Tele-seismic coseismic well temperature changes and their interpretation

Coseismic water level oscillation and correlated deep water temperature changes have been observed in a water well at Tangshan City by high sensitivity measurement. Amount of water temperature changes depend on ampli-tude of water level oscillation. Coseismic water temperatures always decrease as water level oscillates, drop of temperature ranges from 0.001 °C to 0.01 °C corresponding to amplitude of water level oscillation from several centimeters to about one meter. Temperatures usually recover one to several hours after the oscillation. We suggest that the temperature drop is produced by dispersive transfer of heat as the water oscillates, and follow-up thermal conduction makes temperature recovery. Our finite element calculations support quantitatively the idea. High ac-curacy measurements of water temperature at different depths in the future may test our interpretation.     

The relationship between three-dimensional coseismic displacement and distribution of coseismic landslides

In mountainous areas,landslides induced by destructive earthquakes are one of the main causes of human casualties,which is an important link in the chain of earthquake hazards.Earthquake-triggered landslides are mainly controlled by three factors,namely seismic property,topography,and geology.Many studies have been conducted on these controlling factors of earthquake-triggered landslides.However,little is known about the effect of coseismic displacement on the distribution of landslides under di...     

Electrical resistivity changes in rocks during frictional sliding and fracture

Significant changes of electrical resistivity of saturated rocks and water pressure along sliding surface occurred during stick-slips in our direct shear experiment. Two types of changes of electrical resistivity occurred. In the first, resistivity decreased with increasing shear stress, reached minimum together with a sudden release of shear stress and returned to a higher value immediately afterwards. In the second, resistivity again decreased with increasing stress but, in contrast to the first type of changes, it decreased further upon the sudden drop of shear stress. The magnitude and the direction of the changes of water pressure on the sliding surface during stick-slip were not uniform, indicating local variations of surface deformation.     

承压介质电阻率变化的方向性与主应力的关系

依据塘沽台所在区域的水文环境、地质构造和动力学等特征,讨论了地表储水能力、地下静水位及构造变形等因素对视电阻率的影响和塘沽台直流视电阻率下降异常的成因机制。结果表明,长期构造位移变化与地下静水位下降是视电阻率呈下降趋势的主要原因;短期降水量大且雨季长,加之有利的地表储水能力是造成塘沽台视电阻率破年变的主要原因。虽然对于塘沽视电阻率异常中是否掺有前兆信息尚缺少确凿证据,但本文采取的分析方法仍可为典型的异常形态提供有益的参考。     

承压介质电阻率变化的方向性 与主应力的关系

介质电阻率变化的方向性与主应力方向的关系研究一直受到岩石电阻率实验和地震学等学科的关注. 本文在电极面4 cm×8 cm, 6 cm×12 cm, 8 cm×16 cm和4.6 cm×10 cm的10块分别为花岗岩、 凝灰质粗砂岩(夹砾石)和细晶花岗岩标本上及100 cm×100 cm的1块人工合成模型上, 采用对称四极法布设多位置、 多极距、 多方位电阻率测线, 用单轴压缩、 二维约束差应力压缩、 低围压三轴差应力压缩和真三轴差应力压缩方式加载. 其中在单轴压缩和低围压三轴压缩加载方式下, 7块岩石标本中的4块被压破, 3块没有被压破, 只压到电阻率出现明显的下降异常时卸压. 另在上述10块岩石标本中, 还有3块是电阻率原始各向异性标本. 将这样11块标本所获得的多位置、 多极距、 多方位测线电阻率资料, 按从开始加载到破裂的过程, 分成30%附近破裂应力段、 50%附近破裂应力段、 80%附近破裂应力段和100%破裂应力段, 研究同一测点多方位测道中该4个破裂应力段视电阻率变化最大(最小)幅度测线方位与力源最大主压应力方位的关系, 结果未发现二者之间有确定的关系.     

重力卫星GRACE检测出2010年智利Mw8.8地震的同震重力变化

本研究通过对重力卫星GRACE观测数据的处理,采用去相关加300 km半径的高斯平滑,成功地提取了2010智利M_w8.8地震所产生的重力变化信号,最大变化幅度达到7 μGal,并且与位错理论计算结果具有较好的一致性.这是继GRACE检测出2004苏门答腊M9.3地震重力变化后的又一个卫星观测地震的例证,说明GRACE具有检测出M<9.0量级地震的能力,为利用GRACE研究地震以及其更广泛的应用提供了可靠的依据.     

