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Volume 31 Issue 3 June 2018

ISSN 1674-4519 CN 11-5695/P

Adminidrated by: 
China Association of Science and Technology

Sponsored by: 
The Seismological Society of China and Institute of Geophysics, China Earthquake Administration

Editor-in-Chief: Yun-tai Chen


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Topography effect on ambient noise tomography using a dense seismic array
Shuang Wang, Xinlei Sun
Accepted Manuscript
[Abstract](218) [PDF 2432KB](32)
With the rapid increase of dense seismic array deployment, more and more ambient noise studies have been applied on short period surface waves tomography. For arrays with inter-station distance of several hundred meters, the effect of surface topography has to be considered. In this study, we investigate topography effect on ambient noise surface wave tomography using synthetic data from different topographic models. Our travel times are synthetized considering surface topography, and shear wave inversions are performed by incorporating and not incorporating topography respectively. Our inversion results suggest that topography does affect subsurface shear wave velocity inversion. If topography is not considered, although the pattern of the structure may be recovered reasonably well, the depth distribution of velocity structure can be distorted. The maximum distortion depth is generally correlated with the relief of the topography and the amplitude of the velocity anomalies. Finally, our example of real data inversion in a mountain area demonstrates good correlation between shear velocity and the geological settings, as well as the core sample in that area.
Upper crustal anisotropy observed around the Longmenshan fault in the 2013 MS7.0 Lushan earthquake region
Lijun Chang, Zhifeng Ding, Chunyong Wang
Accepted Manuscript
[Abstract](173) [PDF 1062KB](13)
Based on the shear wave splitting analysis of the seismic recordings at 17 temporary stations and three permanent stations, we measured the shear wave splitting parameters (i.e., the polarization direction of fast shear wave and the time delay of slow wave) to perform a systematic analysis of the crustal seismic anisotropy around the Longmenshan fault in the 2013 MS7.0 Lushan earthquake region. We observed apparent spatio-temporal characteristics in the shear wave splitting parameters. The spatial distribution of fast polarization directions showed a clear partitioning in the characteristics from northwest to southeast in the focal region, which changed from NW-SE to NE-SW. In the northwest of the focal region, the fast polarization direction was oriented to NW-SE, which was parallel to the maximum horizontal compressive stress direction. However, the NE-SW fast polarization direction in the southeast of the focal region was parallel to the Longmenshan fault strike. For station BAX on the Central fault in the middle of the focal region, the distribution of fast polarization directions showed a bimodal pattern, with one dominant in the NE-SW direction and the other in the NW-SE direction. With regard to the temporal variation, the time delays were large in the initial stage after the mainshock but then gradually decreased over time and tended to be stable in the later period. This indicated that stress in the focal region increased to a maximum when the main shock occurred, with the stress release caused by the mainshock and aftershock activity, and the stress gradually decreased after a period of time. The scatter of fast polarization directions was large after the main shock, but over time the scatter gradually decreased, indicating that the Lushan earthquake caused a large perturbation in the local stress field. As the stress gradually decreased and was adjusted by the aftershock activity, the perturbation gradually weakened.
Source parameter and rupture process of the MW6.3 early strong aftershock immediately following the 2016 MW7.8 Kaikoura earthquake (New Zealand)
Qingjun Meng, Aizhi Guo, Xiangteng Wang, Shuofan Wang
Accepted Manuscript
[Abstract](6) [PDF 1655KB](0)
The 2016 MW7.8 Kaikoura (New Zealand) earthquake was the most complex event ever instrumentally recorded and geologically investigated, as it ruptured on more than 12 fault segments of various geometries. To study the mainshock rupture characteristics, geodetic methods like InSAR and GPS play an essential role in providing satisfactory spatial resolution. However, early strong aftershocks may cause extra ground deformation which bias the mainshock rupture inversion result. In this paper, we will focus on studying the MW 6.3 aftershock, which is the only M6+ thrust slip aftershock that occurred only 30 minutes after the Kaikoura mainshock. We will relocate the hypocenter of this event using the hypo2000 method, make the finite fault model (FFM) inversion for the detailed rupture processes and calculate the synthetic surface displacement to compare with the observed GPS data and figure out its influence on the mainshock study. Although we are not able to resolve the real ruptured fault of this event because of limited observation data, we infer that it is a west-ward dipping event of oblique slip mechanism, consistent with the subfault geometries of the Kaikoura mainshock. According to the inverted FFM, this event can generate 10–20 cm ground surface displacement and affect the ground displacement observation at nearby GPS stations.
