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991.
破裂时间法在云南小江断裂带强震预测中的应用 总被引:1,自引:1,他引:0
使用云南区域台网记录的地震目录资料,采用破裂时间法模拟前兆地震序列能量加速释放过程,系统研究了能量加速释放模型参数估计方法及强震位置搜索技术,详细推导了预测模型基本公式,对小江断裂带上发生的19次主震事件进行了模拟,结果表明:强震前能量加速现象是一种普遍现象,主震前,与主震相差2.5个震级单位的前兆序列出现能量加速释放所占的比例为80%;由前兆序列模拟得到的预测时间和震级与实际值很接近,主震震级预测精度约为±0.5级,预测主震发生时间误差约为±0.5年;搜索半径和搜索时间与主震震级成线性正相关关系;采用主震位置搜索技术,对小江断裂带未来中期6~7级强震可能危险区进行了预测。 相似文献
992.
Self-organized Fractal Seismicity and b Value of Aftershocks of the 2001 Bhuj Earthquake in Kutch (India) 总被引:1,自引:0,他引:1
— The devastating intraplate earthquake of Mw 7.7 of 26 January, 2001 took place along the south-dipping reverse fault in the lower crust ( 23 km) of Kutch, Gujarat, India, obliterating some 14,000 people. The aftershock activity has ensued for three years. We analyzed 997 aftershocks of M 3.0 to study the b value and fractal correlation dimensions in time and space. The b value is found to be 0.8 ± 0.03 from the Gutenberg-Richter relation and 0.76 ± 0.02 from the maximum-likelihood, suggesting a typical value for the intraplate region. The spatial correlation is 1.71 ± 0.02, indicating that events are approaching a two-dimensional region. Further, the temporal correlation dimension is estimated to be 0.78 ± 0.02, confirming the structure is mono-fractal in time domain. The depth section of b value shows a peak at 15–35 km depth range coinciding with the maximum occurrence of aftershocks ( 47%), which is inferred as a fluid-filled highly fractured rock matrix with fractures of high density. It will be important to note that tomographic results also suggest a low Vp, low Vs and a large Poissons ratio for the same depth range, further confirming this inference. Additionally, we have studied the variation of D2s and b value with time. During the first two months of aftershock activity the results show a marked negative correlation between spatial correlation dimension D2 (large) and b value (low), indicating the predominance of large events associated with weak clustering. The negative correlation means the stress release along faults of a larger surface area. After two months the fractal dimension (D2s) and b value suggests a positive correlation implying more numerous smaller shocks with stress release along faults of a smaller surface area. This would indicate a reduced probability of large magnitude earthquakes due to fragmentation of the fault zone.Acknowledgement. The authors thank Dr. V.P. Dimri, Director, NGRI for his encouragement and kind permission to publish this work. The Department of Science and Technology, New Delhi supported this study. 相似文献
993.
ZHENG Yanpeng LIU Baohua WU Jinlong LIANG Ruicai LIU Chenguang & ZHANG Zhengmin . Marine Geology College Ocean University of China Qingdao China . First Institute of Oceanography State Oceanic Administration Qingdao China . Key Laboratory of Marine Sedimentology Environmental Geology State Oceanic Administration Qingdao China 《中国科学D辑(英文版)》2005,48(8):1165-1174
The sea area east of Taiwan Island is a unique ac-tive arc-continental collision margin in China and even in the world with the NE-nearly EW extending Ryu-kyu trench-arc-backarc basin system on the east and the nearly N-S extending Manila trough and Luzon arc on the west. The sea area east of Taiwan Island differs from the Ryukyu trench-arc-basin system in structural property, but there is a certain genetic relation between the two (Fig. 1)[1-3]. Since the 1990s, many investigations and … 相似文献
994.
