Do some shallow earthquakes have a tensile source component?

The laboratory tests carried out in studying shear and tensile seismogenic displacements occurring in compressed samples, led us to search for earthquakes with a tensile source component. To determine this component in the seismic focus, a special procedure based on the construction of radiation patterns of the combined shear-tensile type is introduced.The criteria for selecting the events produced by the combined source mechanism are listed, and their limitations are mentioned. From the seismic zones with good azimuthal distribution of stations in the world seismic network nine earthquakes which occurred in the 6-year period 1976–1981 were analyzed; for these events better agreement of the observed and theoretical patterns was found for the combined shear-tensile source mechanism than for the pure double-couple mechanism. However, the share of the tensile component was always, found to be relatively small, ranging from 1 to 13 percent of the shear component. The comparison of the two solutions (double-couple vs. combined shear/tensile) is based on the first onset signs statistics.The results obtained indicate that tensile fracturing does not play a substantial role in the total amount of released seismic energy; on the orther hand, it is expected to be more important in the creation and development of focal zone morphology from both the instantaneous and long-term point of view.     

地震供能、储能与放能机制的研究

“双向应变结构”是孕育和发生地震可能的供能、储能与放能的一种机制。它既能缓慢地储存弹性势能与重力势能而孕育地震,又能突然快速地释放弹性势能与重力势能而发生地震。由于地震发生时孕震体是(原应变的)应变逆转的自破裂过程,因此当初始破裂发生后,在破裂的前沿区,地震剪切应力在原有应力集中的基础上,还会再次产生一个突然快速增强(使破裂加速)到快速减弱(使破裂减速到停止)的过程,即产生“应变逆转破裂效应”。这样,不仅会大大增加地震破裂前沿区的剪切应力,而且还会大大减小破裂中的阻力,可能使地震发生中的断层强度佯谬得到解释。“双向应变结构”破裂发生地震的具体物理机制,印证了震源体断层运动的复杂性：既产生了剪切错动的破裂运动,又产生了单向的拉张与单向的压缩运动,同时还产生了整体的膨胀与整体的压缩运动。     

Using Active Faults and Seismicity for the Strong Motion Modeling in the Eastern Rif (Northern Morocco)

— The aim of this study is to conduct a probabilistic seismic hazard analysis for the eastern Rif area in northern Morocco. The source zones were established on the basis of the seismicity database, the fracturing analysis deduced from Landsat7 ETM digital enhancement and marine seismic reflection profiles. By the use of this information together with the selected attenuation relationship, the peak ground acceleration contour maps are produced for specific return periods. The map has been divided into intervals of 0.1 degrees in both latitude and longitude to calculate the values at each grid point and draw the seismic hazard curves. The results of seismic hazard assessment are displayed as iso-acceleration contours expected to be exceeded during typical economic life times of structures.     

A double seismic antenna experiment at teide Volcano: existence of local seismicity and lack of evidences of Volcanic tremor

Data analyzed in the present work correspond to a 40 days field experiment carried out in Teide Volcano (Canary Islands, Spain) with two short-period small-aperture dense seismic antennas in 1994. The objective of this experiment was to detect, analyze and locate the local seismicity. We analyzed also the background seismic noise to investigate the possible presence of volcanic tremor. From a set of 76 events, we selected 21 of them in base of their good signal-to-noise ratio and their possibility to locate their seismic source by using the seismic antennas. A visual classification based on the S–P time and seismogram shape has permitted to establish three groups of events: local seismicity (S–P time between 3 and 5 s), very local earthquakes (S–P time smaller than 3 s) and artificial explosions. These earthquakes have been located by applying the Zero Lag Cross-Correlation technique and the inverse ray-tracing procedure. Those earthquakes that were recorded simultaneously by both seismic antennas were also located by intersecting both back-azimuths. The analysis of the seismicity has revealed that the amount of seismicity in Teide Volcano is moderate. This seismicity could be distributed in three main areas: inside the Caldera Edifice (below the Teide–Pico Viejo complex), in the eastern border of the Caldera Edifice and offshore of the island. At present, this activity is the only indicator of the volcano dynamics. The analysis of the back-ground seismic noise has revealed that at frequencies lower than 2 Hz, the Oceanic Load signal is predominant over other signals, even over local earthquakes with a magnitude of 2.0. Due to this, although if in the Teide area were present a weak volcanic tremor, or other volcanic signals with predominant peaks below 2 Hz, to observe them would be a very difficult task.     

