基于遥感和重力多源数据研究沂沭断裂带南段及周边断裂的交切关系

以沂沭断裂带南段(沂水县—郯城县)及周边地区为研究对象,收集该地区的遥感影像、数字高程模型和布格重力数据,研究区域构造地貌和地壳深部构造特征,进一步对沂沭断裂带南段与周边断裂的交切关系予以分析.研究结果显示:在遥感影像中,蒙山山前断裂和苍尼断裂的构造地貌特征明显,断裂沿线发育水系转弯、河流错断、断层陡坎、断层崖、断层三角面等地貌现象,反映了两断裂正断兼左旋走滑的活动性质,其中蒙山山前断裂向东延伸至莒南县附近,苍尼断裂向东延至郯城一带,两条断裂在地貌上均截切了沂沭断裂带;在重力细节场中,两断裂形成了不同尺度上的重力梯度带,切割至下地壳深度,在地壳浅层至深层均交切于沂沭断裂带,且交切处出现扭曲、断折、串珠状等重力异常现象,证实其在地壳深部切穿沂沭断裂带.因此,两条断裂的遥感和重力场解译结果具有明显的一致性,在地貌及深部均截切沂沭断裂带南段,使其出现分段性特征.此外,在临沭县附近发现了一条新断裂,即相庄—沙岭断裂,该断裂在地貌上呈北高南低,沿线水系发生左旋同步转弯,且在1—3阶重力细节场中形成线性梯度条带,故推测该断裂下切至中地壳深度,在临沭县附近截切沂沭断裂带交切于东地堑,并未延伸至西地堑.      

郯庐断裂带沂水-汤头断裂南段晚第四纪活动新证及构造意义

以往对郯庐断裂带沂沭段各条断层第四纪活动性研究工作都集中在有历史地震记录的东地堑断层,而对断裂带西地堑断层却极少涉及,仅有的关于西地堑2条断层活动性的研究也至今没有定论。针对沂沭断裂带南段西地堑2条断层开展系统的浅层地震勘探和钻孔联合剖面相结合的研究,明确了鄌郚-葛沟断裂(F4)可能是第四纪早期断裂,而活动性较强的沂水-汤头断裂(F3)属于晚更新世活动断裂,其最新1次活动时间发生在距今(91.2±4.4)~(97.0±4.8)ka。结合断裂带其他断层的最新研究成果,对比东、西地堑活动断层最新活动时间,揭示出沂沭断裂带南段晚第四纪活动是断裂带对来自两侧应力的构造响应。沂水-汤头断裂可能是该区域未来中强地震的潜在发震构造。     

沂沭断裂带地壳结构特征

本文利用1986年在鲁南地区开展深地震测深所获资料,反演了沂沭带南段的地壳速度结构和地壳层构造。结果发现:(1)沿沂沭断裂带的地壳低速层是不连续的,在相当于1668年8(1/2)级地震的发震段(河阳—郯城)的中、下地壳内都有低速层,在该段南、北分别缺失下地壳与中地壳的低速展;(2)沂沭断裂带深部存在四条超壳断裂,西侧二条深断裂东倾,东侧二条深断裂直立,形成上宽下窄斗形超壳断裂带;(3)沂沭带地壳的破裂程度明显存在由北向南逐渐减弱的现象;(4)沂沭带莫霍面为向下拗曲带,其两侧地壳层形变程度具有西强东弱、下强上弱和由郯城向南、向北减弱的特征;(5)郯城附近,沂沭带东地堑浅层断裂相对于深部断裂向西位错约6公里。     

基于布格重力数据研究郯庐断裂带江苏段深部构造

本文收集了郯庐断裂带江苏段及周边地区的布格重力数据,利用小波多尺度分析方法对该地区的重力场进行场源分离,深入研究了区域地壳密度结构及断裂空间展布特征,并应用Parker密度界面模型对区域莫霍界面进行反演分析.研究结果如下:(1)布格重力场显示,郯庐断裂带江苏段形成了NNE走向的大型的重力高异常条带及重力梯度带,分隔了华北板块和苏胶块体、下扬子板块,成为区域内重要的地球物理分界线;(2)小波重力细节场揭示,沉积层及上地壳结构复杂,郯庐断裂带的5条主干断裂形成线性异常条带贯穿区域中央,控制着断裂带内部的隆凹构造单元.中下地壳结构相对简单,郯庐断裂带形成宽缓的高、低异常区,洪泽-沟墩断裂、淮阴-响水口断裂、宿迁-无锡断裂等深大断裂与之交切,而邵店-桑墟断裂不与之相交.受宿迁-无锡断裂交切的影响,郯庐断裂带江苏段在宿迁地区出现明显的分段性;(3)区域莫霍面形态东高西低,郯庐断裂带形成了莫霍面陡变带,造成了东西分异的格局,泗洪地区出现莫霍面局部上隆区,可能由于软流层或上地幔高密度物质上隆所致.     

