Fault rupture directivity of large earthquakes in Himalaya

The paper examines the predominant fault rupture directivity during large earthquakes in different sectors of the Himalaya which influences strong ground motion and damage scenario. The nature of the faulting of earthquakes vis-à-vis their rupture directivity has been discussed. It is found that the rupture directivity near the Indo-Eurasian plate boundary varies from place to place i.e. either along the strike direction of the faults or at right angles to it. The secondary meizoseismal areas as observed for 1505 Dharchula, 1803 Uttarakhand, 1905 Kangra earthquakes in the Himalaya and 2001 Bhuj earthquake in stable continental region suggest that they are a fairly good indicator of predominant rupture directivity since the latter accentuates the site response up to a longer distance. The resulting larger ground motions, therefore, need to be incorporated in the design of engineering structures by suitable modifications in the BIS code.     

Xu-White模型预测横波速度的不确定因素分析

自Xu和White (1995)建立了泥质砂岩的有效介质模型(Xu-White模型)后,很多地球物理学者用此模型或改进的模型进行岩石横波速度的预测,但至今很少有人对Xu-White模型预测过程中的不确定因素进行详细总结。介绍了Xu-White模型及其预测流程;对预测过程中的不确定因素进行了分析,指出在使用中可以根据实际地质情况合理的取舍参数,从而能更有效地进行纵横波速度预测。     

Multi-segment earthquakes and tsunami potential of the Aleutian megathrust

Large to great earthquakes and related tsunamis generated on the Aleutian megathrust produce major hazards for both the area of rupture and heavily populated coastlines around much of the Pacific Ocean. Here we use paleoseismic records preserved in coastal sediments to investigate whether segment boundaries control the largest ruptures or whether in some seismic cycles segments combine to produce earthquakes greater than any observed since instrumented records began. Virtually the entire megathrust has ruptured since AD1900, with four different segments generating earthquakes >M8.0. The largest was the M9.2 great Alaska earthquake of March 1964 that ruptured ～800 km of the eastern segment of the megathrust. The tsunami generated caused fatalities in Alaska and along the coast as far south as California. East of the 1964 zone of deformation, the Yakutat microplate experienced two >M8.0 earthquakes, separated by a week, in September 1899. For the first time, we present evidence that earthquakes ～900 and ～1500 years ago simultaneously ruptured adjacent segments of the Aleutian megathrust and the Yakutat microplate, with a combined area ～15% greater than 1964, giving an earthquake of greater magnitude and increased tsunamigenic potential.     

Fine structure beneath the Tohoku district, northeastern Japan arc, as derived by an inversion of P-wave arrival times from local earthquakes

Three dimensional P-wave velocity structure beneath the Tohoku district, northeastern Japan arc, is investigated by an inversion of arrival times from local earthquakes using the method originally due to Aki and Lee (1976).In the crust (0–32 km depth) a low-velocity region is found along the volcanic front and its vicinity. Velocities at depths of 32–65 km are low beneath the regions where many Quaternary volcanoes and geothermal areas are distributed. In the region deeper than 65 km, the subduction of the Pacific plate is clearly revealed, and the mantle structure above the descending plate is rather uniform. These features suggest that volcanic activities have relation to the upper mantle structure. The results obtained in this study will be helpful in investigating the mechanism of magma generation in a subduction zone.     

Method of calculating tsunami travel times in the Andaman Sea region

A new model to calculate tsunami travel times in the Andaman Sea region has been developed. The model specifically provides more accurate travel time estimates for tsunamis propagating to Patong Beach on the west coast of Phuket, Thailand. More generally, the model provides better understanding of the influence of the accuracy and resolution of bathymetry data on the accuracy of travel time calculations. The dynamic model is based on solitary wave theory, and a lookup function is used to perform bilinear interpolation of bathymetry along the ray trajectory. The model was calibrated and verified using data from an echosounder record, tsunami photographs, satellite altimetry records, and eyewitness accounts of the tsunami on 26 December 2004. Time differences for 12 representative targets in the Andaman Sea and the Indian Ocean regions were calculated. The model demonstrated satisfactory time differences (<2 min/h), despite the use of low resolution bathymetry (ETOPO2v2). To improve accuracy, the dynamics of wave elevation and a velocity correction term must be considered, particularly for calculations in the nearshore region.     