P. H. REYNOLDS N. G. CULSHAW R. A. JAMIESON S. L. GRANT K. J. McKENZIE 《Journal of Metamorphic Geology》1995,13(2):209-221
Abstract 40Ar/39Ar data (on hornblende, muscovite and K-feldspar) are presented for samples from the western Grenville Province taken along a 140-km traverse from the Grenville Front into the Britt domain. Our interpretation is based on 28 new analyses, synthesized with 20 previously reported from the traverse area. In regions where comparisons are possible, muscovite and (large domain) K-feldspar apparent ages appear similar (at c. 920–930 Ma), but throughout the traverse, these are c. 60–70 Myr younger than the hornblende ages. The inferred cooling rate over the c. 350–500°C temperature range, c.2°C Myr-1, is appropriate for exhumation controlled by post-orogenic erosional unroofing. At the Grenville Front Tectonic Zone (GFTZ) — Britt domain boundary there is a c. 25-Myr offset in both hornblende and muscovite/K-feldspar ages. We interpret the lower ages in the Britt domain to reflect variations in crustal thickness and geothermal gradient between the flank and interior of a thick orogen. The argon data from the GFTZ are interpreted in the context of an asymmetric crustal-scale antiformal structure developed during a late episode of convergence. Hornblende from rocks on either side of the core of the antiform has an apparent age of c. 990 Ma, our estimate of the age of the compressional event. In the west, we infer that these date the short-lived thermal event associated with the development of the crustal-scale antiform previously postulated. In the east, the ages reflect the cooling of material brought toward the surface in the flank of the antiform. Hornblendes from the antiform core appear to contain excess radiogenic argon. We suggest that this was the ambient argon in rocks transported from depth that was subsequently trapped when the rocks cooled rapidly. 相似文献
995.
996.
Models of fluid/rock interaction in and adjacent to the Alpine Fault in the Hokitika area, South Island, New Zealand, were investigated using hydrogen and other stable isotope studies, together with field and petrographic observations. All analysed samples from the study area have similar whole‐rock δD values (δDWR = ?56 to ?30‰, average = ?45‰, n = 20), irrespective of rock type, degree of chloritization, location along the fault, or across‐strike distance from the fault in the garnet zone. The green, chlorite‐rich fault rocks, which probably formed from Australian Plate precursors, record nearly isothermal fluid/rock interaction with a schist‐derived metamorphic fluid at high temperatures near 450–500°C (δD of water in equilibrium with the green fault rocks (δDH2O, green) ≈ ?18‰; δD of water in equilibrium with the greyschists and greyschist‐derived mylonites (δDH2O, grey) ≈ ?19‰ at 500°C; δDH2O, green ≈ ?17‰; δDH2O, grey ≈ ?14‰ at 450°C). There is no indication of an influx of a meteoric or mantle‐derived fluid in the Alpine Fault Zone in the study area. The Alpine Fault Zone at the surface shows little evidence of late‐stage retrogression or veining, which might be attributed to down‐temperature fluid flow. It is probable that prograde metamorphism in the root zone of the Southern Alps releases metamorphic fluids that at some region rise vertically rather than following the trace of the Alpine Fault up to the surface, owing to the combined effects of the fault, the disturbed isotherms under the Southern Alps, and the brittle–ductile transition. Such fluids could mix with meteoric fluids to deposit quartz‐rich, possibly gold‐bearing veins in the region c. 5–10 km back from the fault trace. These results and interpretations are consistent with interpretations of magnetotelluric data obtained in the South Island GeopHysical Transects (SIGHT) programme. 相似文献
997.
JANE D. SILLS DIETRICH ACKERMAND RICHARD K. HERD BRIAN F. WINDLEY 《Journal of Metamorphic Geology》1983,1(4):337-351
Sapphirine occurs in a 3-5 m wide zone between amphibole-lherzolite and garnetiferous metagabbro at Finero in the Ivrea Zone, NW Italian Alps. Layers consisting of plag + hb + sa + cpx + opx + sp + gt are interbanded with spinel pyroxenites, which may contain sapphirine replacing spinel. All minerals are very magnesian, with XMg between 0.78 and 0.92. Bulk rock analyses suggest that precursors to the sapphirine-bearing rocks were igneous cumulates of plagioclase + olivine + hornblende + spinel. Up to 16wt% CaO does not inhibit sapphirine formation and it is the unusually Mg-rich nature of the host rocks which allows sapphirine development. The early igneous assemblage was replaced by one of cpx + sa + hb +± plag at a pressure of 9 ± 1 kbar and temperatures of 900 ± 50°C. Subsequent rapid uplift caused the instability of gt, gt + hb, hb and sa + cpx to form opx + plag ± sp ± sa symplectites. 相似文献
998.
The Palghat Cauvery Shear Zone (CSZ) is a major shear zone that possibly extends into different fragments of Gondwanaland.
In the present study mafic granulites occurring on either side of the CSZ in Namakkal area, southern India are examined. Textural
features recorded in the mafic granulites are crucial in elucidating the metamorphic history of the southern granulite terrane
(SGT).
