground motions; however, measurements are often limited. In this study, we develop a κ0 dataset for the San Francisco Bay area (SFBA) by estimating κ0 for 228 stations, and we produce a continuous regional map of κ0. We find κ0 to range between 0.003 and 0.072 s, with larger values concentrating on the east, north, and south sides of the bay, and lower values concentrating on the west side. We also evaluate the robustness of κ0 as a site parameter and find it to correlate with peak ground acceleration. These estimates of κ0 can add predictive power to GMMs, thus increasing the accuracy of predicted ground motion and improving the robustness of ground-motion studies in the SFBA.

Feasibility study of a practical causal rate-independent damping device for the improved performance of seismic isolated structures

When rate-independent linear damping (RILD) is incorporated into a base-isolated structure, it achieves a similar response displacement reduction effect with significantly lower floor response acceleration compared with linear viscous damping (LVD) with the same loss factor. To address the undesirable stiffness added to an isolation layer when we adopt a mechanical device comprising a few branches of a spring–dashpot link to realize RILD, this study proposes canceling the undesirable isolator stiffness by adding an inerter element and reducing the isolator stiffness. A heuristic optimization method was developed to design the proposed mechanical device to mimic the behavior of RILD. A parametric survey on the optimal designs of the device suggests that the three branches of the spring–dashpot link work best in terms of practicality and feasibility. Seismic control performance was assessed using a 10-story base-isolated building mounted on linear and nonlinear isolation systems. Analyses of linear systems revealed that the proposed device achieved lower inter-story drifts and approximately 40% reduction in floor response acceleration with a 4% increase in isolator displacement compared with the LVD when subjected to high-frequency ground motions. Analyses of the nonlinear systems showed that the displacement control performance of the proposed model was slightly compromised when subjected to ground motions dominated by low-frequency components, thereby identifying a further challenge in developing a causal RILD device for nonlinear structures.

A frequency-dependent variable-damping control based variable-orifice damper for smart base-isolation system

Damping devices are commonly used in the isolation layer of base-isolated structures to provide additional damping to control the structural responses. Normally, large damping for the isolation layer is useful to minimize the resonance effect under low-frequency ground motions, and small damping is preferred to reduce the structural responses under high-frequency ground motions. Traditional passive dampers are widely adopted in engineering practice. Due to the fixed damping characteristic, however, the performance of base-isolated structures with passive dampers cannot stay optimal under ground motions with different frequency characteristics. To realize an optimal control of base-isolated structures, semi-active control has been increasingly valued for its efficiency and intelligence to adjust the damping characteristics based on feedback information from the structure and ground motion. However, a large number of sensors and hardware components are required for semi-active control, which makes the system complex and costly. In this paper, a variable-orifice damper (VOD) for smart base-isolation system is proposed. The VOD can adaptably adjust its damping according to the frequency of the ground motion without additional power or sensors. Theoretical analysis and sinusoidal excitation tests were carried out to investigate the behavior of VOD. A series of numerical simulations of a base‐isolated structure equipped with the proposed VOD and subjected to ground motions are also conducted. A semi-active control with ON-OFF strategy, and two passive controls (passive-on and passive-off) are conducted for comparison. The simulation results show that the VOD can result in an optimal control under ground motions with different frequency characteristics.

Improvement of Stochastic Green’s Function Method for 3D Broadband Ground-Motion Simulation

Abstract Inputting reasonable ground motion is very significant in the seismic design of engineering structures under near-fault earthquakes. At present, the stochastic Green’s function method has been successfully applied to the simulation of moderate to high-frequency ground motions, but its accuracy is poor for low-frequency ground-motion simulation. In this study, an improved stochastic Green’s function method that is used to simulate broadband ground motion is established by considering the variation of the correlation of phase spectra among small earthquakes in different subfaults with the frequency and distance as well as the variation of the radiation pattern with the frequency and distance. Taking the 1994 Northridge earthquake in America and the 2013 Lushan earthquake in China as examples, the simulation results by the improved stochastic Green’s function method are compared with observed ground-motion records. The results show that considering the influence of near-field and intermediate-field terms has a little effect on the accuracy of ground-motion simulation. The directivity effect of near-fault ground motion can be reflected to a certain extent by considering the variation of the correlation of phase spectra among small earthquakes in different subfaults with the frequency and distance. Considering both the variation of the correlation of phase spectra among small earthquakes in different subfaults with the frequency and distance and the variation of the radiation pattern with the frequency and distance, the simulated acceleration response spectra generally show good agreement with the observed records. Therefore, the improved stochastic Green’s function method proposed in this study can simulate the broadband ground motion effectively.

