Pseudo-acceleration Response Spectra Research Articles

The 2023 New Zealand Ground-Motion Database

ABSTRACT This article summarizes the development of the 2023 New Zealand ground-motion database (NZGMDB). A preceding version was formally used as the central ground-motion database in the ground-motion characterization modeling for the 2022 New Zealand (NZ) National Seismic Hazard Model (NSHM) revision. The database contains ground motions for events with a moment magnitude greater than ∼3.0 from the years 2000 to the end of 2022. Several challenges associated with NZ earthquake source metadata are explained, including determination of earthquake location, magnitude, tectonic classification, and finite-fault geometry, among others. The site table leverages the site database developed as a part of the 2022 NZ NSHM revision, and several definitions of source-to-site distance are computed for the propagation path table. The ground-motion quality classification was initially assessed using a neural network. Subsequent waveform quality verification was conducted and additional quality criteria were enforced to ensure a sufficiently high-quality database. Standard processing techniques were applied to the ground motions before intensity measure (IM) calculation. IMs in the database include peak ground acceleration, 5%-damped pseudoacceleration response spectra, smoothed Fourier amplitude spectra, and other cumulative and duration-related metrics. The NZGMDB is publicly available and routinely updated as new and higher quality data become available.

AN INVESTIGATION OF DAMPING MODIFICATION FACTORS CORRESPONDING TO DIFFERENT DAMPING RATIOS FOR SDOF SYSTEMS

The damping ratio is an important parameter in dynamic analyses and plays a key role in the design of building structures. Elastic response spectra are widely used in this design to describe the earthquake action in specific site classes. 5% damped response spectra are generally used for most of the conventional structures. However, other types of structures may not have a damping ratio of 5% and can have much lower or much larger damping ratios. The damping ratio is recommended at about 5% for concrete structures whereas it is estimated at 2% for steel structures. Tall slender buildings may have much lower damping ratios and low-rise buildings may be designed using much larger damping ratios than 5%. For these types of buildings, elastic response spectra are modified to account for different levels of damping ratios. This study proposes the damping modification factors (DMFs) which are computed using 5% response spectra as the benchmark. A strong ground motion set has been selected for the stiff site class and the displacement, pseudo-velocity, and pseudo-acceleration response spectra have been computed for single-degree-of-freedom (SDOF) systems. DMFs have been obtained in terms of these spectra and 3%, 10%, 20%, and 30% damping ratios have been considered. Variations of DMFs with different damping ratios have been obtained graphically. It can be seen from the results that DMFs are sensitive to the vibration period of the SDOF system.

Numerical investigation of the effects of symmetric and eccentric earthquake-induced pounding on accelerations and response spectra for two-storey structures

This paper examines the effect of symmetric and eccentric earthquake-induced pounding between adjacent two-storey structures on acceleration times histories and response spectra. The numerical investigation benefits from experimental data issued from an extensive shake table campaign involving two-storey steel–concrete composite structures and slab-to-slab contacts. It is shown that the high frequency branch of the floor response spectra is only related to the maximum impulse acceleration spike during time, characterized by its impulse value, contact duration and time-history shape. In order to numerically reproduce such characteristics, three-dimensional detailed models are set up by modeling the slab with shell elements or hexahedral elements. Previously developed explicit time integrator, the CD-Lagrange scheme, based on non-smooth contact dynamics approach, is employed for ensuring the contact conditions expressed in terms of velocity and impulse, without requiring any contact parameters such as contact stiffness, local damping and restitution coefficient. The eccentric pounding between slabs is simulated by introducing a small defect in the parallelism of the colliding slabs, as measured in experience. The results of eccentric and symmetric pounding, assuming elastic behavior for the steel and concrete materials, are compared with the experimental data in terms of acceleration time-histories and pseudo-acceleration response spectra. It is concluded that symmetric pounding predicts an acceleration spike with a more abrupt increase to the peak and a shorter contact duration in comparison to experimental data, leading to an overestimation of the high frequency branch of the response spectra. On the contrary, eccentric pounding achieves to reproduce the experimental acceleration time histories as well as response spectra up to 400 Hz. Finally, the robustness of the proposed numerical approach is underlined by changing the time step size of the CD-Lagrange scheme as well as by prospecting different relevant choices concerning the Rayleigh damping.