二滩水库蓄水前天然地震活动本底背景

了解和掌握水库蓄水前库坝区及其周围天然地震背景水平,不仅可以对水库诱震的危险性进行前期评价,而且还可以为蓄水后的诱发地震活动监测提供可靠的鉴别依据。二滩水库遥测地震台网在水库蓄水前已连续运行了６年,积累了世界水库地震监测史上极为难得的资料。为此,本文利用这批资料提供了表征库坝区蓄水前天然地震活动水平确切实用的各项指标,同时也使用四川台网的地震资料,给出了包括库区的较大区域地震活动背景,讨论了对库区可能的影响。     

Radiated seismic energy and strain energy release in laboratory dynamic tensile fracture

Tensile dynamic fractures were propagated under two experimental congifurations for the purpose of assessing the relative amount of strain energy release that is consumed as fracture energy and radiated as seismic waves. The configurations used were (1) application of localized thermal stresses to 2.29 mm-thick plates of soda-lime glass and (2) double cantilever beam (DCB) experiments in 12.7 mm-thick glass plates, in which a fracture is propagated from a notch at one end of the specimen by application of a transverse load. Fracture propagation velocities of 0.35–2 mm/s were obtained for fractures in the first configuration. A capacitance transducer with a point-like probe was used for measuring the seismic displacement waveforms from propagating fracture sources. This transducer is capable of measuring absolute surface displacements with a resolution of 0.01 nm. It has a flat frequency response in the range 10 kHz to 6 MHz. Measured seismic efficiencies, or the ratio of radiated seismic energy to strain energy released, are in the range 10^–5 to 10^–3.     

Fault interaction and seismicity: Laboratory investigation and its seismotectonic interpretation

Systems of two parallel linear faults of the same length with the angle of inclination =45° were investigated under uniaxial linearly increasing load. Perspex plates were used as models. For each treated fault configuration the morphology of tensile cracks and the sequence of seismoacoustic events of shear and tensile origin were studied.It is shown that the seismic regime of a fault system is strongly influenced by the contact conditions on a fault plane; it is different in the faults with the aseismic contact, represented by open slits, and in the faults with the seismoactive contact, represented by filled slits, respectively.The experiments proved the dominating role of a fast shear displacement of the stick-slip type in the regime of seismic energy release of a fault system. The tensile crack generation seems to be only of little—if not negligible—importance. On the other hand, the existence of tensile cracks in a fault system can play an important role in the course of subsequent loading cycles because the stick-slip displacements can take place not only along the primary faults but also along the planes of tensile cracks.A comparison of some results of model experiments and the already published results of geological and seismological investigations indicated that the way of seismic energy relase on faults in nature and in the laboratory could be of the same character. Several analogies between the seismic regime of a fault model and of real seismic regions were found concerning the morphology of faults, off-fault fore- and aftershocks, and earthquake doublets, respectively.     

基于特大地震发生率的川滇地区地震危险性预测新模型

川滇地区是我国地震危险性较高的地区之一.本文基于对特大强震的风险性考虑,使用全球地震模型OpenQuake软件,建立了川滇地区地震危险性预测新模型.首先根据构造特征划分多个震源分区,并整理出这些震源分区内断层活动特征与滑动速率;基于震源分区和断层模型,使用GPS应变率转换成的锥形古登堡-里克特关系作为整个区域的地震积累率,并允许超过历史最大震级的特大地震的出现,结合活动断层滑动速率所积累的地震发生率,给出震源分区内断层地震源和背景地震源的地震发生率的比率分配关系;在活动断层分段上,保留了大型断裂或其主要部分,没有根据小的阶区来对断层进行详细分段,以便分配特大地震发生率;并使用地震率平滑方法分配背景地震发生率.最后在OpenQuake中加入地震动预测方程,计算出了川滇地区的PGA分布图,为区域地震危险性提供科学依据.     