Kathmandu Basin as a local modulator of seismic waves: 2-D modelling of non-linear site response under obliquely incident waves

SUMMARYThe 2015 Mw 7.8 Gorkha, Nepal earthquake is the largest event to have struck the capital city of Kathmandu in recent times. One of its surprising features was the frequency content of the recorded ground motion, exhibiting a notable amplification at low frequencies (&amp;lt;2 Hz) and a contrasting depletion at higher frequencies. The latter has been partially attributed to the damper behaviour of the Kathmandu basin. While such weak high-frequency ground motion helped avoiding severe damage in the city, the catastrophic outcomes of earlier earthquakes in the region attest to a contrasting role of the Kathmandu basin as a broad-band amplifier, in addition to possible source effects. Given the possibility of future strong events in the region, our main objective is to elucidate the seismic behaviour of the Kathmandu basin by focusing on site effects. We numerically model 2-D P–SV wave propagation in a broad frequency band (up to 10 Hz), incorporating the most recent data for the Kathmandu basin geometry, soil stratigraphy and geotechnical soil properties, and accounting for the non-linear effect of multidimensional soil plasticity on wave propagation. We find that: (1) the Kathmandu basin generally amplifies low frequency ground motion (&amp;lt;2 Hz); (2) waves with large incidence angles relative to vertical can dramatically amplify the high frequency ground motion with respect to bedrock despite the damping effect of soil non-linearity and (3) the spatial distribution of peak ground motion amplitudes along the basin is highly sensitive to soil non-linearity and wave incidence (angle and direction), favouring larger values near the basin edges located closer to the source, as observed during the 2015 event. Our modelling approach and findings can support the ongoing resilience practices in Nepal and can guide future seismic hazard assessment studies for other sites that feature similar complexities in basin geometry, soil stratigraphy and dynamic soil behaviour.

Attenuation Characteristics of High-Frequency Ground Motions from Local Sources Caused by Great Subduction Zone Earthquakes in Northeast Japan

AbstractGround-motion attenuation characteristics are examined for the peak ground accelerations (PGAs) and peak ground velocities (PGVs) from strong-motion generation areas (SMGAs), which emit strong high-frequency waves during great subduction zone earthquakes. A conventional ground-motion prediction equation (GMPE) for earthquakes is designed based on the magnitude and distance from the source fault to predict the peak ground motion amplitude. For great subduction zone earthquakes, significant wavetrains of high-frequency ground motions are often separately observed in seismograms, and the corresponding rupture areas are estimated as SMGAs along the plate interface. In this case, although the advantages of using the shortest distances measured from the closest SMGAs rather than the shortest fault distance have been confirmed in previous studies, it is more physically reasonable to examine the ground-motion attenuation characteristics of individual SMGAs based on their magnitudes and locations. Therefore, we examine the attenuation characteristics of the PGAs and PGVs of individual SMGAs of the 2003 Mw 8.2 Tokachi–Oki earthquake and 2011 Mw 9.1 Tohoku earthquake in the Japan subduction zone considering data at stations within an SMGA distance of 300 km and the SMGA moment magnitude. The model, exhibiting the same functional form as a conventional GMPE, is fitted to the PGA and PGV data pertaining to each SMGA that are normalized to a bedrock site with VS30=760 m/s at any distance. The uncertainties in the obtained PGAs and PGVs are 3.6% and 13.5% lower, respectively, in terms of the residual in logarithmic units than those in the results of previous approaches considering only the SMGA distance. This result could help develop GMPEs for SMGAs to more appropriately predict the strong motions generated during great subduction zone earthquakes.

