Stochastic Finite-fault Simulation Research Articles

Studying Past Earthquakes with Modern Techniques: Ground-Motion Simulations for the 11 January 1693 Noto Earthquake in Italy

Abstract The 1693 Noto earthquake, which struck on 11 January, is one of the Italy’s largest and most devastating earthquakes. According to the Italian Parametric Earthquake Catalogue, it reached a maximum intensity of 11 on the Mercalli–Cancani–Sieberg scale and had an estimated magnitude of M 7.3. Nevertheless, its precise location and source definition remain subjects to debate due to the complexity of the seismic sequence and lack of geological evidence. A series of potential seismic sources differing in location, dimension, and kinematics have been proposed in the literature based on seismotectonic data and interpretations. The goal of this work is to perform a retrospective experiment to verify which of the proposed seismic sources have a better fit with the observed intensity data. To do so, novel simulation techniques are used to study this historical earthquake. We generated ground-motion scenarios for each proposed source model through a stochastic finite-fault simulation approach. Then, the simulated ground-motion parameters were converted to intensities using two different ground-motion intensity conversion equations for Italy. Finally, we compared these converted intensities with the observed intensity data in terms of normalized root mean square errors and converted intensities from ground-motion models. Our results generally show good consistency between converted intensities from the simulated and predicted ground motions, whereas the observed intensities fit better to converted ones from the peak ground velocity rather than peak ground acceleration. Our analysis reveals that the source model reproducing the best of the macroseismic data of the Noto earthquake is the Canicattì–Villasmundo fault system with a magnitude of 7.1.

Impact of Multiple Faults on the Maximum Credible Ground-Motion Parameters of Large Earthquakes at a Near-Field Site

The ground-motion simulation of regional-specific earthquake scenarios is crucial for the seismic design of key facilities. Herein, we considered parameter uncertainty in ground-motion simulations and the impact of multiple faults when determining the maximum credible ground-motion parameters of large earthquakes at a near-field dam. The source models of the Daju–Lijiang, Xiaozhongdian–Daju, and Longpan–Qiaohou faults were established based on geological and geophysical data. Although the method for identifying asperity is not yet mature and still faces many difficulties, it provides an opportunity to identify the non-uniform slip distribution on the rupture plane by earthquake scenarios. A multi-scheme stochastic finite-fault simulation method was then used to estimate the minimum; mean; maximum; and 50th-, 84th-, and 95th-percentile values of the peak ground acceleration and pseudo-spectral acceleration response spectra. The results showed that the Longpan–Qiaohou fault can generate the largest ground-motion parameters compared with the other two faults. Moreover, this result was supported by the statistical analysis of the results of twelve thousand simulations of these three faults. Thus, it can be concluded that the maximum credible ground-motion parameters are represented by the 84th-percentile pseudo-spectral acceleration response spectrum of the Longpan–Qiaohou fault. This finding will benefit the seismic safety design of the target dam. More importantly, this multi-scheme method can be applied to other key facilities to obtain reasonable ground-motion parameters.

Ground-Motion Model for Significant Duration Constrained by Seismological Simulations

ABSTRACT A duration ground-motion model for crustal earthquakes based on the normalized Arias intensity (IA) is developed. Two sets of seismological simulations are used to constrain the form and scaling of the duration model. Simulations using a 3D crustal model show that an additive model for the source, path, and site terms captures the physical behavior of duration better than a multiplicative model for the site term. Stochastic finite-fault simulations are used to constrain the saturation of the large-magnitude scaling at short distances. The duration model is developed in two parts: a duration model for the time interval between 5% and 75% of the normalized Arias intensity (D5−75) and a duration model for the ratio of the D5−X/D5−75 duration for X values from 10 to 95. Together, these two models provide a more complete description of the evolution of the seismic energy with time than a single duration metric. A new aspect of the statistical model for duration is the inclusion of a random effect for the path term in addition to random effects for the source and site terms. The source and site random effects are modeled as scale factors on the duration, whereas the path-term random effect is a scale factor on the distance slope. The distribution of the duration residuals has a skewness that is between the skewness of a lognormal distribution and the symmetry of a normal distribution. The final duration aleatory variability is modeled by a power-normal distribution with an exponent of 0.3, which accounts for the amplitude dependence of the aleatory variability of the duration with smaller aleatory variability for large-magnitude events and larger aleatory variability for small-magnitude events as compared to the variability from a lognormal distribution.

