Three-dimensional crustal P-wave velocity structure in the Yangbi and Eryuan earthquake regions, Yunnan, China

A magnitude 5.5 earthquakes occurred in Eryuan County,Dali Bai Autonomous Prefecture,Yunnan Province,China,on March 3.And a magnitude 5.0 earthquake occurred in the same place on April 17,2013,i.e.,45 days later.Then,on May 21,2021,multiple earthquakes,one with magnitude 6.4 and several at 5.0 or above,occurred in Yangbi County,Dali Bai Autonomous Prefecture,Yunnan Province,China.All of these occurred in the Weixi-QiaohouWeishan fault zone.In this study,1,874 seismic events in Yangbi and Eryuan counties were identified by automatic micro-seismic identification technology and the first arrivals were picked up manually.Following this,a total of 11,968 direct P-wave absolute arrivals and 73,987 high-quality Pwave relative arrivals were collected for joint inversion via the double difference tomography method.This was done to obtain the regional three-dimensional fine crustal P-wave velocity structure.The results show that the travel time residuals before and after inversion decreased from the initial–0.1–0.1 s to–0.06–0.06 s.The upper crust in the study area,which exhibited a low-velocity anomaly,corresponded to the basin region;this indicated that the low-velocity anomaly in the shallow part of the study area was affected by the basin.Results also showed some correlation between the distribution of the earthquakes and velocity structure,as there was a lowvelocity body Lv1 with a wide distribution at depths ranging from 15–20 km in the Yangbi and Eryuan earthquake regions.In addition,earthquakes occurred predominantly in the highlow velocity abnormal transition zone.The low-velocity body in the middle and lower crust may be prone to concentrating upper crustal stress,thus leading to the occurrence of earthquakes.

Inversion of source mechanism of 1989 Loma Prieta earthquake by three-dimensional FEM Green's function

Based on three-dimensional joint finite element, this paper discusses the theory and methodology about inversionof geodetic data. The FEM and inversion formula is given in detail; also a related code is developed. By use of theGreen's function about 3-D FEM, we invert geodetic measurementS of coseismic deformation of the 1989 Ms=7. 1Loma Prieta earthquake to datermine itS source mechanism. The result indicates that the slip on the fault plane isvery heterogeneous. The maximum slip and shear stress are located about 10 kin to northwest of the eathquakesource, the stress drop is about more than 1 MPa.

Exploration of fault-zone trapped waves at Pingtong Town,in Wenchuan earthquake region

Pingtong Town is located on the fractured zone of the Wenchuan 8.0 earthquake, and is seriously damaged by the earthquake. Our observation line is centered at an earthquake exploration trench across the fractured zone in the NW-SE direction, and is about 400 m long. The results reveal trapped waves in the rup- tured fault zone of the earthquake, and indicate a great difference in physical property between the media inside and outside the fault zone. The predominant frequency of the fault-zone trapped waves is about 3 -4 Hz. The wave amplitudes are larger near the exploration trench. The width of the fault zone in the crust at this location is estimated to be 200 m. In some records, the waveforms and the arrival times of S waves are quite different between the two sides of the trench. The place of change coincides with the boundary of uplift at the surface.

Three-dimensional coseismic deformation of the 2016 MW7.8 Kaikuora,New Zealand earthquake obtained by InSAR and offset measurements

The 2016 MW7.8 Kaikoura earthquake struck the northern part of south Island,New Zealand,within the active and complex Australia-Pacific plate boundary system.Firstly,we used the InSAR method to obtain coseismic LOS deformation fields based on SAR images and applied offset tracking methods to obtain offset measurements based on optical satellite images.The maximum displacement of about 6 m is detected in the direction away from the satellite on the south-west side and also towards the satellite on the north-east side.The 3D deformation field is then resolved by the combination of these measurements with a least-square solve method,and comparisons with 3 components of GPS stations show good consistency.Despite complex features demonstrated in the 3D deformation field,there are still clear spatial correlations between surface deformation and faults distribution.It reveals that more than ten faults were ruptured during the earthquake,including some faults were previously understudies for their tectonic activities.The maximum horizontal deformation of about 10 m occurs along the Kekerengu fault with the vertical deformation up to 2 m.The 3D deformation shows that the mainshock is a multisegments faulting with a rupture process of strike-slip,compression,transpressional rupture and strike-slip in space along the NE direction.

