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1.
Average source parameters for three large North Aegean events are obtained from body wave inversion for the moment tensor. The parameters for the events are as follows: The events exhibit dextral strike-slip faulting with theT axis striking NS and nearly horizontal, implying extension in this direction. The focal mechanisms obtained are in agreement with the seismotectonic regime of the North Aegean. It is known that the region is tectonically controlled by the existence of the strike-slip Anatolian fault and its westward continuation in the Aegean, as well as the NS extension the whole Aegean area undergoes.The components of the moment tensor show that the region is dominated by compression in the EW direction which is encompassed by extension in the NS direction. All the events were found to be shallow (10 km) with a source time function of approximately 8 s duration and small stress drop values.The teleseismic long period verticalP-waves exhibited distortions, that could be attributed to lateral inhomogeneities in the source structure or more probably to a nonflat water-crust interface.  相似文献   

2.
TeleseismicP-waves of some large earthquakes that occurred in the eastern Mediterranean region have been analysed by using an iterative maximum entropy technique in order to obtain the independent spectral parameters, the long-period spectral level 0 and the corner frequencyf 0 of the far-field displacement spectra.Based on these parameters, the seismic source parameters seismic momentM 0, source dimensionr, fault lengthl, average displacement u, shear stress drop , radiated energyE s and apparent stressn are calculated for the considered earthquakes by using Brune's and Madariaga's models.The striking feature of the source parameters obtained in this study is the low stress drop value which varies between 5 and 15 bars. If Madariaga's model had been used, higher stress drop values would have been obtained.The low stress drop earthquakes in the eastern Mediterranean region might be interpreted either by the possible presence of low strength material near the source or by the partial stress drop model.  相似文献   

3.
华北上地幔各向异性研究   总被引：9，自引：6，他引：3       下载免费PDF全文
对华北地震科学台阵的200个宽频带和甚宽带地震台站所记录的远震SKS(SKKS)波形资料作偏振分析,采用最小切向能量的网格搜索法和叠加分析方法求得每一个台站的SKS(SKKS)快波偏振方向和快、慢波的时间延迟,并结合已发表的固定台站的结果,获得了华北上地幔各向异性图像.从得到结果看,华北东部各向异性快波方向基本为NWW-SEE方向,而西部的快波方向转到NW-SE或NNW-SSE.快、慢波时间延迟范围是0.50~1.47 s,华北西部的平均快、慢波时间延迟小于华北东部.在华北东部,快波方向与绝对板块运动(APM)方向基本一致,预示了NWW向的软流圈地幔流是引起该区域上地幔各向异性的主要原因,它使得上地幔橄榄岩等晶体的晶格优势取向沿地幔物质流动方向,从而导致了NWW趋向的快波方向.然而,在稳定的西部,快波方向既不与绝对板块运动方向一致,也不与构造走向一致,这种弱各向异性很可能是遗留在古老克拉通的厚的岩石圈内的"化石"各向异性.  相似文献   

4.
We calculated focal mechanisms for 30 of the strongest events (1.5 M L 3.3) in distinct subregions of Vogtland/Western Bohemia between 1990 and 1998. Our investigations are concerned with events of the swarms near Bad Elster (1991), Haingrün (1991), Nový Kostel (1994 and 1997) and Zwickau (1998), two events from a group of earthquakes near Klingenthal (1997) and eight single events. Seismograms were provided by the digital station networks of the Geophysical Observatory of the University of Munich, the Technical University of Freiberg, the Academy of Sciences of the Czech Republic in Prague, the Masaryk University in Brno and some nearby stations of the German Regional Seismic Network (GRSN). To calculate focal mechanisms two inversion methods were applied. The inferred focal mechanisms do not show a simple, uniform pattern of seismic dislocation. All possible dislocation types – strike-slip, normal and thrust faulting - are represented. The prevailing mechanisms are normal and strike-slip faulting. Considerable differences in the fault plane solutions are noted for the individual subregions as well as in some cases among the individual events of a single swarm. For the Nový Kostel area we succeeded to resolve a change in the orientations of the nodal planes for the two successive swarms of December 1994 and January 1997. Besides this we also observe a change in the mechanisms, namely from strike-slip and normal faulting (December 1994) to strike-slip and thrust faulting (January 1997). Based on the inferred focal mechanisms the stress field was estimated. It turned out, that the dominating stress field in the region Vogtland/Western Bohemia does not substantially differ from the known stress field of West and Central Europe, being characterized by a SE-NW direction of the maximum compressive horizontal stress. We conclude that the seismicity in the Vogtland/Western Bohemia region is not predominantly caused by an independent local stress field, but rather controlled by the dominating stress regime in Central Europe.  相似文献   