Stochastic Model to Characterize High-Frequency Ground Motion at Depth Validated by KiK-Net Vertical Array Data

ABSTRACT The design of important structures for earthquake resistance requires an assessment of the local seismic hazard. One of its essential components is a site response that evaluates the amplification and attenuation of ground motion on a local scale. The shaking on the ground surface (in which it is generally measured) differs from the one at a depth; therefore, there is a need to characterize the ground motion at depth for important underground structures and buildings with deep foundations. In this study, we introduce a method to characterize the high-frequency (&amp;gt;1 Hz) ground motion at depth. The method makes use of a novel stochastic model (SM) that relates the ground motion at depth and on the surface in the Fourier domain. The SM is physics-based, its spectral amplification resembles an empirical 1D site response, and it allows reliable full-waveform ground-motion predictions. The method is validated through the comparison with empirical surface-to-borehole amplification curves observed in 144 selected KiK-net vertical arrays in Japan. Using a frequency range of 0.1–50 Hz, we identified 36 and 83 sites with a good and partially good mutual fit of theoretical and empirical amplification curves, respectively. Finally, we demonstrate the performance of the method in two diverse applications. First, we design a Bayesian inversion of the empirical surface-to-borehole amplification to retrieve the S-wave velocity model and an effective value of t* (the path-integrated effect of the quality factor). This inversion is applied to all selected KiK-net sites. Second, we perform a full-waveform prediction of the ground motion at depth from surface recordings of the 2018 northern Osaka Mw 5.6 earthquake. Both of these applications demonstrate a good performance of our SM in a broad frequency range.

XYtracker: a new approach to estimate fault rupture extent in real time for large earthquakes

We present a methodology for estimating fault geometry and utilizing the distance to the fault for the shaking estimation to improve the accuracy of real-time shaking estimates for large earthquakes. Most of the earthquake early warning system currently estimates the seismic intensity with the ground-motion prediction equations (GMPE) as a function of the hypocenter distance. However, using the fault distance computed from a finite source model can improve the accuracy of the shaking intensity estimation for large earthquakes. This study proposes a novel methodology, XYtracker, to estimate the surface projection of the fault extent and real-time seismic intensity. For large earthquakes, high-frequency ground motions tend to saturate over the magnitude range and strongly correlate with fault distance. As a result, this work can achieve the fault extent using seismic intensity and GMPE. We considered three types of fault models: point-source, line-source, and rectangle-source model. We found the most probable model parameters for each model by minimizing the residual sum of squares between the observed and estimated seismic intensities. The Akaike Information Criterion selected the most probable model among them. The strong motion data set of the 2008 Wenchuan, 2011 Tohoku, and 2016 Kumamoto earthquakes was used to test our methodology. The new method for estimating the fault geometry can obtain the ongoing rupture length and direction using the strong motion data. The model selection scheme with the Akaike Information Criterion selected the finite-source model to explain the shaking distribution. Results revealed that this new approach performed well in estimating the fault dimension. The method can promote the accuracy of the seismic intensity estimation for future large earthquakes, including the subduction earthquakes.Graphical

Parameter estimation for predicting near-fault strong ground motion and its application to Lushan earthquake in China

Stochastic finite-fault modelling based on the dynamic corner frequency is a widespread method for synthesizing near-fault ground motions at high frequencies and has been proven to be an effective tool for simulating high-frequency ground motions by researchers globally. Many input parameters, which have more uncertain effects on the synthetic result, are related to sources, sites, and paths in stochastic modelling. Thus, it is difficult to select reasonable parameters. When using the stochastic finite-fault method, the key link to be considered is how to determine the input parameters required for simulations. Based on this fact, this study analyzed and discussed the sensitivity of several important input parameters in the simulation parameters to the simulation results. The results showed that for the Class C site stations, the crustal and local site amplification factors must be considered simultaneously. Overall, the influence of the high-frequency attenuation parameter κ0 calculated using different methods on the simulation results is not evident. For near-fault stations, various upper-edge buried depth synthetic response spectra can be set, and finally, the average value is taken as the final synthetic recorded response spectra. When using the random source model, the rupture starting point must be set on the subfault in the middle along the fault length and width direction. The conclusions provide a basis for a highly reasonable and rapid determination of input parameters when using the stochastic finite-fault simulations for practical engineering applications and can improve the simulation accuracy to apply the finite source modelling to rebuild scenarios and evaluate and alleviate the seismic hazard in the target region.

