Downward migration of seismic activity prior to some great shallow earthquakes in Japanese subduction zones—a possible intermediateterm precursor

Before the 1944 Tonankai earthquake along the Nankai Trough, seismic activity increased in the shallow depths, and then the activity gradually migrated downwards. When it reached its limit (a depth of approximatelty 70 km), the main shock occurred. Several deep earthquakes, including one ofM5.3, occurred several months prior to the Tonankai earthquake. A similar downward migration pattern also can be recognized regarding the 1952 Tokachi-oki earthquake. In this case the deepest earthquakes reached about 400 km. This may be one of the intermediate-term precursory phenomena of great thrusttype earthquakes in subduction zones. Recent observations in the Tokai district along the Suruga Trough, where a large earthquake is expected to occur in the future, suggest a similar downward migration pattern in the land area.     

多元统计在大洋玄武岩同位素端元识别中的应用

应用因子分析的方差最大旋转和非线性映射方法对海洋玄武岩Sr、Nd、Pb同位素数据在多维空间中的数据结构进行了分析。研究表明这些数据在多维空间中呈五角形结构,并对应5个地幔端元:大西洋中脊玄武岩(DMM),圣埃伦那(HIMU),鲸岛(EMI),萨摩亚岛(EMII)和DS(EMIII).其中DS(EMIII)是本文新识别出来的一个地幔端元,它可能是重新进入地幔的具有较高古老放射成因Pb的下地壳物质。     

The Zoulang Nanshan Caledonian Subduct ion Complex in the Northern Qilian Mountains and Its Dynamics

The front of the Zoulang Nanshan Caledonian volcanic island arc zone in the northern Qilian Mountains is a forearc accretionary terrane, composed of multiple accretionary volcanic island arcs, flysch accretionary wedges,high-pressure metamorphosed detachment zones and remnants of ophiolites. It resulted from the northeastward subduction of the Early Palaeozoic Qilan oceanic crust beneath the Alxa block. High-pressure metamorphism, which occurred during the subduction, progressed through three stages: the initial stage of medium T-high P,the main stage of temperature decrease and pressure increase, and the lag stage of pressure decrease and temperature increase. Finally the paper presents a retrotrench subduction dynamic model indicative of northward subduction of the central Qilian block and southward accretion of the Alxa block during the period of 450-500 Ma.     

A constitutive law for thermo-plastic behaviour of rocks: an analogy with clays

Thermo-mechanical behaviour of rocks shows phenomenological similarities to that of clays. A visible decrease of strength and an increase in ductility at elevated temperatures are observed in drained conditions. In undrained conditions the strength decrease is even more dramatic and the ductile behaviour may turn into the brittle one, if a sufficient amount of water is present in the pores at the start of the process, or it is being released during heating by the decomposition of hydrous minerals. Applying a thermo-plasticity theory, major macroscopical characteristics of behaviour may be modelled. According to this theory, the elastic domain is postulated as temperature dependent, shrinking with temperature; heating may result in expansive or compactive volumetric irreversible strain depending on the confining stress. This theory is applied to the modelling of heat effects on local changes of permeability around a cylindrical nuclear waste container disposed off in deep clays, and in the numerical simulation of a mechanism initiating deep earthquakes.     

P-waveform analysis for local subduction geometry south of Puget Sound,Washington

The local subduction geometry at a site south of Puget Sound in western Washington is investigated using teleseismicP-waveforms recorded on a three-component event triggered seismograph. The data are processed using source equalization deconvolution in order to isolate locally convertedP-to-S arrivals and stacked to improve the signal-to-noise ratio. Stable arrivals in the radial component indicate an oceanic Moho within the subducted slab at a depth of about 53 km beneath the station. Observed amplitude variations with azimuth in the radial data, as well as qualitative aspects of the tangential data, are used to establish a slab dip of 16° to the southeast. Our results are compatible with previous results from a site 60 km to the west, and further confirm a substantial warp in the regional geometry of the subducted Juan de Fuca plate.