Precursory seismic quiescence

Seventeen cases of precursory seismic quiescence to mainshocks with magnitudes fromM _L=4.7 toM _S=8.0 are summarized. The amount of rate decrease ranges from 45% to 90%. The significance of these changes varies between 90% and 99.99%. The assumption that the background rate is approximately constant is fulfilled in most crustal volumes studied. All quiescence anomalies seem to have abrupt beginnings, and the rate during the anomalous period is fairly constant. The duration of the precursors ranges from 15 to 75 months, and it is not clear what factors determine that time. At least three successful predictions have been based on seismic quiescence. These cases have shown that mainshocks can be predicted based on quiescence, but they have also shown that the interpretation of the data in real time is difficult and nonunique. If a false alarm is defined as a period of quiescence with a significance level larger than a precursory quiescence in the same tectonic area, then we estimate, based on searches in four areas, that the false alarm rate may be on the order of 50%. Failure to predict may be expected in perhaps 50% of mainshocks, even in carefully monitored areas. Quiescence cannot be used as a precursor in tectonic environments with low seismic activity. Most characteristics of the phenomenon are still poorly defined, but data exist which probably permit at least a doubling of the presently available data on case histories.     

与强地震有关的视电阻率各向异性变化

研究了27个强地震附近41个地电台的视电阻率变化,在震前1年尺度～短临阶段,在超过95％的台站上与震源机制解最大主压应力方位正交（或近于正交）测道的视电阻率变化最显著,主应力方位（或近于该方位）的测道的变化反之,其物理机理比较清晰.可以确认：在强地震孕育晚期阶段,在震源区及附近视电阻率各向异性变化是客观存在的地球物理现象,在震中区附近可观测之.     

岩石电阻率图像及各向异性变化的实验研究

在两组人工样品自由表面以中心点为基准对称布设3条辐射状测线,对样品实施单轴应力加载和卸载后,利用电阻率层析成像方法构建了相应的视电阻率相对变化图像,并计算和绘制了表征裂隙产生和发展速率的视电阻率各向异性系数λ*以及表征裂隙产生和发展方位的各向异性主轴方位角α随应力和深度的变化曲线.结果表明:所有测线所对应的RRC图像均随着应力的变化呈现出相同的变化趋势,即在加载阶段,随着应力的增加,视电阻率相对变化图像中电阻率降低区域逐渐收缩,而电阻率升高区域逐渐扩张,在卸载阶段,随着应力的减小,电阻率降低区域继续收缩,电阻率升高区域继续扩张;样品中的高阻体对其所在部位及附近区域的电阻率增幅有较大影响,而对横越高阻体测线的视电阻率相对变化图像的趋势性变化无影响;对于原始电性为各向异性的样品,随着应力的增加,其各向异性程度降低;裂隙主要在岩样的浅部产生和发展,而在较深部位的裂隙产生和发展的速率相对较低.上述结果有助于解释和理解地震、火山活动和大型地质构造运动引起的视电阻率及其各向异性的变化特征,电阻率层析成像方法可能成为目前地震电阻率观测方法的有益补充.      

Inversion of relative changes in the magnetotelluric apparent resistivity into the relative changes in the resistivity of the elements of a geoelectric structure

The earlier proposed method for inversion of the observed changes in the magnetotelluric apparent resistivity ρ_a into the relative changes in the resistivity of the elements of a well-studied geoelectric structure is tested on a 2D model representing one of the geodynamic research areas of Kamchatka. The possibilities are demonstrated for choosing the most effective observation conditions of the study, and for the determination of the source of the observed variations in γ_a. It is concluded that, if a number of conditions are met, it is possible to estimate the amplitude of relative changes in the electrical resistivity of rocks with sufficient accuracy for monitoring these changes with the aforementioned changes used as a prognostic parameter.     

基于同震库仑应力变化的汶川地震余震频次研究

在计算同震静态库仑应力变化的基础上,基于速率-状态摩擦定律及与同震静态库仑应力变化相关的余震频次估计方法,研究了汶川地震余震区及附近不同区域内与汶川地震同震静态库仑应力触发相关的"直接"余震频次。结果显示,主震破裂带尤其是主震破裂带南段的"直接"余震频次明显低于实际情况,其原因在于同震库仑应力变化导致的主震破裂面上应力水平的降低。研究结果还显示,汶川地震同震库仑应力变化对余震活动持续时间的影响与震级下限有关。对ML4.0以上余震,持续时间约为震后15~16个月;对ML3.5以上余震,接近60个月。在上述时段内,与同震库仑应力变化相关的"直接"余震占全部余震的比例为44.7%~48.6%。这表明,即使在同震库仑应力变化的"有效"作用时段内,主震破裂面上也大约有50%的余震活动不是缘于同震库仑应力变化的影响,这可能与震后余滑及粘弹松弛等时间相关因素的影响有关。     

