Empirical Green’s Function Research Articles

Crustal deformation of intermontane basins beneath Central Tien Shan revealed by full-wave ambient noise tomography

The Tien Shan is one of the highest, youngest, and most active orogenic belts at the southwestern margin of Central Asia. The topography of the Central Tien Shan is characterized by nearly parallel orogenic belts with interposing intermontane basins, providing an ideal site to investigate the formation and modification of the intermontane basins. Here we construct a high-resolution crustal velocity model of the Central Tien Shan using full-wave ambient noise tomography based on high-quality Rayleigh wave empirical Green's functions at periods of 5–50 s extracted from the dense seismic array during 1997–2000. Our new model reveals two high-velocity crustal anomalies beneath the Naryn Basin and Issyk-Kul Basin, respectively, and widespread low-velocity anomalies in the middle to lower crust beneath the mountain belts. We propose that the high-velocity crustal features of the Naryn Basin and Issyk-Kul Basin may be the remnant structures from the Palaeozoic collision. Widespread low-velocity anomalies reflect mantle upwelling and partial melting due to the Kazakh and Tarim lithosphere underthrusting, which have been consequently modifying the overlying crustal structures. High-resolution tomographic results reveal significant crustal heterogeneities beneath the Central Tien Shan, which shed new light on the continental growth mechanism and the crustal deformation of intermontane basins.

Sensing Shallow Structure and Traffic Noise with Fiber-optic Internet Cables in an Urban Area

Distributed acoustic sensing (DAS) is a novel seismic observation system developed in recent years that can realize ultrahigh density observations and has attracted extensive attention in the field of seismology. DAS uses fiber-optic cables as sensing units, which are easy to incorporate with urban telecommunication fiber-optic cables for seismological observations. Compared with seismometers, DAS has the advantages of being rapidly deployed and recyclable, being able to acquire dense observations at low cost, and convenient data collection. In this study, a 5.2 km long telecom fiber-optic internet cable was utilized as a DAS array in an urban area to record ambient noise, and the noise cross-correlation function (NCF) was calculated. There are two different distribution types of ambient noise sources along the cable, regular along-road trucks (Taihe Road) and complex ambient noise, including human activities and traffic sources along and across the Jinniu road. In the first case, we constructed a 2D S-wave velocity model down to 100 m depth and a low-velocity zone was revealed. The S-wave model well explained the traffic signal along the Taihe road and the low-velocity zone is also consistent with the results obtained from co-located geophone arrays. In the second case, due to the complexity of the traffic noise distribution, empirical Green’s functions were barely achieved. Therefore, we performed a synthetic test obtaining different NCFs with different source distributions, and two specific cases that dominate the NCF results were matched. Finally, we obtained the traffic noise distribution along the road, which is consistent with the power spectra density of the ambient noise. In conclusion, by combining DAS and urban fiber-optic internet cables with urban traffic noise, we can effectively reveal the traffic activities and image shallow structures with high resolution, which could offer a reference for urban construction and disaster prevention.Article Highlights DAS turns the urban fiber-optic internet cables into ultra-dense permanent seismic observation arraysWe revealed a high-resolution shallow structure using urban fiber-optic internet cablesWe obtained the distribution of traffic activities along the road

Rapid estimation of empirical Green’s functions for passive acoustic characterization of dynamic shallow-water environments

Applications of acoustic noise interferometry to passive remote sensing of the ocean and improvement of sonar performance predictions rely on retrieval of empirical Green’s functions (EGFs) from synchronized measurements of ambient sound at spatially separated points. Usually, long noise averaging times of many hours and even days are employed to achieve precise passive measurements of the ray or normal mode travel times that are necessary for evaluation of the environmental parameters with oceanographically relevant accuracy. However, long averaging times have limited utility when nonlinear internal waves or other oceanographic processes cause significant, time-dependent variations in sound propagation conditions. Analysis of the noise records acquired during the Shallow Water 2006 experiment reveals that EGFs can be reliably retrieved from cross-correlations of noise recorded over periods as short as 10 minutes by moored receivers horizontally separated by 40–50 ocean depths. The measured EGFs evolve as the sound speed field changes. Suitability of the EGFs for acoustic remote sensing is demonstrated by geoacoustic inversion of the passively measured normal mode dispersion curves and comparison to previously reported inversion results. Rapid EGF retrieval has far-reaching implications for operational use of the underwater acoustic noise interferometry. [Work supported by NSF.]

