Regional stresses along the Eurasia-Africa plate boundary derived from focal mechanisms of large earthquakes

The focal mechanism solutions of 83 European earthquakes withM>6, selected from a total of 140, have been used to derive the directions of the principal axes of stress along the plate boundary between Eurasia and Africa from the Azores islands to the Caucasus mountains. Along most of the region, the horizontalP-axes are at an angle of 45° to 90° with the trend of the plate boundary. HorizontalT-axes are concentrated in central Italy and northern Greece in association with normal faulting. Large strike-slip motion of right-lateral character takes place at the center of the Azores-Gibraltar fault and the North Anatolian fault. From Gibraltar to the Caucasus the boundary is complicated by the presence of secondary blocks and zones of extended deformations with earthquakes spread over wide areas. Intermediate and deep earthquakes are present at four areas with arc-like structure, namely, Gibraltar, Sicily-Calabria, Hellenic arc and Carpathians.     

Space-time model for migration of weak earthquakes along the northern boundary of the Amurian microplate

In this paper, we aimed to investigate the statistical distributions of shallow earthquakes with 2 ≤ М ≤ 4, located in 13 rectangular areas (clusters) bounded by 120°E and 144°E along the northern boundary of the Amurian microplate. As a result of our study, the displacement of seismicity maxima has been determined and three recurrent spatial cycles have been observed. The clusters with similar distribution of earthquakes are suggested to alternate being equally spaced at 7.26° (360–420 km). A comparison of investigation results on the structure of seismicity in various segments of the Amurian microplate reveals the identity between the alternation pattern observed for meridional zones of large earthquakes and a distinguished spatial period. The displacement vector for seismicity in the annual cycles is determined, and the correspondence between its E-W direction and the displacement of the fronts of large earthquakes is established. The elaborated model of seismic and deformation processes is considered, in which subsequent activation of clusters of weak earthquakes (2 ≤ М ≤ 4), tending to extend from the Japanese-Sakhalin island arc to the eastern closure of the Baikal rift zone, is initiated by the displacement of the strain wave front.     

龙门山断裂带中北段大震复发特征与复发间隔估计

汶川MS8.0地震发生在青藏高原东缘著名的龙门山断裂带上,造成了从映秀、北川至南坝长约240km的同震地表破裂带．然而目前关于龙门山断裂带的大震复发特征研究较少．通过地震地质科学考察和断层断错地貌的差分GPS测量,发现第一级河流阶地、河床和河漫滩上的垂直断距大致相当,均代表汶川地震的位错,而第二级河流阶地上的累计位移大致是最新地震垂直位移的2倍．利用断错地貌、地震矩率和滑动速率3种方法,分别估算了龙门山断裂带大地震的复发间隔．结果表明:龙门山断裂带中北段可能发生与汶川大地震相当的地震,大震复发符合特征地震模型;大震复发间隔为3000——6000a．该结果可为龙门山断裂带的大震预测和地震危险性评价等研究提供重要的定量数据．      

郯庐断裂带沈阳段地震精定位研究

作为中国东部的一条巨型走滑型断裂带,郯庐断裂带具有漫长的发育历史,对我国东部矿产资源分布、地质灾害的形成与发生具有重要的控制作用.作为辽宁地区主要区域性活动构造断裂,郯庐断裂沈阳段与周围其他构造体系相互作用,共同控制着该地区强震活动.2013年灯塔MS 5.1地震打破了辽宁地区长达13年的5级地震平静,标志着该区进入新...     

Space-time migration of earthquakes along the North Anatolian fault zone and seismic gaps

The North Anatolian fault is a well-defined tectonic feature extending for 1400 km across Northern Turkey. The space-time distribution of seismicity and faulting of this zone has been examined with a particular emphasis on the identification of possible seismic gaps. Results suggest several conclusions with respect to the temporal and spatial distribution of seismicity. First, the earthquake activity appears not to be stationary over time. Periods of high activity in 1850–1900 and 1940 to the present bracket a period of relatively low activity in 1910–39. Second, there appears to have been a two-directional migration of earthquake epicenters away from a central region located at about 39°E longitude. The migration to the west has a higher velocity (>50 km/yr) than the migration to the east (10km/yr). The faulting associated with successive earthquakes generally abuts the previous rupture. Some existing gaps were filled by later earthquakes.At present there are two possible seismic gaps along the North Anatolian fault zone. One is at the western end of the fault, from about 29° to 30°E. Unless this is a region of ongoing aseismic creep, it could be the site of a magnitude 6 or greater earthquake. The other possible gap is at the eastern end, from about 42° to 43°E, to the west of the unexpected M=7.3 event of 24 November 1976.     

