Precursory Seismic Quiescence Before July 28, 1976 Great Tangshan,China, Earthquake

The earthquake catalog with the minimum completeness magnitude of ML=1.7 during the period from 1970 to 1976 has been used as basic data for the research on seismic quiescence in source area before occurrence of the July 1976 great Tangshan earthquake in China,based on the analyses of the detectability of seismic network,and of relation between earthquake magnitude and frequency in Northern China.The temporal processes of seismicity rate(January 1970-July 1976)before the July 1976 great Tangshan earthquake for four researched sub regions in and around source area(aftershock area)of the mainshock are investigated and compared.It is found that there exist abnormal seismic quiescence with duration of 38 months in the subregion located in western part of aftershock area,which is significant statistically at confidence level of 0.99 and larger by beta-test.In addition,because there were no false alarms except for the above abnormal interval in the west aftershock area,and the Tangshan mainshock and 85% of af     

DILATION-CREEP MODEL OF EARTHQUAKE SOURCE DEVELOPMENT AND THE TANGSHAN EARTHQUAKE

The article has made a simple exposition of the dilation-creep modelof earthquake source development(abbreviated DC).Inelastic volumetricdilation of rock masses and fault creep are considered as two basic phys-ical processes in the DC model.The physical mechanism of precursors ofthe M=7.8 Tangshan earthquake has also been analysed and discussed inthis paper.The results show that the precursors of the Tangshan earth-quake were not caused by only one factor and the precursors observed in-and around the epicentral area prior to this earthquake can be grouped into three types.TypeⅠprecursors may be causally related to rock dilata-ncy.TypeⅡprecursors result from fault creep.TypeⅢprecursors maybe associated with some sort of upward migration of mass in the crust orthe upper mantle,and/or may be attributed to large scale stressing processaccommodated by some combination of stable slippage and discontinuousbrittle rupture(namely small earthquake)along the faults.It was sug-gested that repeated dilation and discontinuous creep occurred during theprocess of source development of the Tangshan earthquake and the prepa-ratory process of the Tangshan earthquake can be divided into the follo-wing six phases:elastic stress accumulation(from 1954 to 1967);earlyinelastic dilation(from 1968 to 1969);early fault creep(from 1970 to1973);the second dilation(from the end of 1973 to the first half of 1975);the second evident fault creep(from the second half of 1975 to the end ofApr.1976);and fault creep just before the main shock(from the end ofApr.1976 to the occurrence of the Tangshan earthquake).It is regardedhat the preparatory process of the Tangshan earthquake,as one of the intraplate events,may be controlled jointly by the upward migration ofdeep mass and large scale intraplate stress field.This characteristic prob-ably is different from that of earthquake along plate boundary.     

THE VERTICAL DEFORMATION BEFORE AND AFTER THE TANGSHAN EARTHQUAKE OF 1976

Based on the data of 18 times of leveling carried out during 1954 to 1985, the relative stability degrees of relevelling nodes in the Tangshan area are calculated. According to these results, the characteristics of crustal vertical deformation before and after the Tangshan Earthquake(28/7/1976,Ms =7.8) are studied. Our study indicates that probably since 1968 significant relative movements between structural blocks in that area had appeared and continued till 1978. The seismic fault wa     

The Residual Deformation and Equivalent Source of the Lancang-Gengma Earthquake

In this paper,the equivalent force inside the source which produces a large area deformation field is calculated by using the records of residual deformation,S wave radiation of shear dislocation source and Green function of the Lancang-Gengma earthquake.Under the action of equivalent force whose intensity was 1.103×1013 N,the crust derm of northeast segment of seismic fault had finished the upraise and formed permanent irrecoverable deformation within 20.46 min after the major earthquake occurred.The authors indicate that the residual deformation would split into 2 deformation fields with different propagation velocities and different directions in the Song-distance propagation process.     