Simulating the strong ground motion of the 2022 MS6.8 Luding, Sichuan, China Earthquake

Stochastic finite-fault simulations are effective for simulating ground motions and are widely used in engineering to determine the impacts of ground motion and develop relevant predictive equations. In this study, the source, path, and site amplification coefficient of western Sichuan Province, China, and stochastic finite-fault simulations were used to simulate the acceleration time series, Fourier amplitude spectra, and 5% damped response spectra of 28 strong-motion stations with rupture distances within 300 km of the 2022 MS6.8 Luding earthquake. The simulation results of 14 stations at rupture distances of 45–185 km match the observation. However, the simulation results of 3 near- and 6 far-field stations at rupture distances of 12–36 km and 222–286 km, respectively, were obviously deviated from the observations. Simulation results of the near-field stations are larger than the observations at high frequencies (>6 Hz). The discrepancy likely comes from the nonlinear site effect of near-field stations, which reduced the site amplification at high frequencies. Simulation result of the far-field stations is smaller than the observation at frequencies above 1 Hz. As these stations are located close to the Longmenshan Fault Zone (LFZ), thus, we obtained a new quality factor (Q) from data of historical events and stations located around LFZ. Using the new Q value, the discrepancies of the high-frequency simulation results of the far-field stations were corrected. This result indicated that the laterally varying Q values can be used to address the impact of strong crustal lateral heterogeneity on simulation.

Stochastic Ground-Motion Simulation of the 2021 Mw 5.9 Woods Point Earthquake: Facilitating Local Probabilistic Seismic Hazard Analysis in Australia

ABSTRACT The 2021 Mw 5.9 Woods Point event is the largest onshore earthquake that has occurred in the recorded history of southeastern Australia since European settlement. To study its source and ground-motion characteristics and to extract information for local seismic hazard analysis, we employ a stochastic finite-fault simulation approach to simulate ground motions for this event based on the observations collected from 36 onshore stations. We determine the regional distance-dependent attenuation parameters using the horizontal Fourier acceleration amplitude spectrum in the frequency range of 0.1–20 Hz. We parameterize path parameters using different models to consider uncertainties and sensitivities. To investigate local site effects, we construct a VS30-based site amplification model. Source parameters are then determined by fitting the theoretical Brune’s ω2 model with a reference Fourier source spectrum at 1.0 km. The κ0 value for the reference rock site is estimated as κ0=0.01 s, and dynamic stress drop is found to be 41.0 MPa by minimizing the overall absolute residual of 5% damped pseudospectral acceleration. We validate the simulations by comparing simulated and observed ground motions in terms of various intensity measurements; analyses of residuals show that the simulations are in good agreement with observations (average residual is close to 0). To facilitate future probabilistic seismic hazard analysis, six selected ground-motion models are ranked using the deviance information criteria based on an independent data set consisting of field observations and simulated ground motions.

Simulation of the 2022 Mw 6.6 Luding, China, earthquake by a stochastic finite-fault model with a nonstationary phase

We adopted stochastic finite-fault simulations considering the nonstationary frequency characteristics for the 2022 Mw 6.6 Luding, China, earthquake at 16 strong-motion stations using the stochastic finite-fault model based on a dynamic corner frequency (EXSIM) code. The average simulated response spectra of earthquake ground motions were obtained and then compared with response spectra of 5% damped single degree of freedom system under recorded ground motion excitation to verify whether the finite-fault model that considers the nonstationary frequency characteristics can reproduce the critical characteristics of ground motions and improve the simulation accuracy. The high-frequency spectral decay is calculated by the real recordings and the stress drop is determined by the trial-and-error method. In addition, the local site amplifications are estimated from the horizontal-vertical spectral ratio (H/V). An improved EXSIM that considers the slip-related corner frequency is also used in this study. The results indicate that the nonstationary phase can significantly improve the simulation accuracy of EXSIM in the short period and has little effect on the long-period simulation results. However, the effect of the nonstationary phase on the simulation results obtained from the improved EXSIM is not significant. Regarding the peak ground acceleration (PGA), the nonstationary phase can produce waveform characteristics that are more similar to the real waveform, and the synthetic PGA is closer to the observed ones.