郯庐断裂带南段重力异常及不同深度的横向构造特征

为研究郯庐断裂带南段及周边构造的深部特征、空间展布、交切关系,文中利用小波多尺度分析方法对该地区的布格重力场进行场源分离,剖析不同深度下的地壳横向构造;同时采用Parker变密度模型对莫霍面深度进行反演分析。研究表明,郯庐断裂带南段表现为NNE走向的大型重力区域场梯度条带,切割深度达岩石圈地幔,其两侧密度结构及构造特征差异明显。沉积层及上地壳密度结构复杂,郯庐带东支2条断裂形成线性异常纵贯区域;而西支2条断裂线性异常较弱,截切EW向的重力异常体断续延展。中、下地壳密度结构简单,断裂带形成宽缓的低异常条带,反映了白垩纪—古近纪伸展环境下造成的地堑式构造。西支2条主干断裂纵穿合肥市,沿肥东凹陷西缘向S延伸,由于舒城以南的高密度圈闭体遮挡,尖灭于舒城县一带;而肥中、六安—合肥、肥西—韩摆渡EW向的深大断裂交切于西支,未延伸至东支。该区近代小震多发生于断裂构造所对应的重力高、低异常转化带之间,尤其是断裂之间的交会处与郯庐构造带内部,郯庐断裂带南段为历史强震空区段,考虑到断裂带莫霍面陡变及与多条深大断裂交切等深部环境,中强震危险性不容忽视。     

鲁西隆起重磁异常特征及其构造活动性分析

通过对鲁西隆起区重磁资料的分析和反演计算,研究了沂沭断裂带、齐河—广饶断裂带、聊城—兰考断裂带、丰沛断裂带以及地块内部断裂的重磁异常、莫霍面和居里面深度特征,并讨论了鲁西隆起的地质构造特征和构造活动性.结果显示:鲁西隆起基底广泛出露,沉积层主要分布在由断裂下降盘控制的凹陷内,区内断裂深度达20 km以上,其中蒙山断裂深入至上地幔,控制了蒙山金伯利岩型金刚石矿的产出;鲁西隆起区莫霍面深度为30—35 km,整体呈向西开口的箕形,地块中部地壳厚度较厚,除西侧地壳呈阶梯状增厚外隆起地块四周地壳逐渐减薄;居里面深度介于20—33 km之间,中部地区较深,为整体稳定的地块,断裂带分布位置对应于居里面梯度带;地震活动主要集中于断裂带与莫霍面梯度带交会区以及断裂带上的居里面突变区.      

沂沭断裂带及周边区域Pn波速度结构与各向异性

沂沭断裂带位于鲁西隆起、胶辽隆起与苏鲁褶皱带之间,是郯庐断裂带地震活动最为强烈的部分,上地幔顶部Pn波速度结构与各向异性研究对于认识沂沭断裂带对地震活动的控制作用具有重要意义.本研究通过手工拾取与观测报告相结合的方法,最终挑选出研究区域2008—2019年期间290个台站记录到的1665个地震事件的26598条Pn走时数据,通过反演获得一个新的沂沭断裂带及周边区域上地幔顶部Pn波速度结构及各向异性结构模型.结果显示,研究区域上地幔顶部Pn波速度结构存在显著的横向变化,华北盆地南端、太行山造山带、鲁西隆起、胶辽隆起表现为低速异常,华北盆地内部速度结构存在显著的横向不均匀性,其强高速异常表现出明显的分块现象.沂沭断裂带呈现明显的分段特征,其两侧速度结构存在明显差异,表明沂沭断裂带可能延深至上地幔顶部.强震多数发生在上地幔顶部具有高速和低速异常变化特征区域上方的地壳内,说明强震的孕育发生与上地幔顶部结构有关.太行山造山带、胶辽隆起区Pn波各向异性快波方向与地质构造的伸展方向趋于一致,但与SKS波分裂结果不同,说明在岩石圈形变一定深度范围可能存在局部解耦现象.南黄海盆地Pn波各向异性快波方向与断裂走向基本一致,暗示扬子块体北向挤压在南黄海地区形成的部分断裂的深度已达上地幔成为岩石圈尺度的断裂.     