The effect of uneven noise source and/or station distribution on estimating the azimuth anisotropy of surface waves
Laiyu Lu, Kaiming Wang, Zhifeng Ding
Uncorrected proof
[Abstract](288) [PDF 1544KB](65)
With the development of the dense array, the surface wave velocity and azimuthal anisotropy under the array can be directly obtained by beamforming the noise cross-correlation functions (NCFs). However, the retrieval of the Green's function by cross-correlating the seismic noise requires that the noise source has a uniform distribution. For the case with uneven noise source, the azimuthal dependence on the sources in the expression for the spatial coherence function, which corresponds to the NCF in the time domain, has the same form as the azimuthal dependence of the surface wave velocity in weakly anisotropic media. Therefore, the uneven noise source will affect the surface wave anisotropy extraction. In this study, three passive seismic methods, i.e., beamforming, SPAC (Spatial autocorrelation), and NCF, are compared to demonstrate that an uneven source distribution and uneven station distribution have equivalent effects on the outcome from each method. A beamforming method is proposed to directly extract the velocity and azimuthal anisotropy of surface waves. The effect of uneven noise source and/or station distribution on estimating the azimuth anisotropy of surface waves was investigated using data from the ChinArray Phase II. A method for correcting the apparent anisotropy in beamforming results caused by an uneven station distribution is suggested.
Rupture imaging of the 25 April 2015 MW7.8 Nepal earthquake from back-projection of teleseismic P waves
Yong Qiu, Qiaoxia Liu
Uncorrected proof
[Abstract](306) [PDF 2257KB](15)
The MW7.8 Nepal earthquake of 25 April 2015 had over 8, 500 fatalities and was the most destructive earthquake in Nepal since the Bihar-Nepal earthquake in 1934. In this study, we imaged the rupture process of this Nepal event by back-projecting the teleseismic P-wave energy recorded at the three regional networks in Alaska, Australia and Europe. The back-projection images of the three subarrays revealed that the Nepal earthquake propagated along the strike in a southeast direction over a distance of ~ 160–170 km with the duration of ~ 50–55 s. The rupture process was found to be a simple, unilateral event with a near constant velocity of 3.3 km/s. The beam power was mainly distributed in the geographic region just north of Kathmandu and the peak intensity for the source time function curve occurred at about 30 s. The earthquake was destructive due to its occurrence at shallow depth (~ 12–15 km) and the fact that the capital lies in a basin of soft sediment. Additionally, the resonance effect for the longer period waves that occurred in the Kathmandu valley led to destructive aggravation, impacting mainly the taller buildings.
Study of crustal thickness and poisson’s ratio of the south of erenhot area by teleseismic receiver function
Lei Jiang, Yonghong Duan, Yanna Zhao, Yong Qiu, Cheng Li
Uncorrected proof
[Abstract](254) [PDF 1214KB](27)
The Xing’an Mongolian Orogenic Belt (XMOB) and the northern margin of North China Craton (NCC) have undergone multistage tectonic superimposition and the tectonic evolution is extremely complicated. We collect the teleseismic data of 44 temporary broadband seismic stations deployed in the XMOB and the northern margin of NCC to calculate the P wave receiver functions. The crustal thickness and average crustal ratio as well as the Poisson’s ratios beneath 33 stations are estimated using the H-κ stacking method. The results show: (1) the crustal thickness of the study area ranges from 38.7 to 42.7 km, with an average thickness of 41.2 km. There is a great difference in crustal thickness on both sides of Solonker suture zone. The characteristics of crustal thickness support the geodynamic model that the Paleo-Asian Ocean subducted and closed at the Solonker suture zone. (2) The Poisson’s ratios in the study area are low, ranging from 0.215 to 0.277, with an average value of 0.243, suggesting that the rock composition of the area is dominated by felsic-acid rocks. (3) There exists a negative correlation between the Poisson’s ratio and the crustal thickness. Combined with the lower values of Poisson’s ratio, we speculate that the delamination is the major mechanism in crustal extension and thinning in the study area.
