RESTUDY ON HYPOCENTRAL LOCATION AND SEISMOGENIC TECTONIC OF THE JIUJIANG-RUICHANG MS5.7 EARTHQUAKE SEQUENCE,JIANGXI PROVINCE

We collected seismic records of 228 M_L≥1.0 Jiujiang-Ruichang M_S5.7 earthquake sequence from Dec.26, 2005 to Jun. 30, 2006. By using double-difference method combined with waveform cross-correlation, those earthquakes were relocated and finally the accurate source parameters of 224 earthquakes were obtained. The errors are about 0. 5km in horizontal and less than 2km in vertical direction, respectively. It was found that the depth of earthquake sequence concentrates in 8~14km range, and the epicenters are distributed along both NW and NE direction, and dominantly along NW direction. Combined with the focal mechanism, the distribution direction and the tectonic setting, we infer that the rupture of the NW-trending fault caused the M_S5. 7 main shock, and then the rupture probably encountered an asperity and triggered the M_S4. 8 strong aftershock. The NE-trending fault came into a seismically quiet period by stress adjustment in a short time, while the NW-trending fault released stress for a long time which caused a series of aftershocks. The M_S5. 7 main shock is caused by the NW striking Yangjisshan-Wushan-Tongjiangling Fault and the M_S4. 8 aftershock occurred on the NE striking Liujia-Fanjiapu-Chengmenshan Fault.     

2014年四川康定MS6.3和MS5.8地震的应力触发研究

以2014年四川康定M_S6.3和M_S5.8地震为研究对象,计算了2次主震在近场和周围断层造成的库仑破裂应力变化,研究主震与余震的触发关系,以及2次主震对周围断层施加的应力负荷作用。结果表明:2次主震的共同作用控制了后续地震活动的演化趋势,其中康定M_S6.3地震产生的库仑破裂应力变化对后续余震事件的触发占主导作用。鲜水河断裂带南段和安宁河断裂带受到了一定的应力加载作用,未来地震活动的趋势可能会加强。     

2014年新疆于田7.3级地震发震构造和震前地震活动过程讨论

2014年2月12日在新疆于田发生7.3级地震,震中位于阿尔金断裂西段,这是继2008年3月21日于田7.3级地震后在塔里木盆地南侧发生的第2次7级地震。这次于田7.3级地震的余震主体沿NE向分布,余震区的西南段呈近SN向分布;绝大部分余震与前震在余震区西南密集分布,强余震(全部的5级以上地震和81%的4级地震)绝大多数都分布在这个区域,第1天的余震主要在这个区域呈近SN向分布,余震由西向东扩展。在这次于田地震的近SN方向上曾在1982、2011以及2012年先后发生过几次6级左右的地震,而这次地震填补了其中的空段。文中从区域构造环境、地震震源机制解和余震分布特征等方面,分析这次地震的发震过程,认为地震发生在硝尔库勒盆地南缘的分支断裂,受阿尔金断裂带构造应力影响,硝尔库勒盆地受到局部近EW向的拉张作用力,首先沿近SN向破裂,这个构造部位的解锁,促进阿尔金断裂左旋错动,产生NE向破裂,应力向东传递;文中还对有历史记录以来,阿尔金断裂上7级地震的发震构造及其对阿尔金断裂带的影响进行了讨论。     

2014年8月3日云南鲁甸6.5级地震序列破裂过程研究

本文利用主地震相对定位法,对2014年鲁甸MS6.5地震序列中的8月3日—9月30日地震进行了重新定位,借助于时空图像分析方法,对本次地震破裂过程进行了分析,得到如下结果:(1)2014年鲁甸MS6.5地震主要沿NW向破裂,存在沿NE向破裂的成分,但是NE向破裂并不明显;(2)地震破裂时,主要从主震震中处往ES方向传播,破裂带长度大约为10km,破裂面近乎直立;(3)余震活动主要集中于主震上方区域,震源深度大于主震的余震稀少.根据上述结果,结合当地的地震构造情况和本次地震的震源机制解,分析表明,本次地震的破裂面为NW向,其发震断层为包谷垴—小河断裂的可能性很大.     

