祁连山地震带地震活动特征及序列类型

祁连山地震带中强地震具有成组活动特征,震前地震活动图象表现为地震空区、前兆地震、区域地震活动增强等。地震空区以中期异常为主,中期向短期过渡的标志为空区边缘或内部出现前兆地震、空区停止围空1~3个月或空区内发生小地震而导致空区解体。前兆地震一般于震前1~6个月出现,多数前兆地震与主震之间距离小于30 km,震级差1.5~3.0。通过序列类型及参数的统计分析,为震后趋势判定提供了依据。     

江西北部及邻区地震活动特征

利用不同时间尺度、不同震级下限的资料,分析了江西北部及邻区的地震活动时空分布特点。结果显示：本区地震活动成组现象明显．中强地震前中短期阶段的地震学异常特征主要表现为地震空区、地震条带或中等地震集中增强区。未来数年内本区还有可能发生中强地震,目前应当高度重视区域的震情发展趋势。     

甘肃及邻近地区中强地震前弱震空区的时空参数研究

用R-T图方法研究了甘肃及邻近地区发生的19次中强地震前震源及附近地区出现空区的现象以及空区的时空参数.结果表明其中16次震前都出现了明显的空区.根据空区时空参数拟合出了空区的最大空间尺度和持续时间与主震震级的关系,为研究"区域-时间-长度"算法(RTL算法)对甘肃及邻近地区中强地震前的地震活动异常特征(平静和活跃)提供了依据.     

山东苍山5.2级地震活动背景及震前地震活动

分析了山东苍山５２级地震前的历史地震活动、现今中小地震活动的时空分布及地震活动的异常特征。表明,苍山５２级地震强度和空间分布与历史地震活动强度和空间分布特征相一致,并具有某些大震的异常特征,震前出现了长达１２ａ的背景空区,出现了“信号震”和孕震空区,出现了多种测震学指标异常等。     

2017年九寨沟7.0级地震前不同级别地震空区的演化过程分析

结合传统地震学方法与数字地震学方法,回顾总结了日常分析预报中针对九寨沟7.0级地震开展的相关工作,梳理总结了地震前提出的甘东南地震异常信息,进一步讨论了九寨沟地震前中期、短期及临震异常特征。通过分析九寨沟7.0级地震之前区域范围内不同震级活动图像,发现甘青川交界地区5级空区在震前向震中一侧收缩、4级地震超长平静458天、3级空区在震前4天被打破的现象,同时发现,震中附近区域震源机制一致性较高,反映了震源区的高应力水平。     

西南地区强震前后地壳变形异常强度场特征研究

采用标准化的F统计量给出了异常强度的定量描述方法,在变形监测台点比较密集的西南地区构造了变形异常强度场。通过对异常特征空间分布随时间演化图像及强地震前后变形异常分布图像的研究,讨论了地震成组活动模式下变形空间分布的特点及变形异常分布的非均匀性特征。探讨了变形异常集中区与平静区(变形空区)与地震活动空区之间的关系,以及地震活动空区内外小震活动与变形场分布之间的微动态联系。     

四川中强地震特征性活动图象与当前地震趋势讨论

本文从四川Ｍ≥４级地震时、空、强活动图象分析中，提取了有重复性前兆意义的异常图象，称为特征活动图象，如条带状、丛集、环状围空、缺震异常时段、盆地及周缘区前震等。并根据当前的地震活动图像，讨论了四川地震趋势。     

数字图象处理方法在确定地震空间分布中的应用

本文提出了用数字图象处理方法确定地震空间分布图象的实用方法及其计算结果。该方法能克服通常在确定地震空间分布时难免的人为随意性。还讨论了地震空区的表现形式和识别孕震空区的方法。必须指出, 本方法是个创新。但确定地震空区还不唯一, 要识别真正的孕震空区还需要综合分析其它有关参量(例如b值等)的变化。结果表明:被低地震活动区包围起来的高地震活动区现象──地震丛集图象, 是地震空间分布的普遍形式；而被高地震活动区围起来的地震活动弱区──地震空区图象, 只是地震空间分布的一种重要形式。地震往往发生在某一地震丛集区内或其边缘地带。      