井喷关井水击分析及水击波速计算

油气井发生溢流或井喷后,立即关闭防喷器,关井期间防喷器需经受关井产生的水击压力作用。本文在对井喷关井水击的传播物理特征分析的基础上,推导直接关井水击压力计算公式,综合考虑井筒流体含气率、气液两相流型、固相类型等对水击波速的影响,推导水击波在气、液、固三相混合流体中传播速度计算公式,并通过算例对水击波速影响因素进行分析。计算表明水击波速对含气率变化非常敏感,钻井液含气率在0～0.001的时候,水击波速几乎是发生了突变,水击波速在含气率为0.001时仅为385 m/s,相比纯钻井液情况下降了67%,而钻井液固相含量、套管的内径和壁厚对水击波传播速度影响较小。     

Auto-correlation analysis of wave heights in the Bay of Bengal

Time series observations of significant wave heights in the Bay of Bengal were subjected to auto-correlation analysis to determine temporal variability scale. The analysis indicates an exponential fall of auto-correlation in the first few hours with a decorrelation time scale of about six hours. A similar figure was found earlier for ocean surface winds. The nature of variation of auto-correlation with time lags was also found to be similar for winds and wave heights     

Evaluation of tsunami impacts on shallow marine sediments: An example from the tsunami caused by the 2003 Tokachi-oki earthquake, northern Japan

A combined approach of field geology and numerical simulation was conducted for evaluating the tsunami impacts on the shelf sediments. The 2003 Tokachi-oki earthquake, M 8.0, that occurred on 25 September 2003 off southeastern Hokkaido, northern Japan, generated a locally destructive tsunami. Maximum run-up height of the tsunami waves reached 4 m above sea level. In order to estimate the tsunami impacts on shallow marine sediments, we compared pre- and post-tsunami marine sediments in water depths of 38–112 m in terms of grain size, sedimentary structure, and microfossil content. Decreases of fine fractions, especially finer than very fine sand, which led to coarsen the mean grain size, were detected in the inner shelf of the northern part of the study area. Foraminiferal assemblages also changed in the coarsened sediments. On the other hand, the other shelf sediments largely unchanged or slightly fined. We also simulated the tsunami wave velocity and direction, and grain size entrained by the modeled tsunami. The numerical simulation resulted in that the 2003 tsunami could transport very fine sand in water depths shallower than 45–95 m at the northern part of the study area. This is comparable with the actual grain-size changes after the tsunami had passed. However, some storms and tidal currents might also be possible to stir the surface sediments after the pre-tsunami survey, so we could not conclude that the grain-size changes had been caused only by the tsunami. Nevertheless, a combined approach of sampling and modeling was powerful for estimating the tsunami impacts under the sea.     

Distribution of the geochemical signature caused by the 2011 Tohoku-oki tsunami in Misawa harbor,northern Japan

Natural Hazards - The 2011 Tohoku-oki tsunami left a characteristic geochemical signature in the sediments of the Misawa harbor on the Aomori coastline (northern Japan), not only in vertical...     

西藏扎西康铅锌银矿床含矿走滑断裂地球物理特征及找矿方向

扎西康铅锌银矿床位于北喜马拉雅大陆边缘褶冲带内,是西藏扎西康整装勘查区中大型铅锌锑银共生矿床之一。矿床赋存于下侏罗统日当组,容矿岩石为含碳钙质板岩、钙质板岩和构造角砾岩,矿体严格受近SN向和NE向断裂控制,呈脉状、透镜状产出。通过岩石物性分析,认为矿体具有中低电阻和中低极化的物性特征。在矿集区开展了音频大地电磁法(AMT)和激发极化法试验,结合地质地球物理初始模型,从地质背景、勘查模型角度分析,含矿走滑断裂的围岩具有中低极化率、断裂走向与极化率条带一致的特征;认为在中低极化率区域存在断裂,且断裂走向的垂直方向存在高极化、低电阻区,赋矿可能性较大。     