泰安—诸城剖面重力异常和地壳密度结构特征

泰安—诸城高精度重力观测剖面,位于华北克拉通的东部,是中国东部重要的构造带且是地震活动的重要地段,利用剖面相对重力联测与同址GNSS三维坐标测量数据,得到剖面的重力异常,结合区域布格重力场、地球深部探测及地质构造成果,对剖面进行密度结构反演和剩余密度相关成像研究.结果表明:剖面布格异常变化范围为(-30.1～7.3)×10-5m·s-2,从西往东总体呈上升(泰安至沂源以低背景平缓逐渐上升与相应位置的高程呈现"同步型"变化;马站至张家楼以高背景逐渐上升与地形高程呈现"镜像型"变化)趋势,高低异常背景转折在沂沭断裂带西段150 km左右位置;背景异常是地壳厚度变化的反映,整体形态西深东浅,在沂沐断裂带地壳厚度隆起可能是地壳下地幔物质沿断裂向上入侵时,地壳产生挤压和膨胀隆起所致;剖面地壳密度呈现上、中和下三层结构,鲁西隆起、沂沭断裂带西部中下地壳局部存在低密度体,可能是上地幔物质上涌岩石含部分高温流体和熔融岩体所致,胶东地区保持了比较完整和均一的地壳结构特征,反映了胶东地区地壳活动不强;剖面的主要断裂带位置均能看到布格重力异常和剩余密度的变化,可能是断裂控制了地下介质的发育所致;剖面密度模型显示地下密度的分段特征和深浅构造差异,沂沭断裂带是鲁西隆起和胶东隆起的分界带;本文通过对布格重力异常和密度结构的特征研究,分析了地震活动、动力学背景与地壳密度结构的关系.     

沂沭带形变、重磁场时空变化特征与地震活动

综合利用流动水准和流动重磁复测资料,分析沂沭断裂带上地壳形变场和地球物理场的时空强变化特征,揭示沂沭断裂带活动特征是以鲁中隆起的继承性活动为主,沂沭带北端的双山-李家庄断裂两盘的垂直形变受区域断裂控制并与区域地震活动有关,与1983年荷泽地震和1995年苍山地震有较好对应关系;沂水附近地磁场有“窗口效应”,尤其与南黄海地震活动及沂沐断裂带上的小震活动有关。     

应用布格重力异常研究郯庐断裂构造

使用布格重力资料对郯庐断裂带的中段部分（沂沭断裂带）进行了研究. 结果表明， 郯庐断裂带莫霍面及地壳内界面均发生错断，断裂带两侧地壳各界面起伏平缓. 该结果与前人的郯庐断裂带是切穿地壳的深大断裂的认识相一致. 在郯庐断裂带两侧，地壳结构明显不同，西侧沉积层较薄，平均在5 km以下；东侧多数在6 km以上；在断裂带中央沉积物最薄，大约为3～4 km. 断裂带东侧莫霍面埋深浅，大约为33～34 km；西侧莫霍面埋深明显增加，达到36～38 km．反映了莫霍面深度在断裂带附近整体是向西增加的. 郯庐断裂带在重力场分布中则表现为一条宽度较大的线性布格重力异常梯度带.      

TRANSVERSE STRUCTURES FEATURES OF DIFFERENT DEPTHS DERIVED FROM BOUGUER GRAVITY ANOMALIES IN THE SOUTHERN SEGMENT OF TAN-LU FAULT ZONE