Response spectral relationships for rock sites derived from the component attenuation model

The seismological model was developed initially from the fundamental relationship between earthquake ground motion properties and the seismic moment generated at the source of the earthquake. Following two decades of continuous seismological research in the United States, seismological models which realistically account for both the source and path effects on the seismic shear waves have been developed and their accuracy rigorously verified (particularly in the long and medium period ranges). An important finding from the seismological research by Atkinson and Boore and their co‐investigators is the similarity of the average frequency characteristics of seismic waves generated at the source between the seemingly very different seismic environments of Eastern and Western North America (ENA and WNA, respectively). A generic definition of the average source properties of earthquakes has therefore been postulated, referred to herein as the generic source model. Further, the generic ‘hard rock’ crustal model which is characteristic of ENA and the generic ‘rock’ crustal model characteristic of WNA have been developed to combine with the generic source model, hence enabling simulations to be made of the important path‐related modifications to ground motions arising from different types of crustal rock materials. It has been found that the anelastic contribution to whole path attenuation is consistent between the ENA and WNA models, for earthquake ground motions (response spectral velocities and displacements) in the near and medium fields, indicating that differences in the ENA and WNA motions arise principally from the other forms of path‐related modifications, namely the mid‐crust amplification and the combined effect of the upper‐crust amplification and attenuation, both of which are significant only for the generic WNA ‘rock’ earthquake ground motions. This paper aims to demonstrate the effective utilization of the latest seismological model, comprising the generic source and crustal models, to develop a response spectral attenuation model for direct engineering applications. The developed attenuation model also comprises a source factor and several crustal (wave‐path modification) component factors, and thus has also been termed herein the component attenuation model (CAM). Generic attenuation relationships in CAM, which embrace both ENA and WNA conditions, have been developed using stochastic simulations. The crustal classification of a region outside North America can be based upon regional seismological and geological information. CAM is particularly useful for areas where local strong motion data are lacking for satisfactory empirical modelling. In the companion paper entitled ‘response spectrum modelling for rock sites in low and moderate seismicity regions combining velocity, displacement and acceleration predictions’, the CAM procedure has been incorporated into a response spectrum model which can be used to effectively define the seismic hazard of bedrock sites in low and moderate seismicity regions. This paper and the companion paper constitute the basis of a long‐term objective of the authors, to develop and effectively utilize the seismological model for engineering applications worldwide.     

Ambiguous Moment Tensors and Radiation Patterns in Anisotropic Media with Applications to the Modeling of Earthquake Mechanisms in W-Bohemia

Anisotropic material properties are usually neglected during inversions for source parameters of earthquakes. In general anisotropic media, however, moment tensors for pure-shear sources can exhibit significant non-double-couple components. Such effects may be erroneously interpreted as an indication for volumetric changes at the source. Here we investigate effects of anisotropy on seismic moment tensors and radiation patterns for pure-shear and tensile-type sources. Anisotropy can significantly influence the interpretation of the source mechanisms. For example, the orientation of the slip within the fault plane may affect the total seismic moment. Also, moment tensors due to pure-shear and tensile faulting can have similar characteristics depending on the orientation of the elastic tensor. Furthermore, the tensile nature of an earthquake can be obscured by near-source anisotropic properties. As an application, we consider effects of inhomogeneous anisotropic properties on the seismic moment tensor and the radiation patterns of a selected type of micro-earthquakes observed in W-Bohemia. The combined effects of near-source and along-path anisotropy cause characteristic amplitude distortions of the P, S1 and S2 waves. However, the modeling suggests that neither homogeneous nor inhomogeneous anisotropic properties alone can explain the observed large non-double-couple components.The results also indicate that a correct analysis of the source mechanism, in principle, is achievable by application of anisotropic moment tensor inversion.     