Frequency-dependent site amplification functions for key geological units in Iceland from a Bayesian hierarchical model for earthquake strong-motions

A fundamental modeling assumption in engineering seismology is that seismic ground motion is affected by source, propagation path and site effects. Contrary to the first two, comprehensive studies on site effects, the localized amplification effects on seismic ground motion due to the uppermost geological structure, have been limited in Iceland. As a result, site amplification has been modeled in ground motion models in a simplistic and qualitative way. In this study, we address this limitation by using the new Bayesian hybrid GMM proposed by Kowsari et al. (2020) to model the recorded ground motions at regional strong-motion stations in southwest Iceland, specifically the peak ground acceleration and pseudo-acceleration response spectra as a function of oscillator period (i.e., PGA and PSA at T=0.01-3.0 s), earthquake magnitude, distance and hypocentral depth, and at first, we omit a site-term. Then, we incorporate the GMM in a Bayesian Hierarchical Model (BHM) that allows prior distributions of model parameters to be used and we partition the model residuals into various terms associated with the source, propagation path and station terms. Through Bayesian inference of the model parameters the results show that the station term residuals reveal a frequency-dependent site amplification at the recording sites. By inspecting this systematic behaviour, we propose four groups of station terms by their characteristics. We then carefully inspect the station locations in terms of their associated geological unit and observe that the geological units and station term groups are qualitatively correlated. By combining the two, we propose the first frequency-dependent average site amplification functions associated with key geological units in Iceland based on age and formation. Provisionally we refer to them as hard rock, rock, (Holocene) lava rock, and (unspecified) soil site classes, respectively. We then update the GMM by incorporating these amplification functions so that it now enables the prediction of PGA and PSA at these four site classes. As a result, the new model standard deviations are considerably lower, with those associated with the event term being τ = 0.05, station terms φS2S = 0.10, intra-event φ = 0.17 and total σ = 0.18 in common logarithm units for PGA. The new site class-specific GMM now enables the site-specific ground motion for separate geological units which in turn improves site-specific seismic hazard assessment and informed site-specific engineering decision making in Iceland.

Evaluation of random-vibration procedures to estimate response spectral ordinates on soft soil sites from fourier amplitude spectra

This study evaluates Random-Vibration Procedures (RVP) to estimate 5% damped response spectra of ground motions recorded on soft soil sites from Fourier Amplitude Spectra (FAS). The study makes use of 136 ground motions recorded in the soft soil Mexico City region during two recent large magnitude earthquakes. Seven different RVP are considered in this investigation consisting of four peak factor models, three strong ground motion duration models, and four non-stationary empirical correction factors. Spectral ordinates computed through conventional time integration were compared to approximate spectral ordinates computed using the FAS of the recorded acceleration time series in combination with RVP. A statistical error analysis was then conducted to determine how effective the various RVP were at estimating the exact pseudo-acceleration response spectra computed with response history analyses. It is found that most RVP commonly used today produce significant systematic overestimations of spectral ordinates at the fundamental mode of the soil deposit. Modifications to durations are proposed to be used in conjunction with various peak factors to reduce overestimations at the site period.

Statistical calibration of two-dimensional seismic aggravation effects for homogeneous basins