MECHANISM OF THE 2015 PISHAN,XINJIANG, MS6.5 MAINSHOCK AND RELOCATION OF ITS AFTERSHOCK SEQUENCES

Using the digital broadband seismic data recorded by Xinjiang network stations, we obtained focal mechanism of the July 3 Pishan, Xinjiang, M_S6.5 earthquake with generalized Cut and Paste(gCAP)inversion method. The strike, dip and rake of first nodal plane are 97°, 27°, 51°, and the second nodal plane are 318°, 70°, 107°. The centroid depth and moment magnitude are calculated to be 12km and 6.4. Combining with the distribution of aftershocks, we conclude that the first nodal plane is the seismogenic fault, and the main shock presents a thrust earthquake at low angle. We relocated 1014 earthquakes using the double-difference algorithm, and finally obtained 937 relocated events. Our results show that the earthquake sequences clearly demonstrate a unilateral extension about 50km nearly in NWW direction, and are mainly located above 25km depth, especially the small earthquakes are predominately located at the shallow parts. Furthermore, the focal depth profile shows a southwestward dipping fault plane at the main shock position, suggesting listric thrust faulting, which is consistent with the dip of the mainshock rupture plane. The spatial distribution of aftershocks represents that the Tarim block was thrust under the West Kunlun orogenic belt. In addition, the dip angle of the fault plane gradually increases along the NWW direction, possibly suggesting a gradual increase of strike-slip component during the NWW rupturing process. From above, we conclude that the Pishan M_S6.5 earthquake is the result of Tibet plateau pushing onto the Tarim block from south to north, which further confirms that the continuous collision of India plate and Eurasia plate has strong influence on the seismic activity in and around the Tibet plateau.     

THE PRELIMINARY STUDY ON THE SEISMOGENIC STRUCTURE OF THE HUTUBI MS6.2 EARTHQUAKE

Based on the phase report of Xinjiang Seismic Network, the Hutubi M_S6.2 earthquake sequence M_L ≥ 1.0 was relocated by the HypoDD method. The results show that the aftershocks were distributed along NE and NW direction. The aftershocks were in the depths of 5~15km. In addition, by using the digital waveforms of Xinjiang Seismic Network, the best double-couple focal mechanism of the main shock and some aftershocks of M_S ≥ 3.8 were determined by the CAP method. Based on the above studies, the source depth, focal mechanism and aftershock distribution of the Hutubi M_S6.2 earthquake were analyzed and the seismogenic structure was discussed. The nodal plane parameters of the best double-couple focal mechanism are strike 144°, dip 26°, rake 118°, and strike 293°, dip 67°, rake 77°, respectively. The moment magnitude M_W is about 5.9, with centroid depth of 15.2km. These show that the main shock was a thrust type. Most focal mechanism solutions of the aftershocks were shown as a thrust type, which are similar to the main shock. It is speculated that the possible seismogenic fault of this earthquake is the Huorgosi-Manas-Tugulu Fault.     