Precursory seismic quiescence: A preliminary assessment of the hypothesis

Numerous cases of precursory seismic quiescence have been reported in recent years. Some investigators have interpreted these observations as evidence that seismic quiescence is a somewhat reliable precursor to moderate or large earthquakes. However, because failures of the pattern to predict earthquakes may not, in general, be reported, and because numerous earthquakes are not preceded by quiescence, the validity and reliability of the quiescence precursor have not been established.We have analyzed the seismicity rate prior to, and in the source region of, 37 shallow earthquakes (M 5.3–7.0) in central California and Japan for patterns of rate fluctuation, especially precursory quiescence. Nonuniformity in rate for these pre-mainshock sequences is relatively high, and numerous intervals with significant (p<0.10) extrema in rate are observed in some of the sequences. In other sequences, however, the rate remains within normal limits up to the time of the mainshock. Overall, in terms of an observational basis for intermediate-term earthquake prediction, no evidence is found in the cases studied for a systematic, widespread or reliable pattern of quiescence prior to the mainshocks.In earthquake sequences comprising full seismic cycles for 5 sets of (M 3.7–5.1) repeat earthquakes on the San Andreas fault near Bear Valley, California, the seismicity rates are found to be uniform. A composite of the estimated rate fluctuations for the sequences, normalized to the length of the seismic cycle, reveals a weak pattern of a low rate in the first third of the cycle, and a high rate in the last few months. While these observations are qualitative, they may represent weak expressions of physical processes occurring in the source region over the seismic cycle.Re-examination of seismicity rate fluctuations in volumes along the creeping section of the San Andreas fault specified by Wyss and Burford (1985) qualitatively confirms the existence of low-rate intervals in volumes 361, 386, 382, 372 and 401. However, only the quiescence in volume 386 is found by the present study to be statistically significant.     

Delineation of Potential Seismic Source Regions of Moderately Strong Earthquakes Without Seismogenic Tectonics

In this paper,approaches are developed to delineate the potential seismic source regions of moderately strong earthquakes that do not have clear seismotectonic settings.Based on comprehensive analysis of regional tectonic backgrounds and seismicity,the data,such as isoseisrnals,spatial distribution of after shocks,regional tectonic stress field,and focal mechanisms,are employed for the delineation of the potential seismic source regions.The reliability of such potential seismic source regions is also discussed.     

菏泽5.9级地震前后三项地震活动性参数的空间动态演化

用地震活动性参数（地震频度Ｎ，地震能量Ｅ和地震ｂ值）的二维空间等值线计算机扫描技术，系统地追踪了１９８３年菏泽５．９级地震前后三项地震活动性参数的空间动态演化情况。结果表明，菏泽５．９级地震前１－４年，河北平原断陷南部地区特别是其东界聊考断裂带地震活动异常增强，地震频度增高，地震能量释放加速而地震ｂ值降低；地震活动增强区的长度约２００余公里，未来主震发生于地震活动增强区的边缘；震后约一年左右震中周     

Application of digital image processing to the determination of spatial distribution of earthquakes

（陈锦标，沈萍，郑治真）Ａｐｐｌｉｃａｔｉｏｎｏｆｄｉｇｉｔａｌｉｍａｇｅｐｒｏｃｅｓｓｉｎｇｔｏｔｈｅｄｅｔｅｒｍｉｎａｔｉｏｎｏｆｓｐａｔｉａｌｄｉｓｔｒｉｂｕｔｉｏｎｏｆｅａｒｔｈｑｕａｋｅｓ￥Ｊｉｎ－ＢｉａｏＣＨＥＮ；ＰｉｎｇＳＨＥＮａｎｄＺ...     

Seismicity of Morocco for the Period 1987–1994

— A seismic data file of 3,740 earthquakes from January 1987 to December 1994 has been elaborated for Morocco and the border regions, with 10 main events registering magnitudes from 5 to 5.6. Such seismicity is particularly important for Morocco as the released seismic energy constitutes a considerable part of the total energy radiated during the 20th century. Relative seismicity maps confirm the persistence of the major features of the seismicity of Morocco. An important seismic activity is observed in the Alboran region continental crust, which absorbs the maximum deformation resulting from the convergence of the African and Iberian plates. However, in the longitude window 3.5°– 6° W at depths of 25 to 50 km, a seismic gap zone seems to take place. An explanation of this phenomenon may be provided by the slab breakoff model. Even if the seismicity of Morocco remains moderate, heavy damage is observed when the magnitude of earthquakes exceeds 4.5, especially in the case of traditional buildings.     

Numerical simulation of the earthquake generation process

A numerical algorithm is proposed for the simulation of the earthquake process during a seismic cycle. The algorithm is based on a heterogeneous discrete model of the fault plane and assumes there are two kinds of seismicity: background crack-like earthquakes and asperity-like events. An active zone of the fault contains an asperity distribution with a characteristic elementary area. The background seismicity randomly develops shear stress-free surfaces which tend to surround the asperities as in a 2D percolation process. The model parameters are taken from observations on the Vrancea (Romania) intermediate depth seismic region. The results emphasize the significant role of the geometry in the mechanism of the seismic failure. The algorithm predicts the nonlinear behavior in the frequency-magnitude distribution, the decrease of theb-slope associated with the asperity-like events, the magnitude range of major earthquakes, and their recurrence times.     