滇西南地震活动特征分析

对滇西南1970年以来所有M≥4.6级地震前的空间活动图像进行研究,用地震频度作为参数描述地震前区域的地震活动增强与平静现象,用空区、条带描述地震的孕震过程,得到以下认识:①地震频度增加是滇西南地区5级以上地震前的共性特征;②滇西南地区M≥4.6级地震前出现空区的比例随着地震震级的增加而有所增加,4.6~4.9级地震前只有60%的地震出现空区,5.0~5.9级地震前有90.9%的地震出现空区,而6.0级以上地震前全部出现空区;③空区平均持续时间也随着震级的加大而增加,空区平均持续时间为13.58个月,最短为3个月,最长为37个月;④主震位置(27/33)多发生在空区边缘(含长轴端12次、短轴端8次)。震前出现条带的规律则不明显。     

大华北地块区地震大形势研究

分析了大华北区历史地震活动分期与现代地震活动特点,讨论了大华北区及其相关区带不同地震活动幕地震空间活动特征。认为大华北地块区目前处在第四活动期的调整活动阶段,未来几年可能进入新的地震活跃幕,地震活动以5-6级地震为主,主要活动地区为燕渤带和鲁东—南黄海地块区。     

On the theory of the axially-symmetric,time-average,state of the atmosphere

Summary A possible formal approach to a closed steady-state theory of the mean axially-symmetric variables is outlined. The approach involves alternating iterative solutions of the energy and momentum equations. In these equations the effects of transient eddy phenomena of all frequencies are assumed to be parameterized in terms of the mean symmetric variables.     

Erosional unloading,hillslope geometry,and the height of the Cascade Range,Washington State,USA

Peaks in the Cascade Range in northern Washington State are on average ～800 m higher than in southern Washington. The influences of differential valley excavation and variations in hillslope length and average slope on these altitudinal trends were tested using a 3‐dimensional model for isostatic rock uplift and calculations of hillslope length and slope respectively. The magnitude of isostatic peak uplift calculated by the model is highly dependent on the flexural rigidity (D) and the related effective elastic thickness (T_e) of the crust of this region. Crustal rigidity was constrained using published estimates and by estimating the depth of the seismogenic zone in the area (D > 1 × 10²³ Nm and T_e > 24 km). With these constraints, isostatic compensation due to differential erosion added < 700 m and 300 m, or < 25% overall, of height to peaks in the northern and southern Cascades, respectively. Deeper valley incision in the northern Cascades accounts for < 300 m of the 800 m difference in peak altitudes between north and south. Similarly, variation in valley spacing and slope account for < 350 m of the difference in mean altitude between northern and southern regions. Hence, at least several hundred m difference in altitude between the northern and southern regions of the Cascades in Washington must be due to tectonic, geologic, or geophysical factors rather than surficial and geomorphic effects like isostatic response to valley incision and hillslope geometry. Copyright © 2009 John Wiley & Sons, Ltd.     

A quantitative study of the energy release in the aftershocks of the Bhuj earthquake, 2001, India, using Lg phase

The devastating earthquake on 26 January 2001 at Bhuj, India, resulted in large-scale death and destruction of properties of several million US dollars. The moment magnitude of the earthquake was 7.7 and its maximum focal intensity exceeded X in MM scale. The rate of aftershocks of this earthquake, recorded at Gauribidanur seismic array station (GBA), shows a monotonic decay with time superposed with oscillations. For the Indian continent the Lg phase is a prominent arrival at regional distances. The estimate of Lg amplitude is obtained by optimally fitting the Lg wave train to a exponential decay curve. The logarithm of these amplitudes and logarithm of root mean square (rms) value of actual amplitudes of the Lg are calibrated with USGS m_b to create a local m_bLg magnitude scale. The energy released from these aftershocks is calculated from the rms value of Lg phase. The plot of cumulative energy release with time follows the power law of the form t^p, superposed with oscillations. The exponent of the power law, p, is estimated both by a time-window scanning method and by an interpolation method. The value of p is 0.434 for time-window scanning method and 0.432 for the interpolation method. The predominant periods found in the oscillatory part of the cumulative energy, obtained by differencing the observed from the power law fit, are 10.6, 7.9, 5.4, 4.6 and 3.5 h for time-window scanning method. The corresponding periods for interpolation method are 13.4, 11.5, 7.4, 4.2, 3.5, 2.6 and 2.4 h.     