Passive acoustic characterization of sub-seasonal sound speed variations in a coastal ocean

Acoustic noise interferometry is applied to retrieve empirical Green's functions (EGFs) from the ambient and shipping noise data acquired in the Shallow Water 2006 experiment on the continental shelf off New Jersey. Despite strong internal wave-induced perturbations of the sound speed in water, EGFs are found on 31 acoustic paths by cross-correlating the noise recorded on a single hydrophone with noise on the hydrophones of a horizontal linear array about 3.6 km away. Datasets from two non-overlapping 15-day observation periods are considered. Dispersion curves of three low-order normal modes at frequencies below 110 Hz are extracted from the EGFs with the time-warping technique. The dispersion curves from the first dataset were previously employed to estimate the seabed properties. Here, using this seabed model, we invert the differences between the dispersion curves obtained from the two datasets for the variation of the time-averaged sound speed profile (SSP) in water between the two observation periods. Results of the passive SSP inversion of the second dataset are compared with the ground truth derived from in situ temperature measurements. The effect of temporal variability of the water column during noise-averaging time on EGF retrieval is discussed and quantified.

Evidence for short temporal atmospheric (tropospheric) variations observed by infrasonic signals

Infrasound monitoring is used in the forensic analysis of events, to study the physical processes of sources of interest and to probe the atmosphere. The dynamical nature of the atmosphere and the use of infrasound as a forensic tool lead to the following questions: (1) what is the time-scale of atmospheric variability that affects infrasonic signals? (2) can we link variations of infrasound signals to specific atmospheric phenomena? This study addresses these questions by monitoring a repetitive infrasound source and its corresponding tropospheric returns 54 km away. Source-receiver empirical Green's functions are obtained every 20 s and used to demonstrate the effect of atmospheric temporal variability on infrasound propagation. In addition, observations are compared to predicted simulated signals based on realistic atmospheric conditions. It is shown that infrasound properties change within tens of seconds. Particularly, phases can appear and disappear, the propagation time decreases, and the signals' energy fluctuates. Furthermore, the similarity between the predicted and observed signals varies significantly within a few minutes. The observed changes are related to variations in temperature and wind, which are coupled to dynamic processes such as radiation, gravity waves, and turbulence. Therefore, this study highlights the potential of high temporal infrasound-based atmospheric sounding.

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.

Simulation of strong ground motions onshore the southeastern coast of the Red Sea, Saudi Arabia

In the southernmost part of the Arabian Shield, southeastern onshore of the Red Sea, damage earthquakes were historically reported implying that the region may be susceptible to have a potential seismic hazard in future. To understand the potential seismic hazard in the region, we present an analysis to simulate the peak ground accelerations (PGA) from future large earthquakes of Mw 6.0, and 6.5 in the region using an empirical Green's function (EGF) summation technique. The waveform recordings for a small-sized earthquake of Mw 4.4, that occurred in 23rd of January 2014, were used as an element earthquake. The estimated PGA values are consistent with the corresponding values of ground motion prediction equations (GMPEs) that derived for California, Japan, and Western Saudi Arabia, indicating that these equations are applicable for rock site conditions in the region. The results of response spectra reveal periods vary from 0.2 s to 3.0 s, implying potential hazard impacts for short and tall buildings.

Towards real-time assessment of convergence criteria in seismic interferometry: Selective stacking of cross-correlations at the San Emidio geothermal field

Seismic field logistics, particularly recording geometry and duration, can be made significantly more efficient in ambient-noise interferometry if strategies for ensuring timely convergence of Empirical Green's Functions (EGFs) can be devised. Quantitative measures of each cross-correlation panel's constructive contribution to the final result are needed to ensure the best result with the shortest recording duration. We investigate the utility of a variety of seismic data features to improve the convergence of EGFs in ambient-noise interferometry with a dataset acquired at the San Emidio Geothermal Field, Nevada. We identify two features, “rms energy” and “kurtosis”, that succeed in distinguishing cross-correlation panels that contribute to EGF convergence from those that degrade the stacked result. Using either of these features allows us to selectively stack data and enhance signal quality at greater offsets and recover weaker arrivals than is possible by stacking all available data. The data used in this study were acquired in May 2019 by a 144-element array of 4.5 Hz geophones recorded by Reftek 130 digitizers in a configuration that was augmented by embedded processors to perform real-time, in-field processing. Such capabilities, coupled with selective stacking strategies, may help minimize the time, cost, and effort required to determine the best possible EGFs.