In the mafic granulites occurring to the south of CSZ, evidence of garnet breaking down during near isothermal decompression
(ITD) is indicated by the development of orthopyroxene + plagioclase moats in between quartz and garnet. The presence of comparatively
small elongated second generation garnet embedded in pyroxenes from the mafic granulites occurring to the north of CSZ is
indicative of the garnet formation via reaction between pyroxenes and plagioclase, which occurred during isobaric cooling
(IBC).
Rocks occurring to the south of CSZ have recorded comparatively higher temperature and pressure (849‡C and 9.6kbar) than those
occurring to the north of the CSZ (731‡C and 8.6kbar) using conventional geothermobarometry. The rocks occurring to the north
of CSZ have suffered more complex metamorphic histories in comparison to the southern part. Integrating the results of the
present field and metamorphic studies with the earlier investigations and available geochronological data we suggest that
the CSZ could represent a suture zone between two different continental blocks that underwent distinct metamorphic evolution. 相似文献
999.
ZHENG Yanpeng LIU Baohua WU Jinlong LIANG Ruicai LIU Chenguang ZHANG Zhengmin 《中国科学D辑(英文版)》2005,48(8)
Based on compositive analysis and interpretation of the observed and historical data, the geophysical field characters and structural properties of the Gagua "Wedge" Zone of the sea area east of Taiwan Island and the primary tectonic stress direction and its variabilities of backarc spreading in the southern Okinawa Trough are studied. It is concluded from the study results that the Gagua "Wedge" Zone is structurally consistent with the Gagua ridge and two fault basins on both sides of the Gagua ridge, and adjusts the moving direction and distance of the western Philippine Sea plate to make the northwestward motion of the plate on its east side change to the northward subduction of the plate on its west side so that the primary tectonic stress direction of the Okinawa Trough changed from NW-SE to nearly N-S, which provided the stress source for the Okinawa Trough to enter the second spreading stage. 相似文献
1000.
Platinum group elements zoning and mineralogy of chromitites from the cumulate sequence of the Nurali massif (Southern Urals, Russia) 总被引:1,自引:0,他引:1
The Nurali lherzolite massif is one of the dismembered ophiolite bodies associated with the Main Uralian Fault (Southern Urals, Russia). It comprises a mainly lherzolitic mantle section, an ultramafic clinopyroxene-rich cumulate sequence (Transition Zone), and an amphibole gabbro unit.The cumulate section hosts small chromitite bodies at different stratigraphic heights within the sequence. Chromitite bodies from three different levels along a full section of the cumulate sequence and two from other localities were investigated. They differ in the host lithology, chromitite texture and composition, and PGE content and mineralogy. Chromitites at the lowest level, which are hosted by clinopyroxenite, form cm-scale flattened lenses. They have high Cr# and low Mg# chromites and are enriched in Pt and Pd relative to Os and Ir. At a higher, intermediate level, the chromitites are hosted by dunite. They form meter thick lenses, contain low Cr# and high Mg# chromites, have high PGE contents (up to 26,700 ppb), and are enriched in Os, Ir and Ru relative to Pt and Pd, reflecting a mineralogy dominated by laurite–erlichmanite and PGE–Fe alloys. At the highest level are chromitites hosted by olivine–enstatite rocks. These chromitites have high Cr# and relatively low Mg# chromites and very low PGE content, with laurite as the dominant PGE mineral.The platinum group minerals (PGMs) show extreme zoning, with compositions ranging from erlichmanite to almost pure laurite and from Os-rich to Ru-rich alloys, with variable and irregular zoning patterns.Two chromitite bodies up to 6 km from the main sequence can be correlated with the latter based on geochemistry and mineralogy, implying that the variations in chromitite geochemistry are due to processes that operated on the scale of the massif rather than those that operated on the scale of the outcrop.Pertsev et al. [Pertsev, A.N., Spadea, P., Savelieva, G.N., Gaggero, L., 1997. Nature of the transition zone in the Nurali ophiolite, Southern Urals. Tectonophysics 276, 163–180.] propose that the Transition Zone formed by solidification of a series of small magma bodies that partially overlapped in time and space. The magmas formed by successive partial melting of the underlying mantle. We suggest that this process determined the changing PGE geochemistry of the successive batches of magma. The PGE distribution fits a model of selected extraction from the mantle, where monosulphide solid solution–sulphide liquid equilibrium was attained until complete melting of the monosulphide solid solution. Later and localized variations in fS2 resulted in the formation of different PGMs with complex zoning patterns. 相似文献