Dextral-Slip Thrust Faulting and Seismic Events of the Ms 8.0 Wenchuan Earthquake,Longmenshan Mountains,Eastern Margin of the Tibetan Plateau

Dextral-slip thrust movement of the Songpan-Garze terrain over the Sichuan block caused the Ms 8.0 Wenchuan earthquake of May 12, 2008 and offset the Central Longmenshan Fault （CLF） along a distance of -250 km. Displacement along the CLF changes from Yingxiu to Qingchuan. The total oblique slip of up to 7.6 m in Yingxiu near the epicenter of the earthquake, decreases northeastward to 5.3 m, 6.6 m, 4.4 m, 2.5 m and 1.1 m in Hongkou, Beichuan, Pingtong, Nanba and Qingchuan, respectively. This offset apparently occurred during a sequence of four reported seismic events, EQ1-EQ4, which were identified by seismic inversion of the source mechanism. These events occurred in rapid succession as the fault break propagated northeastward during the earthquake. Variations in the plunge of slickensides along the CLF appear to match these events. The Mw 7.5 EQ1 event occurred during the first 0-10 s along the Yingxiu-Hongkou section of the CLF and is characterized by 1.7 m vertical slip and vertical slickensides. The Mw 8.0 EQ2 event, which occurred during the next 10-42 s along the Yingxiu-Yanziyan section of the CLF, is marked by major dextralslip with minor thrust and slickensides plunging 25°-35° southwestward. The Mw 7.5 EQ3 event occurred during the following 42-60 s and resulted in dextral-slip and slickensides plunging 10° southwestward in Beichuan and plunging 73° southwestward in Hongkou. The Mw 7.7 EQ4 event, which occurred during the final 60-95 s along the Beichuan-Qingchuan section of the CLF, is characterized by nearly equal values of dextral and vertical slips with slickensides plunging 45°-50° southwestward. These seismic events match and evidently controlled the concentrations of landslide dams caused by the Wenchuan earthquake in Longmenshan Mountains.

Volumetric changes in the focal areas of seismic events corresponding to destress blasting

The typical development of total volumetric change in the focal areas of seismic events,corresponding to destress blasting,is characterized as an explosive phase followed by an implosive phase and with alternating additional phases following on from that.In a few cases,a non-typical development of volumetric change was identified,where the first phase was implosive and the second phase,explosive.This development is mainly typical for induced seismic events recorded during mining,not for destress blasting.Seismic events were recorded during longwall mining in the Czech part of the Upper Silesian Coal Basin,where the destress blasting technique is used as a rockburst prevention active measure.Kinematic source processes in the focal areas of selected seismic events were analyzed by the seismic moment tensor inversion method,as well as by studying geomechanical rock mass conditions at the localities of the seismic events.The main goal of the analysis was to attempt to identify the reasons for non-typical development of volumetric changes in these cases.Volumetric changes were analyzed for seismic events with energy greater than 104 J,recorded in the period of time from 1993 to 2009(1109 events).80%(891)of the recorded seismic events were induced seismic events that were registered during longwall mining and 20%(218)corresponded to destress blasting events.Research shows that the main reason for the non-typical development of volumetric changes in the focal areas of seismic events is an association with destress blasting in the rock mass,which is very close to rock mass overstressing.The detonation of explosives in boreholes,which would dominate the first phase of volumetric changes,probably obscured stress release in the rock mass,as manifested in the first implosion phase of the volumetric changes in this case.

Classification of clustered microseismic events in a coal mine using machine learning

Discrimination of seismicity distributed in different areas is essential for reliable seismic risk assessment in mines.Although machine learning has been widely applied in seismic data processing,feasibility and reliability of applying this technique to classify spatially clustered seismic events in underground mines are yet to be investigated.In this research,two groups of seismic events with a minimum local magnitude(ML) of-3 were observed in an underground coal mine.They were respectively located around a dyke and the longwall face.Additionally,two types of undesired signals were also recorded.Four machine learning methods,i.e.random forest(RF),support vector machine(SVM),deep convolutional neural network(DCNN),and residual neural network(ResNN),were used for classifying these signals.The results obtained based on a primary dataset showed that these seismic events could be classified with at least 91% accuracy.The DCNN using seismogram images as the inputs reached the best performance with more than 94% accuracy.As mining is a dynamic progress which could change the characteristics of seismic signals,the temporal variance in the prediction performance of DCNN was also investigated to assess the reliability of this classifier during mining.A cascaded workflow consisting of database update,model training,signal prediction,and results review was established.By progressively calibrating the DCNN model,it achieved up to 99% prediction accuracy.The results demonstrated that machine learning is a reliable tool for the automatic discrimination of spatially clustered seismicity in underground mining.

