由环境应力值寻找地震危险区的尝试

作者根据位错理论,采用二维矩形破裂模式,在导出了平均震源速度谱的基础上,导出了用地震波计算环境剪应力τ_0的公式。计算了1987—1991年中国和邻区大约1000多次4级以上中强地震的环境剪应力值τ_0,并把τ_0≥10MPa的地震看作是高剪应力值地震。自1987—1991年5年内,我国及邻区共发生M_s≥6.0的强震45次,和160余次高剪应力中强地震(M=4.0-5.0,τ_0≥10MPa),大约有1/4的强震和1/3的高应力值地震发生在台湾地区。说明台湾地区是一个高地震活动区和高应力值地区。新疆的喀什—乌恰—乌什地区也是高地震活动区和高应力值地区。我们对发生在其地区的强震,向前追溯了环境应力值τ_0的变化。发现有11个强震事件(约占全部事件80%)在震前1—5年内,在以强震为圆心,半径为200km范围内(个别略远),均有1—2个或多个高应力值地震出现。说明当高应力值地震发生时,该地区环境应力在增高,它可以作为将要发生强震的前兆。据此,推测了今后3年内大陆强震发生的可能地区是:①中印缅三国边境地区;②西藏波密地区;③青海北部到宁夏北部地区;④新疆伊宁地区;⑤纵贯新青藏90°E±2°线地带;⑥晋北忻州地区;⑦苏北及黄海地区;⑧山东长岛地区。     

任意点震源τ-P域地震图的解及AVO分析

用严格的数学推导得出τ-P域地震图的解析解。分析表明,τ-P域中地震图的子波可表示为激发震源子波的积分形式,决定反射波振幅的因素是平面波反射系数、反射界面上覆介质的波速以及方向因子。这些结果说明:1.在τ-P域进行AVO分析是可行的,并具有更高的精度;2.从一种新的途径可以进行速度函数的反演计算。     

2015年8月12日天津化学爆炸产生的多模式面波分析及其应用研究

2015年8月12日天津滨海新区发生的强烈化学品爆炸造成了巨大的经济损失和社会影响.天津爆炸产生了清晰的大振幅面波信号,分析结果表明这组信号由基阶和高阶面波组成,可以追踪到约135 km外的远处台站.利用这组面波信号分别开展了以下研究：（1）利用附近三个台站记录的四个单频基阶Rayleigh波信号对爆破事件的绝对位置进行了网格搜索,结果与利用GPS测量的位置相差仅0.498 km;（2）分别利用网格搜索和主事件定位法,对两次子事件的相对位置进行了确定,距离约75 m左右,与前人研究结果吻合;（3）从面波记录中测量到36条基阶Rayleigh波、49条第一高阶Rayleigh波、9条基阶Love波和29条第一高阶Love波的频散曲线,并进一步反演获得研究区域地下4 km内的S波速度结构.反演结果显示地表处S波速度低至0.375 km·s^-1,在小于1 km的浅地表速度梯度较大,符合典型的盆地结构特征.本文的研究结果为类似爆炸等突发事件快速定位提供了新的思路,有助于灾后救援的迅速展开;同时得到天津滨海新区及周边浅层精细的速度结构,对于地震灾害评估有很大帮助.     

山西断陷带上地幔顶部Pn波速度结构与各向异性成像

本研究拾取了中国数字测震台网固定台站记录的2008—2019年期间发生在山西断陷带及邻区2级以上天然地震事件及陕西神木、府谷等3级以上非天然地震事件共25304条高质量Pn到时数据,反演了山西断陷带及邻区上地幔顶部高分辨率Pn波速度结构与各向异性.研究结果显示,山西断陷带及邻区Pn波速度结构差异较大,大同火山及以南区域、忻定盆地、太行山造山带、华北盆地南部和吕梁山局部地区表现为显著的低波速异常,而运城盆地、临汾盆地北部、太原盆地、大同盆地北部、华北盆地北部和鄂尔多斯块体呈现明显的高波速异常.大同火山下方上地幔顶部的低波速异常与Pn快波方向呈现以火山为中心的近发散状结构特征,结合已有的远震上地幔成像结果,暗示大同火山岩浆可能来源于地幔深部,岩浆的底侵或热侵蚀作用造成了该地区岩石圈的破坏以及整个华北克拉通的"活化",这一推论符合克拉通的热-化学侵蚀破坏模型.山西断陷带上地幔顶部速度异常形态较好的对应了研究区的地质构造,Pn快波速方向与地质构造的展布方向和SKS波各向异性的特征基本一致,说明变形形式以简单剪切为主,表明其形成和演化过程与上地幔物质运移过程有密切关系.     