RELOCATION OF THE 23 NOVEMBER 2017 WULONG MS5.0 EARTHQUAKE SEQUENCE AND ANALYSIS OF ITS SEISMOGENIC FAULT

The Wulong M_S5.0 earthquake on 23 November 2017, located in the Wolong sap between Wenfu, Furong and Mawu faults, is the biggest instrumentally recorded earthquake in the southeastern Chongqing. It occurred unexpectedly in a weak earthquake background with no knowledge of dramatically active faults. The complete earthquake sequences offered a significant source information example for focal mechanism solution, seismotectonics and seismogenic mechanism, which is helpful for the estimation of potential seismic sources and level of the future seismic risk in the region. In this study, we firstly calculated the focal mechanism solutions of the main shock using CAP waveform inversion method and then relocated the main shock and aftershocks by the method of double-difference algorithm. Secondly, we determined the seismogenic fault responsible for the M_S5.0 Wulong earthquake based on these calculated results. Finally, we explored the seismogenic mechanism of the Wulong earthquake and future potential seismic risk level of the region. The results show the parameters of the focal mechanism solution, which are:strike24°, dip 16°, and rake -108° for the nodal plane Ⅰ, and strike223°, dip 75°, and rake -85° for the nodal plane Ⅱ. The calculations are supported by the results of different agencies and other methods. Additionally, the relocated results show that the Wulong M_S5.0 earthquake sequence is within a rectangular strip with 4.7km in length and 2.4km in width, which is approximately consistent with the scales by empirical relationship of Wells and Coppersmith(1994). Most of the relocated aftershocks are distributed in the southwest of the mainshock. The NW-SE cross sections show that the predominant focal depth is 5~8km. The earthquake sequences suggest the occurrence features of the fault that dips northwest with dip angle of 63° by the least square method, which is largely consistent with nodal planeⅡof the focal mechanism solution. Coincidentally, the field outcrop survey results show that the Wenfu Fault is a normal fault striking southwest and dipping 60°~73° by previous studies. According to the above data, we infer that the Wenfu Fault is the seismogenic structure responsible for Wulong M_S5.0 earthquake. We also propose two preliminary genetic mechanisms of "local stress adjustment" and "fluid activation effect". The "local stress adjustment" model is that several strong earthquakes in Sichuan, such as M8.0 Wenchuan earthquake, M7.0 Luzhou earthquake and M7.0 Jiuzhaigou earthquake, have changed the stress regime of the eastern margin of the Sichuan Basin by stress transference. Within the changed stress regime, a minor local stress adjustment has the possibility of making a notable earthquake event. In contract, the "fluid activation effect" model is mainly supported by the three evidences as follows:1)the maximum principle stress axial azimuth is against the regional stress field, which reflects NWW-SEE direction thrusting type; 2)the Wujiang River crosscuts the pre-existing Wenfu normal fault and offers the fluid source; and 3)fractures along the Wenfu Fault formed by karst dissolution offer the important fluid flow channels.     

2021年西藏比如MS6.1地震序列发震构造分析

利用双差定位方法对西藏比如M_S6.1地震序列141次M_L≥2.0地震进行重新定位,采用CAP波形反演方法获得主震的震源机制解,并运用最小空间旋转角方法比较不同机构发布的震源机制解的差异。重新定位后主震震中位置为(31.924°N,92.824°E),靠近余震区中心,震源深度为12.8 km;余震分布沿NE向展布,长约18 km。沿NE向深度剖面结果显示,在主震右上方存在5 km×10 km的近椭圆形地震破裂空区。主震的震源机制解为正断兼走滑型,最佳矩心深度为9.3 km,矩震级为5.98。结合重新定位后余震分布、主震与历史地震震源机制解及地质构造背景等分析,认为具有左旋运动性质的安多南缘断裂可能是该次地震序列的主要发震构造。     

2014年2月12日新疆于田MS7.3地震序列及其构造背景研究

2014年2月12日新疆于田发生M_S7.3地震,该震前1天曾发生M_S5.4前震,震后余震活动频繁.截止到2月20日12时,该地震序列记录到4000多次余震,最大余震为2月12日M_S5.7地震,序列类型为前震—主震—余震型.该地震前震的b值明显低于该区域正常活动的b值和余震的b值.这次地震位于西昆仑断裂带与阿尔金断裂带的交汇区域的阿什库勒断裂北段,震源机制解为走滑型.余震区NE向长70 km、宽20 km,分为主余震分布区和次余震分布区,其中M_L4.0以上强余震基本位于NE向主余震分布区,N--S向的次余震分布区则以M_L3.0左右地震分布为主,显示该部分可能受到主震的触发作用.于田地区曾发生的2008年3月21日M_S7.3地震的震源机制解为正断型,距这次地震约100 km;2012年8月12日发生的M_S6.2地震的震源机制解为正断型,距这次地震约10 km.该地区的发震构造背景是:在NE向阿尔金断裂带尾端向SW方向延伸过程中,左旋走滑作用逐渐转换为拉张作用,形成多条左旋走滑兼具拉张作用的断裂. 2014年于田M_S7.3地震的发震模式表现为:左旋走滑的阿什库勒断裂北段与南段因速率差异而产生的小型构造盆地,在区域拉张作用力下顺时针旋转;2008年M_S7.3张性地震后区域的伸展作用增强,导致盆地南侧的苦牙克断裂发生2012年M_S6.2张性地震,该地震引起2014年M_S5.4前震,两者激发其后在盆地北侧阿什库勒断裂发生了2014年M_S7.3主震.      