Relocation and seismogenic structure of the 1998 Zhangbei-Shangyi earthquake sequence

Introduction The January 10, 1998 Zhangbei-Shangyi, Hebei Province, earthquake has been the third large event of magnitude 6.0 and greater since the 1976 great Tangshan earthquake of magnitude 7.8 in the northern China (33皛42癗, 110皛124癊). Before this event, there were only two events of magnitude 6.0 and greater occurred in or around the Tangshan area since 1976: the M=6.9 Ninghe, Tianjin, earthquake of November 15, 1976 and the M=6.2 Hangu, Tianjin, earthquake of May 12, 1977. The …     

Aseismic negative dislocation model and deformation analysis of crustal horizontal movement during 1999~2001 in the northeast margin of Qinghai-Xizang block

Introduction So small is the hypocenter area of strong earthquake, but its formation is controlled by time-space evolution of present-day crustal movement in wider-range area, and related to motion and deformation of active blocks and their boundary faults. Aseismic negative dislocation model presented by Matsuura, et al (1986) is that, the relative motion between blocks driven by present-day crustal movement may be partly locked at the block …     

Deformation Evolution Characteristics Revealed by GPS and Cross-fault Leveling Data before the MS8.0 Wenchuan Earthquake

Based on GPS velocity during 1999-2007, GPS baseline time series on large scale during 1999-2008 and cross-fault leveling data during 1985-2008, the paper makes some analysis and discussion to study and summarize the movement, tectonic deformation and strain accumulation evolution characteristics of the Longmenshan fault and the surrounding area before the M_S8.0 Wenchuan earthquake, as well as the possible physical mechanism late in the seismic cycle of the Wenchuan earthquake. Multiple results indicate that:GPS velocity profiles show that obvious continuous deformation across the eastern Qinghai-Tibetan Plateau before the earthquake was distributed across a zone at least 500km wide, while there was little deformation in Sichuan Basin and Longmenshan fault zone, which means that the eastern Qinghai-Tibetan Plateau provides energy accumulation for locked Longmenshan fault zone continuously. GPS strain rates show that the east-west compression deformation was larger in the northwest of the mid-northern segment of the Longmenshan fault zone, and deformation amplitude decreased gradually from far field to near fault zone, and there was little deformation in fault zone. The east-west compression deformation was significant surrounding the southwestern segment of the Longmenshan fault zone, and strain accumulation rate was larger than that of mid-northern segment. Fault locking indicates nearly whole Longmenshan fault was locked before the earthquake except the source of the earthquake which was weakly locked, and a 20km width patch in southwestern segment between 12km to 22.5km depth was in creeping state. GPS baseline time series in northeast direction on large scale became compressive generally from 2005 in the North-South Seismic Belt, which reflects that relative compression deformation enhances. The cross-fault leveling data show that annual vertical change rate and deformation trend accumulation rate in the Longmenshan fault zone were little, which indicates that vertical activity near the fault was very weak and the fault was tightly locked. According to analyses of GPS and cross-fault leveling data before the Wenchuan earthquake, we consider that the Longmenshan fault is tightly locked from the surface to the deep, and the horizontal and vertical deformation are weak surrounding the fault in relatively small-scale crustal deformation. The process of weak deformation may be slow, and weak deformation area may be larger when large earthquake is coming. Continuous and slow compression deformation across eastern Qinghai-Tibetan Plateau before the earthquake provides dynamic support for strain accumulation in the Longmenshan fault zone in relative large-scale crustal deformation.     

Relationship between the earthquake sequences of Tangshan and Xingtai and the three dimensional velocity structure

In this paper, based on the results of tomographic image of Tangshan and Xingtai areas, the relations between the characteristics of the two strong earthquake sequences and their three-dimensional velocity structures are studied. The research results indicate that:① Mosaic distribution of low-velocity bodies and high-velocity bodies, especially the existence of high-velocity bodies with large size in crust are the common basis of development of the two earthquake sequences. ②Scale, depth, and heterogeneity of high-velocity and low-velocity bodies are the important factors to effect the characteristic of earthquake sequences.③ The depth of the high-velocity body in Tangshan area is less than that in Xingtai area, which is the principal reason why the dominant focal depth and the biggest focal depth of Tangshan earthquake sequence are less than Xingtai's.④The depth of the high-velocity bodies in Ninghe area is more than that in Tangshan-Luanxian area, which lead to the biggest magnitude and epicentral intensity are lower. These results could be helpful for predicting the main shock of strong swarm-type earthquakes and later strong aftershocks.     