Two-dimensional basin effects often play a relevant role in defining earthquake ground motion at sites with deep morphologies. The quantitative estimation of two-dimensional effects requires a numerical approach which is typically complex and computationally demanding. Due to modelling complexities, basin effects are not routinely accounted for and have not received significant attention in seismic design codes. One-dimensional seismic response can be modeled with greater ease but does not allow their confident investigation. The use of simplified approaches which allow the estimation of two-dimensional effects from one-dimensional analyses is thus warranted. Past studies have defined and investigated aggravation factors, which can be applied as multiplicative coefficients to one-dimensional ground response outputs to account for two-dimensional basin effects. This paper proposes a novel framework for estimating basin effects by assigning aggravation factors for a vast range of symmetric basins overlain by a single-layer, homogeneous soil deposit and for varying basin sizes, dynamic geotechnical properties, and seismic inputs. In the study, aggravation factors are defined with reference to pseudo-acceleration response spectrum (5% of critical damping) and are calibrated statistically using a large set of dimensionless parameters from the outputs of an extensive set of one-dimensional and two-dimensional numerical analyses. The calibration process is described along with notable observations regarding the spatial variability of the magnitude of two-dimensional aggravation along the surface of the deposit. The proposed method is amenable to current design code formats involving tabulated partial factors. In principle, aggravation factors obtained in this study can be applied to 1D response spectra developed either from the provisions of the design codes themselves or from site-specific ground response analyses. A discussion focusing on the validation of the proposed approach using experimental and empirical measurements is provided. An example application attests to the simplicity of use of the method.

An updated stochastic finite fault modeling: Application to the Mw 6.0 earthquake in Jiashi, China

On January 19, 2020 at 21:27 local time, the Jiashi County in China witnessed an Mw 6.0 earthquake. The earthquake event was recorded by 42 strong ground-motion observation stations and recorded a total of 126 sets of acceleration time history records. In this study, we simulated the Jiashi earthquake based on the updated finite-fault modeling. The zero-distance high-frequency attenuation parameter κ0 and local site amplification factor were calculated based on the recordings, and the other input parameters were obtained from the previous studies. Then the Fourier amplitude spectrum, 5%-damped pseudo-acceleration response spectrum, peak ground acceleration, and accelerograms at 12 selected stations were synthetized using an updated finite-fault modeling. The results indicated that the synthetic results had no evident differences at most of the selected stations, which showed that the updated approach can be adopted to predict the ground motion. In addition, the simulated accelerograms are similar to the observed records at most stations, which indicated that the updated modeling can be used to generate the seismic input records required for structural analysis. In the future research, we should more analyze the interconnection between each input parameter and establish the relationship between parameters. If so, even though we cannot obtain the detailed information of several parameters, we can still accurately estimate the seismic field of imminent earthquakes in the future, which will help to establish the disaster maps of the target region.

A novel approach to improve accuracy in seismic fragility analysis: The modified intensity measure method

Seismic fragilities, i.e., probabilities that structural systems exceed specified limit states, are used in Performance-Based Earthquake Engineering to characterize structural behavior during seismic events. Generally, seismic fragilities are constructed from structural responses to scaled accelerograms described by single/multiple ordinates of the pseudo-acceleration response spectrum, peak ground acceleration or other intensity measures. Recently, it was shown that the resulting fragilities provide limited if any information on the structural seismic performance if the dependence between intensity measures and demand parameters is weak. Yet, the method is used in Performance-Based Earthquake Engineering since the statistical uncertainty caused by the limited numbers of recorded ground accelerations is overcome by scaling these records.This study develops a method for estimating the fragility of linear systems for cases in which intensity measures and demand parameters are weakly correlated, i.e., situations in which the current methodology is inaccurate. The method is based on a linear transformation of samples of a given intensity measure which is designed to improve the correlation between demand and intensity parameters. The effectiveness of the proposed method for linear system is demonstrated by an elementary oscillator, a multi-degree of freedom system and a real complex multi-degree of freedom structural system.

High spatial-resolution loss estimation using dense array strong-motion near-fault records. Case study for Hveragerði and the [formula omitted]6.3 Ölfus earthquake, South Iceland