STUDY ON THE SEISMOGENIC FAULT AND DYNAMICS PARAME-TERS OF THE 2014 MS6.6 JINGGU EARTHQUAKE IN YUNNAN

On October 17, 2014, a M_S6.6 earthquake occurred in Jinggu, Yunnan. The epicenter was located in the western branch of Wuliang Mountain, the northwest extension line of Puwen Fault. There are 2 faults in the surrounding area, one is a sinistral strike-slip and the other is the dextral. Two faults have mutual intersection with conjugate joints property to form a checkerboard faulting structure. The structure of the area of the focal region is complex. The present-day tectonic movement is strong, and the aftershock distribution indicates the faulting surface trending NNW. There is no obvious surface rupture related to the known fault in the epicenter, and there is a certain distance from the surface of the Puwen fault zone. Regional seismic activity is strong. In 1941, there were two over magnitude 7.0 earthquakes in the south of the epicenter of Jinggu County and Mengzhe Town. In 1988, two mainshock-aftershock type earthquakes occurred in Canglan-Gengma Counties, the principal stress axes of the whole seismic area is in the direction of NNE. Geological method can be adopted to clarify the distribution of surficial fracture caused by active faults, and high-precision seismic positioning and spatial distribution characteristics of seismic sequences can contribute to understand deep seismogenic faults and geometric features. Thus, we can better analyze the three-dimensional spatial distribution characteristics of seismotectonics and the deep and shallow tectonic relationship. The focal mechanism reveals the property and faulting process to a certain extent, which can help us understand not only the active property of faults, but also the important basis for deep tectonic stress and seismogenic mechanism. In order to study the fault characteristic of the Jinggu earthquake, the stress field characteristics of the source area and the geometric parameters of the fault plane, this paper firstly uses the 15 days aftershock data of the Jingsuo M_S6.6 earthquake, to precisely locate the main shock and aftershock sequences using double-difference location method. The results show that the aftershock sequences have clustering characteristics along the NW direction, with a depth mainly of 5~15km. Based on the precise location, calculations are made to the focal mechanisms of a total of 46 earthquakes including the main shock and aftershocks with M_L ≥ 3.0 of the Jinggu earthquake. The double-couple(DC)component of the focal mechanism of the main shock shows that nodal plane Ⅰ:The strike is 239°, the dip 81°, and the rake -22°; nodal plane Ⅱ, the strike is 333°, the dip 68°, and the rake -170.31°. According to focal mechanism solutions, there are 42 earthquakes with a focal mechanism of strike-slip type, accounting for 91.3%. According to the distribution of the aftershock sequence, it can be inferred that the nodal plane Ⅱ is the seismogenic fault. The obtained focal mechanism is used to invert the stress field in the source region. The distribution of horizontal maximum principal stress orienation is concentrated. The main features of the regional tectonic stress field are under the NNE-SSW compression(P axis)and the NW-SE extension(T axis)and are also affected by NNW direction stress fields in the central region of Yunnan, which indicates that Jinggu earthquake fault, like Gengma earthquake, is a new NW-trending fault which is under domination of large-scale tectonic stress and effected by local tectonic stress environment. In order to define more accurately the occurrence of the fault plane of the Jinggu earthquake, with the precise location results and the stress field in the source region, the global optimal solution of the fault plane parameters and its error are obtained by using both global searching simulated annealing algorithm and local searching Gauss-Newton method. Since the parameters of the fault plane fitting process use the stress parameters obtained by the focal mechanism inversion, the data obtained by the fault plane fitting is more representative of the rupture plane, that is, the strike 332.75°, the dip 89.53°, and the rake -167.12°. The buried depth of the rupture plane is 2.746km, indicating that the source fault has not cut through the surface. Based on the stress field characteristics and the inversion results of the fault plane, it is preliminarily believed that the seismogenic structure of the Jinggu earthquake is a newly generated nearly vertical right-lateral strike-slip fault with normal component. The rupture plane length is about 17.2km, which does not extend to the Puwen fault zone. Jinggu earthquake occurred in Simao-Puer seismic region in the south of Sichuan-Yunnan plate. Its focal mechanism solution is similar to that of the three sub-events of the Gengma earthquake in November 1988. The seismogenic structure of both of them is NW-trending and the principal stress is NE-SW. The rupture plane of the Jinggu main shock(NW direction)is significantly different from the known near NS direction Lancang Fault and the near NE direction Jinggu Fault in the study area. It is preliminarily inferred that the seismogenic structure of this earthquake has a neogenetic feature.     

