Study of the Seismicity of Strong Earthquakes in the Yunnan Area

Yunnan is located in the east margin of the collision zone between the India Plate and the Eurasian Plate on the Chinese Continent, where crustal movement is violent and moderate-strong earthquakes are frequent. In addition, the area features marked active block movement. Therefore, Yunnan is a perfect place for research on strong earthquake activity. Through the study on the temporal and spatial distribution of the M≥6.7 earthquakes and the related earthquake dynamics in Yunnan in the last century, we conclude that the four seismically active periods, which are characterized by alternative activity in the east and the west part of Yunnan, possibly result from a combination of active and quiescent periods in each of the east and west part. And for every 100 years, there may be a period in which strong earthquakes occur in the east and west parts simultaneously. In addition, the seismicity of strong earthquakes in Yunnan corresponds well to that in the peripheral region. The seismicity of the great earthquakes in the Andaman-Myanmar Tectonic Arc belt indicates, to some extent, the beginning of a seismically active period in Yunnan. The seismicity of strong earthquakes in east Yunnan is closely related to that in Sichuan. Strong earthquakes in Sichuan often occur later than those in Yunnan. Furthermore, in the east part of Yunnan, the three procedures including continuous occurrence of moderate-strong earthquake, quiescent period, and the occurrence of the first strong earthquake may be the style of the beginning of the earthquake active period. The above cognition is helpful to the study of earthquake prediction, seismogenic mechanism, and the dynamics of the plate margin in Yunnan.

Analysis of Tectonic Conditions for Strong Earthquakes and Prediction of Seismic Risk of Seismogenic Fault Zones in the Northern Part of North China

The tectonic conditions for strong earthquakes are complicated in various aspects.The genetic conditions for strong earthquakes were studied from the angle of various disciplines.About 20 conditions belonging to different fields have been described by predecessors.In this paper,the authors try to study and evaluate all the tectonic condition factors for strong earthquakes by the methods of fuzzy mathematics and hierarchical analysis.Taking the northern part of North China as an example,the authors make a comprehensive digital analysis of all the quantitative and semi-quantitative tectonic factors.The credibility values of all strong earthquakes with different magnitudes are given after statistical analysis and calculation.Forty-one fault zones in the study region are quantitatively analyzed,and the potential seismogenic fault zones,maximum magnitude,and risk of earthquake occurrence in the near future are assessed.The result of synthetic evaluation,based on all tectonic conditions of different

Discussion on Deep Tectonic Background of Moderately Strong Earthquake in Anhui Province and Its Neighboring Areas Using Results of Seismic Tomography

Using the latest results of seismic tomography, we studied the deep tectonic settings of the moderate and strong earthquakes in Anhui Province and its neighboring areas (28°～39°N, 112°～124°E). The results are as follows: (1) There exists a certain correlation between the location of moderate-strong earthquake, the geologic structure of the surface and the partitioning of active tectonic elements with the upper-crust velocity structure. Most earthquakes recording M≥6.0 occur in high-velocity zones or in the transitional areas between high-velocity and low-velocity zones in the upper crust. Seismicity in the low-velocity zone has a lesser impact. Earthquakes occuring in the high-velocity zone are distributed mainly in the velocity variation area. The boundary belts and the interior of the North China plain fault block are the main active sites of moderate-strong earthquakes. Beneath the fault basins in the western and northern sides of the block, the upper crust is characterized by a wide discontinuous distribution in the low-velocity zone, and in the transition zone from the low- to high velocities, the moderate strong seismicity shows a zonal distribution where active faults are developed. The NW-extension Zhoukou-Hefei-Xuancheng low-velocity zone separates the high-velocity zones of Dabieshan Mountains and west Shandong-Anhui, and moderate-strong earthquakes on its northern side bordering the high-velocity zones are relatively frequent. This low-velocity zone is probably an important and deeply structured boundary between the North China and the South China tectonic provinces. (2) The frequent moderate-strong earthquake recorded in the past and the recent small earthquake activities in the Huoshan-Lu’an area are the result of a low-velocity zone in the middle crust beneath the central part of Dabieshan and the two sets of deep faults that cut through the crust.(3) In terms of deep structures, the distribution of moderate-strong earthquake in Anhui Province has an obvious regional feature. Based on historical earthquake recurrence intervals and analogue principles of deep seismogenic tectonics, the potential earthquake risk zones recording M_S5.0 in Anhui Province are determined.