The most recent and costliest damaging earthquake in Iceland is the Mw6.3 29-May-2008 Ölfus earthquake to date. In particular, Hveragerði town located in the extreme near-fault region, suffered intense horizontal peak ground accelerations (PGA) of ∼ 40–90%g and large amplitude and long-period near-fault pulses, recorded on a dense urban strong-motion array in the town. In this study we collated a high-spatial resolution exposure database (building-by-building) complete with actual reported losses and classified the buildings by building materials and construction year according to the code design requirements in place at the time.We took advantage of the array data and evaluated a set of well-known ground motion intensity measures (IM), including PGA, pseudo-acceleration response spectra at short-to-long periods, Arias Intensity and Cumulative Absolute Velocity. We applied empirical Bayesian kriging geostatistical analyses to generate high-resolution shakemaps and provide IM estimates for each building. The shakemaps showed a significant and systematic variation of the IMs across the small study area, with the lowest ground motions observed centrally and highest values in the outskirts. Furthermore, correlation analysis was carried out for the damage ratio and the exposure data IMs, but only low-to-moderate correlations were observed. A key reason is the incurred losses were primarily due to damage to non-structural components, to which the code design requirements do not apply. We carried out a seismic loss assessment in Hveragerði for the earthquake scenario of the Ölfus earthquake both on building-by-building, and municipality levels of spatial resolution. We applied both local and global fragility models associated with detail building typologies identified based on the SERA taxonomy scheme.The results show that the global fragility functions severely underestimate the seismic performance of the building stock, except for one-story reinforced concrete buildings, while overall the masonry buildings were associated with the most predicted and observed losses. On the other hand, the local models predicted losses that conformed well with the observed damages to timber and concrete buildings. The high-spatial-resolution predictions of losses gave results that better correlated with the observed losses in most typologies.

Simulation of Pulse-Like Ground Motions during the 2015 Mw 8.3 Illapel Earthquake with a New Source Model Using Corrected Empirical Green’s Functions

Abstract Pulse-like near-source ground motions were observed by the local network during the 2015 Mw 8.3 Illapel, Chile earthquake. Such ground motions can be quite damaging to a wide range of infrastructures. The primary objective of this study is to provide a source model that can explain such ground motions. The isolated nature of the pulses indicated that the rupture of some small isolated region on the fault contributed to the generation of the pulse. Therefore, we considered such regions and termed them as Strong Motion Pulse Generation Areas (SPGAs). We used the corrected empirical Green's function (EGF) method because this method has been successfully applied to near-source pulse-like ground motions in previous studies. We simulated synthetic waveforms using the frequency dependent quality factor Q=239f0.71 and empirical site amplification factors, which we obtained by applying a generalized inversion technique to local weak-motion data. The result indicated that the observed ground motions from the Mw 8.3 Illapel earthquake can readily be explained with a source model that involves two SPGAs with dimensions of several kilometers in spite of the huge rupture zone of the earthquake. The source model can reproduce velocity waveforms, acceleration Fourier amplitude spectra (FAS) and pseudoacceleration response spectra. It also reproduces the duration of strong ground motions quite accurately. No significant bias was found with respect to distance and frequency. In conclusion, the corrected EGF method is a very efficient tool to simulate near-source ground motions of a subduction earthquake when it is combined with higher stress-drop subevents whose sizes are adjusted to the observed pulse widths.

Strong Ground Motion Prediction Model for PGV and Spectral Velocity for the Chilean Subduction Zone

ABSTRACTGround-motion prediction models (GMPMs) are a critical tool in performing seismic hazard analyses; in turn, these studies condition structural designs. Consequently, new research has appeared not only with a regionalization focus but has also explored the prediction of intensities other than acceleration. We present a GMPM for peak ground velocity (PGV) and spectral velocity (Sv) for the Chilean subduction zone. Because of the limitations of VS30 as site proxy, the proposed model adds the site’s fundamental frequency (f0) as an explanatory variable for the site term in the GMPM. We developed the model for PGV and spectral response periods between 0.06 and 10 s. The total error (σ) of the model shows a slight reduction with the inclusion of the fundamental frequency (f0) compared with a similar model for the pseudoacceleration response spectrum in the same zone. We used the proposed model to predict structural damage during the 2010 Mw 8.8 Maule earthquake, showing a good fit with the geographical distribution of damage, and this creates an opportunity to characterize the seismic behavior of soil deposits, including basins, for urban planning.