2010年4月玉树M_S7.3地震序列的断层结构

利用双差定位方法对玉树地震序列2010年4月14日至10月31日间发生的ML≥1.0地震进行双差定位,得到1545个地震的重定位结果.综合分析地震双差定位结果和玉树地震序列中强地震震源机制解,发现玉树MS7.3地震发震构造由北西向和北东东向两条相交断层组成,主震发生在北西走向的甘孜—玉树断裂带上,5月29日的MS5.9余震序列发生在北东东走向的一条隐伏断裂上,两条断裂均接近直立.甘孜—玉树断裂是羌塘地块和巴彦喀拉地块的构造边界,由于羌塘地块和巴颜喀拉地块的差异运动使甘孜—玉树断裂强耦合段应力高度积累,在应变能超过岩石强度时破裂失稳发生了MS7.3地震.主震断层的左旋滑动导致北东东向断层的正应力减小,库伦应力增加,45天后触发了MS5.9余震序列的活动.     

2014年8月3日云南鲁甸MS6.5级地震序列重定位与震源机制研究

本研究采用双差定位法对2014年 8月3日至7日期间鲁甸M_S6.5级主震及647个余震序列进行重新定位,得到471个重定位结果.结果显示,主震的震源深度为13.3 km,与破裂过程显示的初始破裂深度较为接近,余震序列呈现出近东西向-北西向的不对称共轭状分布,近东西向长约17 km,而北西向长约22 km,小震优势分布深度为10 km以上,且由主震处沿共轭断层分别向东南向和近东西向逐渐往10 km深度以上的浅部迁移.小震分布还展示出发震断层高倾角分布,且与昭通-鲁甸断裂分支断裂包谷垴-小河断裂活动相关.由于主震破裂的质心深度可为深入认识本次地震灾害严重提供重要证据,为此我们采用gCAP（generalized Cut And Paste）方法反演了包括主震在内共5个4.0级以上地震的震源机制解,结果显示主震质心深度仅约5.0 km,与已有破裂过程显示的较大滑移量处于2~8 km之间的深度一致.本次主震错断了互为共轭的两条断裂,这种共轭破裂模式与矩心深度较浅,可能为本次地震致灾严重的重要原因.     

FOCAL MECHANISM SOLUTIONS AND STRESS FIELD OF THE 2019 CHANGNING,SICHUAN MAINSHOCK AND ITS MODERATE-STRONG AFTERSHOCKS(MS≥4.0)