Study on Potential Strong Earthquake Risks Around the Mabian Area,Southern Sichuan

Based on seismic data from the regional network for the last 34 years,we analyzed the present fault behavior of major fault zones around the Mabian area,southern Sichuan,and identified the risky fault-segments for potential strong and large earthquakes in the future.The method of analysis is a combination of spatial distribution of b-values with activity background of historical strong earthquakes and current seismicity.Our results mainly show:(1) The spatial distribution of b-values indicates significant heterogeneity in the studied area,which reflects the spatial difference of cumulative stress levels along various fault zones and segments.(2) Three anomalously low b-value areas with different dimensions were identified along the Mabian-Yanjin fault zone.These anomalies can be asperities under relatively high cumulated stress levels.Two asperities are located in the north of Mabian county,in Lidian town in western Muchuan county,and near Yanjin at the south end of the fault zone.These two areas represent potential large earthquake seismogenic sites around the Mabian area in the near future.Besides them,the third relatively smaller asperity is identified at southern Suijiang,as another potential strongearthquake source.(3) An asperity along the southwestern segment of the Longquanshan fault zone indicates the site of potential moderate-to-strong earthquakes.(4) The asperity along the segment between Huangmu town in Hanyuan county and Longchi town in Emeishan city on Jinkouhe-Meigu fault has potential for a moderate-strong earthquake.

On seismic strengthening area before strong and great shocks and its mechanism

By systematically studying seismic strengthening areas before 85 earthquakes with M>=6.0 in China, some results have been extracted. 1) Earthquake active strengthening area exists universally before strong shock or great earthquake; 2) The size of the strengthening area and its appearing time will increase when the earthquake magnitude increases; 3) The rate between the size of seismic strengthening area and the size of the source region decreases when earthquake magnitude increases; 4) The appearing time of the earthquake active strengthening region in the eastern part of China is longer than that in the western part of China. The above characteristics have been preliminarily explained qualitatively and half-quantitatively by applying the strong body earthquake generating model and the hard inclusion theory. Then applying the seismic strengthening area, we have obtained long-term predictions of 2 earthquakes, so the seismic strengthening area before strong earthquake or great earthquakes is a universal phenomenon, which has some mechanical base.

An Inhomogeneous Distribution Model of Strong Earthquakes along Strike-Slip Act ive Fault Segments on the Chinese Continent and Its Implication in Engineering Seismology

Through the statistical analysis of earthquake distribution along 51 strike-sli p active fault segments on the Chinese continent, we found that strong earthquak e distribution along the seismogenic fault segments is inhomogeneous and the dis tribution probability density p(K) can be stated as p(K)=1.1206e -3.947K in which K=S/(L/2), S refers to the distance from earthquake epicenter to the center of a fault segment, L is the length of the fault segment. The above model can be utilized to modify the probability density of earthquake occurrence of t he maximum magnitude interval in a potential earthquake source. Nevertheless, it is only suitable for those potential earthquake sources delineated along a sing le seismogenic fault. This inhomogeneous model has certain effects on seismic risk assessment, especia ll y for those potential earthquake sources with higher earthquake reoccurrence rat es of the maximum magnitude interval. In general, higher reoccurrence rate of th e maximum magnitude interval and lower exceeding probability level may bring lar ger difference of the results in seismic risk analysis by adopting the inhomogen eous model, the PGA values increase inner the potential earthquake source, but r educe near the vicinity and out of the potential earthquake source. Taking the T angyin potential earthquake source as an example, with exceeding probability of 10% and 2% in 50 years, the difference of the PGA values between inhomogeneous m odel and homogenous models can reach 12%.

Study of Finite Element Modeling of Strong Earthquake Activities and Its Preliminary Application—Taking Southwest China as an Example

Based on research result concerning the preparation and activity of strong earthquakes in groups and using the finite element method, a finite element dynamic model for Southwest China is established in this paper. Using this model, the stress adjustment in the whole of the Southwest China region in response to the stress change due to strong earthquake occurrence is studied. The preliminary result shows that many strong earthquakes occurred in areas where the stress heightened after the last strong earthquake. So, the finite element model set up in this paper is useful for judging the regions where strong earthquakes are likely to occur in future.

Retrospection on the Conclusions of Earthquake Tendency Forecast before the Wenchuan M_S8.0 Earthquake

The reason for the failure to forecast the Wenchuan M_S8.0 earthquake is under study, based on the systematically collection of the seismicity anomalies and their analysis results from annual earthquake tendency forecasts between the 2001 Western Kunlun Mountains Pass M_S8.1 earthquake and the 2008 Wenchuan M_S8.0 earthquake. The results show that the earthquake tendency estimation of Chinese Mainland is for strong earthquakes to occur in the active stage, and that there is still potential for the occurrence of a M_S8.0 large earthquake in Chinese Mainland after the 2001 Western Kunlun Mountains Pass earthquake. However the phenomena that many large earthquakes occurred around Chinese Mainland, and the 6-year long quietude of M_S7.0 earthquake and an obvious quietude of M_S5.0 and M_S6.0 earthquakes during 2002～2007 led to the distinctly lower forecast estimation of earthquake tendency in Chinese Mainland after 2006. The middle part in the north-south seismic belt has been designated a seismic risk area of strong earthquake in recent years, but, the estimation of the risk degree in Southwestern China is insufficient after the Ning’er M_S6.4 earthquake in Yunnan in 2007. There are no records of earthquakes with M_S≥7.0 in the Longmenshan fault, which is one of reasons that this fault was not considered a seismic risk area of strong earthquakes in recent years.