Petrology of the dyke rocks of the western portions of Rajpipla hills,Broach district,Gujarat, India

The paper embodies the field, petrographic and petrochemical studies of the dykes occuring within the Deccan basalts, in the Western portions of Rajpipla hills. Major and minor dykes with different trends occur in the area varying in thickness from 2′ to 75′ and traceable lengthwise from few to several miles. The density of the dyke distribution is two per mile. The composition of the minor dykes ranges from teschenite to trachyte with dominant basaltic types and they seem to be coeval with the flows of the area. The major dolerite dykes are found to be post-lava. Both alkali-olivine basalt and tholeiitic types occur. The former phase preceeds the latter and includes the minor alkaline dykes. A differentiation trend based on new chemical analyses is proposed.     

Cd, Cr, Cu, Ni and Pb in the water column and sediments of the Ob-Irtysh Rivers, Russia

Concentrations of Cd, Cr, Cu, Ni and Pb were determined in filtered water, suspended particulate matter, and bottom sediments from a 2000 km section of the Ob and Irtysh Rivers. Dissolved Cd, Cr, Cu and Ni concentrations are similar to, or higher than, results from other Russian Arctic and large world river-estuaries. Concentrations of Cd, Cr, Cu, Ni and Pb in suspended particulate matter are generally comparable to results from other Russian Arctic and large world rivers and estuaries. Comparison of trace metal ratios in crustal material and suspended particulate matter and bottom sediment suggests that the source of Cr, Cu and Ni is continental weathering. Particulate Cd and Pb are elevated relative to their crustal abundance, suggesting a source of these metals to the Ob-Irtysh in addition to continental weathering.     

Mechanism of the June 4, 2000 Southern Sumatra, Indonesia, earthquake

Introduction The June 4, 2000 southern Sumatra, Indonesia, earthquake (16h28min26.2s UTC, 4.72, 102.09, 33 km, MS=8.0 [IRIS]) occurred under the Indian Ocean, near the Mentawai fault, along the well-known Sumatran subduction zone and the great Sumatran fault, all of which trend northwest-southeast. 1 800 houses were totally destroyed, 10 196 were heavily damaged, and 18 378 were slightly damaged by the earthquake. At least 97 people were killed, 1 900 were injured, and 122 000 were left…     

Eyewitness Accounts of the Impact of the 1998 Aitape Tsunami, and of Other Tsunamis in Living Memory, in the Region from Jayapura, Indonesia, to Vanimo, Papua New Guinea

Field investigations in 1999 confirmed that the tsunami that struck the Aitape coast of Papua New Guinea on 17 July, 1998 caused damage at points as far as 230 km to the west-northwest, particularly at locations where the coast is indented. Eyewitnesses saw the sea withdraw (in most cases), then surge to levels around 2 m higher than normal in a series of three waves. In some cases the time of arrival of the waves is known approximately by reference to the onset of darkness and to felt earthquakes. Seiche waves followed in some bays, notably in Yos Sudarso Bay, Indonesia, where waves persisted for 3–5 days. Damage was caused by the backwash from the waves. Bodies presumed to be those of Aitape victims were seen floating at sea off Jayapura five days after the tsunami. We record the recollections of people in the Yos Sudarso Bay area who experienced a number of tsunamis in the past 60 years; people that we interviewed on the Papua New Guinea side of the border recollected few or none.     