Improvements in seismic event locations in a deep western U.S. coal mine using tomographic velocity models and an evolutionary search algorithm

Methods of improving seismic event locations were investigated as part of a research study aimed at reducing ground control safety hazards. Seismic event waveforms collected with a 23-station three-dimensional sensor array during longwall coal mining provide the data set used in the analyses. A spatially variable seismic velocity model is constructed using seismic event sources in a passive tomographic method. The resulting three-dimensional velocity model is used to relocate seismic event positions. An evolutionary optimization algorithm is implemented and used in both the velocity model development and in seeking improved event location solutions. Results obtained using the different velocity models are compared. The combination of the tomographic velocity model development and evolutionary search algorithm provides improvement to the event locations.

Neogene Paleoseismic Events and the Shanwang Biota's Burial in the Linqu Area,Shandong Province,China

Several paleoseismic events are recorded in the Neogene Linqu Group, exposed in the Linqu area, Shandong Province, China. The events were interpreted on the basis of fleldwork and laboratory analysis, which showed the presence of seismites with plastically deformed soft-sediment deformation structures in the Shanwang Formation, and of seismic volcanic rocks in the Yaoshan Formation which show brittle deformation. The earthquake-triggered soft-sediment deformations in the seismites include load structures, ball-and-pillow structures, flame structures, piUow-like beds, boudinage structures, slump folds, syn-depositional faults, veins of liquefied sand, and dikes of liquefied sandy lime-mud. The seismic activity is also reflected in what might be called ＇brittle seismites＇; these originated when, under the influence of seismic vibrations, semi-consolidated conglomerate was shattered. Moreover, volcanic activity is related to intense earthquakes that affected basalts intercalated with sand layers; these successions are known as ＇seismic volcanic rocks＇, which are characterized by veins of liquefied sand intruding the basalts. AH above traces of paleoseismic activity were left from one single time span of 4 Ma with active seismicity that took place 14-10 Ma. This time span is known as ＇the Linqu Neogene Paleoseismic Active Period＇, which is divided into four paleoseismic episodes, which were responses to tectonic extension and basin rifting in this area. It even includes the activity of the Yishu Fault Zone during the Miocene and the Neogene. The ratios of trace elements in the seismites, w（La）/w（Sc） and w （La）/w（Th） are higher than the average value of the upper crust, but w（Th）/w（Sc） is lower; this is geochemical evidence for the basin rifting that resulted in a high sedimentation rate. The intense and frequent paleo-earthquakes are held responsible for the rapid burial of the Shanwang Biota. Secondary earthquake-induced processes （e.g. slumping of a lake shore and the strongly increased lacustrine sedimentation rate） contributed to the rapid burial of the biota.

Calibration of P/S amplitude ratios for seismic events in Xinjiang and its adjacent areas based on a Bayesian Kriging method

Correction maps of P/S amplitude ratios for seismic events distributed in Xinjiang, China and its adjacent areas were established using a Bayesian Kriging method for the two seismic stations WMQ and MAK. The relationship between correction maps and variations of along-path features was analyzed and the validity of applying the correction maps to improve performances of P/S discriminants for seismic discrimination was investigated. Results show that obtained correction maps can generally reflect event-station path effects upon corresponding P/S discriminants; and the correction of these effects could further reduce scatters of distance-corrected P/S measurements within earthquake and explosion populations as well as improve their discriminating performances if path effects are a significant factor of such scatters. For example, as corresponding Kriging correction map was applied, the misidentification rate of earthquakes by Pn（2-4 Hz）/Lg（2-4 Hz） at MAK was reduced from 16.3% to 5.2%.