高压气体诱发煤岩动力破坏的实验研究

为了研究高压气体对煤岩材料变形破坏的作用,自主设计并制造了含高压气体煤岩实验装置,通过测量气体泄压作用诱发的煤岩动力破坏现象,研究了高压气体对煤岩材料变形破坏的作用。实验表明,当气体泄压速率小时,煤岩仅会出现轻微变形;当气体泄压速率大时,煤岩会产生破裂和破碎现象。同时发现煤岩的破坏程度不仅取决于气体的泄压速率,还取决于孔隙气体的压强。当气体泄压速率和气体压强都达到一定临界值时,煤岩才会发生剧烈破碎。通过气体压强与应变之间的关系,确定了煤岩发生破裂和破碎的临界气体压强。      

Migration of large earthquakes along the great fault zones in Middle Asia

An analysis of the distribution (both spatial and temporal) of large earthquakes (M 6.5) along the Gissar—Kokshaal and the Hindu-Kush—Darvaz—Karakul fault zones in Middle Asia has revealed the linear character of migration from the ends to the centre of the Pamir arcs at a rate of 1–2 km/year to 3–6 km/year. Migration of large earthquakes at a similar rate has also been found in some of the other great fault zones. An attempt has been made to evaluate the duration of a migration cycle.The regularity found, although it needs further confirmation, has been used to tentatively predict the possible sites of future large earthquakes likely to occur in the present century.     

Studies on the determination of shear wave velocity in sands

Laboratory investigations such as resonant column, cyclic triaxial, and torsional shear tests are usually conducted on undisturbed and reconstituted sand samples in order to determine their shear modulus, which can be used to compute shear wave velocity. However, these methods are extremely cumbersome and indirect, they employ cost-intensive instrumentation, and they require trained manpower. These problems can be overcome by using bender elements, which yield the shear wave velocity of sands directly, easily, and quite rapidly. In this context, efforts made by earlier researchers in developing empirical relationships (based on bender elements and resonant column techniques) which can be employed to compute shear wave velocity in sands are worth consideration. Although these relationships take into account parameters like effective stress and void ratio, they do not directly incorporate fundamental characteristics of sands such as their state (i.e. dry or saturated) and the particle size. Thus a generalized relationship, which can be used to compute shear wave velocity in sands in either the dry or saturated state, has been developed. Experiments were conducted on samples of sand and glass beads of different sizes and the results were used to demonstrate the utility and efficiency of the relationship. Details of the methodology developed for this purpose are also presented in this paper.     

Occurrence patterns of large earthquakes in the seismic zones along the Hellenic Arc

Occurrence patterns of large shallow and intermediate depth earthquakes in the seismic zones along the Hellenic Arc have been investigated. It is shown that throughout this active region the earthquakes tend to occur in a rather systematic manner. At each time-period earthquakes occur within a discrete activated segment of the arc. These occurrence patterns are considered to offer an insight into identifying where large earthquakes are expected to occur in the future.     

Seismic wave velocity and anisotropy of serpentinized peridotite in the Oman ophiolite

Shallow seismic measurements in harzburgite from the Oman ophiolite performed in a zone where the maximum horizontal anisotropy is expected (vertical foliation and horizontal lineation) point to a dominant dependence of seismic properties on fracturing.

Optical microscopy studies show that microcracks are guided by the serpentine (lizardite) penetrative network oriented subparallel to the harzburgite foliation and subperpendicular to the mineral lineation, and that serpentine (lizardite) vein filling has a maximum concentration of (001) planes parallel to the veins walls. The calculated elastic properties of the oriented alteration veins filled with serpentine in an anisotropic matrix formed by oriented crystals of olivine and orthopyroxene are compared with seismic velocities measured on hand specimens.

Laboratory ultrasonic data indicate that open microcracks are closed at 100 MPa pressure, e.g. (J. Geophys. Res. 65, (1960) 1083) and (Proc. ODP Sci. Results Leg 118, (1990) 227). Above this pressure, laboratory measurements and modeling show that P-compressional and S-shear wave velocities are mainly controlled by the mineral preferred orientation. Veins sealed with serpentine are effective in slightly lowering P and S velocities and increasing anisotropy. The penetrative lizardite network does not affect directly the geometry of seismic anisotropy, but contributes indirectly in the fact that this network controls the microcrack orientations.

Comparison between seismic measurements of peridotite and gabbro in the same conditions suggest that P- and S-waves anisotropies are a possible discriminating factor between the two lithologies in the suboceanic lithosphere.