Frequency-dependent site factors for the Icelandic strong-motion array from a Bayesian hierarchical model of the spatial distribution of spectral accelerations

The earthquake ground motions of over 1700 earthquakes recorded on a small-aperture strong-motion array in south Iceland (ICEARRAY I) that is situated on a relatively uniform site condition characterized as rock, exhibit a statistically significant spatial variation of ground-motion amplitudes across the array. Both earthquake and microseismic horizontal-to-vertical spectral ratios (HVSR) have been shown to exhibit distinct and in some cases, bimodal peaks in amplification, indicating site resonance at periods of 0.1–0.3 s, a phenomenon that has been attributed to a surface layer of lava rock lying above a sedimentary layer, a structure that is then repeated with depth under the array. In this study, we implement a Bayesian hierarchical model (BHM) of the seismic ground motions that partitions the model residuals into earthquake event term, station term, and event–station term. We analyzed and compared peak ground acceleration (PGA) with the 5% damped pseudo-acceleration response spectrum (PSA) at oscillator periods of T = 0.05–1.0 s. The results show that the event terms, dominate the total variability of the ground-motion amplitudes over the array. However, the station terms are shown to increase in the period range of 0.1–0.3 s on most stations and to different extents, leading to an increase in the overall variability of ground motions at those periods, captured by a larger inter-station standard deviation. As the station terms are a measure of how much the ground motions at those stations deviate from the array average, they act as proxies for localized site effects and amplification factors. These results, improve our understanding of the key factors that affect the variation of seismic ground motions across the relatively small area of ICEARRAY I. This approach can help to improve the accuracy of earthquake hazard assessments on local scales, which in turn could contribute to more refined seismic risk assessments and engineering decision-making.

Simulation of response spectrum-compatible ground motions using wavelet-based multi-resolution analysis

This study proposed a simple and effective response spectrum-compatible ground motions simulation method to mitigate the scarcity of ground motions on seismic hazard analysis base on wavelet-based multi-resolution analysis. The feasibility of the proposed method is illustrated with two recorded ground motions in El Mayor-Cucapah earthquake. The results show that the proposed method enriches the ground motions exponentially. The simulated ground motions agree well with the attenuation characteristics of seismic ground motion without modulating process. Moreover, the pseudo-acceleration response spectrum error between the recorded ground motion and the average of the simulated ground motions is 5.2%, which fulfills the requirement prescribed in Eurocode 8 for artificially simulated ground motions. Besides, the cumulative power spectra between the simulated and recorded ground motions agree well on both high- and low-frequency regions. Therefore, the proposed method offers a feasible alternative in enriching response spectrum-compatible ground motions, especially on the regions with insufficient ground motions.

Application of horizontal to Vertical Spectral Ratio microtremor technique in the analysis of site effects and structural response of buildings in Querétaro city, Mexico

Querétaro city is situated in the north central part of the Trans-Mexican Volcanic Belt, and it was considered by several previous studies as an aseismic region. Since that, as well as considering the surrounding seismotectonic setting, it was significant to assess the seismic hazard across the city which aids in the development and updating the building codes and the seismic design criteria. In this regard, the Horizontal to Vertical Spectral Ratio (HVSR) Nakamura's technique has been applied to investigate the local-site effects across the city. Microtremor measurements were performed at 76 free-field selected sites, through the implementation of portable seismic stations equipped with three-component velocity sensors of Trillium Compact 120 s broadband seismometer. Then, acceleration values were estimated by generating synthetic accelerograms based on a recently-developed algorithm that combines the HVSR technique and the theory of random vibrations, considering as a reference the September 19, 2017 (Mw 7.1) intraslab earthquake recorded at the Centro de Geociencias (CGEO) Seismic Station, in Juriquilla, Querétaro. Finally, by computing the pseudoacceleration response spectra through the linear acceleration method at structural periods of 0.1, 0.5, 1.0, 2.0, and 5.0 s, corresponding seismic hazard maps were established for the studied region. Based on these values and delimiting the areas with higher risk considering the interaction of the soil with the buildings on it, this article is the basis for delimiting seismic parameters that define the city, with future applications in the seismic engineering considering both linear and nonlinear structural behaviors.