5.
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6.
The South Iceland seismic zone is, roughly speaking, situated between two sections of the mid-Atlantic ridge, i.e., the Reykjanes Ridge southwest of Iceland and the Eastern Volcanic Zone on the island. It is a transform zone, where earthquakes are expected to occur on E-W-trending left-lateral shear faults, equivalent to conjugate, N-S-oriented right-lateral, rupture planes. In fact, earthquakes take place on en-échelon N-S-oriented faults, which is indicated by the distribution of main shock intensities, aftershocks as well as by surface fault traces. The stress field continuously generated in the fault zone by opening of the adjacent ridges is computed and superimposed on the stress field changes induced by a series of 13 earthquakes (M 6) between 1706 and 2000. The level of the pre-seismic stress field is analysed as well as the size of the area under high stress. Finally, the post-seismic stress field of June 2000 is analysed, to see where high stresses might have accumulated. The modelling indicates that the rupture planes located on separated parallel N-S-striking zones are dense enough to lead to an area-wide stress release by the series of events. The obtained pre-seismic stress level for most events is high and stable with the exception of situations when several strong shocks occur over a time span of several days, i.e., display typical main shock-aftershock patterns. The size of areas under high stress aside from of the rupture plane, i.e., where no event occurs at the specific time, is of medium to small size.  相似文献   

7.
Source parameter scaling relations are examined for microseismic events (–2.4M–0.3) occurring within higyly and moderately stressed and fractured rock masses at Strathcona mine, Sudbury, Canada. Insight into scaling is provided by waveform complexities, calculated rupture velocities, and maximum shear stresses based onin situ and numerical modelling data. The importance of normal stress on the failure process is also considered. Our results show that a strong dependence exists between stress release and seismic moment. An observed positive scaling in excess stress release (/2– a ) is consistent with the concept of overshoot. Rupture velocities ranging from 0.2 to 0.5 and waveform complexities less than 1.5 suggested that overshoot was related to healing behind a slowly advancing rupture front. Scaling in seismic efficiency paralleled that in apparent stress, implying that seismic stress release estimates are quasi-independent of the maximum shear stress. High levels of normal stress further supported the importance of high resisting stress in the observed overshoot behaviour and its role in the failure process.  相似文献   

8.
The rates and configuration of seismic deformation in the North Aegean trough-North Anatolian fault are determined from the moment tensor mechanisms of the earthquakes that occurred within this region. The analysis is based onKostrov's (1974) formulation. The fault plane solutions of the earthquakes of the period 1913–1983 withM s 6.0 are used. The focal mechanism of some of the past events (before 1960) is assumed, based on the present knowledge of the seismotectonics as well as on the macroseismic records of the area studied. The analysis showed that the deformation of the northern Aegean is dominated by EW contraction (at a rate of about 15 mm/yr) which is relieved by NS extension (at a rate of about 9 mm/yr). It was also shown that the northern part of North Anatolia (north of 39.7°N parallel) undergoes contraction in the EW direction (at a rate of about 9 mm/yr) and NS extension as the dominant mode of deformation (at a rate of about 5 mm/yr). It may be stated therefore, that the pattern of deformation of the northern Aegean and the northern part of North Anatolian fault is controlled by the NS extension the Aegean is undergoing as a whole, and the dextral strike-slip motion of the North Anatolian fault. The southern part of North Anatolia is undergoing crustal thinning at a rate of 2.3 mm/yr, NS extension (at a rate of 5 mm/yr) as well as EW extension (at a rate of 4 mm/yr), which are consistent with the occurrence of major normal faulting and justify the separation of North Anatolia into two separate subareas.  相似文献   