Slow earthquakes and great earthquakes along the Nankai trough

We have reexamined reports indicating that slow deformation occurred before the great Japan earthquakes of 1944 (Tonankai) and 1946 (Nankaido) and find that the observations are well founded. Although no quantitative models have previously been proposed to explain all of the relevant data we show that they are satisfied by a simple model for both earthquakes. The model, based on known properties of subduction zones, has slow slip on the subduction interface in an area deeper than the seismic rupture. If this model is correct and a similar physical situation holds for an anticipated Tokai earthquake, existing instruments will be able to reveal the pre-slip in real time. While differences among the deformation time series at different sites will provide strong constraints on the slow rupture propagation, these differences could result in delaying the recognition of a coherent event.     

On similarity solution of an unsteady laminar boundary layer along a flat plate

Summary A unified analysis has been made to obtain all possible similarity solutions of the steady and unsteady, forced flow, inside a boundary layer along a flat plate. Though previously, attempts were made to obtain similarity solutions of a steady boundary layer flow neglecting viscous dissipation term in the energy conservation equation but the treatments were not complete. Here we have taken account of the viscous dissipation term. In the steady case it has been shown that for a similarity solution of both velocity and temperature, there should be a relation between the undisturbed flow outside the boundary layer and the temperature of the plate. It has been shown that the similarity solution exists in the unsteady case if we neglet the viscous dissipation term in the energy equation.     

板块地震迁移链与汶川地震

从1956年开始沿印度洋—亚洲和太平洋板块边界的地震迁移触发了2008年5月12日M8.0中国汶川地震。迁移链为东西两条,彼此相向迁移,在缅甸弧附近的中国大陆汇合,并触发地震。迁移持续约50年。与此相类似的迁移发生在1902—1957年,迁移触发了1950年西藏东部M8.5地震和1957年蒙古M8.3地震。预计1956—2008地震迁移可能尚未结束,可能会继续触发大地震。     

The fissure swarm of the Askja volcanic system along the divergent plate boundary of N Iceland

Divergent plate boundaries, such as the one crossing Iceland, are characterized by a high density of subparallel volcanic fissures and tectonic fractures, collectively termed rift zones, or fissure swarms when extending from a specific volcano. Volcanic fissures and tectonic fractures in the fissure swarms are formed during rifting events, when magma intrudes fractures to form dikes and even feeds fissure eruptions. We mapped volcanic fissures and tectonic fractures in a part of the divergent plate boundary in northern Iceland. The study area is ~1,800 km², located within and north of the Askja central volcano. The style of fractures changes with distance from Askja. Close to Askja the swarm is dominated by eruptive fissures. The proportion of tectonic fractures gets larger with distance from Askja. This may indicate that magma pressure is generally higher in dikes close to Askja than farther away from it. Volcanic fissures and tectonic fractures are either oriented away from or concentric with the 3–4 identified calderas in Askja. The average azimuth of fissures and fractures in the area deviates significantly from the azimuth perpendicular to the direction of plate velocity. As this deviation decreases gradually northward, we suggest that the effect of the triple junction of the North American, Eurasian and the Hreppar microplate is a likely cause for this deviation. Shallow, tectonic earthquakes in the vicinity of Askja are often located in a relatively unfractured area between the fissure swarms of Askja and Kverkfjöll. These earthquakes are associated with strike-slip faulting according to fault plane solutions. We suggest that the latest magma intrusions into either the Askja or the Kverkfjöll fissure swarms rotated the maximum stress axis from being vertical to horizontal, causing the formation of strike-slip faults instead of the dilatational fractures related to the fissure swarms. The activity in different parts of the Askja fissure swarm is uneven in time and switches between subswarms, as shown by a fissure swarm that is exposed in an early Holocene lava NW of Herðubreið but disappears under a younger (3500–4500 BP) lava flow. We suggest that the location of inflation centres in Askja central volcano controls into which part of the Askja fissure swarm a dike propagates. The size and amount of fractures in the Kollóttadyngja lava shield decrease with increasing elevation. We suggest that this occurred as the depth to the propagating dike(s) was greater under central Kollóttadyngja than under its flanks, due to topography.     