Simulation of Strong Ground Motion in the National Capital Region, India, from a Future 8.5 Magnitude Earthquake Using Two-Step Empirical Green’s Function Method

ABSTRACT In this study, I simulate high-frequency ground motions at five stations in the National Capital Region (NCR) of India for a large hypothetical Mw 8.5 earthquake in the Himalayan central seismic gap, at fault-distances of about 200–300 km. A smaller magnitude earthquake (22 July 2007 Mw 4.9 Kharsali) is used as the first-step empirical Green’s function (EGF) for the synthesis of an intermediate-sized earthquake of magnitude Mw 6.8 (1991 Uttarkashi earthquake). In the second step, the records of Mw 6.8 synthetics are further used as the EGF in the simulation of the postulated Mw 8.5 earthquake. Because the target region for the postulated earthquake is devoid of the necessary information on the geophysical constraints, I perform a suite of simulations for plausible scenarios of fault dimensions, stress-drop ratios, C, and scaling factor, N (between the EGF and target earthquake). This article uses heterogeneous slip distributions and variable stress drops on the rupture plane to simulate the target earthquake, based on the power spectral density of the von Karman correlation function. The estimated values of the ground-motion intensity measure (GMIM) such as peak ground acceleration, along with the engineering parameters such as the 5% damped, pseudospectral acceleration (Sa), Arias intensity (IA), and significant duration (TD), are compared for both the recorded and the simulated time histories. The estimated GMIMs of the Mw 6.8 synthetics are examined with those of the 1991 Mw 6.8 Uttarkashi earthquake, whereas the Mw 8.5 simulations are compared with those predicted by prevalent ground-motion prediction equations for rock sites. The Mw 8.5 earthquake scenarios indicate higher GMIMs and seismic hazard in the NCR, principally due to the area being underlain by sediment layers and fluvial deposits.

Near-surface structure of the Sodankylä area in Finland, obtained by passive seismic interferometry

Abstract. Controlled-source seismic exploration surveys are not always possible in nature-protected areas. As an alternative, the application of passive seismic techniques in such areas can be proposed. In our study, we show results of passive seismic interferometry application for mapping the uppermost crust in the area of active mineral exploration in northern Finland. We utilize continuous seismic data acquired by the Sercel Unite wireless multichannel recording system along several profiles during XSoDEx (eXperiment of SOdankylä Deep Exploration) multidisciplinary geophysical project. The objective of XSoDEx was to obtain a structural image of the upper crust in the Sodankylä area of northern Finland in order to achieve a better understanding of the mineral system at depth. The key experiment of the project was a high-resolution seismic reflection experiment. In addition, continuous passive seismic data were acquired in parallel with reflection seismic data acquisition. Due to this, the length of passive data suitable for noise cross-correlation was limited from several hours to a couple of days. Analysis of the passive data demonstrated that dominating sources of ambient noise are non-stationary and have different origins across the XSoDEx study area. As the long data registration period and isotropic azimuthal distribution of noise sources are two major conditions for empirical Green function (EGF) extraction under the diffuse field approximation assumption, it was not possible to apply the conventional techniques of passive seismic interferometry. To find the way to obtain EGFs, we used numerical modelling in order to investigate properties of seismic noise originating from sources with different characteristics and propagating inside synthetic heterogeneous Earth models representing real geological conditions in the XSoDEx study area. The modelling demonstrated that scattering of ballistic waves on irregular shape heterogeneities, such as massive sulfides or mafic intrusions, could produce a diffused wavefield composed mainly of scattered surface waves. In our study, we show that this scattered wavefield can be used to retrieve reliable EGFs from short-term and non-stationary data using special techniques. One of the possible solutions is application of “signal-to-noise ratio stacking” (SNRS). The EGFs calculated for the XSoDEx profiles were inverted, in order to obtain S-wave velocity models down to the depth of 300 m. The obtained velocity models agree well with geological data and complement the results of reflection seismic data interpretation.