Three dimensional velocity structure and accurate earthquake location in Changning–Gongxian area of southeast Sichuan

In order to understand the crustal structure and tectonic background of the Changning–Gongxiang area, southeastern Sichuan Province, where a series of moderate-to-strong earthquakes occurred in recent years, we utilized the seismic phase data both from a local dense array and from the regional seismic networks;we used the tomoDD program to invert for the high-resolution three-dimensional velocity structure within the depth range of 0–10 km and for accurate hypocentral locations in this area. We analyzed the seismogenic structures for the events of Xingwen M5.7 in 2018 and Gongxian M5.3 and Changning M6.0 in 2019. The results show that:(1) widespread lateral inhomogeneity exists in the velocity structure of the study area, and the location of the velocity anomaly is largely consistent with known structures. In the range of distinguishable depth, the inhomogeneity decreases with increasing depth, and the velocity structure anomalies in some areas are continuous in depth;(2) earthquakes occurred in clusters, showing the characteristics of zonal folding trends in the NW-SE and NE-SW directions;the focal depth in the area is generally shallow in both the sedimentary cap and the crystalline basement. The seismogenic structures of small earthquake clusters are different in size and occurrence in different sections, and the clusters occurred mostly in regions with high P-or S-wave velocities;(3) synthesis of a variety of data suggests that the seismogenic structures of the Xingwen M5.7 and Changning M6.0 earthquakes are associated with slip faults that trend NW-SE in, respectively, the south wing and the axis of the Changning–Shuanghe anticline, while that of the Gongxian M5.3 earthquake is associated with thrust faults that trend N-S in the Jianwu syncline region. The dynamic sources of the three earthquakes are all from the SE pushing of the Qinghai–Tibet block on the Sichuan basin;(4) the risk of future strong earthquakes in this area must be reevaluated in light of the facts(a)that in recent years, moderate-to-strong earthquake swarms have occurred frequently in southeast Sichuan;(b) that the complex structural area exhibits the easy-to-trigger characteristic, and(c) that the small-scale faults in this area are characterized by the phenomenon of stress "lock and release".

Deep Seismogenic Environment in the Southern Section of the Longmenshan Fault Zone on the Eastern Margin of the Tibetan Plateau and Lushan M_s 7.0 Earthquake

The 2,026 earthquake events registered by the Sichuan regional digital seismic network and mobile seismic array after the April 20th, 2013 Lushan earthquake and 28,188 pieces of data were selected to determine direct P waves arrival times. We applied the tomographic method to inverse the characteristics of the velocity structure for the three-dimensional （3D） P wave in the mid-upper crust of the seismic source region of the Lushan earthquake. The imaging results were combined with the apparent magnetization inversion and magnetotelluric （MT） sounding retest data to comprehensively study the causes of the deep seismogenic environment in the southern section of the Longmenshan fault zone and explore the formation of the Lushan earthquake. Research has shown that there are obvious differences in velocity structure and magnetic distribution between the southern and northern sections of the Longmenshan fault zone. The epicenter of the Lushan earthquake is located near the boundary of the high and low-velocity anomalies and favorable for a high-velocity section. Moreover, at the epicenter of the Lushan earthquake located on the magnetic dome boundary of Ya＇an, the development of high velocity and magnetic solid medium favors the accumulation and release of strain energy. Low- velocity anomalies are distributed underneath the are of seismogenic origin, The inversion results of the MT retest data after the April 20th Lushan earthquake also indicate that there a high-conductor anomaly occurs under the area of seismogenic origin of the Lushan earthquake, Therefore, we speculated that the presence of a high-conductivity anomaly and low-velocity anomaly underneath the seismogenic body of the Lushan earthquake could be related to the existence of fluids. The role of fluids caused the weakening of the seismogenic layer inside the mid-upper crust and resulted in a seismogenic fault that was prone to rupture and pIayed a triggering role in the Lushan earthquake.

Tectonic and geological setting of the earthquake hazards in the Changning shale gas development zone, Sichuan Basin, SW China