Estimating the site effects in Luoyang basin using horizontal-to-vertical spectral ratio method from a short-period dense array
Yujuan Tan, Yunhao Wei, Yonghong Duan, Fuyun Wang
Uncorrected proof
[Abstract](208) [PDF 1643KB](8)
The influence of local site effects on seismic ground motions is an important issue in seismic hazard assessment and earthquake resistant design. Determining site effects in densely populated cities built on basins can help to reduce the earthquake hazard. Site effects of Luoyang basin are estimated by the horizontal-to-vertical spectral ratio (HVSR) method using ambient noise records from a short-period dense array. The sites in Luoyang basin are sorted into three types according to the pattern of the HVSR curves. There are cases with a single clear peak, two clear peaks, and an unclear low frequency peak or multiple peaks, which correspond to there being one large impedance contrast interface, two large interfaces, and a moderate one beneath the sites, respectively. The site effects characterized by fundamental frequency from HVSR curves are affected by underlying sedimentary layers and depth of sedimentary basement. According to our results, the existence of thick sediment layer obviously lowers the fundamental frequency to the period range from 2 to 4 s in the downtown area of Luoyang city. The ground motion will amplify when through the sites and the buildings with height of 20–50 floors can resonate at the similar frequency domain. Site effects estimation using HVSR method from a short-period dense array is an effective technique in areas of moderate seismic risk where strong motion recordings are lacking, such as the Luoyang basin.
Shallow velocity structure of the Luoyang basin, derived from dense-array observations of urban ambient noise
Ming Zhou, Xiaofeng Tian, Fuyun Wang, Yunhao Wei, Hailiang Xin
Uncorrected proof
[Abstract](156) [PDF 2629KB](23)
Determining the shallow structure of a sediment basin is important when evaluating potential seismic hazards given that such basins can significantly amplify seismic energy. The Luoyang basin is located in the western He'nan uplift and is a Meso-Cenozoic depression basin. To characterize the shallow structure of the basin, we develop a model of the shallow high-resolution three-dimensional (3D) shear-wave velocity structure of the basin by applying ambient noise tomography to a dense array of 108 portable digital seismometers deployed over the basin. More than 1,400 Rayleigh-wave dispersion curves for periods in the range 0.5–5 s are extracted. The 3D variations of shear-wave velocity in the shallow crust are inverted using a direct surface-wave tomographic method with period-dependent ray tracing, with all the surface-wave group-velocity dispersion data being inverted simultaneously. The results show that in the shallow crust of the study area, the velocity distribution corresponds to surface geology and geological features. The Luoyang basin exhibits a low shear-wave velocity feature that is consistent with the distribution of sediment in the region, while the Xiongershan and Songshan uplifts exhibit higher shear-wave velocity structures. The results provide a shallow high-resolution 3D velocity model that can be used as a basis for simulation of strong ground motion and evaluation of potential seismic hazards.
Multiple seismological evidences of basin effects revealed by Array of Binchuan (ABC), northwest Yunnan, China
Yihe Xu, Baoshan Wang, Weitao Wang, Bo Zhang, Tianwei Sun
Uncorrected proof
[Abstract](139) [PDF 1967KB](12)
Dense array seismology, which is characterized by large number, densely deployed autonomous geophone/seismographs, has received great concerns worldwide recently, especially after the great success of dense array in Long Beach. One of the biggest curiosity is that if the great success in Long Beach is replicable in China. Hence, we analyze the seismic records from a dense array in Binchuan basin, Yunnan province, which consists of three-component short-period seismographs of three most common domestic models. The Binchuan basin is located near the intersection between the Chenghai-Binchuan fault and the Red River fault, with the latter being the major fault accommodating significant tectonic deformation resulting from eastern extrusion of the Tibetan Plateau. Both faults pose serious seismic threats to local residents in Binchuan basin. Basin-range differences, faults, local earthquakes, and a Fixed Airgun Seismic Transmitting Station (FASTS), makes the Binchuan basin a perfect experiment site for dense array experiment. The array is named Array of Binchuan (ABC) and the main target is imaging the shallow crustal structure, especially the structure of the basin. To examine the monitoring capability of ABC, we analyze the seismograms to check if they can reveal the basin, the most significant geological feature in the area. Power spectral density analysis, travel time and amplitude analysis of FASTS signals, and amplitude analysis of earthquakes and noise cross-correlation functions are used in the analysis. All the results show correlation with the basin and clear difference between basin and non-basin area. Therefore, the preliminary results support that the ABC has the potential to provide constraints on local structures.