Seismicity and Seismic Hazard in Alexandria (Egypt) and its Surroundings

— Alexandria City has suffered great damage due to earthquakes from near and distant sources, both in historical and recent times. Sometimes the source of such damages is not well known. Seismogenic zones such as the Red Sea, Gulf of Aqaba-Dead Sea Hellenic Arc, Suez-Cairo-Alexandria, Eastern-Mediterranean-Cairo-Faiyoum and the Egyptian costal area are located in the vicinity of this city. The Egyptian coastal zone has the lowest seismicity, and therefore, its tectonic setting is not well known. The 1998 Egyptian costal zone earthquake is a moderate complex source. It is composed of two subevents separated by 4 sec. The first subevent initiated at a depth of 28 km and caused a rupture of strike (347°), dip (29°) and slip (125°). The second subevent occurred at a shallower depth (24 km) and has a relatively different focal parameter (strike 334°, dip 60° and slip 60°). The available focal mechanisms strongly support the manifestation of a complex stress regime from the Hellenic Arc into the Alexandria offshore area. In the present study a numerical modeling technique is applied to estimate quantitative seismic hazard in Alexandria. In terms of seismic hazard, both local and remote earthquakes have a tremendous affect on this city. A local earthquake with magnitude M_s = 6.7 at the offshore area gives peak ground acceleration up to 300 cm/sec². The total duration of shaking expected from such an earthquake is about three seconds. The Fourier amplitude spectra of the ground acceleration reveals that the maximum energy is carried by the low frequency (1–3 Hz), part of the seismic waves. The largest response spectra at Alexandria city is within this frequency band. The computed ground accelerations due to strong earthquakes in the Hellenic Arc, Red Sea and Gulf of Aqaba are very small (less than 10 cm/sec²) although with long duration (up to 3 minutes).     

埃及东部沙漠Gasco天然气公司工程场地大地构造参数和地震灾害评估

根据埃及Gasco天然气公司工程场地的现场调查资料、地震活动性和地震灾害评估结果,对该场地进行了评估.通过浅源地震波折射勘探,对8个地震剖面上的土壤和岩石的弹性动态常数进行了分析.场地地面模型包括两层,分析表明它们具有从中值到高值的弹性动态常数,说明场地有良好的土壤条件和基岩岩性.同时,我们搜集和分析了研究区周围150 km半径范围内的历史和仪器记录地震,从构造环境、地震活动性、地质构造和主要地震的断层面解等方面进行了研究,认为场地受4个震源区的影响:苏伊士中部海湾、开罗-苏伊士地区、开罗西南和Beni-Suef地区.我们采用地震活动性统计方法并结合专家判定,确定了这些震源区内最大地震的矩震级(MW).另外,依据场地峰值加速度(PGA)和反应谱,用随机方法对场地进行了地震灾害评估.4个震源区中,Beni-Suef地区距离工程场地最近,其PGA最大值为9.34cm/s2,但这个值很小,表明地震不会对工程场地产生破坏性影响.最后,我们还模拟了阻尼为1%、3%和5%时的伪加速度场地反应谱.     

Seismic regime of a single fault model

A special experimental technique enabled us to study in detail seismic events on a fault model in a uniaxial stress field. The recording system used made it possible to investigate the radiation pattern for all the events observed including the precise determination of the dislocation origin.The aim of the present paper is (1) to find the relation between shear and tensile displacements, (2) to check a possible influence of the process of tensile crack generation on the seismic energy release and (3) to compare the seismic regime of a single fault before and after the tensile crack generation, respectively.Results prove the dominating importance of shear mechanism for the seismic energy release. The tensile displacement can be seismoactive only under special contact conditions on the fault plane.The existence of tensile cracks at the fault tips changed the pattern of seismic energy radiation. This feature is probably caused by subsequent changes in contact conditions on the fault plane and in the stress field around the fault.A comparison of some results of the present model experiments with the already published results of geological and seismological measurements and investigations shows the analogous character in laboratory and in nature of the process of tectonic earthquake preparation, the displacement course on the fault during the earthquake and the manner of seismic energy release on faults.