Hydraulic fracturing is a key technology in shale gas extraction,whether hydraulic fracturing induces earthquakes has become a hot topic in the public and the focus of scholars’research.The urgency of shale gas mining and the catastrophic nature of earthquakes highlight the urgent need to study this issue.The Changning anticline at the southern margin of the Sichuan Basin is a key area for shale gas exploitation.Taking this as an example,this paper applies the velocity model of the study area to reposition the M5.7 magnitude earthquake on December 16,2018 and the M5.3 magnitude earthquake on January 03,2019 and their aftershock sequence in this area.Using shale gas exploration drilling and reflection seismic data to carry out structural analysis,and recovering the tectonic geological setting of earthquake occurrence by restoring the formation process of the Changning anticline,to further explore the seismic mechanism.Our results show that the Changning anticline is a large basement fault-bend fold,and the displacement of the fault forming the anticline is 18 km,and the Changning anticline absorbs 33%of the fault slip.The Silurian Longmaxi Formation of the Changning anticline experienced larger-parallel shearing along underlying basement faults,forming a micro-fracture system.The footwall ramp of the basement fault is reactivated at present,earthquakes in this area mostly occur along the footwall ramp of the basement fault and above and below it.The anticlinal and synclinal hinge zones are also the earthquake concentration areas,but the earthquake magnitude decreases upwards along the kink-band,and small earthquakes below M2.0 occur in the Silurian Longmaxi Formation.So far,the earthquake in the Changning anticline mainly occurred in the southern limb of the anticline,which is a natural earthquake formed along the footwall ramp of the basement fault.The earthquakes in the Changning area are possible related to the geo-tectonic setting for the southeast outward compression of the Qinghai-Tibet Plateau at present,the moderate or large-scale earthquakes in the southwest Sichuan Basin are mainly due to the reactivation during late Quaternary of the earlier formed faults.It is suggested to carry out scientific monitoring of seismic activities in shale gas development zones.

Using Grey System Theory for Earthquake Forecast

By combining conventional grey correlation analysis, grey clustering method and grey forecasting methods with our multi-goal forecast thoughts and the techniques of grey time series processing, we develop six different grey earthquake forecast models in this paper. Using the record of major earthquakes in Japan from 1872 to 1995, we forecast future earthquakes in Japan. We develop an earthquake forecast model. By using the major earthquakes in Japan from 1872 to 1984, we forecast earthquakes from 1985 to 1995 and check the precision of the grey earthquake models. We find that the grey system theory can be applied to earthquake forecast. We introduce the above analysis methods and give a real example to evaluate and forecast. We also further discuss the problems of how to improve the precision of earthquake forecast and how to strengthen the forecast models in future research.

Tectonic setting of the seismogeny of the 1976 Tangshan,ChinaM=7.8earthquake

Based on the analysis of multi-temporal and multi-spectral satellite images for North China region, we have found that one year and more before the occurrence of 1976 Tangshan earthquake, the anomalies of electro-magnetic radiation on the satellite images indicated that the NE-trending Tangshan fault zone was dissected by the NNW-trending Nantai-Tangshan fault, and the Changping-Fengnan fault was dragged to form an arcuate bending at Fengnan. All these indicate the right-lateral translation along the Tangshan fault in NE direction. In order to gain an insight into the features of these faults, a shallow seismic exploration along the Tangshan and Changping-Fengnan faults has been carried out. The results have indicated that the NE-trending Tangshan fault is a high angle right-lateral strike-slip normal fault, dipping northwest, while the NWW-trending Changping-Fengnan fault is a southwest-dipping left-lateral strike-slip normal fault.

Three-dimensional gravity and magnetic data acquisition and study on its joint gradient Euler deconvolution method

Three-dimensional(3 D)gravitational and magnetic exploration is performed using aerial measurement tools,however,this has difficulties with measuring-height design and the construction of a joint-interpretation scheme.At present,the height in such experiments is set according to the measurement scale,and the distribution characteristics of anomalies are not fully considered.Here,we present the idea of using the attenuation characteristics of a singular-value spectrum to evaluate the contributions of various measurement heights and multi-height combinations for inversion to correctly and reasonably design appropriate measuring heights and the number of various measurement heights to be set.The jointgradient Euler-deconvolution method can accurately obtain the distribution of geological bodies from 3 D gravitational and magnetic data at an improved resolution,and experimental tests confirm these findings.Therefore,an actual 3 D aeromagnetic-data-acquisition and inversion test were carried out in the vicinity of the Zhurihe Iron Mine in Inner Mongolia.The fl ight-height diff erence was set to 60 m,and the specifi c distribution of lodes was obtained by the joint-gradient Euler-deconvolution method.This provides a reliable basis for future detailed exploration and proves that the methods presented in this paper have good practicalapplication eff ects and prospects.