Rupture process of the Miyagi-Oki,Japan, earthquake of June 12, 1978

The faulting mechanism and multiple rupture process of the M = 7.4 Miyagi-Oki earthquake are studied using surface and body wave data from local and worldwide stations. The main results are as follows. (1) P-wave first motion data and radiation patterns of long-period surface waves indicate a predominantly thrust mechanism with strike N10° E, dip 20°W, and slip angle 76°. The seismic moment is 3.1 × 10²⁷ dyne-cm. (2) Farfield SH waveforms and local seismograms suggest that the rupture occurred in two stages, being concordant with the two zones of aftershock activity revealed by the microearthquake network of Tohoku University. The upper and lower zones, located along the westward-dipping plate interface, are separated by a gap at a depth of 35 km and have dimensions of 37 × 34 and 24 × 34 km², respectively. Rupture initiated at the southern end of the upper aftershock zone and propagated at N20°W subparallel to the trench axis. About 11 s later, the second shock, which was located 30 km landward (westward) of the first, initiated at the upper corner of the lower aftershock zone and propagated down-dip N80°W. Using Haskell modelling for this rupture process, synthetic seismograms were computed for teleseismic SH waves and nearfield body waves. Other parameters determined are: seismic moment $\text{M}{}_0\text{= 1.7 × 10}{}^{27}\text{dyne-cm, slip dislocation}\text{u}\text{= 1.9}\text{m}\text{, Δσ = 95}\text{bar, rupture velocity}\text{ν = 3.2}\text{km s}{}^{\mathrm{?1}}\text{,}\text{rise time}\text{τ = 2}\text{s}$, for the first event; $\text{M}{}_0\text{= 1.4 × 10}{}^{27}\text{dyne-cm}\text{,}\text{u}\text{= 2.4}\text{m}\text{, Δσ = 145}\text{bar}$, for the second event; and time separation between the two shocks ΔT = 11 s. The above two-segment model does not explain well the sharp onsets of the body waves at near-source stations. An initial break of a small subsegment on the upper zone, which propagated down-dip, was hypothesized to explain the observed near-source seismograms. (3) The multiple rupture of the event and the absence of aftershocks between the two fault zones suggests that the frictional and/or sliding characteristics along the plate interface are not uniform. The rupture of the first event was arrested, presumably by a region of high fracture strength between the two zones. The fracture energy of the barrier was estimated to be 10¹⁰ erg cm^?2. (4) The possible occurrence of a large earthquake has been noted for the region adjacent to and seaward of the area that ruptured during the 1978 event. The 1978 event does not appear to reduce the likelihood of occurrence of this expected earthquake.     

Water contents, temperatures and diversity of the magmas of the catastrophic eruption of Nevado del Ruiz, Colombia, November 13, 1985

The petrology of the highly phyric two-pyroxene andesitic to dacitic pyroclastic rocks of the November 13, 1985 eruption of Nevado del Ruiz, Colombia, reveals evidence of: (1) increasingly fractionated bulk compositions with time; (2) tapping of a small magma chamber marginally zoned in regard to H₂O contents (1 to 4%), temperature (960–1090°C), and amount of residual melt (35 to 65%); (3) partial melting and assimilation of degassed zones in the hotter less dense interior of the magma chamber; (4) probable heating, thermal disruption and mineralogic and compositional contamination of the magma body by basaltic magma “underplating”; and (5) crustal contamination of the magmas during ascent and within the magma chamber. Near-crater fall-back or “spill-over” emitted in the middle of the eruptive sequence produced a small pyroclastic flow that became welded in its central and basal portions because of ponding and thus heat conservation on the flat glaciated summit near the Arenas crater. The heterogeneity of Ruiz magmas may be related to the comparatively small volume (0.03 km³) of the eruption, nearly ten times less than the 0.2 km³ of the Plinian phase of Mount St. Helens, and probable steep thermal and P_H2O gradients of a small source magma chamber, estimated at 300 m long and 100 m wide for an assumed ellipsoidal shape.     

Composition,temperature, and thickness of the lithosphere of the Archean Kaapvaal craton

A new model is proposed for the structure of the Kaapvaal craton lithosphere. Based on chemical thermodynamics methods, profiles of the chemical composition, temperature, density, and S wave velocities are constructed for depths of 100–300 km. A solid-state zone of lower velocities is discovered on the S velocity profile in the depth interval 150–260 km. The temperature profiles are obtained from absolute values of P and S velocities, taking into account phase transformations, anharmonicity, and anelastic effects. The examination of the sensitivity of seismic models to the chemical composition showed that relatively small variations in the composition of South African xenoliths result in lateral temperature variations of ～200°C. Inversion of some seismic profiles (including IASP91) with a fixed bulk composition of garnet peridotites (the primitive mantle material) leads to a temperature inversion at depths of 200–250 km, which is physically meaningless. It is supposed that the temperature inversion can be removed by gradual fertilization of the mantle with depth. In this case, the craton lithosphere should be stratified in chemical composition. The depleted lithosphere composed by garnet peridotites exists to depths of 175–200 km. The lithospheric material at depths of 200–250 km is enriched in basaltoid components (FeO, Al₂O₃, and CaO) as compared with the material of garnet peridotites but is depleted in the same components as compared with the fertile substance of the underlying primitive mantle. The material composing the craton root at a depth of ～275 km does not differ in its physical and chemical characteristics from the composition of the normal mantle, and this allows one to estimate the thickness of the lithosphere at 275 km. The results of this work are compared with data of seismology, thermal investigations, and thermobarometry.