Probing the Damage Zone at Parkfield

AbstractRocks are heterogeneous materials that exhibit nonlinear elastic (anelastic) behavior at scales ranging from the laboratory to Earth. In the laboratory, typical, complex relationships exist between stress and strain that include hysteresis, finite relaxation times, strain rate, and history dependence. These behaviors are linked to important characteristics such as stress, porosity, permeability, material integrity, and material failure. We adopted a “pump‐probe” type experiment common in laboratory studies, using solid earth tides as the low‐frequency pump and empirical Green's function as the high‐frequency probe. By probing the velocity at different points in the pump cycle, we constrained important information about the strain‐modulus relationship. Near the San Andreas Fault, we observed strongly nonlinear elastic behavior that characterizes the damage zone. We also constrained important aspects of hysteretic behavior that are related to damage properties and possibly pore pressure. Away from the fault, the nonlinear behavior is diminished.

Future magnitude 7.5 earthquake offshore Martinique: spotlight on the main source features controlling ground motion prediction

SUMMARY The eastern offshore of Martinique is one of the active areas of the Lesser Antilles Subduction Zone (LASZ). Although its seismicity is moderate compared to other subduction zones, LASZ is capable of generating a M 7+ interplate earthquake and recent studies and historical events, such as the M 8 1839 and M 7–7.5 1946 earthquakes, confirm this possibility. Given the high risk that Martinique can face in case of unpreparedness for such a M 7+ earthquake, and the lack of a regional seismic hazard study, we investigated through numerical modelling how ground motion can vary for a hypothetical Mw 7.5 interplate earthquake. Our main objective is to highlight the major factors related to earthquake source that can cause the highest variation in ground motion at four broad-band seismic stations across Martinique. For this purpose, we generated 320 rupture scenarios through a fractal kinematic source model, by varying rupture directivity, source dimension, slip distribution. We computed the broad-band ground motion (0.5–25 Hz) by convolution of source–time functions with empirical Green’s functions (EGFs), that we selected from the analysis of moderate events (M 4–4.5) recorded in the area of interest since 2016 by the West Indies network. We found that the fault geometry and the spatial extension of the largest slip patch are the most influential factors on ground motion. The significance of the variation of the predicted ground motion with respect to ground motion prediction equations (GMPEs) depends on the evaluated frequency of ground motion and on the station. Moreover, we concluded that the EGF selection can be another significant factor controlling the modelled ground motion depending on station. Our results provide a new insight for the seismic source impact on ground motion across Martinique and can guide future blind seismic hazard assessment studies in different regions.

Adjoint Tomography of Ambient Noise Data and Teleseismic P Waves: Methodology and Applications to Central California

AbstractAdjoint tomography has been recently applied to ambient seismic noise and teleseismic P waves separately to unveil fine‐scale lithospheric structures beyond the resolving ability of traditional ray‐based traveltime tomography. In this study, we propose an inversion scheme that alternates between frequency‐dependent traveltime inversions of ambient noise surface waves and waveform inversions of teleseismic P waves to take advantage of their complementary sensitivities to the Earth's structure. We apply our method to ambient noise empirical Green's functions from 60 virtual sources, direct P and scattered waves from 11 teleseismic events recorded by a dense linear array (∼7 km station spacing) and other regional stations (∼40 km average station spacing) in central California. To evaluate the performance of the method, we compare tomographic results from ambient noise adjoint tomography, full‐waveform inversion of teleseismic P waves, and the alternating inversion of the two data sets. Both applications to practical field data sets and synthetic checkerboard tests demonstrate the advantage of the alternating inversion over individual inversions as it combines the complementary sensitivities of the two independent data sets toward a more unified model. The three dimensional model from our alternating inversion not only shows major features of velocity anomalies and discontinuities in agreement with previous studies, but also reveals small‐scale heterogeneities which provide new constraints on the geometry of the Isabella Anomaly and mantle dynamic processes in central California. The proposed alternating inversion scheme can be applied to other regions with similar array deployments for high‐resolution lithospheric imaging.