Regional Spectral Characteristics Derived Using the Generalized Inversion Technique and Applications to Stochastic Simulation of the 2021 Mw 6.1 Yangbi Earthquake

ABSTRACTIn this study, 334 accelerograms of 42 small-to-moderate earthquakes recorded at 36 strong-motion stations were used to investigate the ground-motion characteristics of the southwestern margin of the Sichuan–Yunnan rhombic block (SWM-SYRB), one of the most active seismic regions in China. The high-frequency attenuation parameter was estimated using the spectral decay method, and the site component (κ0) was fitted. The κ0 estimates decrease with the increasing time-averaged S-wave velocity of the uppermost 30 m (VS30). Using the generalized inversion technique, the source spectra, quality factor (Q), and site amplification were derived from the Fourier amplitude spectra (FAS). The obtained average stress drop for earthquakes occurred in SWM-SYRB was the second largest among various boundary areas of SYRB. The inverted Q model was Q(f)=115.1f0.616. The low Q structure that extends southwestward from the Songpan–Garze block to SWM-SYRB could be responsible for the strong regional attenuation of ground motion with distance. At frequencies above 10 Hz, the average site amplifications were influenced by the high-frequency attenuation effect. The site amplification of site class D reached a factor of 6 at 0.7 Hz. Moreover, it was observed that site amplification factors can be even higher when peak ground acceleration is larger than 0.8 m/s2. Finally, the obtained parameters were used in the stochastic finite-fault simulation method to reproduce the FAS, 5%-damped pseudospectral acceleration, and time series of the 2021 Mw 6.1 Yangbi earthquake. The simulated spectra properly matched the observations in a broad frequency band of 0.1–25 Hz. Furthermore, the simulated time series could generally represent the amplitude of the S-wave portion of observed ones.

Seismic hazard analysis for engineering sites based on the stochastic finite-fault method

Seismic hazard analyses are mainly performed using either deterministic or probabilistic methods. However, there are still some defects in these statistical model-based approaches for regional seismic risk assessment affected by the near-field of large earthquakes. Therefore, we established a deterministic seismic hazard analysis method that can characterize the entire process of ground motion propagation based on stochastic finite-fault simulation, and we chose the site of the Xiluodu dam to demonstrate the method. This method can characterize earthquake source properties more realistically than other methods and consider factors such as the path and site attenuation of seismic waves. It also has high computational efficiency and is convenient for engineering applications. We first analyzed the complexity of seismogenic structures in the Xiluodu dam site area, and then an evaluation system for ground motion parameters that considers various uncertainties is constructed based on a stochastic finite-fault simulation. Finally, we assessed the seismic hazard of the dam site area comprehensively. The proposed method was able to take into account the complexity of the seismogenic structures affecting the dam site and provide multi-level parameter evaluation results corresponding to different risk levels. These results can be used to construct a dam safety assessment system of an earthquake in advance that provides technical support for rapidly and accurately assessing the post-earthquake damage state of a dam, thus determining the influence of an earthquake on dam safety and mitigating the risk of potential secondary disasters.

Spectral Analysis of Subduction Zone Earthquakes for Coastal Stations and Applications to Stochastic Finite-Fault Simulation Method

Accelerograms of 58 subduction zone earthquakes recorded by 25 K-NET coastal stations located in Sagami Bay and its adjacent regions were used to analyze the spectral properties of the source, propagation path, and site effects by using the generalized inversion technique. The inverted [Formula: see text] values vary from 1.7[Formula: see text]MPa to 17.4[Formula: see text]MPa, with an average value of 4.89[Formula: see text]MPa and a standard deviation of [Formula: see text]. Although the depths of the selected earthquakes are confined to 30 km, which is similar to those of the crustal earthquakes, this study obtains an average [Formula: see text] which coincides with the plate-boundary earthquakes rather than the crustal earthquakes. It implies that the stress condition of the plate-boundary region varies from the inland crust. The obtained [Formula: see text] is similar to the previous studies. The [Formula: see text] of the selected stations varies from 0.0479[Formula: see text]s to 0.0904[Formula: see text]s, indicating high-frequency attenuation for the coastal area. The inverted site response shows the systematic tendency for different site classes. The amplification levels of class A sites fluctuate around one between 1[Formula: see text]Hz and 10[Formula: see text]Hz. Class B sites indicate peak amplification at the resonant frequency distributed between 3[Formula: see text]Hz and 10[Formula: see text]Hz. Most of the Classes D and E sites have peak amplification below about 4[Formula: see text]Hz. Using the inversion results as the input parameters of the stochastic finite-fault method, an [Formula: see text] subduction zone earthquake was simulated. The resulting response spectra corresponded to the observations and confirmed that the inverted parameters are reasonable.