Rockfall seismic features analysis based on in situ tests:frequency, amplitude, and duration

In the seismic event classification,determining the seismic features of rockfall is significantly important for the automatic classification of seismic events because of the huge amount of raw data recorded by seismic stations in continuous monitoring. At the same time, the rockfall seismic features are still not completely understood.This study concentrates on the rockfall frequency content, amplitude(ground velocity), seismic waveform and duration analysis, of an artificial rockfall test at Torgiovannetto(a former quarry in Central Italy). A total of 90 blocks were released in the test, and their seismic signals and moving trajectories were recorded by four tri-axial seismic stations and four cameras, respectively. In the analysis processing,all the artificial rockfall signal traces were cut separately and the seismic features were extracted individually and automatically. In this study, the relationships between a) frequency content and impacted materials, b) frequency content and the distance between block releasing position and seismic station(source-receiver distance) were discussed. As a result, we found that the frequency content of rockfall focuses on 10-60 Hz and 80-90 Hz within a source-receiver distance of 200 m, and it is well correlated with impacted material and source-receiver distance. To evaluate the difference between earthquake and rockfall, 23 clear earthquake signals recorded in a seven month-long continuous seismic monitoring, carried out with the four seismic stations, were picked out, according to the Italian national earthquakes database(INGV). On these traces we performed the same analysis as in the artificial rockfall traces, and two parameters were defined to separate rockfall events from earthquake noise. The first one, the amplitude ratio, is related to the amplitude variation of rockfall between two stations and is greater than that of earthquakes, because of the higher attenuation occurring for rockfall events, which consists in high frequencies whereas for earthquakes it consists in low frequencies. The other parameter, the shape of waveform of signal trace, showed a significant difference between rockfall and earthquake and that could be a complementary feature to discriminate between both. This analysis of artificial rockfall is a first step helpful to understand the seismic characteristics of rockfall, and useful for rockfall seismic events classification in seismic monitoring of slope.

Poststack internal multiples attenuation based on virtual events

The attenuation of prestack internal multiples based on virtual seismic events is computationally costly and hinders seismic data processing. We propose a multiples attenuation method for poststack seismic data by approximating conventional virtual events. The proposed method is iterative. The proposed method is tested using 2D synthetic and the field poststack seismic datasets. Compared with the conventional virtual events method, the proposed method does not require data regularization and offers higher computation efficiency. The method requires to know the travel time of the primary reflection waves. The results of the application to 2D field datasets suggest that the proposed method attenuates the internal multiples while highlighting the deep primaries.

Anomalies of geomagnetic field related to natural and technogenic events in Charvak area

Based on the long-term geomagnetic observations in the area of the Charvak reservoir,the analysis results of geomagnetic variations are presented in the article.It is revealed that local anomalies of geomagnetic field related to the change process of water volume in reservoir and local seismicity happen simultaneously.It is also considered that these results will be used for earthquake forecasting.

Seismic evaluation of the destress blasting efficiency

In this paper, selected methods of destress blasting efficiency assessment are presented, and novel quantitative methods based on in situ seismic measurements are proposed. The newly formulated solution combines two different approaches. The first, which is useful mostly for the near-field seismic analyses, is based on the analysis of seismic amplitude characteristics, and the second, relevant for farfield evaluation, is extended by the duration and frequency of the seismic wave. Both approaches are based on the seismic analyses of the waveforms generated by blasting recorded by the local seismic network. The proposed solutions are tested and validated in deep underground mines in Poland in which the room-and-pillar mining method is applied. Based on performed analysis, it is shown that both methods may be used as a rockburst hazard control in underground mines. However, developed methods may also be successfully implemented in other engineering practices, including the assessment of seismic vibrations in open pits and quarries.