To research the faults distribution and deep structures in the southern segment of Tan-Lu fault zone(TLFZ) and its adjacent area, this paper collects the Bouguer gravity data and makes separation by the multi-scale wavelet analysis method to analyze the crustal transverse structure of different depths. Meanwhile Moho interface is inversed by Parker variable density model. Research indicates that the southern segment of TLFZ behaves as a NNE-directed large-scale regional field gravity gradient zone, which separates the west North China-Dabie orogen block and the east Yangtze block, cutting the whole crust and lithosphere mantle. There are quite differences of density structures and tectonic features between both sides of this gradient belt. The sedimentary and upper crustal density structure is complex. The two east branches of TLFZ behave as linear gravity anomalous belt throughout the region, whereas the two west branches of TLFZ continue to extend after truncating the EW-trending gravity anomaly body. The lower crustal density structure is relatively simple. TLFZ behaves as a broad and gentle low abnormal belt, which reflects the Cretaceous-Paleogene extension environment caused graben structure. The two west branches of TLFZ, running through Hefei city, extend southward along the west margin of Feidong depression and pinch out in Shucheng area due to the high density trap occlusions in the south of Shucheng. The Feizhong Fault, Liu'an-Hefei Fault, and Feixi-Hanbaidu Fault intersect the two west branch faults of TLFZ without extending to the east. Recent epicenters are mainly located in conversion zones between the high-density and the low-density anomaly, especially in TLFZ and the junction of the faults, where earthquakes frequently occurred in the upper and middle crust. As strong earthquakes rarely occur in the southern segment of TLFZ, considering its deep feature of abrupt change of the Moho and intersections with many EW-trending faults, the hazard of strong earthquake cannot be ignored.     

郯庐断裂带莒县胡家孟晏地震破裂带的发现

郯庐断裂带是中国东部最主要的一条活动断裂带。在该断裂带中部,沂沭断裂东地堑的潍坊—嘉山段中发育了1条长360km的全新世活动断裂带(F5),在该全新世断裂带的北段和中段分别发生了公元70年的安丘地震和公元1668年的郯城地震。2003年底我们考察沭河断裂带时,在莒县境内发现了1条长约7km的地震破裂带,作为活动断层应该归属于F5断裂带,但其是一条独立的地震破裂段还是归属于1668年郯城8.5级地震破裂带有待于进一步的研究。尽管如此,探槽揭示出的上覆未经破坏的地层的14C年代表明,该破裂带在(2140±190)aBP以来没有过活动,因此我们认为其作为1条独立破裂段的可能性较大     

THE SEISMOGENIC ENVIRONMENT ANALYSIS OF LUDIAN MS6.5 EARTHQUAKE USING GRAVITY DATA

The main rupture of Ludian M_S6.5 earthquake is directed to the northwest, which occurred in the east of Xianshuihe-Xiaojiang fault zone. The epicenter is in the transitional zone of the Sichuan-Yunnan block and the South China block, where there are many slip and nappe structures. Some controversy still remains on the earthquake tectonic environment. So, Bouguer gravity anomalies calculated by EGM2008 were broken down into 1-5 ranks using the way of Discrete Wavelet Transform(DWT), then we get the lateral heterogeneity in different depths of the crust. The distribution characteristics of Bouguer gravity anomaly are analyzed using measured gravity profile data. We also get its normalized full gradient(NFG)picture, and study the differences between different depths in crust. The results show that: (1)the characteristic of Buoguer gravity anomaly in southwest to northeast is high-low-high between the Lianfeng Fault(LFF)and Zhaotong-Ludian Fault(ZLF). The mainshock and aftershocks are distributed in the middle of the low-value zone, which means that the east moving materials of Qinghai-Tibet plateau broke through the southern section of Lianfeng Fault(LFF), moving along the Baogunao-Xiaohe zone(low-value belt)to the southeast, stopped by the Zhaotong-Ludian Fault(ZLF), and then earthquake occurred.(2)The third-order discrete wavelet transform(DWT)details show that: there is a good consistency between the negative gravity anomaly in upper crust and the distribution of major faults, which reflects that the rupture caused by the movements of the faults in crust has reduced gravity anomaly. There is a NW-trending negative anomaly belt near the epicenter, which may has some relationship to the southward development of the Daliangshan Fault(DLSF). So we speculate that the southward movement of Daliangshan Fault is the main direct force source of Ludian earthquake.(3)In the picture of the fourth-order DWT details, there is an obvious positive gravity anomaly under the epicenter of Ludian earthquake, which confirms the presence of a high-density body in the middle crust. While the fifth-order DWT details show that: A positive anomaly belt is below the epicenter too, which may be caused by mantle material intruding to the lower crust. Tensile force in crust caused by mantle uplift and extrusion-torsion force caused by Indian plate push are the main force source in the tensile and strike slip movement of the Ludian earthquake.(4)The normalized total gradient of Bouguer gravity anomalies of Huili-Ludian-Zhaotong profile shows that: there is obvious ‘deformation’ in the Xiaojiang fault zone which dips to the east and controls the local crust movement. There is a local ‘constant body’ at the bottom of the epicenter. The stable constant body in density has limiting effects to the earthquake rupture, which is the reason that the earthquake rupture' scale in strike and in depth are limited.(5)The ability of earthquake preparation in Zhaotong-Ludian Fault is lower than the Xianshuihe-Xiaojiang fault zone, and the maximum earthquake capacity in this area should be around magnitude 7.     