9.
A number of uncertainties of forecasts of changes in the annual runoff depths at global scale, obtained using information on results of integration of 21 IPCC climate models is studied. Following possible errors of these forecasts are calculated: errors of models; differences between main (IPCC) scenarios of emission of greenhouse gases in the atmosphere and resultant changes of global temperatures; mistakes in estimates of average long-term observed values of the runoff depths for the “control” period. Global maps of a “significance index” of forecasted changes in the runoff depths (estimations of changes in the annual runoff depths divided by mean square root values of errors of these estimations) for 2025, 2050, 2075 and 2100 are presented. It is shown that the most significant global changes of the runoff depths (growth in the north of Eastern Siberia, of the Russian Far East, of North America, falling in the “Greater Mediterranean Region”) are predicted for the second quarter of 21st century. Further changes of the runoff amplify only in the Amazon basin (reduction, by 2075). Almost everywhere else (including almost all European territory of Russia, Western Siberia, south of Eastern Siberia and of the Far East) the significance of changes in the runoff depths during 21st century is negligible.  相似文献   

10.
Summary Among various trial models ofP-wave travel times, there exists at least one model which best predicts the times of first arrivals from a certain region at a set of recording stations even if no attempt is made to correct the travel-time standards against known station errors and source bias. In teleseismic distance range (25°100°) and surface-focus case, the possibility of statistically establishing which of the twoP models, viz. Jeffreys-Bullen and Herrin, is more appropriate for each of the four source regions, viz. Southern Nevada, Aleutian Islands, Eastern Kazakh and Novaya Zemlya, is discussed in this paper. Data corresponding to a set of underground explosions from these regions form the basis of such an analysis. The Herrin model is found to be better applied to Aleutian Islands region while the Jeffreys-Bullen model seems more appropriate for each of the remaining three regions.As a result of the study of the travel-time models, numerical estimates of space and time corrections pertaining to the above mentioned regions, based on the most appropriate model and directly applicable to the computed source parameters, are obtained. On applying these corrections, the size of source location error ellipse and the source-time error reduce to very small representative values, viz. 4 km×6 km (area 75 km2) and ±0.2 sec respectively.  相似文献   

11.
In a previous paper (Makropoulos andBurton, 1983) the seismic risk of the circum-Pacific belt was examined using a whole process technique reduced to three representative parameters related to the physical release of strain energy, these are:M 1, the annual modal magnitude determined using the Gutenberg-Richter relationship;M 2, the magnitude equivalent to the total strain energy release rate per annum, andM 3, the upper bound magnitude equivalent to the maximum strain energy release in a region.The risk analysis is extended here using the part process statistical model of Gumbel's IIIrd asymptotic distribution of extreme values. The circum-Pacific is chosen being a complete earthquake data set, and the stability postulate on which asymptotic distributions of extremes are deduced to give similar results to those obtained from whole process or exact distributions of extremes is successfully checked. Additionally, when Gumbel III asymptotic distribution curve fitting is compared with Gumbel I using reduced chi-squared it is seen to be preferable in all cases and it also allows extensions to an upper-bounded range of magnitude occurrences. Examining the regional seismicity generates several seismic risk results, for example, the annual mode for all regions is greater thanm(1)=7.0, with the maximum being in the Japan, Kurile, Kamchatka region atm(1)=7.6. Overall, the most hazardous areas are situated in this northwestern region and also diagonally opposite in the southeastern circum-Pacific. Relationships are established between the Gumbel III parameters and quantitiesm 1(1),X 2 and , quantities notionally similar toM 1,M 2 andM 3 although is shown to be systematically larger thanM; thereby giving a physical link through strain energy release to seismic risk statistics. Inall regions of the circum-Pacific similar results are obtained forM 1,M 2 andM 3 and the notionally corresponding statistical quantitiesm 1(1),X 2 and , demonstrating that the relationships obtained are valid over a wide range of seismotectonic enviroments.  相似文献   

12.
ＣｏｍｐｒｅｈｅｎｓｉｖｅａｎａｌｙｓｅｓｏｆｓｅｉｓｍｉｃｓｏｕｒｃｅｌａｙｅｒｉｎＸｉｎｇｔａｉａｎｄＴａｎｇｓｈａｎｓｅｉｓｍｉｃｒｅｇｉｏｎｓａｎｄｔｈｅｃｏｎｄｉｔｉｏｎｓｏｆｍｅｄｉａａｂｏｖｅａｎｄｂｅｌｏｗｔｈｉｓＬａｙｅｒＴＯＮＧ...  相似文献   