A M_S6.0 earthquake with shallow focal depth of 16km struck Changning County, Yibin City, Sichuan Province at 22:55: 43(Beijing Time)on 17 June 2019. Although the magnitude of the earthquake is moderate, it caused heavy casualties and property losses to Changning County and its surrounding areas. In the following week, a series of aftershocks with M_S≥4.0 occurred in the epicentral area successively. In order to better understand and analyze the seismotectonic structure and generation mechanism of these earthquakes, in this paper, absolute earthquake location by HYPOINVERSE 2000 method is conducted to relocate the main shock of M_S6.0 in Changning using the seismic phase observation data provided by Sichuan Earthquake Administration, and focal mechanism solutions for Changning M_S6.0 main shock and M_S≥4.0 aftershocks are inferred using the gCAP method with the local and regional broadband station waveforms recorded by the regional seismic networks of Sichuan Province, Yunnan Province, Chongqing Municipality, and Guizhou Province. The absolute relocation results show that the epicenter of the main shock is located at 28.35°N, 104.88°E, and it occurred at an unusual shallow depth about only 6.98km, which could be one of the most significant reasons for the heavier damage in the Changning and adjoining areas. The focal plane solution of the Changning M_S6.0 earthquake indicates that the main shock occurred at a thrust fault with a left-lateral strike-slip component. The full moment tensor solution provided by gCAP shows that it contains a certain percentage of non-double couple components. After the occurrence of the main shock, a series of medium and strong aftershocks with M_S≥4.0 occurred continuously along the northwestern direction, the fault plane solutions for those aftershocks show mostly strike-slip and thrust fault-type. It is found that the mode of focal mechanism has an obvious characteristic of segmentation in space, which reflects the complexity of the dislocation process of the seismogenic fault. It also shows that the Changning earthquake sequences occurred in the shallow part of the upper crust. Combining with the results from the seismic sounding profile in Changning anticline, which is the main structure in the focal area, this study finds that the existence of several steep secondary faults in the core of Changning anticline is an important reason for the diversity of focal mechanism of aftershock sequences. The characteristics of regional stress field is estimated using the STRESSINVERSE method by the information of focal mechanism solutions from our study, and the results show that the Changning area is subject to a NEE oriented maximum principal stress field with a very shallow dipping and near-vertical minimum principal stress, which is not associated with the results derived from other stress indicators. Compared with the direction of the maximum principal compressive stress axis in the whole region, the direction of the stress field in the focal area rotates from the NWW direction to the NEE direction. The Changning M_S6.0 earthquake locates in the area with complex geological structure, where there are a large number of small staggered fault zones with unstable geological structure. Combining with the direction of aftershocks distribution in Changning area, we infer that the Changning M_S6.0 earthquake is generated by rupturing of the pre-existing fault in the Changning anticline under the action of the overall large stress field, and the seismogenic fault is a high dip-angle thrust fault with left-lateral strike-slip component, trending NW.     

2021年云南漾濞MS6.4地震动态应力对后续余震活动的触发作用

选取IRIS远震台站波形数据,反演了云南漾濞M_S6.4地震震源破裂过程,计算了断层破裂在近场产生的动态库仑破裂应力变化,并讨论了主震对近场余震活动的动态应力触发作用。结果显示:动态库仑应力演化过程与震源破裂特征反演结果一致,其大小分布与地震序列分布的疏密程度也具有较好的相关性。主震产生的静态和动态库仑破裂应力均促进余震的发生,但相比静态应力,余震位于库仑破裂应力正值区域的比例提高了21%,余震与动态库仑应力变化的正负区域有更好的一致性,动态应力能更好地解释震后余震分布的空间特征。垂直于地震序列主干10 km处出现小震丛集,该现象可能是由主震产生的动态库仑破裂应力占主导作用所致。定量分析主震对余震的动态应力触发结果显示,主震后一周内M_S4.0以上的8次余震接收点均受到了动态库仑破裂应力的触发作用。      

芦山MS7.0级地震余震序列重新定位及构造意义

利用四川省地震台网的震相数据和双差定位方法对芦山M_S7.0级地震及其余震序列进行了精确定位,根据余震分布确定了发震断层的位置和断层面的几何特征,并对余震活动进行了分析.结果显示,芦山M_S7.0级地震的震中位于30.28°N、102.99°E,震源深度为16.33 km.余震沿发震断层向主震两侧延伸,主要分布在长约32 km、宽约15~20 km、深度为5~24 km的范围内.地震破裂带朝西南方向扩展范围较大,东北方向略小,余震震级随时间迅速衰减.震源深度剖面清晰地显示出发震断层的逆冲破裂特征,推测发震断层为大川—双石断裂东侧约10 km的隐伏断层.该断层走向217°、倾向北西,倾角约45°,产状与大川—双石断裂相比略缓,它们同属龙门山前山断裂带的叠瓦状逆冲断层系.受发震断裂影响,部分余震沿大川—双石断裂分布,西北方向的余震延伸至宝兴杂岩体的东南缘,与汶川地震的破裂带之间存在50 km左右的地震空区,有可能成为未来发生强震的潜在危险区.     