Study on Dynamic Characteristics of Crustal Deformation in the Yunnan Area Using GPS

Based on the GPS velocity field data of 1999-2007 and 2011-2013,we used the least squares configuration method and GPS velocity profile results to synthetically analyze the dynamic evolution characteristics of crustal deformation in the Yunnan area before and after the Wenchuan earthquake. The dynamic evolution of GPS velocity field shows that the direction is gradually changed from the south in the southern part of the Sichuan-Yunnan block to the south-west in the southern Yunnan block and there is a clear relative motion characteristic near the block boundary fault zone. Compared with the GPS velocity of 1999-2007, the results of 2011-2013 also reflect segmental deformation characteristics of the block boundary fault zone. Southeast movement shows a significant increase, which may be related to crustal deformation adjustment after the Wenchuan earthquake. The dynamic evolution of strain parameters shows a pattern of "extension in the middle and compression at both ends" in the whole area and the distribution of deformation (shear, extension or compression) is closely related to the background motion and deformation characteristics of the main fault zone. Compared with the results of the period of 1999-2007, the extensional deformation zone of 2011-2013 is expanded eastward and southward. The compressional deformation of the eastern boundary (the Xiaojiang fault zone) of the Sichuan-Yunnan block is no longer significant, which is mainly concentrated in the northern section of the Xiaojiang fault zone and may be related to the post-seismic deformation adjustment of the Wenchuan earthquake. The GPS velocity profile results show that the left-lateral slip velocity of the Xiaojiang fault zone reduced gradually from north to south (10mm/a-5mm/a), and the width of the northern section is wider. The right-lateral slip rate of the Honghe fault zone is about 4mm/a, and the deformation width is wider. The dynamic results show that the Wenchuan earthquake has little effect on the deformation modes of these two fault zones.     

The Spatial and Temporal Variatiion of Modern Tectonic Stress Field in North CHina before and after the 1976 Tangshan Earthquake

By using 126 earthquake focal mechanism solutions (M S≥4.7) during the period of 1963～1998, modern tectonic stress field in North China is inverted by means of the step by step convergence. The inversion results indicate that the tectonic stress field in the research region is clearly variational in space and time: (1) The middling principal stress axis σ 2 is basically vertical. The maximum and minimum principal stress axes σ 1 and σ 2 are nearly horizontal, but the azimuths of σ 1 and σ 3 are inconsistent in different districts and periods. (2) Before the Tangshan earthquake in 1976, the three principal stress axes are uniform. The azimuth of maximum principal stress axis σ 1 is 68° (striking in a NEE-SWW direction). (3) After the Tangshan earthquake, the maximum principal stress axis σ 1 and minimum principal stress axis σ 3 have variations in different districts. In the northern area of North China and on the eastern side of the Tancheng-Lujiang fault zone, the maximum principal stress axis σ 1 is also striking in a NEE-SWW direction. Its azimuth is 68°. It is the same as that before the Tangshan earthquake. In the southern area of North China, the maximum principal stress axis σ 1 is striking in a E-W direction and its azimuth is 87°.     

A study on relation between acoustic emission and characteristic displacement field on the sample with multi en echelon structures — The theoretic and experimental explorations of strain gap

Based on the phenomena that the deformation gap was observed before the great Tangshang earthquake, this paper discusses the strain gap according to test and theory. The (strain) patterns were recorded photographically by realtime holographic interferometry and shadow optical method of caustics, as soon as the loading process started. In the meantime, the AE (acoustic emission) signals were recorded by a micro crack information storage-analysis system. According to damage theory and location of micro fracture, we have studied the stain gap and gained: a) It is necessary that strain gap appears under the condition of linear elasticity theory, and its situation is relatively stable, corresponding to stress concentration. b) Micro fractures, which appear initially at area of high stress, occur rarely at the strain gap, and their locations are finally in the zone between the stress concentration area and the strain gap, which indicate the clusters or groups. However, the major macro fracture (final rupture) started from the shadow areas, and then grew quickly towards the strain gaps, which resulted in failure of sample. Foundation item: The Dual Project of China Seismological Bureau (9691309020301), the Specialized Funds for National Key Basic Study (G1998040704), the project for the MOST under contract (2001BA601B02) and Youth Funds for applied basic study of the Science and Technology Bureau of Yunnan Province (98D019Q).     