Phase velocity maps of Rayleigh waves in the Ordos block and adjacent regions
Shaoxing Hui, Wenhua Yan, Yifei Xu, Liping Fan, Hongwu Feng
Uncorrected proof
[Abstract](125) [PDF 1295KB](12)
We analyze continuous waveform data from 257 broadband stations of the portable seismic array deployed under the "China Seismic Array-northern part of NS seismic belt" project as well as data from a permanent seismic network from January 2014 to December 2015. The phase velocity dispersion curve of 7,185 Rayleigh waves is obtained with a method based on the image analysis method of phase velocity extraction, and the inversion is obtained. The period of Rayleigh wave phase velocity distribution has a range of 5–40 s, and minimum resolution close to 20 km. The results show that the phase velocity structure image well reflects the geological structural characteristics of the crust and uppermost mantle, and that the phase velocity distribution has obvious lateral heterogeneity. The phase velocity of the 5–15 s period is closely linked to the surface layer and sedimentary layer, the low-velocity anomalies correspond to loose sedimentary cover, and the high-velocity anomalies correspond to orogenic belts and uplifts and the boundary between high and low velocity anomalies is consistent with the block boundary. The phase velocity of the 5–15 s period is strongly affected by the crust layer thickness, the northeastern Tibetan plateau is low-velocity anomalies in the middle to lower crust, the west side of the Ordos block is consistent with the northeastern Tibetan plateau, which may imply the material exchange and fusion in this area. The velocity variation is inversely related to the Moho depth in the 40 s period of S-wave, and the lateral velocity heterogeneous represents the lateral heterogeneous of the Moho depth, the Ordos block and the northern margin of Sichuan basin are located at the uppermost mantle at this depth, and the depth in the transition zone is still located in the lower crust.
Imaging mantle transition zone discontinuities in southwest China from dense array ambient noise interferometry
Jikun Feng, Huajian Yao, Weitao Wang
Uncorrected proof
[Abstract](155) [PDF 2090KB](16)
Body waves retrieved from ambient noise cross-correlation functions (NCFs) have been reported by more and more recent studies in addition to the dominant recovered surface waves. And one of important applications of these recovered body waves is to investigate the structure of discontinuities within the mantle transition zone (MTZ). In this study, clear body wave phases reflected from the MTZ discontinuities at 410 km and 660 km have been observed on the NCFs in the frequency band of 0.1–0.2 Hz from a dense regional seismic array in southwest China. The original time-domain reflected signals in the NCFs were first converted to the depth-domain NCFs based on a velocity model before they were further stacked spatially within different bins. Then the depth-domain NCFs were stacked to investigate the lateral variations of the MTZ discontinuities, that is, the 410-km and 660-km discontinuities. Our results exhibit a simple and lateral coherent P410P phase and a much more complicated P660P phase along two profiles, which are in good agreement with mineralogical prediction and recent receiver function studies in the same area. This interferometric method can provide stable reflected body wave phases mainly in the frequency band 0.1–0.2 Hz due to the secondary microseism noise, which can be potentially used for high-resolution mantle interface imaging. This approach is also a good complement to traditional imaging methods, such as receiver function imaging.
Public cloud computing for seismological research: calculating large-scale noise cross-correlations using ALIYUN
Weitao Wang, Xiufen Zheng, Baoshan Wang
Uncorrected proof
[Abstract](122) [PDF 1878KB](6)
The amount of seismological data is rapidly increasing with accumulating observational time and increasing number of stations, requiring modern technique to provide adequate computing power. In present study, we proposed a framework to calculate large-scale noise cross-correlation functions (NCFs) using public cloud service from ALIYUN. The entire computation is factorized into small pieces which are performed parallelly on specified number of virtual servers provided by the cloud. Using data from most seismic stations in China, five NCF databases are built. The results show that, comparing to the time cost using a single server, the entire time can be reduced over a magnitude of two depending number of evoked virtual servers. This could reduce computation time from months to less than 12 hours. Based on obtained massive NCFs, the global body waves are retrieved through array interferometry and agree well with those from earthquakes. This leads to a solution to process massive seismic dataset within an affordable time and is applicable to other large-scale computing in seismological researches.