由地壳速度结构判断郯庐断裂带江苏段未来大震位置

用多震相地震走时成像法反演郯庐断裂带鲁苏皖段及邻区三维地壳速度结构。一些地区如郯庐断裂带临沭到定远及以东地区在中地壳的20~25km出现低速层,一些地区莫霍面埋深有变化。浅层速度结构的分段与断裂活动的分段相一致,表明新沂到泗洪是活动断裂的闭锁段。对比1668年山东郯城8级地震区和研究区的深部速度结构,结合与郯庐带相交的断裂、地震活动、活动断裂的闭锁段、中地壳低速层及莫霍面深度变化,综合判断郯庐断裂带江苏段未来可能发生大震的地区为33.4°~34.1°N,118.2°~118.8°E,重点是宿迁、沭阳、泗阳和泗洪。震级估计可达8级。     

STUDY ON THE ELECTRICAL STRUCTURE OF THE ANQIU AND JUXIAN ELECTROMAGNETIC STATIONS IN THE TANLU FAULT ZONE

The Yishu fault zone is one of the branch faults of the Tanlu fault zone in its central part. Moderate and strong earthquakes occurred in the Yishu fault zone repeatedly. Due to its complex structure, the Yishu fault zone attracts much attention from earthquake researches. The Anqiu and Juxian electromagnetic stations in Shandong Province locate near the Anqiu-Juxian Fault and Changyi-Dadian Fault, which are branches of the Yishu fault zone, respectively. Geoelectric field and geomagnetic field observation were carried out in these two stations. The Wudi electromagnetic station is in the west of Tanlu fault zone in the Jidong-Bohai block and 230km from Anqiu electromagnetic station. This paper firstly describes the crustal structure near the electromagnetic stations by using magnetotelluric(MT)method. By processing the data carefully, we obtain the MT data in good quality near the stations. The MT data of each electromagnetic station and its nearby area suggests that the electrical structure and geological structure of the station are comparable. This paper applied 1-D and 2-D inversion for MT data and obtained the crustal electrical structure model beneath the Anqiu and Juxian seismic station. The shallow electrical structure from the MT method was compared with the results of symmetrical quadrupole electrical sounding. The model suggests that the electrical structure beneath the Anqiu and Juxian electromagnetic stations is complex and shows the feature of block boundary. The Wudi electromagnetic station is located inside a basin, the crustal structure shows layered feature typical for the stable blocks. Beneath the Anqiu electromagnetic station, there is a 1km-thick relative low resistivity layer in the shallow crust and a high resistivity body beneath it with a depth of 13km. There is a high resistivity structure in the crust beneath the Juxian electromagnetic station. The crustal structures are divided into two different parts by Anqiu-Juxian Fault and Changyi-Dadian Fault, respectively. More conductive layers appear to the west of the two faults. Plenty of fluid possibly exists within the conductive body to the west of Changyi-Dadian Fault, which plays important role in the earthquake generation. There is a relative low resistivity layer in the crust within 1~2km beneath the Wudi electromagnetic station. Beneath the relatively low resistivity layer, a relatively high resistivity layer extends to a depth of around 15km, and the resistivity value decreases with the increase of depth. The electrical resistivity model suggests the seismic activity of the Yishu fault zone around the Anqiu and Juxian electromagnetic stations should be taken into account seriously, and monitoring and research on it need to be strengthened. The results of this paper provide a certain reference value for the crustal structure research to similar stations.     