Spatial Heterogeneity of Fault Slip and the Radiated Spectra of Ground Motions

ABSTRACTKinematic models of simulating earthquake radiation in seismic hazard analysis typically require prescribing the distribution of final slip over the hypothetical fault planes. The spatial spectra of heterogeneous slip affect the frequency spectra of the seismic body waves. The representation integral of elasticity provides a convenient analytical tool by which the relationships between the slip spectra in the wavenumber domain and the wave spectra in the frequency domain can be scrutinized. In the limit of the waves from a small source in the far field, the Fourier spectrum of wave displacement is the spectrum of the slip-rate function multiplied by the spatial slip spectrum representing fault directivity. A popular model for the latter is the k-square slip distribution. Classic results prescribe that for a typical ω-square source time function, such multiplication, conversely to a common assumption that the k-square slip distribution always leads to the ω-square decay of the high-frequency seismic spectra, can result in the ω−4 power-law decay. Such steep fall-off rates are highly unusual in observations, suggesting that the k-square heterogeneous slip in certain cases may significantly underpredict the realistic high-frequency ground motions, including peak velocities and accelerations. An alternative use of heterogeneous slip distributions would be to explain the additional high-frequency diminution of the observed spectra that is usually attributed to ad hoc cutoff (“fmax” or “kappa”) filters. The simple asymptotic relationships between heterogeneous fault slip and body wave spectra may not hold true in the vicinity of large earthquakes, at distances of main interest to hazard calculations.

Spectral decay parameter (κ) analysis for a single fault source and an automatic selection procedure for spectral flattening frequency (fx)

Lately the spectral decay parameter, κ, which essentially represents the decay of the high frequency ground motion amplitudes, is highly utilized and also debated in the scientific community due to the uncertainties and the inconsistencies of different evaluation processes. To reduce source-based uncertainties, a dataset with a good spread of magnitude variety and near as well as far site recordings is needed. Furthermore, the selection of the frequency where the exponential decay of the acceleration spectrum becomes flat, fx, should be performed based on a consistent procedure to minimize the contamination introduced by the noise spectrum. In this study, to reduce the source effect on κ evaluations, only the events generated from Sivrice-Pütürge segment of East Anatolian Fault are considered. This also provided a regional Q correction that is not influenced by different source generated events during the estimation of the zero epicentral spectral decay parameter, κ0 for stations under consideration. An automated selection procedure for fx is developed which yielded a good fit to the dataset. The stations were also examined for the peak frequencies of horizontal-to-vertical spectral ratios (HVSR) whether these peak frequencies intercepted with the frequency band considered in the κ evaluations. The HVSR calculations additionally provided information on the soil properties at the station locations. The findings are considered to be useful for understanding the effect of κ versus epicentral distance (Repi) fit on the estimation of station-based κ0 values. Furthermore, the proposed procedure for obtaining spectral flattening frequency (fx), may improve compatibility among κ evaluations, and reduce selected frequency band related uncertainties.

Effect of Boundary Conditions on Seismic Response of Electrical Equipment subjected to High-Frequency Ground Motions

The updated ground motion hazard in Central and Eastern United States contains high frequency motions. Such motions do not cause structural damage. However, safety-related electrical equipment are sensitive to high-frequency accelerations. Seismic qualification of electrical equipment requires an in-cabinet response spectrum (ICRS) which depends on the dynamic response of electrical cabinets and buildings. A linear analysis to generate ICRS can give excessively high spectral accelerations. Cabinet models with geometric nonlinearities such as a gap in the connection with building floor show that high-frequency accelerations may not propagate to the electrical equipment. Unlike linear analysis, results from nonlinear analysis of a building-cabinet model with a gap show a periodic pattern of secondary peaks and valleys at higher frequencies. In this research, the unique observation of secondary peaks is explained through fundamental principles by considering the sliding response of a cabinet representation with gap subjected to harmonic excitation. It is observed that a periodic pattern of peaks and valleys exist due to a significantly large contribution from transient response, due to localized impacts at the gap in cabinet base, and its interaction with the steady-state response.

Reproducing the Spatial Characteristics of High-Frequency Ground Motions for the 1850M 7.5 Xichang Earthquake