A Ground-Motion Prediction Model for Shallow Crustal Earthquakes in Greece

ABSTRACT Using a recently completed database of uniformly processed strong-motion data recorded in Greece, we derive a ground-motion prediction model (GMPM) for horizontal-component peak ground velocity, peak ground acceleration, and 5% damped pseudoacceleration response spectra, at 105 periods ranging from 0.01 to 10 s. The equations were developed by modifying a global GMPM, to account for more rapid attenuation and weaker magnitude scaling in the Greek ground motions than in the global GMPM. Our GMPM is calibrated using the Greek data for distances up to 300 km, magnitudes from 4.0 to 7.0, and time-averaged 30 m shear-wave velocities from 150 to 1200 m/s. The GMPM has important attributes for hazard applications including magnitude scaling that extends the range of applicability to M 8.0 and nonlinear site response. These features are possible because they are well constrained by data in the global GMPM from which our model is derived. An interesting feature of the Greek data, also observed previously in studies of mid-magnitude events (6.1–6.5) in Italy, is that they are substantially overpredicted by the global GMPM, which may be a repeatable regional feature, but may also be influenced by soil–structure interaction. This bias is an important source of epistemic uncertainty that should be considered in hazard analysis.

Análisis probabilístico de peligro sísmico y terremoto de diseño para Santiago-República Dominicana

Este artículo presenta los resultados de un Análisis Probabilístico de Peligro Sísmico (APPS) realizado para un sitio ubicado en el centro de la ciudad de Santiago, localizada en la República Dominicana. Para realizar el APPS se utilizaron datos sismológicos recientes de la falla Septentrional (fuente sísmica) con el fin de calibrar un modelo de recurrencia de terremotos, y para calcular las curvas de peligro sísmico de varios parámetros de terremotos, movimiento del suelo y respuesta estructural de interés en aplicaciones de ingeniería sísmica. Los parámetros estudiados incluyen aceleración horizontal pico del suelo y ordenadas del espectro de respuesta de pseudo-aceleración con 5 % de amortiguamiento. El efecto de la incertidumbre de la ubicación de la zona de ruptura en la falla es estudiado usando un análisis de Monte Carlo; con este propósito la ubicación de la ruptura se trata como una variable aleatoria uniforme en la longitud de la falla. Los resultados del APPS son comparados con el espectro de diseño del sitio con 2 % de probabilidad de excedencia en 50 años, especificado en la normativa de diseño de la República Dominicana. Se concluye que hay un 17 % de probabilidad de que el valor actual de las ordenadas espectrales con 2 % de excedencia sea mayor que el valor especificado en el espectro de diseño, debido a incertidumbres relacionadas a la ley de atenuación usada para relacionar la fuente de terremotos y las características del movimiento del suelo en el sitio. Basado en el modelo de atenuación, el espectro de diseño en el sitio es consistente con los espectros generados por terremotos de magnitud (momento) de 7.9, lo cual es cercano al terremoto característico de magnitud de 7.8 estimado en estudios recientes de la falla Septentrional.

Variable vs. invariable elastic response spectrum shapes: impact on the mean annual frequency of exceedance of limit states

In research studies focusing on the evaluation of the fragility function of buildings, numerical analyses are almost always carried out using a single suite of accelerograms, which is scaled in intensity to simulate earthquakes of different magnitude.Therefore, such numerical analyses usually neglect the effects of the variation of important seismological characteristics, e.g. the shape of the pseudo-acceleration response spectrum, on the structural response.To evaluate the impact of the variation of the elastic response spectrum shape with the selected earthquake return period on the mean annual frequency of exceedance of assigned limit state functions, the response of a large set of systems is determined in this paper by two different methods of analysis. In the first method of analysis, a multiple stripe analysis is carried out with accelerograms characterised by median response spectrum shape and ground motion duration variable with the earthquake intensity measure. The information pertaining the probabilistic description of the acceleration spectra has been obtained from the website of the National Institute of Geophysics and Volcanology for the Italian territory, whereas the one regarding the earthquake duration has been derived from semi-empirical relationships reported in the literature. In the second method of analysis, instead, the accelerograms are characterized by median response spectrum shape and ground motion duration invariable with the intensity measure.Finally, to evaluate the influence of the scattering of the spectral responses about their mean value, a third analysis is carried out by means of a set of artificial accelerograms with response spectra that are characterized by virtually zero standard deviation and spectrum shape and ground motion duration invariable with the magnitude of the earthquake.To increase the relevance of the results, the investigated systems include single-degree-of–freedom systems with degrading or non-degrading inelastic response and reinforced concrete and steel multi-degree of freedom systems.