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.

Stochastic Finite-Fault Simulation of the Ms 7.0 Lushan Earthquake Based on Frequency- and Distance-Dependent Radiation Patterns

ABSTRACTThe stochastic finite-fault method (EXSIM) has been extensively used for simulating ground motion at high frequencies. However, its poor performance in low-frequency simulations is a limiting factor that restricts its engineering application. Refining the representation of the radiation pattern in the finite-fault method is an effective strategy to improve low-frequency simulations; to this end, a frequency-dependent radiation pattern has been considered by several researchers. However, this strategy fails to provide an accurate simulation of seismic-wave propagation at distances beyond the near-fault region. Researchers have proposed various approaches for characterizing the radiation pattern variation with distance. This study introduces frequency- and distance-dependent radiation patterns of S waves to the EXSIM. The near-field acceleration records in the east–west and north–south directions of the 2013 Ms 7.0 Lushan earthquake were reconstructed. The proposed method was verified by: (1) comparing broadband simulation results obtained by the improved method with observed results, (2) conducting a misfit analysis to compare the model bias between the improved and original methods, and (3) comparing the observed and simulated peak ground acceleration data with the predicted values of the ground-motion prediction equations (GMPEs) to verify the effectiveness of the GMPEs in describing the regional ground-motion attenuation. The results indicated that the 5%-damped pseudo spectral accelerations at high frequencies (1–20 Hz) and acceleration time history simulated by the improved method were consistent with the observed values. Furthermore, the improved method effectively optimizes the simulation effect at low frequencies (0.05–1 Hz) compared with the original method. Thus, the improvement in the representation of the radiation pattern in EXSIM can better estimate broadband ground motion in the study area.

An assessment of the 3 February 2002 Cay (Turkey) earthquake (Mw=6.6): Modeling of ground motions and felt intensity distribution

Estimation of earthquake ground motions is important for several purposes including seismic design, analysis, risk mitigation and disaster management. For regions with insufficient data, simulations become critical, particularly for earthquake engineering applications requiring full ground motion data. In this study, stochastic finite-fault simulation of the 3 February 2002 Cay event is presented at three available stations within epicentral distances less than 200 km. The simulations are initially validated at these stations. Next, simulations of the earthquake at dummy nodes are performed in Afyon region. The simulated ground motion records for the Cay event are evaluated by comparing them against selected ground motion prediction equations. Then, the distributions of the observed intensity and damage are compared against the distribution of simulated intensities. The results reveal that the earthquake is simulated effectively.

Seismic zoning of Tabriz area by stochastic finite fault model considering site-specific soil effects

Abstract Tabriz, as one of the most earthquake-prone cities in the Iran plateau, has experienced enormous earthquakes that have even destroyed the city altogether. Considering this seismological background and the vicinity of Tabriz's northwestern fault, reducing the possible earthquake losses can be highly useful by scrutinizing the strong ground motion resulting from the fault activation. To this end, a stochastic finite-fault ground motion simulation (EXSIM) method was applied as an important means for predicting the ground motion near the epicenter of the earthquake. EXSIM is an open-source stochastic finite-fault simulation algorithm that generates the time-series of the earthquake's ground motion. Based on the findings, the peak horizontal acceleration reached 0.83 g in the northern parts by creating artificial accelerograms and Tabriz's seismic zonation. In comparison, it reduced by 0.48 g by departing from the fault in the city's southern parts. Additionally, providing a seismic zonation map in Tabriz revealed that stopping the construction in the north parts while extending the settlement construction to the south part of the city are considered vital and unavoidable. Also, by applying the magnification and effects of the soil layers above the bedrock, it was further found that the existence of the loose layer with low strength and compaction intensify the application of seismic acceleration on the near-surface structures in the central, west, and southwest parts of the target area.