Analysis of the relationship between water level fluctuation and seismicity in the Three Gorges Reservoir(China)

The Three Gorges Reservoir is a good site for the further researches on reservoir induced seismicity due to decades＇ seismic monitoring. After the first water impounding in 2003, seismic activity becomes more frequent than that before water impoundment. In order to quantitatively study, the relationship between the water level fluctuation and earthquakes in TGR, we introduced statistical methods to attain the goal. First of all, we relocated the earthquakes in TGR region with double difference method and divided the earthquakes into 5 clusters with clustering analysis method. Secondly, to examine the impacts of water level fluctuation in different water filling stages on the seismic activity in the 5 clusters, a series of statistical analyses are applied. Pearson correlation results show that only the 175 m water level fluc- tuation has significantly positive impacts on the seismic activity in clusters I, II, III and V with correlation coefficients of 0.44, 0.38, 0.66 and 0.63. Cross-correlation analysis demonstrates that 0, ], 0 and 0 month time delay separately for the clusters I, II, III and V exists. It illustrated the influences of the water loading and pore pressure diffusion on induced earthquakes. Cointegration tests and impulse response analysis denoted that the 175 m water level only had long term and significant effects just on the seismic events in the intersection region of the Fairy Mount Fault and Nine-brook Fault. One standard deviation shock to 175 m water level increased the seismic activity in cluster V for the first 3 months, and then the negative influence was shown. After 7 months, the negative impulse response becomes stable. The long-term effect of the 175 m water impoundment also proved the important role of pore pressure diffusion in RIS with time.

The Characteristic Analysis and Seismic Triggering Study of the M6.2 and M6.1 Dayao Earthquake Sequences in 2003

The high-resolution hypocenter locations of the mainshocks on July 21 （M6.2） and October 16, 2003 （M6.1） and their aftershock sequences are determined in Dayao, Yunnan by using a double-difference earthquake location algorithm. The results show that the epicenters of the two mainshocks are very close to each other and the distribution of the aftershock sequence appears to be very linear. The distribution of the earthquake sequence is very consistent with the focal mechanism, and both mainshocks are of nearly vertical right-lateral fault. Unlike most other double earthquakes in the Yunmm area, the aftershock distribution of the M6.2 and M6.1 Dayao earthquakes does not appear to be a conjugated distribution but to be in a line, and there are some stacks in the two earthquake sequences. It can be inferred that they are all controlled by the same fault. The distribution of aftershocks is asymmetrical with respect to the mainshock location and appears to be unilateral. The aftershocks of the M6.2 mainshock centralize in the northwest of M6.2 earthquake and the aftershocks of the M6.1 earthquake are in the southeast of the mainshock, moreover, the M6.1 earthquake appears to be another rupture on the southeastern extensiou of the same fault as the M6.2 earthquake. The results of Coulomb failure static stress changes △σf show that the earthquake on July 21 （M6.2） apparently triggered the earthquake on October 16 （M6.1）, the two mainshocks have stress triggering to their off-fault aftershocks to different extents, and the M6.5 earthquake that occurred in Yao＇an in 2000 also triggered the occurrence of the two Dayao earthquakes.

Explosions and seismic phenomena based on exciting of acoustic-electromagnetic waves

During earthquakes and strong underground explosions it is possible to observe two different effects. The first one is connected with the destruction of media, and this causes acoustic and later hybrid acoustic-electromagnetic waves in an epicenter in the atmosphere and in the ionosphere. Another one is connected with cracks in crystals of rocks, which seems more interesting, because it is possible to recognize the natural earthquakes and industrial explosions. In the first effects due to nonlinear elastic modules the acoustic waves move through the lithosphere and transform their spectra from VLF (very low frequencies ~ 1 - 10 kHz) at the depth of about 30 km into the lower part of ELF (extremely low frequencies, ~ 3 Hz - 1 kHz) on the Earth’s surface, then they pass the atmosphere and penetrate into the ionosphere. During the nonlinear acoustic passage through the atmosphere and the ionosphere, the spectrum transforms from ELF into ULF (ultra low frequencies, < 1 Hz) range. In this review article the classification of spectrum and analysis of two cases of the destruction of rocks in the lithosphere is presented. The rocks possess piezoelectric and piezomagnetic properties. In this case the electromagnetic emission is excited by the fracturing in plates of crystals. The difference of emission from piezoelectric and magnetic plates in cases of industrial explosions and natural seismic events including volcanic phenomena gives a possibility to analyze the method of its identification. The consideration is based on the model of the plate of a finite size with an uniformly moving crack.