地震活动性多参数方法研究华北地区强震危险性

利用华北地区38年中小地震进行b值扫描,作为该区域背景应力水平,结果显示华北平原地震带和郯庐断裂带渤海区b值较低,应力积累水平较高。依据一定分区原则,将研究区划分为6个区域,采用多参数组合分析方法,并结合各分区历史强震活动水平,初步判断出各区的地震危险性,结果表明:山西带、燕山带、郯庐断裂带安徽区、山东区都处于较低或中等偏低的应力背景下,以中小地震滑动为特征,未来发生大震的可能性较小。郯庐断裂带渤海区正处于较高应力状态下,以平均震级较大的频繁中小地震滑动为特征,属于华北地区未来最有可能发生强震的区域。华北平原带正处于高应力背景下以稀疏的中小地震滑动为主要特征,未来存在发生大震的可能性。

A FINITE ELEMENT MODEL FOR SEISMICITY INDUCED BY FAULT INTERACTION

On ths basis of interaction between faults, a finite element model for Southwest China is constructed,and the stress adjustment due to the strong earthquake occurrence in this region was studied. The preliminary results show that many strong earthquakes occurred in the area of increased stress in the model. Though the results are preliminary, the quasi 3D finite element model is meaningful for strong earthquake prediction.

强震区超高拱坝抗震处理措施设计优化

基于大岗山拱坝地质条件复杂、地震烈度高,按照静载设计、动载复核的原则,对大岗山拱坝抗震设计开展了研究。通过计算分析和模型试验,采用布置梁向抗震钢筋、跨缝阻尼器、抗力锚索与坝基薄弱岩体灌浆等工程技术措施,提升了大岗山水电站超高拱坝的抗震稳定安全性。

甘肃地区现今地壳地震活动性研究

利用1996年1月—2019年3月地震资料,计算出甘肃省地震活动性参数b值,得出频度图、蠕变图、lgN-M图、发震时间间隔图。结合地震活动参数值组合、历史强震背景分布特点,分析甘肃省内不同地区的强震危险性。

川南马边地区强震危险性分析

根据最近34a的区域台网地震资料,利用地震活动性参数b值的空间分布,结合历史强震与现今地震活动背景,分析了川南马边地区主要断裂带的现今活动习性,并初步判别出了潜在的强震危险区域。研究结果表明:1)马边地区的b值空间分布存在明显的空间差异,反映了该区域不同断裂带与断裂段应力积累水平的差异;2)马边-盐津断裂带上存在3个尺度不等的异常低b值区,它们可能是该断裂带上的相对高应力区(或凹凸体),其中位于马边北、沐川西部利店镇附近的凹凸体与位于该断裂带南端盐津附近的凹凸体可能是马边地区未来发生大地震的危险场所,而位于绥江南的小尺度凹凸体有可能是潜在强震的发生地点;3)存在于龙泉山断裂带西南段的凹凸体将是未来发生中强地震的场所;4)金口河-美姑断裂上位于汉源县皇木镇与峨眉山市龙池镇之间的凹凸体存在发生中强地震的可能性。

由现今地震活动分析鲜水河断裂带中-南段活动习性与强震危险地段

利用最近25年的区域台网地震资料,基于沿鲜水河断裂带中-南段的b值分布以及多个地震活动参数值的不同组合,结合震源深度分布、历史强震背景等,分析了不同断裂段落的现今活动习性,进而初步判别该断裂带潜在的强震危险地段。结果表明,鲜水河断裂带中-南段目前存在6个不同活动习性的段落。其中,道孚段自1981年强震后已再次趋于闭锁,原因可能与断层面存在"凹凸体"有关,但应变可能还会进一步积累;八美段目前处于中偏高应力下的相对静止状态,推测其断面正处于新的应力积累阶段;塔公段已有255-300年的无强震期,目前正处于高应力下的相对闭锁状态,震源深度剖面上的小震空白区显示出闭锁断层面的轮廓,应属于未来最可能发生强震的危险地段;康定、磨西二个段落表现出中偏低应力下的稀疏小震滑动及较频繁小震滑动的状态,意味着最晚的一次大地震破裂后断层面尚未重新耦合;石棉段表现出偏高应力下的频繁中-小地震活动与该段的多条断裂交汇有关。