Influences of the heterogeneity of viscoelastic medium on postseismic deformation of the 2008 M_(W) 7.9 Wenchuan earthquake

The study of postseismic deformation is important for constraining the viscoelastic properties of the Earth and inverting the post-earthquake process.The levelling survey revealed that the area near Bei-chuan elevated 5.3 cm about two years after the M_(W) 7.9 Wenchuan earthquake(05/12/2008),during which the area underwent significant downward movement.The GPS horizontal displacements showed a non-monotonic variation after the Wenchuan earthquake.In this study,a 3-D viscoelastic finite element model is employed to simulate the coseismic and postseismic deformation of the Wenchuan earthquake.The numerical simulations show that the lateral heterogeneity across the Longmenshan fault plays an important role in the postseismic displacements.The results reveal that the coseismic defor-mation is not sensitive to the horizontal heterogeneity,but the postseismic deformation is sensitive to it.The postseismic deformation of the horizontally heterogeneous model is generally consistent with the observations of all geodetic surveys,such as GPS,InSAR and levelling,but not for the horizontally homogenous model.We also find that the non-monotonous variation of the postseismic deformation of the Wenchuan earthquake could be explained by a viscoelastic relaxation model with lateral heterogeneous medium across the Longmenshan fault.

Concealed porphyry delineation based on nonlinear three-dimensional densitydifference inversion: An example in the Beiya mine area, Western Yunnan, China

Intermediate acid-complex rock masses with low-density characteristics are the most important prospecting sign in the Beiya area, of western Yunnan province, and provide a physical basis for good gravity exploration. It is usually difficult to obtaining solutions in connection with actual geological situations due to the ambiguity of the conventional gravity-processing results and lack of deep constraints. Thus, the three-dimensional (3D) inversion technology is considered as the main channel for reducing the number of solutions and improving the vertical resolution at the current stage. The current study starts from a model test and performs nonlinear 3D density-difference inversion called “model likelihood exploration”, which performs 3D inversion imaging and inversion of the known model while considering the topographic effects. The inversion results are highly consistent with those of the known models. Simultaneously, we consider the Beiya gold mine in Yunnan as an example. The nonlinear 3D densitydifference inversion technology, which is restricted by geological information, is explored to obtain the 3D density body structure below 5 km in the mine area, and the 3D structure of the deep and concealed rock masses are obtained using the density constraints of the intermediate-acid-complex rock masses. The results are well consistent with the surface geological masses and drilling-controlled deep geological masses. The model test and examples both show that the 3D density-difference nonlinear inversion technology can reduce inversion ambiguity, improve resolution, optimize the inversion results, and realize “transparency” in deeply concealed rock masses in ore-concentrated areas,which is useful in guiding the deep ore prospecting.

震源位置-速度模型联合反演在地震勘探野外干扰源调查中的研究及应用

"降本提质增效"是油气二维/三维地震勘探的重要目标,而地震资料高品质采集是三维地震勘探质控的重要环节.四川盆地人口众多,城镇聚集,工业生产活动频繁,使得三维地震勘探野外资料高品质采集面临巨大挑战.人类生活/工程活动激发的地震波与人工震源激发地震波的动力学和运动学特征近似,从而在野外地震勘探质控中降低地震资料品质.面对上千平方公里的勘探区域,快速定位并协停干扰源对地震勘探具有重要意义.本文基于地震波动力学和运动学特征,建立了一种震源位置-速度模型联合反演方法,根据人工单炮震源理论及工区外近/远场干扰源的定位分析,其定位精度最高达10 m级别,可极大地缩小干扰源搜寻范围.同时,联合反演速度亦有利于干扰源类型的确定,从而为二维/三维地震勘探"降本提质增效"提供现场技术支撑.

基于地球物理方法的金属矿床勘探技术研究

随着经济的快速发展,金属矿床的勘探与开发成为许多国家的重要战略目标。然而,金属矿床的勘探具有复杂性和高风险性。因此,寻求高效和准确的勘探技术成为解决这一问题的关键。地球物理方法作为一种有效的勘探手段,具有较高的可靠性和准确性,被广泛应用于金属矿床的勘探。地球物理方法通过测量金属矿床区域的重力、磁力、电磁和地震等物理属性,实现对地下矿床的准确定位和性质分析。本文基于山东省济南市莱芜区角峪地区庄地区富铁矿普查项目野外地质调查形成的矿床地质数据、地球物理勘探资料,详细研究基于地球物理方法的金属矿床勘探技术,分析各种方法的原理和适用范围。通过本研究,期望为金属矿床勘探提供科学可靠的技术支持,促进资源开发与利用的可持续发展。

中国海上油田地震勘探技术研究进展及展望

近些年中国海上油田地震勘探技术快速发展,形成多项研究成果,为中国海上油气勘探开发提供了有力技术支撑。在海上宽频地震勘探技术方面,创新形成了“犁式”缆采集技术,并研发了配套的采集装备系统、采集处理技术体系;在海上时移地震方面,形成了海上时移地震可行性评价技术、叠前叠后一致性处理技术以及综合解释技术。未来中国海上油田地震勘探将会在地震采集装备、宽频宽方位采集、多波多分量勘探以及时移地震等方面进行持续攻关,为中国海上油田地震勘探高质量发展提供有效的技术保障。