High-fidelity broadband prediction of regional seismic response: a hybrid coupling of physics-based synthetic simulation and empirical Green functions

The prediction of the seismic response of critical structures is highly sensitive to many aspects, among which the earthquake source and the geological setting are prominent. The related uncertainty issues must be taken into account in seismic risk mitigation studies, for example through the evaluation of several realizations of a future earthquake scenario. This aspect is crucial when addressing vulnerability studies at a regional scale. When opting for physical-based numerical simulations (PBS), however, the computational burden increases along with the expected degree of fidelity, making it difficult to evaluate more than a few dozens of alternatives. To cope with this disadvantage, in this work an alternative method is proposed, which exploits a rather low number of synthetic earthquake simulations and combines them with the empirical Green function (EGF) method, to finally generate thousands of alternative yet realistic seismic response of the site of interest. This hybrid strong motion predictions benefit of both (i) PBS high fidelity and (ii) data assimilation of strong ground motion records in the seismic area of interest, via EGF method, producing broadband synthetics at a relative cheap computational price. The power of the hybrid method is tested on a real case scenario, embodied by the ground-shaking prediction at the nuclear site of Cadarache, in the surroundings of the fault of Middle Durance, in South-Eastern France. Thousands of broadband (0–15 Hz) hybrid synthetic seismic response are generated, associated with different fault parameters (EGF method) and based upon a few key physics-based simulations, accurate up to 5 Hz.

On the strength of the phase cross-correlation in retrieving the Green\u2019s function information in a region affected by persistent aftershock sequences

Although research on seismic interferometry is now entering a phase of maturity, earthquakes are still the most troublesome issues that plague the process in real applications. To address the problems that arise from spatially scattered and temporally transient enormous earthquakes, preference is usually given to the use of time-dependent weights. However, small earthquakes can also have a disturbing effect on the accuracy of interpretations if they are persistently clustered right next to the perpendicular bisector of the line joining station pairs or in close proximity to one of the stations. With regard to the suppression of these cluster earthquakes, commonly used solutions for dealing with monochromatic microseismic cluster events (e.g., implementing a band-reject filter around a comparatively narrow frequency band or whitening the amplitude spectra before calculating the cross-spectrum between two signals) may not have the necessary efficiency since earthquake clusters are generally a collection of events with different magnitudes, each having its own frequency and energy contents. Therefore, the only solution left in such a situation is to use stronger non-linear time-dependent weights (e.g., square of the running average or one-bit normalization), which may cause Green’s function amplitude information to be lost. In this paper, by simulating the records of a benchmark earthquake MN 5.2 with the help of empirical Green’s functions (EGF) obtained after the Ahar-Varzeghan Earthquake Doublet (MN 6.4 and MN 6.3), it is shown that the amplitude-unbiased phase cross-correlation is a relatively efficient approach in the face of the issues concerning long-standing cluster events.

Extracting surface wave dispersion curves from asynchronous seismic stations: method and application

SUMMARY Correlation of the coda of Empirical Green's functions (EGFs) from ambient noise can be used to reconstruct EGFs between two seismic stations deployed different periods of time. However, such method requires a number of source stations deployed in the area surrounding a pair of asynchronous stations, which limit its applicability in cases where there are not so many available source stations. Here, we propose an alternative method, called two-station C2 method, which uses one single station as a virtual source to retrieve surface wave phase velocities between a pair of asynchronous stations. Using ambient noise data from USArray as an example, we obtain the interstation C2 functions using our C2 method and the traditional cross-correlation functions (C1 functions). We compare the differences between the C1 and C2 functions in waveforms, dispersion measurements, and phase velocity maps. Our results show that our C2 method can obtain reliable interstation phase velocity measurements, which can be used in tomography to obtain reliable phase velocity maps. Our method can significantly improve ray path coverage from asynchronous seismic arrays and enhance the resolution in ambient noise tomography for areas between asynchronous seismic arrays.

Ambient Noise Tomography with Short-Period Stations: Case Study in the Borborema Province

The analysis of seismic ambient noise has been recently proven a viable alternative to the analysis of seismic waves generated by earthquakes. Although seismic ambient noise had traditionally been discarded from earthquake records, it has now been shown that the cross-correlation of the seismic ambient noise allows for the recovery of the Green’s function between the receivers. Furthermore, seismic noise has the ability to propagate continuously and independently of the occurrence of earthquakes, allowing for high-resolution tomographic studies in regions of low seismicity. Over the last two decades, the cross-correlation of continuous seismic noise recordings between pairs of stations has been widely utilized in surface-wave tomography studies. For the Northeast Brazil region, tomographic studies have been exclusively performed with seismic data from broadband stations; however, in addition to those stations, there exists a large volume of short-period data that might potentially improve the resolution of existing ambient noise tomographies. Thus, the goal of this work is to utilize, in addition to broadband data, short-period data recorded by 22 short-period stations in the Borborema Province. Through cross-correlation and stacking of ambient seismic noise, the emergence of the fundamental mode of the Rayleigh waves and their dispersive character was observed. Once the empirical Green's functions were retrieved, the accuracy of (phase and group) dispersion curves for periods up to 10 s and its tomographic inversion were assessed. Our results demonstrate that short-period dispersion measurements can be successfully integrated in regional tomographic studies for improved resolution.