基于R值评分的年度地震预报能力评价

针对中国大陆东、西部地震活动水平的差异,以东经107°为界用二种方案进行地震预报效能检验的R值评分。第一种方案用西部5.5级、东部5.0级为检验震级下限;第二种方案以西部6.0级、东部5.0级为检验震级下限。结果表明第一种方案更为科学,13年来中国大陆地震预报效能检验平均的R值约为0.26。进一步考虑不同危险区预报震级与实际发生地震震级间的差异,文中还分震级档计算了相应的R值评分。结果显示,我国目前对6.0级左右地震的预测水平较其他震级档更为成功。

大陆地下流体对台湾南投7.6级地震的响应研究

将大陆地震地下流体观测在台湾南投 7.6级地震发生前后 5天内出现的显著异常变化命名为响应异常。统计分析了异常的变化过程及其空间分布特点 ,结果是 :出现异常的测项多 ,除水位以外 ,还有水 (气 )氡、水温等 ;异常总体空间分布广 ,但局部相对集中 ;异常形态统一 ,除了水位有水震波变化以外 ,其余以上升阶变异常为主 ;异常幅度大 ,容易识别 ;异常出现时间集中在 9月 2 1～ 2 6日 ,但持续时间相对较长。初步研究认为这种响应异常的空间丛集区 。

汶川8．0级地震前地震趋势分析意见的回顾

该文在系统整理自2001年昆仑山口西8.1级地震到汶川8.0级地震前年度地震趋势预测中提出的地震活动异常和分析意见的基础上,研究汶川地震未能做出中长期预测的原因。结果表明:2001年昆仑山口西8.1级地震发生后,判定中国大陆处于强震连发阶段,仍有发生8级大震危险,但此后的2002—2007年中国大陆周边接连发生大震,而内部连续6年的7级地震平静,以及中国大陆5、6级地震相继出现的显著平静,是导致2006年以后对中国大陆地震活动水平预测明显偏低的原因。南北地震带中段一直是作为近几年可能发生强震的危险地区,但2007年云南宁洱6.4级地震后,对西南地区强震危险的紧迫性估计不足。而汶川地震所在的龙门山地震带历史上没有7级以上地震记录,也是该地震带未作为近几年地震重点危险区的原因之一。

山西地堑系强震的活动规律和危险区段的研究

山西地堑系是我国著名的历史强震活动带之一。作者系统地分析了山西地堑系历史强震的重复和迁移现象、M≥7级地震前中强地震的活动特征、蠕变曲线的线性分段性,并根据近期地震活动的特点,判断了现今地震活动暂态和未来地震活动性;最后,在现有的形变测量资料、历史地震和古地震研究成果的基础上,确定了山西地堑系内可能发生强震的危险区段。

利用地震矩张量与GPS资料推算中国大陆现今地壳运动能量分布特征

在一次地震轮回时间段内,对于某个相对统一和完整的体系,应变能的积累与释放基本等量。地震资料得到的地壳变形场是现今地壳应变能已得到释放后的一种表现,而GPS资料得到的地壳变形场中则既包含地壳应变能量的释放量,也包含积累量。结合中国大陆及其邻区的地质构造与活动地块分布特点,将研究区划分为32个独立的分区。在此基础之上,利用地震和GPS资料分别计算得到了两组单位体积能量积累率,并对其进行了比较分析。研究表明,本次地震轮回中大多数区域的地壳应变能量已基本释放完毕,我国境内只有青藏北部、川滇、昆仑、阿尔金等几个地区的地壳应变能量积累偏高,仍具有发生较强地震的可能性,是今后地震预测研究应该重点关注的地区。

云南地区大震活动规律研究

云南地区处于印度板块与欧亚板块中国大陆碰撞带的东缘,地壳运动剧烈,活动块体特征明显,中强以上地震频发,是研究大震活动规律的理想场所。通过过去一个世纪的6.7级以上地震活动的时空分布以及地震动力分析认为,云南地区存在的4个具东西交替活动特征的地震活跃期,可能是东、西部各自地震活跃与平静过程叠加的结果,100a左右可能出现1次东、西部同时爆发大震的时段;云南地区地震活动与外围地区存在较好的呼应关系,安达曼-缅甸弧形带的巨震活动对云南地区地震活跃期的启动有一定的指示意义,而云南东部强震也与四川西部大震密切相关,四川大震活动往往滞后于云南地区;中强地震连发—平静—首发大震可能是云南以东部为活动主体的地震活跃期的启动模式。这些认识对云南地区大震预测、地震机理以及板缘动力学研究会有所帮助。