STUDY ON DYNAMIC CHANGE OF GRAVITY FIELD IN XIANSHUIHE FAULT ZONE AND THEIR RELATION TO THE OCCURRENCE OF MS ≥ 5.0 EARTHQUAKES

Xianshuihe Fault, a main strong earthquake activity belt in southwest China, begins from Ganzi in the northwest, passes through Luhuo, Daofu, and Kangding, and then extents along the Dadu River valley. The fault is divided into two parts at Shimian, one part turns to south and converses to Anninghe Fault extending further to south, the other part, continuing to extend to southeast, cutting through Xiaoxiangling and then changing to Daliangshan Faults in the north of the Yuexi Basin, has the length of about 400km. Since 1700AD, there have happened 22 earthquakes larger than magnitude 6.0 and 8 earthquakes larger than magnitude 7.0. In this paper, we systematically collated and computed the gravity repetition measurement data along the Xianshuihe fault zone since 1988, and by referring to the anomaly index of gravity field of the predecessor achievements, analyzed the spatial-temporal variation of the regional gravity field and the relation to the occurrence of ≥ M_S5.0 earthquakes. The mechanism of the regional gravity changes is further studied, and also the implication of strong earthquake risk because of the dynamic variation of gravity field in the near future is discussed.The results show that:1)The mobile gravity observation has the ability to detect crustal activity and M_S ≥ 5.0 earthquake events. 2)There is definite correspondence between interannual gravitational field change and the 8 earthquakes among the 13 M_S ≥ 5.0 earthquakes occurring in the surveying area since 1988, which can be determined according to the change of interannual gravitational field. Three M ≥ 6.0 earthquakes occurred 3~4 years after the abnormal image was developed, 4 earthquakes that occurred in the region of no data available were not determined. 3)A significant feature of the spatial-temporal variation of the regional gravity is a north-south run-through image before 2004, and characterized by the alternatively positive or negative variation in different year, the earthquakes of M_S ≥ 5.0 occurring in this period were not distributed along the fault. Gravity variation magnitude indicates that there were two similar crustal material movement waves before 2004, corresponding to the course of earthquake space-time distribution from strong to weak in the study area. After 2010, the variation image shows that the local positive and negative zones are concurrent within a year, different from the image before 2004, and earthquakes of M_S ≥ 5.0 basically occurred on the fault. It is believed that the variation of gravity field since 1988 and the seismic distribution fit with the geodynamic mode of strong and weak stages of the northeast motion of Indian plate. According to the conclusion we can try to optimize gravity anomaly index. After the Kangding earthquake in 2014, the north segment of Moxi Fault was still subject to negative high value changes till 2017 and then the gravity variation was further developed to a four quadrant distribution image. Based on the analysis of this paper and the previous variation trend of gravity field, we believe that the north segment of Moxi Fault has the background of medium-long term, strong or large earthquake risk.     