Source modelling and strong ground motion simulations for the 24 January 2020, Mw 6.8 Elazığ earthquake, Turkey

SUMMARYOn 24 January 2020 an Mw 6.8 earthquake occurred at 20:55 local time (17:55 UTC) in eastern Turkey, close to the town of Sivrice in the Elazığ province, causing widespread considerable seismic damage in buildings. In this study, we analyse the main features of the rupture process and the seismic ground shaking during the Elazığ earthquake. We first use Interferometric Synthetic Aperture Radar (InSAR) interferograms (Sentinel-1 satellites) to constrain the fault geometry and the coseismic slip distribution of the causative fault segment. Then, we utilize this information to analyse the ground motion characteristics of the main shock in terms of peak ground acceleration (PGA), peak ground velocity (PGV) and spectral accelerations. The absence of seismic registrations in near-field for this earthquake imposes major constraints on the computation of seismic ground motion estimations in the study area. To do this, we have used a stochastic finite-fault simulation method to generate high-frequency ground motions synthetics for the Mw 6.8 Elazığ 2020 earthquake. Finally, we evaluate the potential state of stress of the unruptured portions of the causative fault segment as well as of adjacent segments, using the Coulomb stress failure function variations. Modelling of geodetic data shows that the 2020 Elazığ earthquake ruptured two major slip patches (for a total length of about 40 km) located along the Pütürge segment of the well-known left-lateral strike-slip East Anatolian Fault Zone (EAFZ), with up to 2.3 m of slip and an estimated geodetic moment of 1.70 $\,\, \times $ 1019 Nm (equivalent to a Mw 6.8). The position of the hypocentre supports the evidence of marked WSW rupture directivity during the main shock. In terms of ground motion characteristics, we observe that the high-frequency stochastic ground motion simulations have a good capability to reproduce the source complexity and capture the ground motion attenuation decay as a function of distance, up to the 200 km. We also demonstrate that the design spectra corresponding to 475 yr return period, provided by the new Turkish building code is not exceeded by the simulated seismograms in the epicentral area where there are no strong motion stations and no recordings available. Finally, based on the Coulomb stress distribution computation, we find that the Elazığ main shock increased the stress level of the westernmost part of the Pütürge fault and of the adjacent Palu segment and as a result of an off-fault lobe.

Use of simulated ground motions for the evaluation of energy response of simple structural systems

The literature on energy-based seismic design and assessment methodologies of structural systems mostly relies on real ground motion datasets. However, certain bias exists due to lack of homogeneity in available ground motion datasets. In this study, a large set of ground motions is simulated. Next, the effects of various parameters on seismic behavior of SDOF systems in terms of energy are studied using nonlinear time history analyses. The stochastic finite-fault simulations are performed on the western parts of North Anatolian Fault zone in Turkey. Input, damping and hysteretic energies are the considered parameters. Results reveal that seismic energy is a more suitable parameter when compared to the other physical parameters such as seismic displacement and force. However, it is important to dissipate the estimated input energy through damping and inelastic action. Finally, parametric seismic analyses using simulated ground motions yield realistic results since these records provide variability in seismic demand.

Ground-motion simulation for the <i>M</i><sub>W</sub>6.1 Ludian earthquake on 3 August 2014 using the stochastic finite-fault method

The stochastic finite-fault simulation method was applied to synthesize the horizontal ground acceleration seismograms produced by the <i>M</i>_W6.1 Ludian earthquake on August 3, 2014. For this purpose, we produced first a total of 200 kinematic source models for the Ludian event, which are characterized by the heterogeneous slip on the conjugated ruptured fault and the slip-dependent spreading of the rupture front. The results indicated that the heterogeneous slip and the spatial extent of the ruptured fault play dominant roles in the spatial distribution of ground motions in the near-fault area. The peak ground accelerations (PGAs) and 5%-damped pseudospectral accelerations (PSAs) at periods shorter than 0.5 s estimated on the resulting synthetics generally match well with the observations at stations with Joyner-Boore distances (<i>R</i>_JB) greater than 20 km. The synthetic PGVs and PSAs at periods of 0.5 s and 0.75 s are in good agreement with predicted medians by the Yu14 model (Yu et al., 2014). However, the synthetic results are generally much lower than the predicted medians by BSSA14 model (Boore et al., 2014). Moreover, the ground motion variability caused by the randomness in the source rupture process was evaluated by these synthetics. The standard deviations of PSAs on the base-10 logarithmic scale, Sigma[log10(PSA)], are closely dependent on either the spectral period or the <i>R</i>_JB. The Sigma[log10(PSA)] remains a constant approximately 0.55 at periods shorter than 0.1 s, and then increase continuously up to ~0.13 as the period increases from 0.1 to 2.0 s. The Sigma[log10(PSA)] values at periods of 0.1‒2.0 s show the downward tendency as the <i>R</i>_JB values increase. However, the Sigma[log10(PSA)]​​​​​​​ values at periods shorter than 0.1 s decrease as the <i>R</i>_JB values increase up to ~50 km, and then increase with the increasing <i>R</i>_JB. Furthermore, we found that the ground-motion variability shows the significant dependence on the azimuth.

