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Volume 31 Issue 5-6 May 2019

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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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](171) [PDF 1655KB](1)
Abstract:
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.

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Preface to the special issue of Dense Array Seismology
Huajian Yao, Baoshan Wang, Xiaobo Tian, Hongfeng Yang, Xiaofeng Tian
2018, 31(5-6): 225 -226   doi: 10.29382/eqs-2018-0225-1
[Abstract](3) [FullText HTML](3) [PDF 0KB](0)
Abstract:
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Public cloud computing for seismological research: Calculating large-scale noise cross-correlations using ALIYUN
Weitao Wang, Baoshan Wang, Xiufen Zheng
2018, 31(5-6): 227 -233   doi: 10.29382/eqs-2018-0227-2
[Abstract](227) [FullText HTML](1) [PDF 1878KB](6)
Abstract:
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 two orders of magnitude 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.
Phase velocity maps of Rayleigh waves in the Ordos block and adjacent regions
Shaoxing Hui, Wenhua Yan, Yifei Xu, Liping Fan, Hongwu Feng
2018, 31(5-6): 234 -241   doi: 10.29382/eqs-2018-0234-3
[Abstract](235) [FullText HTML](3) [PDF 1295KB](13)
Abstract:
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 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 has 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 heterogeneity represents the lateral variation of the Moho depth. The Ordos block and the northern margin of Sichuan basin are located in the uppermost mantle at this depth, and the depth in the transition zone is still located in the lower crust.
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
2018, 31(5-6): 242 -251   doi: 10.29382/eqs-2018-0242-4
[Abstract](404) [FullText HTML](8) [PDF 10599KB](40)
Abstract:
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 32 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.
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
2018, 31(5-6): 252 -261   doi: 10.29382/eqs-2018-0252-5
[Abstract](264) [FullText HTML](1) [PDF 2629KB](25)
Abstract:
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 107 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.
A pilot project of ambient noise tomography on a dense seismic array in Ji’nan urban area
Feng Liang, Lei Gao, Zhihui Wang, Tao Wang, Kai Liu
2018, 31(5-6): 262 -271   doi: 10.29382/eqs-2018-0262-6
[Abstract](1) [FullText HTML](0) [PDF 0KB](0)
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In this study, we conducted ambient noise tomography (ANT), with 33-day data, to investigate the near-surface shear-velocity structure in downtown Ji’nan, Shandong Province, China. The cross-correlation functions with quiet clear Rayleigh waves are obtained with more symmetrical energy distribution which indicates that strong human activities, such as moving vehicles and municipal engineering construction, can produce approximately isotropic distribution of noise sources for high-frequency signals. The direct surface-wave tomographic method with period-dependent ray-tracing is used to invert all surface-wave dispersion data in the period band 0.2–1.5 s simultaneously for 3D variations of shear-velocity structure. The study result can help us better understand the local geologic structures, evaluate the distribution and geometry of the fracture-karst media, and assess hazards of the concealed active fault and its effect on the springs in the future.
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
2018, 31(5-6): 272 -280   doi: 10.29382/eqs-2018-0272-7
[Abstract](331) [FullText HTML](1) [PDF 1643KB](9)
Abstract:
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.
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
2018, 31(5-6): 281 -290   doi: 10.29382/eqs-2018-0281-8
[Abstract](235) [FullText HTML](4) [PDF 1967KB](12)
Abstract:
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), make 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.
Topography effect on ambient noise tomography using a dense seismic array
Shuang Wang, Xinlei Sun
2018, 31(5-6): 291 -300   doi: 10.29382/eqs-2018-0291-9
[Abstract](349) [FullText HTML](13) [PDF 2432KB](38)
Abstract:
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.
Imaging mantle transition zone discontinuities in southwest China from dense array ambient noise interferometry
Jikun Feng, Huajian Yao, Weitao Wang
2018, 31(5-6): 301 -310   doi: 10.29382/eqs-2018-0301-6
[Abstract](267) [FullText HTML](1) [PDF 2090KB](17)
Abstract:
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.
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