RELOCATION OF THE HUTUBI MS6.2 EARTHQUAKE SEQUENCE ON 8 DECEMBER 2016 AND ANALYSIS OF THE SEISMOGENIC STRUCTURE

Based on the digital waveforms of Xinjiang Seismic Network, the Hutubi M_S6.2 earthquake sequence (M_L ≥ 1.0) was relocated precisely by HypoDD.The best double-couple focal mechanisms of the main shock and aftershocks of M_L ≥ 4.0 were determined by the CAP method. We analyzed the characteristics of spatial distribution, focal mechanisms and the seismogenic structure of earthquake sequence. The results show that the main shock is located at 43.775 9°N, 86.363 4°E; the depth of the initial rupture and centriod is about 15.388km and 17km. The earthquake sequence extends unilaterally along NWW direction with an extension length of about 15km and a depth ranging 5~15km. The characteristics of the depth profiles show that the seismogenic fault plane dips northward and the faulting is dominated by thrusting. The nodal planes parameters of the best double-couple focal mechanisms are:strike 292°, dip 62° and rake 80° for nodal plane I, and strike 132°, dip 30° and rake 108° for nodal plane Ⅱ, indicating that the main shock is of thrust faulting. The dip of nodal planeⅠis consistent with the dip of the depth profile, which is inferred to be the fault plane of seismogenic fault of this earthquake. According to the comprehensive analysis of the relocation results, the focal mechanism and geological structure in the source region, it is preliminarily inferred that the seismogenic structure of the Hutubi M_S6.2 earthquake may be a backthrust on the deeper concealed thrust slope at the south of Qigu anticline. The earthquake is a "folding" earthquake taking place under the stress field of Tianshan expanding towards the Junggar Basin.     

2014年新疆于田MS7.3级地震序列重定位

2014年新疆于田发生MS7.3地震,这一地区6年来连续发生2次强烈地震,震中相距不到110km.由于初始定位误差较大,于田地震的发震断层仍不清楚.本研究的主要目标是利用地震精定位方法对于田地震序列及其背景地震活动进行重新定位,确定于田地震的发震断层.本研究使用双差定位方法对于田地震序列进行重新定位.这一方法假设两个地震的震源距小于事件到台站的距离,两个事件到同一台站的走时差主要归因于其空间位置的偏移,因此可消除由于速度模型不准确引起的定位误差.重定位后得到了435个地震的位置参数.结果表明,2014年于田MS7.3级地震发生在阿尔金断裂带的西端,余震分布的优势方向为北东向,展布长度约33km,震源深度主要集中在4~12km,多数余震位于主震的西南侧.NS,EW和UD方向的定位误差分别为0.5km,1.1km和1.7km.于田地震余震序列总体衰减较慢.根据余震分布特征和震源机制解,认为此次地震的断层面为北东向的节面,阿尔金断裂的西南延伸分支断层是这次地震的主要发震构造.于田地震的发生与巴颜喀拉块体的东南向运动有关.     

2008年3月21日新疆于田MS7.3 地震破裂过程研究

利用IRIS全球地震台网30°—90°的长周期P波记录, 反演了2008年3月21日新疆于田M_S7.3地震的破裂过程, 得到了此次地震的破裂时空图像, 并初步分析了余震分布与主震断层滑动量分布的关系. 结果表明, 此次地震是一个破裂尺度长100 km、 宽20 km的破裂过程; 破裂持续时间约为40 s, 在第13 s时地震矩释放速率达到峰值, 断层面上一次大的破裂行为几乎构成了整个地震的破裂过程. 地震所释放的标量地震矩为4.23×1019 N·m, 其矩震级为M_W7.02. 由主震断层静态滑动量分布图可以看出, 整个破裂区以正断左旋走滑为主, 显示出双侧破裂特征, 最大滑动量为151 cm, 位于初始破裂点沿断层出露地表处. 精定位后的余震在断层面上的投影结果显示, 80%以上M_L4.0—4.9余震和全部M_L≥5.0余震均发生在初始破裂点附近区域及其南西方向, 位于主震破裂滑动位移量迅速减小的区域, 反映了震源区介质强度的不均匀性.      

利用sPn震相测定芦山MS7.0级地震余震的震源深度

利用南北地震带南段密集流动地震台阵的观测数据,采用波形互相关方法拾取Pn波走时,应用滑动时窗相关法识别sPn震相,通过sPn与Pn震相之间的走时差测定了芦山地震序列中28个M_L4.0级以上余震的震源深度.结果表明,震源深度集中在10~20 km范围内,垂直余震带的北西-南东向震源深度剖面揭示,余震分布表现出西深东浅的特点,倾角大约为39°.这些余震在空间上具有较好的线性分布特征,推测可能发生在与主震有关的破裂面上或邻近位置,由此推测主震的破裂面倾角大约为39°.根据余震的空间分布特征,认为芦山地震的发震断层并非双石-大川断裂,可能是其东侧的一条隐伏断层.