Experimental Study of the Process Zone,Nucleation Zone and Plastic Area of Earthquakes by the Shadow Optical Method of Caustics

-- On a plexiglass sample, a penetrating crack was prefabricated by laser. The crack is inclined towards the major principal stress †₁(†_y) at an angle of about 30°. Using this sample and by means of shadow optical method of caustics and microcrack location, the process zone, nucleation zone and plastic area of earthquakes were studied experimentally, and the strain variation in the shadow area was monitored. From the result, we comprehend the following. When the stress †_y was increased to a certain value, shadow areas were formed around the prefabricated crack and at its tips, with that at the tips being larger. These shadow areas become larger with the increase of load and smaller with unloading. In the shadow area the strain was inhomogeneous: it was very large in some places but very small in others. When the shadow area reached a critical state, microcracks appeared at the tips of the prefabricated crack. Microcracks appeared on one side of the prefabricated crack where the strain and the shadow were both smaller. The zone with concentrated microcracks, or the process zone, was always located at the crack tip; this zone together with a half length of the original crack formed the nucleation zone which fell into the shadow area but was smaller than it. The shadow optical area of caustics bears a certain quantitative relation with the plastic area. Therefore, if an appropriate method is available to obtain the shadow optical area of caustics, it would be possible to detect the area with strong differential deformation change, and hence to determine the zone where strong fracture (an earthquake) would take place.     

An experimental study of the dynamic features of shadow areas of caustics in response to loading/unloading fracture

IntroductionCHEN (1981), GENG, et al (1980), YAO (1981) and others have found out that different stress paths can lead to fracture. They have further studied the precursory features of wave velocity ratio, acoustic emission, and so on, in this situation. YIN (1987), YIN and YIN (1991), Yin, et al (1994, 1995, 2000) have put forward the concept of loading-unloading response ratio. When a mechanical test system is in a stable state, the path of its response to loading is the same as that…     

加载破裂和卸载破裂的焦散阴影区动态特征的实验研究

以实时全息干涉计量方法和焦散阴影光学法，用有机玻璃试样直观、形象地研究加载过程和卸载过程破裂时的应力（应变）场、塑性区或成核区（阴影区）不同地动态特征变化．结果发现，在低应力时，预制裂缝端点先出现应变核（条纹密集区），进而形成焦散阴影区．这些核和阴影区会随荷载增加变大，随卸载而减小．在一定的压力范围内重复加载卸载（循环加载），焦散阴影区则反复增大缩小；但在高应力时，尤其在将出现大破裂时，加载和卸载的焦散阴影区变化是不可逆的，其变化也完全不同．加载破裂时的焦散阴影区由慢到快迅速扩展；而卸载时的焦散阴影区则扩展较慢，并且在尖角形前方有一圆形阴影．      

钻孔体应变与面应变观测井孔耦合系数的计算

钻孔应变观测系统（岩石、 膨胀水泥和应变仪钢筒）存在显著的井孔耦合效应,只有确定这一耦合系数,才能得到地壳岩石的真实应变值,从而实现不同测点数据的可比性．本文根据双衬套理论及弹性力学理论,建立了三维空间应力作用下体应变与面应变观测力学模型,并进一步推导各自井孔耦合系数计算式,发现两组系数与井孔的受力状况密切相关,分别与不同力源引起的应变信号相对应．亦即应力比（钻孔轴向应力与平面应力之比）不同,耦合系数也不一样,体应变随应力比的增大而下降,面应变则上升. 本文结果还表明平面应力作用下的耦合系数与外加应力无关,只与观测系统本身有关,故数值保持恒定. 此外,文中对其影响因素也进行了分析,结果表明,体应变和面应变的井孔耦合系数均随岩石弹性模量和泊松系数的增大而增大,且前者的幅度较大,井孔耦合材料水泥对二者影响均很小．      

Observations of remote coseismic stress step-changes

The study on stress triggering and stress shadow is a frontier subject of current seismologic research. One of the problems presently encountered is the lack of reliable observation of real coseismic stress changes at locations remote to the epicenter. At Changping Station near Beijing, coseismic step-changes were recorded for the Mw6.3 Zhangbei quake on 10 January, 1998 and the Mw5.8 Zhangbei quake on 11 March, 1999, by a RZB borehole component strainmeter. Based on analyses of these recordings, this paper shows observed coseismic step-changes of the horizontal component of strain tensor. The strain step-changes reach the order of 10?8 and, correspondingly, stress step-changes reach as high as 10?4 MPa. The results are significantly higher than linear elastic theoretical values. However, as self-checking can be carried out with the 4 groups of data recorded by the 4-component strainmeter, it has proved that the results are reliable.     