同震重力变化对震源滑动参数的敏感性研究：以2004年苏门答腊地震为例

在地震孕育和发生过程中,常伴有地表重力变化.通过同震重力变化反演震源参数,可以为认识地震活动特征奠定基础.为了有效提高同震重力变化反演滑动参数的精度,本文设计了一套定量化表征同震重力变化对震源不同滑动参数敏感性的方法.首先,基于位错理论,通过正演得到震源不同滑动参数下的同震重力变化;随后,利用无量纲化的敏感性分析方法,计算同震重力变化对不同滑动参数的敏感度因子.以2004年苏门答腊地震的断层模型为参考,定量评估了该地震同震重力变化对断层深度、断层倾角、滑移量和滑动角四个独立震源滑动参数的敏感性.评估结果显示,2004年苏门答腊地震同震重力变化对滑移量、滑动角、断层深度和断层倾角这四个独立震源滑动参数的敏感性依次降低.因对滑移量的敏感性远高于对断层倾角的敏感性,滑移量较小的反演偏差可以造成断层倾角较大的反演偏差.所以,在反演过程中,需要更加重视排序靠前的滑移量和滑动角.本文设计的方法虽然适用于其他震例,但在不同震例中,同震重力变化对各滑动参数的敏感性将有所差别.

     

Characteristics of coseismic water level changes at Tangshan well for the Wenchuan MS_8.0 earthquake and its larger aftershocks

Coseismic water level changes which may have been induced by the Wenchuan MS8.0 earthquake and its 15 larger aftershocks(MS≥5.4) have been observed at Tangshan well.We analyze the correlation between coseismic parameters(maximum amplitude, duration, coseismic step and the time when the coseismic reach its maximum amplitude) and earth-quake parameters(magnitude, well-epicenter distance and depth), and then compare the time when the coseismic oscillation reaches its maximum amplitude with the seismogram from ...     

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.     

Sensitivity of transversal apparent resistivity to the changes in electrical resistivity of the elements of a 2D geoelectrical structure

The sensitivity of transversal apparent resistivity to the changes in the electrical resistivity of elements of a two-dimensional (2D) geoelectrical structure is studied by numerical modeling. This sensitivity is found to have a series of specific features, due to which the monitoring of transversal apparent resistivity can serve as a helpful addition to the monitoring of longitudinal apparent resistivity to trace the dynamics of subsurface and shallow crustal elements of the medium. It is shown that the method previously suggested for the inversion of relative changes in apparent resistivity into relative changes in electrical resistivities of the elements of the geoelectrical structure is applicable to the transversal electrical resistivity.     

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.     

Temporal changes in electrical resistivity at Sakurajima volcano from continuous magnetotelluric observations

Continuous magnetotelluric (MT) measurements were conducted from May 2008 to July 2009 at Sakurajima, one of the most active volcanoes in Japan. Two observation sites were established at locations 3.3 km east and 3 km west–northwest of the summit crater. At both observation sites, the high-quality component of the impedance tensor (Zyx) showed variations in apparent resistivity of approximately ± 20% and phase change of ± 2°, which continued for 20–180 days in the frequency range between 320 and 4 Hz. The start of the period of changes in apparent resistivity approximately coincided with the start of uplift in the direction of the summit crater, as observed by a tiltmeter, which is one of the most reliable pieces of equipment with which to detect magma intrusion beneath a volcano. A 2D inversion of MT impedance suggests that the resistivity change occurred at a depth around sea level. One of the possible implications of the present finding is that the degassed volatiles migrated not only vertically through the conduit but also laterally through a fracture network, mixing with shallow groundwater beneath sea level and thereby causing the observed resistivity change.     

地震前兆数据监视与管理系统

在北京十三陵地震台建设了一个高度集成的地震观测监控系统, 将原来分散的观测项目(如测震、 GPS、 气氡、 气汞、 电磁波、 地电场等测项), 改造为集中到一台服务器管理, 在一个监控平台上统一实现了对台站数据的收集、 入库、 管理、 处理、 监控等业务; 对这些观测项目可以在控制台上直接取数并将数据入库; 初步实现了观测数据的收集、 数据处理、 数据管理、 数据监控的软件管理系统, 减少了系统维护工作量, 方便台站工作人员操控。 监控系统的地震前兆数据监管软件系统DataMonitor可准实时监视数据, 检查数据的异常和到达情况, 并向台站数据管理人员告警; 可对数据库数据进行统计, 包括数据到达情况统计、 缺数统计、 数据连续率统计等, 并可按用户定制的测项分类进行统计。 该系统还提供多种前兆数据处理方法, 实现对前兆数据的各种常规分析处理。 地震前兆数据监视与管理系统已在北京十三陵地震台应用, 满足了数据管理人员日常工作需要, 可及时方便地掌握数据的情况并对观测系统进行维护。