Stress Drop Variations in the Region of the 2014 MW8.1 Iquique Earthquake, Northern Chile

AbstractWe compute stress drops from P and S phase spectra for 534 earthquakes in the source region of the 2014 MW8.1 Iquique megathrust earthquake in the northern Chilean subduction zone. An empirical Green's function based method is applied to suitable event pairs selected by template matching of eight years of continuous waveform data. We evaluate the parameters involved in the stress drop estimation, consider the effect of the local velocity structure and apply an empirical linear relation between P and S phase related geometry factors (k values). Data redundancy produced by multiple empirical Green's function and the combination of P and S phase spectra leads to a substantial reduction of uncertainty and robust stress drop estimates. The resulting stress drop values show a well‐defined log‐normal distribution with a median value of 4.36 MPa; most values range between 0.1 and 100 MPa. There is no evidence for systematic large scale lateral variations of stress drop. A detailed analysis reveals several regions of increased median stress drop, an increase with distance to the interface, but no consistent increase with depth. This suggests that fault regime and fault strength have a stronger impact on the stress drop behavior than absolute stresses. Interestingly, we find a weak time‐dependence of the median stress drop, with an increase immediately before the April 1, 2014 MW8.1 Iquique mainshock, a continuous reduction thereafter and a subsequent recovery to average values. Additionally, the data set indicates a relatively strong dependence of stress drop on magnitude which extends over the entire analyzed magnitude range.

Ground-motion simulation for the 2017 Mw7.3 Ezgeleh earthquake in Iran by using the Empirical Green's Function Method

The aim of this study is to investigate the strong ground motion generation of destructive earthquake in Kermanshah with the moment magnitude of 7.3 using Empirical Green’s function (EGF) method. To simulate the ground-motion can be helpful for understanding seismic hazard and reduce fatalities due to lack of real ground motion. We collected the seismograms recorded at seven strong motion stations with good quality to estimate the source parameters at frequencies between 0.1 and 10.0 Hz. By minimizing the root-mean-square (rms) errors to obtain the best source parameters for the earthquake. The earthquake fault was divided into seven sub-faults along the strike and seven sub-faults along the slope. The asperity of 21×10.5 km was obtained. The rupture starting point has been located in the northern part of the strong motion seismic area. The coordinates of the rupture starting point indicate that the rupture propagation on the fault plan was unilateral from north to south. The simulated ground motions have a good correlation with observed records in both frequency and time domain. The results are in well agreement with the Iranian code of practice for seismic resistant design of buildings, however, the calculated design spectrum of Sarpol-e Zahab station is higher than the design spectrum of the Iranian code which suggest that the Iranian code may need to be re-evaluated for this area.

Predicting the near-field strong ground motion based on uncertainties in asperities: an opportunity to reproduce the characteristics of the 1970 Tonghai earthquake (Ms 7.8)

Studying and understanding the characteristic ground motions of destructive earthquakes is very important in the prevention of earthquake disasters. In this paper, two 2018 Ms 5.0 earthquakes are used as an empirical Green’s function to reproduce the ground motion strength of the 1970 Ms 7.8 Tonghai earthquake. The present paper considers the uncertainties in the numerical simulation, the source model, the asperities, and the initial rupture positions. The obtained peak ground acceleration (PGA), the time history of the acceleration, and the acceleration response spectrum at each station are compared with the ground motion strength at corresponding stations for the Ms 8.0 Wenchuan earthquake to test the reliability of the ground motion simulation. The results show that the PGA of the one-asperity source model is the largest. With an increasing number of asperities, the ground motion strength decreases, and the earthquake duration increases, indicating that an increase in the number of asperities disperses seismic energy. The two-asperity source model is consistent with the real source model of the Tonghai earthquake. The acceleration time history and acceleration response spectrum at each station reflect the characteristics of the ground motion strength of the destructive Tonghai Ms 7.8 earthquake. Thus, reproducing the main ground motion strength characteristics of the Tonghai earthquake by combining the empirical Green's function method with two Ms 5.0 earthquakes is feasible.