13.
The composite stereographic projection of orientations of the compression and tension axes using thirty-nine fault-plane solutions of earthquakes from two active seismogenic sources of Nepal and adjoining areas were examined and the nature of stress pattern and their influence on tectonics in the region have been studied. The seismogenic source located in Eastern Nepal region, which has been the site of 1934 Bihar-Nepal great earthquake of M 8.4, is presently experiencing N-S to NE-SW directed compressive stresses. The inferred pattern of compression axes in Western Nepal region suggests a shallow compressive stress, dipping N-S to NE-SW. Approximately similar nature of the stress regime is observed in Western and Eastern regions of Nepal, separated by nearly 700 km; it shows N-S to NNE-SSW direction of compression and underthrusting of the Indian Plate beneath the Himalaya at a shallow angle. Present study indicates that the stress is being released along the strikes of some of the transverse faults present in the region since the compressive stress exerted by the northward movement of the Indian Plate is approximately perpendicular to the Himalayan collision belt. Unilateral stress pattern generated by the northward movement of the Indian Plate in the central part of the Himalaya reveals that the present day collision occurs roughly perpendicular to the local strike of the Himalaya.  相似文献   

14.
To evaluate the tectonic significance of the October 20, 1986 Kermadec earthquake (M w =7.7), we performed a comprehensive analysis of source parameters using surface waves, body waves, and relocated aftershocks. Amplitude and phase spectra from up to 93 Rayleigh waves were inverted for centroid time, depth, and moment tensor in a two-step algorithm. In some of the inversions, the time function was parameterized to include information from the body-wave time function. The resulting source parameters were stable with respect to variations in the velocity and attenuation models assumed, the parameterization of the time function, and the set of Rayleigh waves included. The surface wave focal mechanism derived (=275°, =61°, =156°) is an oblique-compressional mechanism that is not easy to interpret in terms of subduction tectonics. A seismic moment of 4.5×1020 N-m, a centroid depth of 45±5 km, and a centroid time of 13±3 s were obtained. Directivity was not resolvable from the surface waves. The short source duration is in significant contrast to many large earthquakes.We performed a simultaneous inversion ofP andSH body waves for focal mechanism and time function. The focal mechanism agreed roughly with the surface wave mechanism. Multiple focal mechanisms remain a possibility, but could not be resolved. The body waves indicate a short duration of slip (15 to 20 s), with secondary moment release 60s later. Seismically radiated energy was computed from the body-wave source spectrum. The stress drop computed from the seismic energy is about 30 bars. Sixty aftershocks that occurred within three months of the mainshock were relocated using the method of Joint Hypocentral Determination (JHD). Most of the aftershocks have underthrusting focal mechanisms and appear to represent triggered slip on the main thrust interface. The depth, relatively high stress drop, short duration of slip, and paucity of true aftershocks are consistent with intraplate faulting within the downgoing plate. Although it is not clear on which nodal plane slip occurred, several factors favor the roughly E-W trending plane. The event occurred near a major segmentation in the downgoing plate at depth, near a bend in the trench, and near a right-lateral offset of the volcanic are by 80 km along an E-W direction. Also, all events in the region from 1977 to 1991 with CMT focal mechanisms similar to that of the Mainshock occurred near the mainshock epicenter, rather than forming an elongate zone parallel to the trench as did the aftershock activity. We interpret this event as part of the process of segmentation or tearing of the subducting slab. This segmentation appears to be related to the subduction of the Louisville Ridge, which may act as an obstacle to subduction through its buoyancy.  相似文献   

15.
From focal mechanism solutions of the earthquakes in the northern part of North China during the period of 2002～2006,the directions of principal stress axes in 4 stress sub-districts are analyzed using a grid test method.The characteristic of present crustal stress directions is discussed.Based on this result and on the focal mechanism solutions calculated for some events in the period of 1977～1998,in combination with some other study results,the temporal variation of present crustal stress directions in the northern part of North China is investigated.The re-sults confirm that the direction of crustal principal stress in some regions had somewhat rotated after the 1976 Tangshan M7.8 earthquake.The mean P axes of the focal mechanism solutions rotated clockwise not only in Tangshan sub-district,but also in Beijing and Xingtai sub-districts after the Tangshan earthquake.In Beijing and Xingtai sub-districts the orientations of principal stress axes in the period of 2002～2006 are consistent with that before the Tangshan earthquake,implying that the stress orientations has rotated back to the state before the Tang-shan earthquake in these two sub-districts.The directions of the mean P axes are nearly E-W in Tangshan sub-dis-trict since the M7.8 earthquake.The present stress field in the sub-district northwest to Beijing,or in the western part of the Zhangjiakou-Bohai fault zone,is relatively stable during the time period concerned in this study.Because of the limitation of data,this paper only states a possible variation of stress field in the northern part of North China in the recent decades.  相似文献   