High-resolution crustal velocity imaging using ambient noise recordings from a high-density seismic array: An example from the Shangrao section of the Xinjiang basin, China
Gaochun Wang, Xiaobo Tian, Lianglei Guo, Jiayong Yan, Qingtian Lyu
Uncorrected proof
[Abstract](290) [PDF 10599KB](39)
A profile of shallow crustal velocity structure (1–2 km) may greatly enhance interpretation of the sedimentary environment and shallow tectonic deformation. Recent advances in surface wave tomography, using ambient noise data recorded with high-density seismic arrays, have improved the understanding of regional crustal structure. As the interest in detailed shallow crustal structure imaging has increased, dense seismic array methods have become increasingly efficient. This study used a high-density seismic array deployed in the Xinjiang basin in southeastern China, to record seismic data, which was then processed with the ambient noise tomography method. The high-density seismic array contained 203 short-period seismometers, spaced at short intervals (~ 400 m). The array collected continuous records of ambient noise for 33 days. Data preprocessing, cross correlation calculation, and Rayleigh surface wave phase-velocity dispersion curve extraction, yielded more than 16,000 Rayleigh surface wave phase-velocity dispersion curves, which were then analyzed using the direct-inversion method. Checkerboard tests indicate that the shear wave velocity is recovered in the study area, at depths of 0–1.4 km, with a lateral image resolution of ~ 400 m. Model test results show that the seismic array effectively images a 50 m thick slab at a depth of 0–300 m, a 150 m thick anomalous body at a depth of 300–600 m, and a 400 m thick anomalous body at a depth of 0.6–1.4 km. The shear wave velocity profile reveals features very similar to those detected by a deep seismic reflection profile across the study area. This demonstrates that analysis of shallow crustal velocity structure provides high-resolution imaging of crustal features. Thus, ambient noise tomography with a high-density seismic array may play an important role in imaging shallow crustal structure.

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Rupture model of the 2013 MW 6.6 Lushan (China) earthquake constrained by a new GPS data set and its effects on potential seismic hazard
Rumeng Guo, Yong Zheng, Faqi Diao, Xiong Xiong, Jiao Xu
2018, 31(3): 117 -125   doi: 10.29382/eqs-2018-0117-1
[Abstract](406) [FullText HTML](313) [PDF 1175KB](21)
Vertical records are critically important when determining the rupture model of an earthquake, especially a thrust earthquake. Due to the relatively low fitness level of near-field vertical displacements, the precision of previous rupture models is relatively low, and the seismic hazard evaluated thereafter should be further updated. In this study, we applied three-component displacement records from GPS stations in and around the source region of the 2013 MW6.6 Lushan earthquake to re-investigate the rupture model. To improve the resolution of the rupture model, records from both continuous and campaign GPS stations were gathered, and secular deformations of the GPS movements were removed from the records of the campaign stations to ensure their reliability. The rupture model was derived by the steepest descent method (SDM), which is based on a layered velocity structure. The peak slip value was about 0.75 m, with a seismic moment release of 9.89 × 1018 N·m, which was equivalent to an MW6.6 event. The inferred fault geometry coincided well with the aftershock distribution of the Lushan earthquake. Unlike previous rupture models, a secondary slip asperity existed at a shallow depth and even touched the ground surface. Based on the distribution of the co-seismic ruptures of the Lushan and Wenchuan earthquakes, post-seismic relaxation of the Wenchuan earthquake, and tectonic loading process, we proposed that the seismic hazard is quite high and still needs special attention in the seismic gap between the two earthquakes.