Bouguer gravity study of middle section of Tanlu fault

Although Tanlu fault is one of the most important tectonic fault zones and active earthquake belts in eastern China, little is known about its deep structure. In this study, we use the existing Bouguer gravity data to study the middle section of the Tanlu fault zone, which is also known as the Yishu fault zone. Our gravity inversion results indicate that the Moho has an abrupt offset in depth at the Tanlu fault zone and it has a relatively smooth variation away from the fault zone. The crustal structures on both sides are different from each other. Sediment is thin on the west side with an average thickness of less than 5 km, while it is as thick as 6 km on the east side. The thinnest sediment （3-4 km） is at the fault zone. Moho depth increases from 33 to 34 km on east side and from 36 to 38 km on west side. Tanlu fault zone is shown as a wide zone of linear gradient in the Bouguer gravity anomaly.     

NEW EVIDENCES FOR LATE QUATERNARY ACTIVITY IN THE SOUTHERN SEGMENT OF THE YISHU-TANGTOU FAULT,THE TAN-LU FAULT ZONE,AND ITS TECTONIC IMPLICATION

The Tan-Lu Fault Zone(TLFZ), a well-known lithosphere fault zone in eastern China, is a boundary tectonic belt of the secondary block within the North China plate, and its seismic risk has always been a focus problem. Previous studies were primarily conducted on the eastern graben faults of the Yishu segment where there are historical earthquake records, but the faults in western graben have seldom been involved. So, there has been no agreement about the activity of the western graben fault from the previous studies. This paper focuses on the activity of the two buried faults in the western graben along the southern segment of Yishu through combination of shallow seismic reflection profile and composite drilling section exploration. Shallow seismic reflection profile reveals that the Tangwu-Gegou Fault(F₄)only affects the top surface of Suqian Formation, therefore, the fault may be an early Quaternary fault. The Yishui-Tangtou Fault(F₃)has displaced the upper Pleistocene series in the shallow seismic reflection profile, suggesting that the fault may be a late Pleistocene active fault. Drilling was implemented in Caiji Town and Lingcheng Town along the Yishui-Tangtou Fault(F₃)respectively, and the result shows that the latest activity time of Yishui-Tangtou Fault(F₃)is between(91.2±4.4)ka and(97.0±4.8)ka, therefore, the fault belongs to late Pleistocene active fault. Combined with the latest research on the activity of other faults along TLFZ, both faults in eastern and western graben were active during the late Pleistocene in the southern segment of the Yishu fault zone, however, only the fault in eastern graben was active in the Holocene. This phenomenon is the tectonic response to the subduction of the Pacific and Philippine Sea Plate and collision between India and Asian Plate. The two late Quaternary active faults in the Yishu segment of TLFZ are deep faults and present different forms on the surface and in near surface according to studies of deep seismic reflection profile, seismic wave function and seismic relocation. Considering the tectonic structure of the southern segment of Yishu fault zone, the relationship between deep and shallow structures, and the impact of 1668 Tancheng earthquake(M=8(1/2)), the seismogenic ability of moderate-strong earthquake along the Yishui-Tangtou Fault(F₃)can't be ignored.     

大别造山带东段重力异常多尺度分解及其构造意义

文中通过多源数据融合、模型构建、数据试验、二维离散小波变换和功率谱分析等方法获取了大别造山带东段深、浅部场源布格异常及其场源似深度,并结合地壳结构、地质构造、岩石圈有效弹性厚度和地震活动等资料,讨论了地壳深、浅部的结构特征及地震活动构造背景。结果表明,低频布格异常显示大别造山带东段与华北地块间深部构造缝合带在东部应位于青山-晓天断裂前缘,在落儿岭-土地岭断裂和商城-麻城断裂之间向N偏移至梅山-龙河口断裂之下,造山带南侧与扬子地块间深部构造缝合带位于襄樊-广济断裂以北约20km,造山带东侧与扬子地块间的深部构造转换带位于郯庐断裂带之下,造山带东段腹地显著的低频布格异常低值表明对应部位的莫霍面存在明显下凹,造山带内部的布格异常高梯度带表明其深部结构不完整;高频布格异常揭示肥中断裂、六安-合肥断裂、肥西-韩摆渡断裂和郯庐断裂带等主要断裂对地壳中上部密度结构的影响明显,落儿岭-土地岭断裂对地壳中上部密度结构的影响范围向N延伸至肥西-韩摆渡断裂前缘。结合地震活动资料进一步分析认为,大别造山带东段与华北地块在青山-晓天断裂前缘附近接触和相互作用,且大别造山带东段地壳深、浅部结构均不完整,不利于应力积累,趋向于在断裂交错的脆弱部位频繁释放应力,是霍山地区小地震活动频繁的主要原因。