“十四五”西藏地震监测能力提升项目场址初步勘察

以西藏那曲地区地震监测观测站实地选址勘察调研工作为研究对象,依照台站分布基本原则及测震观测站、GNSS观测站的初勘标准,选择地质构造、岩性结构、地形地貌、环境、供电、通信、交通、背景噪声等符合要求的场址,完成了那曲测震观测站拟新建8个点位的初勘工作。

Three-dimensional in situ stress determination by anelastic strain recovery and its application at the Wenchuan Earthquake Fault Scientific Drilling Hole-1(WFSD-1)

In situ stress state becomes more and more significant with in-depth research on geodynamics and energy development.However,there has not been an economic and effective method developed to determine deep three-dimensional in situ stress.The Anelastic Strain Recovery(ASR)method is a newly developed technique that can determine three-dimensional in situ stresses.After the 12 May 2008 Ms8.0 Wenchuan earthquake,the ASR method was used for the first time in China's Mainland to measure the in situ stresses in the WFSD scientific boreholes in Sichuan Province,China.In this paper,the basic procedure of the ASR method is introduced in detail and the compliances of ASR for boring cores are investigated.The results show that the maximum principal stress direction was NW64°at a measured depth(MD)of 1173 m(vertical depth 1151 m)in WFSD-1.The ratio of shear mode to the volume mode compliance of ASR was 2.9.And the three principal stresses at 1173 m MD in WFSD-1are 43,28 and 25 MPa.Combined with stress measurement results determined using other in situ measurement methods along the Longmenshan fault zone,the directions of the maximum horizontal principal stress changes from E-W to NEE-SWW to NWW-SEE when moving from NE to SW along the Longmenshan fault zone.This change is in agreement with the stress regime of the Longmenshan fault zone of the Wenchuan Earthquake,which supports a stress regime consisting predominantly of thrusts in the southwest and strike-slip in the northeast.

Mapping three-dimensional co-seismic surface deformations associated with the 2015 MW7.2 Murghab earthquake based on InSAR and characteristics of crustal strain

Three-dimensional(3 D) co-seismic surface deformations are of great importance to interpret the characteristics of coseismic deformations and to understand the geometries and dynamics of seismogenic faults. In this paper, we propose a method for mapping 3 D co-seismic deformations based on InSAR observations and crustal strain characteristics. In addition, the search strategy of correlation points is optimized by adaptive correlation distance, which greatly improves the applicability of the proposed method in restoring deformations in decorrelation areas. Results of the simulation experiment reveal that the proposed method is superior to conventional methods in both the accuracy and completeness. The proposed method is then applied to map the 3 D co-seismic surface deformations associated with the 2015 MW7.2 Murghab earthquake using ascending and descending ALOS-2 PALSAR-2 images. The results show that the seismogenic fault is the Sarez-Karakul fault(SKF), which is dominated by NE-SW strike slips with an almost vertical dip angle. The north section and the south segment near the epicentre have obvious subsidence along with a southwestward motion in the northwest wall, and the southeast wall has northeast movement and surface uplift trend along the fault zone. The strain field of the earthquake is also obtained by the proposed method. It is found that the crustal block of the seismic area is obviously affected by dilatation and shear forces, which is in good agreement with the movement character of the sinistral slip.

综合物探技术在工作面地质构造探测中的应用

针对5305综采工作面推进距离长、回采区域地质构造复杂特点,提出综合采用无线电透视以及槽波地震探测手段进行综合探测,充分发挥无线电透视在确定地质构造类型、槽波地震探测在确定地质构造范围等方面优势,提高采面构造探测效果。对5305综采工作面内布置方案及探测成果进行综合分析,并通过回采揭露、钻探方式进行验证,发现综合探测确定的地质构造类型、位置等与现场实际相吻合,表明采用的综合物探手段可有效指导5305综采工作面回采。

二维地震勘探在东明露天矿地质研究中的应用

为了研究东明露天矿构造赋存情况,解决富水区域和导水通道对生产造成极大的安全隐患,提高露天矿的安全生产能力,采用浅层二维地震法在划定的区域内进行探测研究,通过合理选取激发震源、道间距离、垂叠次数等技术参数,采用最优参数施工,最终确定了断层的分布范围、延展长度、落差等情况。