16.
Earthquake Triggering along the Xianshuihe Fault Zone of Western Sichuan,China   总被引：19，自引：0，他引：19  
Western Sichuan is among the most seismically active regions in southwestern China and is characterized by frequent strong (M 6.5) earthquakes, mainly along the Xianshuihe fault zone. Historical and instrumental seismicity show a temporal pattern of active periods separated by inactive ones, while in space a remarkable epicenter migration has been observed. During the last active period starting in 1893, the sinistral strike–slip Xianshuihe fault of 350 km total length, was entirely broken with the epicenters of successive strong earthquakes migrating along its strike. This pattern is investigated by resolving changes of Coulomb failure function (CFF) since 1893 and hence the evolution of the stress field in the area during the last 110 years. Coulomb stress changes were calculated assuming that earthquakes can be modeled as static dislocations in an elastic halfspace, and taking into account both the coseismic slip in strong (M 6.5) earthquakes and the slow tectonic stress buildup associated with major fault segments. The stress change calculations were performed for faults of strike, dip, and rake appropriate to the strong events. We evaluate whether these stress changes brought a given strong earthquake closer to, or sent it farther from, failure. It was found that all strong earthquakes, and moreover, the majority of smaller events for which reliable fault plane solutions are available, have occurred on stress–enhanced fault segments providing a convincing case in which Coulomb stress modeling gives insight into the temporal and spatial manifestation of seismic activity. We extend the stress calculations to the year 2025 and provide an assessment for future seismic hazard by identifying the fault segments that are possible sites of future strong earthquakes.  相似文献   

17.
We have determined the rupture history of the March 28, 1964, Prince Williams Sound earthquake (M w=9.2) from long-period WWSSNP-wave seismograms. Source time functions determined from the long-periodP waves indicate two major pulses of moment release. The first and largest moment pulse has a duration of approximately 100 seconds with a relatively smooth onset which reaches a peak moment release rate at about 75 seconds into the rupture. The second smaller pulse of moment release starts at approximately 160 seconds after the origin time and has a duration of roughly 40 seconds. Because of the large size of this event and thus a deficiency of on-scale, digitizableP-wave seismograms, it is impossible to uniquely invert for the location of moment release. However, if we assume a rupture direction based on the aftershock distribution and the results of surface wave directivity studies we are able to locate the spatial distribution of moment along the length of the fault. The first moment pulse most likely initiated near the epicenter at the northeastern down-dip edge of the aftershock area and then spread over the fault surface in a semi-circular fashion until the full width of the fault was activated. The rupture then extended toward the southwest approximately 300 km (Ruff andKanamori, 1983). The second moment pulse was located in the vicinity of Kodiak Island, starting at 500 km southwest of the epicenter and extending to about 600 km. Although the aftershock area extends southwest past the second moment pulse by at least 100 km, the moment release remained low. We interpret the 1964 Prince William Sound earthquake as a multiple asperity rupture with a very large dominant asperity in the epicentral region and a second major, but smaller, asperity in the Kodiak Island region.The zone that ruptured in the 1964 earthquake is segmented into two regions corresponding to the two regions of concentrated moment release. Historical earthquake data suggest that these segments behaved independently during previous events. The Kodiak Island region appears to rupture more frequently with previous events occurring in 1900, 1854, 1844, and 1792. In contrast, the Prince William Sound region has much longer recurrence intervals on the order of 400–1000 years.  相似文献   