Stochastic finite-fault simulation of the 2017 Jiuzhaigou earthquake in China

In this study, the strong ground motion of the Jiuzhaigou Ms7.0 earthquake, which occurred in northern Sichuan, China, was simulated based on the stochastic finite-fault method. The earthquake event was recorded by 66 strong ground-motion stations operated by the China Strong Motion Networks Center. We simulated 11 records selected within 200 km source-to-site distance. According to previous studies and empirical relationships, we estimated the region-specific input parameters. The zero-distance kappa filter obtained had a value of 0.0206 s. Two different source models were applied in this study: the random slip model and specified slip model. Using the stochastic finite-fault method, we simulated the PGA, Fourier spectrum and response spectrum at all stations. The stochastic simulated result based on the specified slip distribution models had no significant bias at most stations. Using a model with a random slip distribution, the simulated response spectra also matched the observed result, which indicated that the stochastic finite-fault method is not very sensitive to the input slip distributions and fault dimensions. We divided the study area into 1116 sites to simulate the spatial distribution of PGA based on the two models. The simulated maximum intensity of the epicentral area reached level IX, which was similar to the observed maximum intensity and indicated that the simulated result could be used in prediction of an imminent earthquake disaster. For future earthquake prediction, seismic hazards could even be estimated quickly without obtaining detailed information about the fault plane.

Sensitivity study for stochastic finite-fault model in eastern North America

The reliability of structures depends on the uncertainties of random characteristics of strong ground motions. This study investigates the sensitivity level of the stochastic finite-fault ground-motion simulation model in response to major uncertain input variables, with special focus on eastern North America. The finite-fault model is one of the most useful methods to simulate ground motion for a large earthquake. The variance-based sensitivity analysis using the Monte Carlo-based procedure is considered to compute the set of sensitivity indices of the input variables to estimate the contribution of each uncertain input parameter. The variables that have a dominant influence on the uncertainty of simulated ground motions are identified. The analytical results show that the stress parameter, fault model variables (including faulting geometry, heterogeneous slip distribution and location of hypocentre within fault plane) and site-amplification factor are the main sources of uncertainty, while path variables, such as distance, anelastic attenuation and upper crust attenuation, have relatively little effect.

Ground motion simulations for seismic stations in southern and eastern Romania and seismic hazard assessment

This research focuses on the evaluation of soil conditions for seismic stations in southern and eastern Romania, their influence on stochastic finite-fault simulations, and the impact of using them on the seismic hazard assessment. First, the horizontal-to-vertical spectral ratios (HVSR) are evaluated using ground motions recorded in 32 seismic stations during small magnitude (MW ≤ 6.0) Vrancea seismic events. Most of the seismic stations situated in the southern part of Romania exhibit multiple HVSR peaks over a broad period range. However, only the seismic stations in the eastern-most part of Romania have clear short-period predominant periods. Subsequently, stochastic finite-fault simulations are performed in order to evaluate the influence of the soil conditions on the ground motion amplitudes. The analyses show that the earthquake magnitude has a larger influence on the computed ground motion amplitudes for the short- and medium-period range, while the longer-period spectral ordinates tend to be influenced more by the soil conditions. Next, the impact of the previously evaluated soil conditions on the seismic hazard results for Romania is also investigated. The results reveal a significant impact of the soil conditions on the seismic hazard levels, especially for the sites characterized by long-period amplifications (sites situated mostly in southern Romania), and a less significant influence in the case of sites which have clear short predominant periods.

Hybrid-Empirical Ground Motion Estimations for Georgia

Ground motion prediction equations are essential for several purposes ranging from seismic design and analysis to probabilistic seismic hazard assessment. In seismically active regions without sufficiently strong ground motion data to build empirical models, hybrid models become vital. Georgia does not have sufficiently strong ground motion data to build empirical models. In this study, we have applied the host-totarget method in two regions in Georgia with different source mechanisms. According to the tectonic regime of the target areas, two different regions are chosen as host regions. One of them is in Turkey with the dominant strike-slip source mechanism, while the other is in Iran with the prevalence of reverse-mechanism events. We performed stochastic finite-fault simulations in both host and target areas and employed the hybrid-empirical method as introduced in Campbell (2003). An initial set of hybrid empirical ground motion estimates is obtained for PGA and SA at selected periods for Georgia.