Severe accident risks from external events

This paper reviews the early development of design requirements for seismic events in USA early developing nuclear electric generating fleet. Notable safety studies,including WASH-1400,Sandia Siting Study and the NUREG1150 probabilistic risk study,are briefly reviewed in terms of their relevance to extreme accidents arising from seismic and other severe accident initiators. Specific characteristic about the nature of severe accidents in nuclear power plant (NPP) are reviewed along with present day state-of-art analysis methodologies (methods for estimation of leakages and consequences of releases (MELCOR) and MELCOR accident consequence code system (MACCS)) that are used to evaluate severe accidents and to optimize mitigative and protective actions against such accidents. It is the aim of this paper to make nuclear operating nations aware of the risks that accompany a much needed energy resource and to identify some of the tools,techniques and landmark safety studies that serve to make the technology safer and to maintain vigilance and adequate safety culture for the responsible management of this valuable but unforgiving technology.

Intensive Observation of Reservoirinduced Seismicity and Preliminary Analysis on the Causes of Earthquakes in the Three Gorges Reservoir Area,Yangtze River

In accordance with the requirements of the National Key Technology R＆D Program of the 11th ＂Five-year Plan＂, a densified seismic network consisting of 26 seismic stations was established at the Three Gorges Reservoir area in the section of Hubei Province in March 2009 （21 short-period seismographs, 5 broadband digital seismographs）. From March to December, 2009, a total of 2,995 ML -0. 8 - 2. 9 earthquakes were detected during the trial impounding of the Three Gorges Reservoir （ water level rose from 145 m to 172.8m）. Using the double difference earthquake location algorithm, 2,837 earthquakes were precisely re-located. The results show that the pattern of small local earthquake swarms in the Three Gorges Reservoir area took on a linear distribution or mass-like cluster distribution, the mass-like clusters of events were generally within a distance of 5 km from waterfront, and the linear distribution of the earthquakes could be extended to a distance of 16 km away from the waterfront. In the Hubei section of the Three Gorges Reservoir, earthquakes were mainly concentrated in the northern end of the Xiannvshan and Jiuwanxi faults near the Xiangxihe River, and along the banks of the Yangtze River at the west of Xietan township and the Shenlongxi area on the northern bank in the Badong region, with focal depths less than 10km, and 4km in average. Earthquake frequency in the reservoir region had a positive correlation with reservoir water level fluctuations, indicating that the seismicity belongs to reservoir induced earthquakes. Along the Shenlong River in the reservoir area, earthquakes showed three linear distributions in the northern Badong county, and distributed according to Karst distribution. There are underground rivers in the carbonate strata. When the reservoir was impounded, water permeated into the underground rivers, thus inducing earthquakes. Earthquakes in the areas on the crossriver segment of Xiannvshan fault, the Jiuwanxi fault and at the areas west of Xietan, Shazhen and Xizhen, may be related to the softening of discontinuities, such as the Nukou fault, the Xiannvshan fault, or the bedding joints, which would lead to failure of rock masses, thus, inducing earthquakes. However, convincing conclusions about the triggering mechanism still need further study. Additionally, near the areas south of Wenhua and Yanglin of Zigui county and at Rangkou town east of Badong county, mininginduced earthquakes occurred at the mines nearby, and on the shores of the reservoir are some collapse earthquakes.

Multi-parametric analysis for forecasting volcanic and seismic phenomena

The history of the Etna volcano (Sicily) is marked by the alternation of brief intervals of eruptive activity and long periods of apparent inactivity. Usually during the periods of pseudo-inactivity, there are evident symptoms that the volcano is still active, e.g. fumaroles, hot springs and gaseous emissions. The variation of previous phenomena may be used to forecast seismic and volcanic- events. The aim of the research has been to evaluate and analyze the variation in emissions of ultra fine dusts and in soil Radon concentration, during the period of pseudo-inactivity, to obtain indexes able to forecast the possible events. Investigations have been carried out near two active faults on the slopes of Mt. Etna. The present paper presents the results of the monitoring survey, the analysis methodology and the development of the index to forecast possible seismic event. Research is currently in progress to refine the index which could be utilized not only to forecast possible seismic volcanic events, but also as a key to give early warning to the Civil Protection Agency so that they may be ready in time as soon as an eruptive event will occur.