Regionality of deep low-frequency earthquakes associated with subduction of the Philippine Sea plate along the Nankai Trough, southwest Japan

The Fukuoka District Meteorological Observatory recently logged three possible deep low-frequency earthquakes (LFEs) beneath eastern Kyushu, Japan, a region in which LFEs and low-frequency tremors have never before been identified. To assess these data, we analyzed band-pass filtered velocity seismograms and relocated LFEs and regular earthquakes using the double-difference method. The results strongly suggest that the three events were authentic LFEs, each at a depth of about 50 km. We also performed relocation analysis on LFEs recorded beneath the Kii Peninsula and found that these LFEs occurred near the northwest-dipping plate interface at depths of approximately 29–38 km. These results indicate that LFEs in southwest Japan occur near the upper surface of the subducting Philippine Sea (PHS) plate. To investigate the origin of regional differences in the occurrence frequency of LFEs in western Shikoku, the Kii Peninsula, and eastern Kyushu, we calculated temperature distributions associated with PHS plate subduction. Then, using the calculated thermal structures and a phase diagram of water dehydration for oceanic basalt, the water dehydration rate (wt.%/km), which was newly defined in this study, was determined to be 0.19, 0.12, and 0.08 in western Shikoku, the Kii Peninsula, and eastern Kyushu, respectively; that is, the region beneath eastern Kyushu has the lowest water dehydration rate value. Considering that the Kyushu–Palau Ridge that is subducting beneath eastern Kyushu is composed of tonalite, which is low in hydrous minerals, this finding suggests that the regionality may be related to the amount of water dehydration associated with subduction of the PHS plate and/or differences in LFE depths. Notable dehydration reactions take place beneath western Shikoku and the Kii Peninsula, where the depth ranges for dehydration estimated by thermal modeling agree well with those for the relocated LFEs. The temperature range in which LFEs occur in these regions is estimated to be 400–500 °C.     

Tectonics of the Nazca-Antarctic plate boundary

We have constructed a new bathymetric chart of part of the Chile transform system, based mainly on an R/V “Endeavor” survey from 100°W to its intersection with the East Ridge of the Juan Fernandez microplate at 34°30′S, 109°15′W. A generally continuous lineated trend can be followed through the entire region, with the transform valley being relatively narrow and well-defined from 109°W to approximately 104°30′W. The fracture zone then widens to the east, with at least two probable en echelon offsets to the south at 104° and 102°W. Six new strike-slip mechanisms along the Chile Transform and one normal fault mechanism near the northern end of the Chile Rise, inverted together with other plate motion data from the eastern portion of the boundary, produce a new best fit Euler pole for the Nazca-Antarctic plate pair, providing tighter constraints on the relative plate motions.     

龙门山断裂带映秀-北川断裂的古地震及其复发间隔初步研究(英文)

The activity of the Longmenshan thrust belt has concentrated mainly on its Wenchuan-Maoxian Fault, Yingxiu-Beichuan Fault, and Guanxian-Jiangyou fault since the Quaternary time.     

Physicochemical mechanics of large crustal earthquakes

We examine the geological factors that control the precursory processes of earthquakes and the occurrence of low-and high-magnitude seismicity. Degassing and mutual displacements of lithospheric blocks are under discussion as the leading factors. Three scenarios for seismotectonic processes are discussed, these scenarios incorporate the occurrence of physicochemical reactions in solids and fluids in three dimensions. The precursory processes of large crustal earthquakes are related to block boundaries being locked due to the physicochemical reactions. Possibilities for earthquake prediction and sesimicity modification are discussed.     