The role of intensity measures on the accuracy of seismic fragilities

Seismic fragilities are the probability that structural response of a system overcomes specified limit values for given seismic intensity measures. These curves are frequently defined as functions of single/multiple ordinates of the pseudo-acceleration response spectrum. Recently it was reported that this approach can lead to inaccurate estimation of the structural performance for complex non-linear multi-degree of freedom systems. This preliminary paper investigates the accuracy of fragility curves for an actual building struck by the 2016 Italian earthquake. Numerical analyses of a multi-degree of freedom structural system subjected to this earthquake are performed, considering both linear and non-linear behaviour, and used to construct scatter plots of intensity measures versus the corresponding actual maximum of the response time history. Finally, an example of the fragility curves estimated using different intensity measures is presented for both linear and nonlinear structural behaviour. It is shown that these fragilities have very large variability depending on the selected scalar or vector-valued intensity measure.

Stochastic finite-fault ground motion simulation for the Mw 6.7 earthquake in Lushan, China

The Ya’an, Sichuan Mw 6.7 earthquake occurred on April 20, 2013. In this article, the stochastic finite-fault method (EXSIM) based on dynamic corner frequency, proposed by Motazedian and Atkinson (Bull Seismol Soc Am 95(3):995–1010, 2005), is validated to be an intelligible and productive approach for the generation of high-frequency strong ground motion. The validated model parameters were considered for the simulation at 31 selected stations, which are less than 200 km away from the fault. The input parameters included site condition, source term, and path term. The calibration of the input parameters, such as the stress drop, was achieved by using misfit functions between the observed PSA (pseudo-acceleration response spectra) and simulated PSA in the time domain. Some of the other parameters, such as distance-dependent duration, high-frequency attenuation parameter kappa, and local amplification functions, were calibrated by considering the observed recordings. In this study, we attempted to employ two different slip models for strong ground motion simulation, so that the influence on the simulation results can be revealed. Our results depicted that although the method cannot combine well, the directivity effects and the soil conditions are not adequately represented at individual stations, the synthetics satisfactorily match with the seismic characteristics regarding peak ground acceleration (PGA), response spectra, Fourier spectrum, and time history, for both the time and frequency range considered. The results also demonstrated that there is a slight difference between the simulation results of the two slip models. Finally, we compared the effects of different distance-dependent duration models for the simulated PGA. It illustrates that it is difficult to find a balance between the ground motion duration and PGA at stations with fault distance less than 20 km, which makes the duration and PGA to coincide well with the observed recordings.

A Numerical Study on the Seismic Site Response of Rocky Valleys with Irregular Topographic Conditions

This paper investigates topographic effects of rocky valleys with irregular topographic conditions subjected to vertically propagating SV waves of Ricker type using a boundary element code. Valleys with two intersecting slopes, [Formula: see text] and [Formula: see text], are modelled in order to study their combined effects on ground motion. Presented in the form of pseudo-acceleration response spectra, results of this work can be extended to similar topographies. The main findings are: (i) [Formula: see text] (the first slope angle) and [Formula: see text] (L is the half width of the valley and [Formula: see text] is its corresponding height) have amplifying effects, and [Formula: see text] (the second slope angle) has de-amplifying effects on the site response. (ii) [Formula: see text] has a straight effect on intensifying the effects of both [Formula: see text] and [Formula: see text]. (iii) The combined effects of slope angles have been found to be important in modifying the response so more than a single slope should be considered for seismic analysis. (iv) Engineers should use the maximum amplification of 2.4 in case of valleys with the first and second slope angles below [Formula: see text].