Fault geometrical model of Dujiangyan section in Longmenshan fault zone
Hu-Rong Duan, Sheng-Lei Chen, Run Li, Quan-Chao Yan
2018, 31(3): 126 -136   doi: 10.29382/eqs-2018-0126-2
[Abstract](245) [FullText HTML](160) [PDF 956KB](14)
The focal mechanism solution on the seismic fault plane can reflect the geometric and kinematic characteristics of faults, and it is an important way to further study the fine structure of fault plane. From the focal mechanism solution of the earthquakes around the Dujiangyan fault in Longmenshan fault zone, we derived the average dip angle of Dujiangyan fault is 45.1° based on the seismic moment tensor theory. In order to refine the fault geometry structure, this paper decomposed it into multiple sub-fault planes along the length and width of the fault plane and forms a number of models A13, B13, A23a, A23b, A23c, B23a, B23b and B23c, then calculated the sub-fault’s dip of each model. In order to clarify exactly which one of the fault models is closest to the real fault model, the fault slip was carried out for each model in turn, then compared the surface displacement of each model with GPS observations. The results show that B23c model with high dip in shallow and small dip in deep is the best model, the lengths of each sub-fault of Dujiangyan fault from south to north are 33 km, 21 km and 46 km, respectively. When the depth of the fault bottom is about 11 km, the dip angles are 70.56°, 67.41° and 45.55°. When the depth of the fault bottom is about 30 km, The fault dip angles are 44.55°, 29.18° and 44.25°.
Time-history responses on the surface by regularly distributed enormous embedded cavities: Incident SH-waves
Mehdi Panji, Saeed Mojtabazadeh Hasanlouyi
2018, 31(3): 137 -153   doi: 10.29382/eqs-2018-0137-3
[Abstract](640) [FullText HTML](283) [PDF 2298KB](106)
The time-history responses of the surface were obtained for a linear elastic half-plane including regularly distributed enormous embedded circular cavities subjected to propagating obliquely incident plane SH-waves. An advanced numerical approach named half-plane time-domain boundary element method (BEM), which only located the meshes around the cavities, was used to create the model. By establishing the modified boundary integral equation (BIE) independently for each cavity and forming the matrices, the final coupled equation was solved step-by-step in the time-domain to obtain the boundary values. The responses were developed for a half-plane with 512 cavities. The amplification patterns were also obtained to illustrate the frequency-domain responses for some cases. According to the results, the presence of enormous cavities affects the scattering and diffraction of the waves arrived to the surface. The introduced method can be recommended for geotechnical/mechanical engineers to model structures in the fields of earthquake engineering and composite materials.
Analytic wave series solution of out-of-plane (SH) waves diffraction by an almost semi-circular shallow cylindrical hill
Chunyang Ji, Vincent Lee
2018, 31(3): 154 -165   doi: 10.29382/eqs-2018-0154-4
[Abstract](473) [FullText HTML](343) [PDF 5016KB](22)
A closed-form wave equation analytic solution of two-dimensional scattering and diffraction of out-of-plane (SH) waves by an almost semi-circular shallow cylindrical hill on a flat, elastic and homogeneous half space is proposed by applying the discrete Fourier series expansions of sine and cosine functions. The semi-circular hill problem is discussed as a special case for the new formulated equation. Compared with the previous semi-circular cases solutions, the present method can give surface displacement amplitudes which agrees well with previous results. Although the proposed equation can only solve the problem of SH-waves diffracted by almost semi-circular shallow hills, the stress and displacement residual amplitudes are numerical insignificantly everywhere. Moreover, the influences of the depth-to-width ratio (a parameter defined in this paper to evaluate the shallowness of the topography of hills) on ground motions are presented and summarized. The limitations and errors of truncation from Graf’s addition theorem and Fourier series equations in the present paper are also discussed.
Dynamic equivalent soil characteristics identification using earthquake records
Asskar Janalizadeh Choobbasti, Saman Soleimani Kutanaei
2018, 31(3): 166 -173   doi: 10.29382/eqs-2018-0166-5
[Abstract](367) [FullText HTML](289) [PDF 825KB](20)
Techniques for soil property estimation can be categorized into two main groups, in-situ and laboratory methods. Previous investigations indicated that strong ground motions record provides a very useful tool to estimating the in-situ characteristics of soil. The main objective of the present work is to utilize the particle swarm optimization algorithm (PSOA) integrated with linear site response method to obtain the equivalent soil profile characteristics from the available surface and bedrock earthquake motion records. To demonstrate the numerical efficiency and the validity of this approach, the procedure is validated against an available case. Then this procedure is utilized to identify the soil properties profiles of the site by using strong ground motions data recorded during the Bam earthquake of December 26, 2003. The magnitude and PGA of Bam earthquake were MW 6.6 and 0.8 g respectively.
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