18.
The western part of the Corinth Gulf attracts attention because of its seismically active complex fault system and considerable seismic hazard. Close to the city of Aegion, damaged by the M L 6.2 earthquake of 1995, a sequence of small earthquakes occurred from February to May 2001. The sequence, comprising 171 events of M L 1.8 to 4.7, was recorded by a short-period network of the University of Patras, PATNET. As most stations have single component-recording, the S-wave arrival time readings were scarce. A sub-set of 139 events was recorded by at least 5 stations, and in this study we limit ourselves just to that sub-set. A preliminary location is performed by a standard linearized kinematic approach, with several starting depths and crustal models. Then the mainshock is re-located, and finally it is used as a master event to locate the remaining events. The mainshock relocation is performed by a systematic 3D grid search, and the trade-off between depth and origin time is eliminated by a special procedure, the so-called station difference (SD) method. In the SD method, instead of inverting arrival times directly, their intra-station differences are employed. The station corrections, determined from the master event, are also used. As a result, the sub-set is imaged as a relatively tight cluster, occupying space of about 5 by 5 km horizontally and 10 km vertically, with the mainshock inside (at a depth of 7 km). The results should be interpreted with caution, mainly as regards the absolute depth position of the cluster. A more accurate location would require a local network with both P and S readings.  相似文献   

19.
中国大陆地壳应力场与构造运动区域特征研究   总被引：41，自引：16，他引：25       下载免费PDF全文
系统研究了1918～2006年间中国大陆及其周缘发生的3115个M4.6以上中、强地震的震源机制解,得到中国大陆地壳区域应力场的压应力轴和张应力轴空间分布的统计结果.探讨了大陆应力场的结构,以及周围板块运动对中国大陆应力场影响作用范围及其界线.结果表明,中国东部的华北地区受到太平洋板块向欧亚板块俯冲挤压的同时,又受到从贝加尔湖经过大华北直至琉球海沟的广阔范围内存在的方位为170°引张应力场的控制.华北地区大地震的震源机制解反映出,该区地震发生为NEE向挤压应力和NNW向张应力的共同作用结果.印度洋板块向欧亚板块的碰撞挤压运动所产生的强烈的挤压应力,控制了喜马拉雅、青藏高原、乃至延伸到天山及其以北的广大地区.在青藏高原周缘地区和中国西部的大范围内,压应力P轴水平分量位于20°~40°,形成了近北东方向的挤压应力场,大量逆断层型强震集中发生在青藏高原的南、北和西部周缘地区以及天山等地区. 本文结果表明,正断层型地震集中发生在青藏高原中部高海拔的地区.证明了青藏高原周缘区域发生南北向强烈挤压短缩的同时,中部高海拔地区存在着明显的近东西向的扩张运动.根据本文最新结果,得到了华北、华南块体之间地壳区域应力场的控制边界线,发现该分界线与大地构造、岩石圈板块构造图等有较大差异,特别是在大别及其以东地区, 该分界线向东南偏转,在沿海的温州附近转向东,最终穿过东海直至琉球海沟.台湾纵谷断层是菲律宾海板块与欧亚板块之间碰撞挤压边界,来自北西西向运动的菲律宾海板块构造应力控制了从台湾纵谷、华南块体,直到中国南北地震带南段东部地域的应力场. 地震震源机制结果还表明,南北地震带南段西侧其P轴大约为NNE方向,与青藏高原的P轴方位一致.南北地震带南段东侧其P轴大约为NWW方向,与华南块体的P轴方位一致.因此,将中〖JP2〗国大陆分成东、西两部分的南北地震带南段是印度洋板块与菲律宾海板块在中国大陆内部影响控制范围的分界线.  相似文献   

20.
Summary The disturbances of the velocity and magnetic fields close to the Earth's core-mantle boundary, caused by sudden irregular changes in the Earth's rotation, are investigated. The problem leads to the investigation of the structure of the Ekman-Hartman hydromagnetic boundary layer, the magnetic diffusive region and the currentless region. Precise Laplacean inversions of the images of all disturbances in the Earth's core-mantle system are obtained for the limiting case of a zero magnetic Prandtl number, =0. The disturbance of the velocity in the direction of the axis of rotation (Ekman suction) in the currentless region has the nature of inertial oscillations with a frequency of 2. Additional disturbances (with respect to the case of =0) of the velocity in the azimuthal and radial directions, particularly for the EHL and MDR region, are determined for the case when 0< « 1. The disturbance in the velocity again has the character of inertial oscillations with the frequency 2, being exponentially damped in EHL asexp (–22t) and in MDR asexp (–2t).  相似文献   

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