Velocity distribution in a gradually accelerating free surface flow

This study investigates the interaction of the vertical velocity v and the streamwise velocity u in a gradually accelerating flow. The analytical result shows that the momentum of uv driven by the mean velocities in a non-uniform flow is not negligible. This additional momentum directly results in the concave profiles of Reynolds shear stress in gradually accelerating flows, a departure from the expected linear profile. Consequently, this momentum causes the maximum velocity to be located below the free surface, i.e., the dip-phenomenon. This paper investigated the interactions of the Reynolds shear stress, non-zero vertical velocity and dip-phenomenon, it is found that the non-zero vertical velocity causes the dip-phenomenon. The approach is tested using the experimental data of Song and others, and good agreements between the predicted and measured velocity profiles have been achieved.     

中部天山地壳活动的GPS初步分析

通过对天山中部地区连续多年的GPS观测分析,得到了该监测区相对欧亚板块的运动速度场及应变场分布情况。计算表明天山中部地区内部断裂两盘的运动方向以婆罗科努断裂为界,北部各断层表现为拉张特征,南部总体表现为聚合特征。     

Discussion on Deep Tectonic Background of Moderately Strong Earthquake in Anhui Province and Its Neighboring Areas Using Results of Seismic Tomography

INTRODUCTIONThe studied areaislocated mostlyinthe southeastern part of the North Chinafault block provinceat 28°~39°N, 112°~124°E, where the geologic tectonics is complex with frequent historicalmoderate-strong earthquakes .The 3Dvelocity images derived fromseismic tomography have enrichedthe knowledge about the deep structures of the crust and the upper mantle ,and by combining thespatial distribution of seismicity patterns ,we canfurther studythe relations of velocitystructure toth…     

Experimental study on the effects of artificial bed roughness on turbidity currents over abrupt bed slope change

A series of experimental observations are presented in the current study to discuss the effects of artificial bed roughness on the turbidity current flowing in a rectangular channel with an abrupt change in bed slope.For this purpose,two different types of elements,sinusoidal and trapezoidal,with various heights and arrangements are considered as artificial bed roughness.A Vectrino velocity meter was used to measure the velocity and sediment concentration profiles.The effects of inlet sediment concentration on front velocity,body velocity,unit discharge,sediment concentration,and suspended load transport rate also were investigated.Accurate equations were developed for estimation of the velocity of a turbidity current over smooth and rough beds.The unexpected experimental results showed that unlike the effect of roughness height,a change in the roughness arrangement has no significant influence on the velocity of a turbidity current.Also,the effect of bed roughness on the front velocity of a denser current is more significant.     

Quasi-stationary flow structure in turbidity currents

A turbidity current is a particle-laden current driven by density differences due to suspended sediment particles. Turbidity currents can transport large amounts of sediment down slopes over great distances, and play a significant role in fluvial, lake and submarine systems. To better understand the sediment transport process, the flow system of an experimentally produced turbidity current in an inclined flume was investigated using video processing. We observed that the current progresses with constant frontal velocity and maintains an unchanged global interface geometry. In addition, the spatio-temporal profiles of the inner mean and turbulence velocity obtained by ultrasound velocity profiler (UVP) showed that similar distributions were maintained, with low dissipation. The results indicate that the turbidity current progressed in a quasi-stationary state, which enabled long-distance sediment transport. To understand the mechanisms behind the quasi-stationary flow, we analyzed the forces acting on the turbidity current. We found that under particular densities of suspended particles, the gravitational force is balanced by the viscous forces along the slope direction. We conclude that this specific force balance induces the quasi-stationary flow structure, enabling the long-distance transport of a substantial amount of sediment downstream with low dissipation.