地壳应变场观测中体应变与面应变转换系数的计算

将钻孔体应变仪与分量式应变仪的观测资料在面应变平台上进行整合,对于解决目前两种应变仪观测数据的评估、分析、处理和地震预报问题有实际意义．根据潘立宙-陈沅俊和Evertson理论及弹性力学知识,分别建立了平面应力作用下体应变和面应变观测的力学模型,推导了观测钻孔、空孔和无孔岩石体应变与面应变转换系数的计算公式,发现它们都可以归结为同一公式描述,差异仅在于k（体应变仪钢筒内壁或无孔岩石面应变与空孔岩石面应变之比）的取值不同．用Evertson理论推导的公式与空孔岩石情形相近,当岩石弹性模量为4times;1010——8times;1010 Pa时,二者都可以看成是对潘-陈公式的一种简化、近似计算;无孔岩石的情况则相当于在岩石弹性模量为1times;1010 Pa时对该式的一种估计. 文中结合实际情况对转换系数的各种影响因素进行了详细分析．      

Observations of remote coseismic stress step-changes

The study on stress triggering and stress shadow is a frontier subject of current seismologic research. One of the problems presently encountered is the lack of reliable observation of real coseismic stress changes at locations remote to the epicenter. At Changping Station near Beijing, coseismic step-changes were recorded for the M_w6.3 Zhangbei quake on 10 January, 1998 and the M_w5.8 Zhangbei quake on 11 March, 1999, by a RZB borehole component strainmeter. Based on analyses of these recordings, this paper shows observed coseismic step-changes of the horizontal component of strain tensor. The strain step-changes reach the order of 10^?8 and, correspondingly, stress step-changes reach as high as 10^?4 MPa. The results are significantly higher than linear elastic theoretical values. However, as self-checking can be carried out with the 4 groups of data recorded by the 4-component strainmeter, it has proved that the results are reliable.     

三向受压状态下地壳岩体的张性变形和张性破裂

本文叙述了处于三向受压状态的地壳岩体产生张性变形和张性破裂的原理,并以珠江三角洲地区为例说明:①岩体内应力集中点的浅部,可产生张性变形和张性破裂;②地质考察所观察到的张性或张剪性断裂,其断裂面上作用的正应力未必是拉应力,极有可能是压应力,但却一定是拉应变;③张性破裂引起的地震,震级较小,一般在4.5级以下,④地震引起的张性地裂缝的走向,就是震源应力场的主压应力方向。     

2004年印尼8.7级强震前昌黎台的远场电性异常特征探讨

分析了在2004年12月26日印尼8.7级地震前河北昌黎台记录到的自然电位及电场异常, 结果显示昌黎台自电位、 电场正常时段观测到的半日、 半月潮汐波周期变化在震前1个多月发生畸变, 周期消失, 变化幅度相应减小, 震后快速恢复, 且潮汐波记录比震前正常时段更加清晰。 该台地电阻率的观测结果显示起始下降变化比较平缓, 震前一周左右, 变化加剧, 出现多次阶变。 究其原因, 在强震发生前断层的蠕滑作用导致孔隙压力变化, 产生一种可以在岩石中传播的应变波, 由于台站下方岩土层对小应变信号具有很好的放大作用, 所以记录到了这种小幅度的应变波, 也就是远场的静态应变效应。 随着地震临近, 应力作用加强, 远程应变波引起台站下方水位、 水化学性质以及岩石孔隙度、 渗透率等的改变, 导致电阻率、 电场大幅度的阶变, 这反映了远场应变波的动态效应。 当然就异常幅度而言, 应变波的效应还嫌弱一些, 因此昌黎台异常变化可能还叠加了震源区激发的电磁波。