AbstractReproducing the spatial characteristics of large historical earthquakes and predicting the strong ground motions of future destructive large earthquakes through actual small earthquakes have high-practical value. The empirical Green’s function method is a numerical simulation method that can impart real seismic information in synthetic ground motions. In this article, we use data from the 2018 M 5.1 Xichang earthquake to reproduce the ground-motion characteristics of the 1850 M 7.5 Xichang earthquake using the empirical Green’s function method. The uncertainties of the parameters, such as the number, area, and locations of asperities, are considered. The synthetic time histories, peak ground accelerations (PGAs), and response spectra are obtained through simulation. The main results are as follows. (1) The synthetic Xichang earthquake (such as the ground-motion intensity and attenuation characteristic of the PGA) matches well with the M 8.0 Wenchuan earthquake and M 7.3 Jiji earthquake. When the number of asperities is 1 or 2, the PGA characteristics of the Xichang earthquake match well not only with the Next Generation Attenuation-West2 (2014) ground-motion model in the range of 100 km but also with the seismic ground-motion parameter zonation map of China in the range of 20–100 km. (2) The prediction results based on the asperity source model are relatively reliable in the range of 20–100 km. The one-asperity and two-asperity models of the Xichang earthquake match better than the three-asperity and four-asperity models. (3) We can speculate that when the M 7.5 earthquake struck the Xichang area, the damage was relatively strong. The PGA may have exceeded 1.0g in the meizoseismal area, and the seismic intensity in the meizoseismal area may have reached or exceeded a degree of X–XI. Therefore, the synthesized M 7.5 Xichang earthquake has the strength characteristics of a large destructive earthquake.

Comparison of Analysis and Design of Regular and Irregular Configuration of Multi Story Building in Seismic Zones

Abstract: This paper is concerned with the effects of various vertical irregularities on the seismic response of a structure. The objective of the project is to carry out Response spectrum analysis (RSA) and Time history Analysis (THA) of vertically irregular RC building frames and to carry out the ductility based design using IS 13920 corresponding to Equivalent static analysis and Time history analysis. Comparison of the results of analysis and design of irregular structures with regular structure was done. The scope of the project also includes the evaluation of response of structures subjected to high, low and intermediate frequency content earthquakes using Time history analysis. Three types of irregularities namely mass irregularity, stiffness irregularity and vertical geometry irregularity were considered. According to our observation, the storey shear force was found to be maximum for the first storey and it decreases to minimum in the top storey in all cases. The mass irregular structures were observed to experience larger base shear than similar regular structures. The stiffness irregular structure experienced lesser base shear and has larger inter-storey drifts. The absolute displacements obtained from time history analysis of geometry irregular structure at respective nodes were found to be greater than that in case of regular structure for upper stories but gradually as we moved to lower stories displacements in both structures tended to converge. . Lower stiffness results in higher displacements of upper stories. In case of a mass irregular structure, time history analysis gives slightly higher displacement for upper stories than that in regular structures whereas as we move down lower stories show higher displacements as compared to that in regular structures. When time history analysis was done for regular as well as stiffness irregular structure, it was found that displacements of upper stories did not vary much from each other but as we moved down to lower stories the absolute displacement in case of soft storey were higher compared to respective stories in regular structure. Tall structures were found to have low natural frequency hence their response was found to be maximum in a low frequency earthquake. It is because low natural frequency of tall structures subjected to low frequency earthquake leads to resonance resulting in larger displacements. If a high rise structure (low natural frequency)is subjected to high frequency ground motion then it results in small displacements. Similarly, if a low rise structure (high natural frequency) is subjected to high frequency ground motion it results in larger displacements whereas small displacements occur when the high rise structure is subjected to low frequency ground motion.

Understanding the seismic response of electrical equipment subjected to high–frequency ground motions

Ground motion studies in Central and Eastern United States (CEUS) show that the ground motion response spectra exceeds design spectrum of nuclear power plants in high–frequency range. To ensure safe shutdown of nuclear power plants in events where design spectrum is exceeded, the safety–related digital control systems and electrical equipment such as relays must be seismically qualified for high–frequency ground motions. Conventionally, uncoupled linear analysis of building and electrical cabinet is conducted and then seismic demands on equipment are evaluated. The conventional analysis ignores effects of various factors such as geometric nonlinearities, mass interaction between primary (building) and secondary (electrical cabinet) systems, etc. Ignoring such factors leads to unnecessarily high seismic demands on equipment. In this study, the seismic demands on equipment are evaluated by conducting both coupled and uncoupled analysis of linear primary system and linear as well as nonlinear secondary system. Various cases are analyzed where at least one mode of primary system is tuned with one mode of secondary systems. Each case is subjected to both low–frequency and high–frequency ground motions to compare the difference in response to each ground motion. The seismic demands on equipment are compared for coupled and uncoupled analysis as well as for linear and nonlinear systems. The results show reduction in seismic demands on electrical equipment for various nonlinear coupled analysis cases. The high seismic demands evaluated from linear uncoupled analysis may lead to exclusion of some electrical equipment which can be avoided by conducting coupled nonlinear analysis of primary–secondary systems and obtaining the realistic seismic demands on equipment.