地震事件分类识别软件

非天然地震事件分类是地震监测业务部门的日常工作之一。本研究主要针对地震、爆炸和矿震的分类问题,在地震波数据处理、特征提取和人工智能综合分类的研究基础上,基于Qt开发框架,结合Python、 Matlab等多种编程语言,开发了一个具有良好的可移植性和可扩展性、具有自主知识产权的地震分类识别软件。该软件可以部署在不同操作系统上,由七个模块组成:地震数据导入模块、数据处理模块、特征提取模块、综合分类模块、特征分析模块、当量估算模块和结果分析模块。软件集成了多种时频特征提取技术和人工智能分类方法,形成了较为完整的地震类型判定流程。软件内置的地震事件分类模型准确率高于90%,适用范围较广,已推广应用于多个地震监测部门,并取得了较好的应用成果,提高了对非天然地震的快速分析能力。

基于格拉姆角场和多尺度残差神经网络的地震事件分类方法

以江苏地震台网中心搜集并标注的天然地震、人工爆破和塌陷事件为试验数据样本,提出了一种基于格拉姆角场和多尺度残差神经网络的新的地震事件分类方法。首先对波形数据进行滤波、归一化等预处理,然后应用格拉姆角场对地震波形数据进行二维编码得到二维图像,再将此经过编码后的图像作为多尺度残差神经网络的输入进行分类模型的训练和测试,从而得出分类结果。采用上述方法对1078个天然地震台站记录、981个爆破台站记录和830个塌陷台站记录进行试验,结果显示:最终以单条波形为单位的地震事件分类准确率为92.55%,以单个台站为单位的分类准确率为96.36%,这表明基于格拉姆角场和多尺度残差神经网络的地震分类方法具有良好的效果。

地震沉积学中90°相位转换技术适用性的讨论

地震沉积学理论及应用技术研究中90°相位转换技术是地震沉积学研究的核心技术之一,但在实际研究与应用中,该技术并未取得如期的成果。在地震勘探原理中,地震反射同相轴代表着具有波阻抗差的地层界面。90°相位转换拟通过相移的方式,将代表着地层界面的地震反射同相轴赋于岩性地层的意义。由于地下地层的复杂多变,多种岩性频繁薄互层。利用薄互层地质模型的地震正演模拟结果,证实地震反射同相轴并不能与地层界面一一对应,因此,地震剖面中的同相轴是多套薄互层组合效应。受地震资料分辨力的限制,90°相位转换后的地震资料既不能将原始地震资料由界面型地震剖面转换成岩性地层剖面,也缺乏明确的地质含义,其在地震构造解释和地震岩性解释中的应用能力均很有限,所以很难称其为地震沉积学的应用技术。因此,将90°相位转换技术作为地震沉积学的核心技术并不合适,该技术无法提高地震资料分辨能力,也不能精确地将地震剖面转化成岩性剖面。

考虑地震事件内空间相关性的区域概率地震危险性分析

一次地震事件不同地点的地震动强度参数与结构响应参数之间是空间相关的,相关的地震效应将导致地震损失急剧集中与积累,从而造成灾难性事件,但是现有的区域尺度地震风险评估并没有考虑地震动的空间相关性。该文采用地统计学方法表征地震事件内地震动强度参数的空间相关性,提出了理论半变异函数的几何稳健估计方法,并采用加权最小二乘法实现对短距离空间相关性的准确高效评估。以新潟越冲地震为例,基于指数半变异函数模型,采用加权最小二乘法对几何稳健估计半变异函数值进行拟合,得到了PGA、PGV和0 s~10 s谱加速度的空间相关性函数,提出了变程b值的预测模型,结果表明:相比于短周期谱加速度,长周期谱加速度的空间相关性衰减速率更小,且在T=0.45 s处存在拐点。将所开发的空间相关性模型应用于区域概率地震危险性分析,采用Monte Carlo方法实现了空间相关随机场的区域地震动强度参数的模拟,改进了传统区域地震危险性分析的方法,发现绝大多数情况下考虑空间相关性的年超越概率大于未考虑空间相关性的年超越概率,表明考虑空间相关性对于建筑群落及基础设施风险评估、韧性城市建设等具有重要的指导意义。