Seismicity in the generalized vicinity of large earthquakes

Earthquake catalogs are used to construct a generalized space-time vicinity of large earthquakes and to investigate the seismicity behavior in such areas. We investigate the character of the inverse cascade (seismicity rate increasing toward the time of the main shock) and of the aftershocks, as well as the lower seismicity increase relevant to the larger vicinity of the main shock. It is shown that the inverse and aftershock cascades are accompanied by several anomalies indicating the development of a precursory strength decrease in this space-time volume; this consists in decreased apparent stresses, an increased relative contribution of low frequency oscillations into the earthquake spectrum, and an increased correlation (homogeneity) of stress and strain. We emphasize the special significance of anomalies that also characterize the physical nature of the ongoing process, in addition to the existence of a strength decrease. An example of such an anomaly is provided     

地震预报首先实现"救人"的战略思考 ——临震预报可能性与实现的途径分析

美国学者盖勒认为地震无法预测,该观点在地震界占据上风.但是,我国曾在1975年海城7.3级地震前成功预报,挽救了数万人的生命.只是,这种地震预报的成功案例是具偶然性还是必然性,值得地震学家深思. 1966年3月8日邢台6.7级地震发生后,毛泽东主席明确指示:不能再让地震杀人.周恩来总理指示,必须加强地震预报研究,做到准确及时,要对地震中的各种现象,任何微弱变化记录下来,综合、辩证的分析,争取从邢台地震中找出地震预报的头绪.在周恩来总理的亲自指导下,新中国地震预报科学事业,以邢台地震为起点,踏上了征程.     

Mechanisms of earthquakes and plate motions in the east Pacific

Mechanisms of earthquakes occurring on plate boundaries have been used to determine the relative motions of lithospheric plates in the east Pacific. By using both P-wave first motions and the radiation pattern of Rayleigh waves in the focal mechanisms determinations, the uncertainty in strike is held to about ± 10°. On the Chile fracture zone and the fracture zone at 4°S on the East Pacific Rise, the direction of motion is not parallel to the trend of epicenters. This supports the hypothesis that these fracture zones are composed of series of en-echelon transform faults.     

Mathematical principles of predicting the probabilities of large earthquakes

A multicomponent random process used as a model for space-time earthquake prediction enables the construction of a consistent estimator for conditional probabilities of large earthquakes with known values of the predictor characterizing seismic prehistory. This paper defines tools for the assessment of the prediction efficiency, including separate determinations of the efficiency for time prediction and location prediction: a generalized correlation coefficient and information gain density. A technique for testing a predictor is proposed to determine whether the hypothesis of no prediction can be rejected.     

兴都库什－贝加尔地震带的地震迁移

应用一种新思路、新方法研究兴都库什－贝加尔地震带内的地震迁移，结果表明，地震沿地震带由南西西向北东东方向定向迁移，平均迁移速度为每年１２９ｋｍ，量级与中国大陆西部地震带（区）的地震迁移速度相当．研究结果为地震动力学和地震预报提供了一定的依据．     

Free vibration of soils during large earthquakes

Free vibration of soils happens frequently during some large earthquakes, perhaps seeming like a paradox. This happens because the energy released from seismic sources in some cases is not stationary in time, allowing relaxation intervals in between without important seismic wave arrivals in which free soil vibration happens. Two techniques to estimate the natural period of the free vibration from accelerograms are presented: autocorrelograms and Fourier spectra. Both techniques sometimes allow measuring higher mode frequencies of the soil for the three first modes as well as modal damping. Free vibration modal periods satisfy the classic 1D equation S-wave theory. The presence of free vibrations corresponds to shear wave soil energy radiation episodes rather than to energy amplification of incoming stationary seismic shear waves suggested by the dynamic soil amplification. These results explain the discrepancies observed between the theoretical soil dynamic amplification and the accelerographic measurement. Observation of free vibration of soils is not always possible, it depends on the duration of the time windows without important seismic waves arrivals compared to the natural period and damping of the soil.