龙门山地震带的地质背景与汶川地震的地表破裂

龙门山位于青藏高原与扬子地台之间, 系由一系列大致平行的叠瓦状冲断带构成, 自西向东发育汶川茂汶断裂、映秀北川断裂和彭县灌县断裂,并将龙门山划分为3个构造地层带,分别为变形变质构造地层带（主要由志留系泥盆系浅变质岩和前寒武系杂岩构成）、变形变位构造地层带（主要由上古生界三叠系沉积岩构成）、变形构造地层带（主要由侏罗系至第三系红层和第四纪松散堆积构成）。 龙门山断裂带属地震危险区,3条主干断裂皆具备发生7级左右地震的能力,其中映秀北川断裂是引发地震的最主要断层,据对彭县灌县断裂青石坪探槽场地的研究结果表明,在该断裂带上最晚的一次强震发生在93040a.B.P.左右,据此,可以初步判定,这3条主干断裂的单条断裂上的强震复发间隔至少应在1000a左右,表明龙门山构造带及其内部断裂属于地震活动频度低但具有发生超强地震的潜在危险的特殊断裂,以逆冲-右行走滑为其主要运动方式。 汶川地震属于逆冲走滑型的地震,地表破裂分布于映秀北川断裂带和彭县灌县断裂带上。根据近南北向的断裂（小鱼洞断层、擂鼓断层和邓家坝断层）和地表断距可将映秀北川断层的地表破裂带划分为两个高值区和两个低值区,两个高值区分别位于南段的映秀-虹口一带和位于中北段的擂鼓北川县城邓家坝一带;两个低值区分别位于中南段的白水河茶坪一带和北段的北川黄家坝至平武石坎子一带,两个高值区分别与小鱼洞断层和擂鼓断层相关。根据保存于破裂面上的擦痕,可将该地震破裂过程划分为两个阶段,早期为逆冲作用,晚期为斜向走滑作用,其与地壳增厚构造模式和侧向挤出摸式在青藏高原东缘的推论具有不吻合性。鉴于龙门山的表层运动速率与深部构造运动速率具有不一致性,初步探讨了龙门山地区的地表过程与下地壳流之间的地质动力模型,认为下地壳物质在龙门山近垂向挤出和垂向运动,从而造成导致龙门山向东的逆冲运动、龙门山构造带抬升和汶川特大地震。在此基础上,根据汶川地震所引发的地质灾害,对地震灾后重建提出了的几点建议。     

Shocked monazite chronometry: integrating microstructural and in situ isotopic age data for determining precise impact ages

Monazite is a robust geochronometer and occurs in a wide range of rock types. Monazite also records shock deformation from meteorite impact but the effects of impact-related microstructures on the U–Th–Pb systematics remain poorly constrained. We have, therefore, analyzed shock-deformed monazite grains from the central uplift of the Vredefort impact structure, South Africa, and impact melt from the Araguainha impact structure, Brazil, using electron backscatter diffraction, electron microprobe elemental mapping, and secondary ion mass spectrometry (SIMS). Crystallographic orientation mapping of monazite grains from both impact structures reveals a similar combination of crystal-plastic deformation features, including shock twins, planar deformation bands and neoblasts. Shock twins were documented in up to four different orientations within individual monazite grains, occurring as compound and/or type one twins in (001), (100), \(\left( 10\bar{1} \right)\), \(~\{110\}\), \(\left\{ 212 \right\},\) and type two (irrational) twin planes with rational shear directions in \([0\bar{1}\bar{1}]\) and \([\bar{1}\bar{1}0]\). SIMS U–Th–Pb analyses of the plastically deformed parent domains reveal discordant age arrays, where discordance scales with increasing plastic strain. The correlation between discordance and strain is likely a result of the formation of fast diffusion pathways during the shock event. Neoblasts in granular monazite domains are strain-free, having grown during the impact events via consumption of strained parent grains. Neoblastic monazite from the Inlandsee leucogranofels at Vredefort records a ²⁰⁷Pb/²⁰⁶Pb age of 2010?±?15 Ma (2σ, n?=?9), consistent with previous impact age estimates of 2020 Ma. Neoblastic monazite from Araguainha impact melt yield a Concordia age of 259?±?5 Ma (2σ, n?=?7), which is consistent with previous impact age estimates of 255?±?3 Ma. Our results demonstrate that targeting discrete microstructural domains in shocked monazite, as identified through orientation mapping, for in situ U–Th–Pb analysis can date impact-related deformation. Monazite is, therefore, one of the few high-temperature geochronometers that can be used for accurate and precise dating of meteorite impacts.     

Noble gas and isotope geochemistry in western Canadian Arctic watersheds: tracing groundwater recharge in permafrost terrain

In Canada’s western Arctic, perennial discharge from permafrost watersheds is the surface manifestation of active groundwater flow systems with features including the occurrence of year-round open water and the formation of icings, yet understanding the mechanisms of groundwater recharge and flow in periglacial environments remains enigmatic. Stable isotopes (δ¹⁸O, δD, δ¹³C_DIC), and noble gases have proved useful to study groundwater recharge and flow of groundwater which discharges along rivers in Canada’s western Arctic. In these studies of six catchments, groundwater recharge was determined to be a mix of snowmelt and precipitation. All systems investigated show that groundwater has recharged through organic soils with elevated P_CO2, which suggests that recharge occurs largely during summer when biological activity is high. Noble gas concentrations show that the recharge temperature was between 0 and 5 °C, which when considered in the context of discharge temperatures, suggests that there is no significant imbalance of energy flux into the subsurface. Groundwater circulation times were found to be up to 31 years for non-thermal waters using the ³?H-³He method.     

Determination of the strain ellipsoid from measurements on any three sections

An ellipsoid is defined by, and may be re-constructed from, any three sections through it. In the field, calculation of the strain ellipsoid from general sections (two-dimensional strain ellipses determined from measured strain markers) is complicated by the fact that, due to experimental error and/or strain inhomogeneity, the three ellipses may not come from the same ellipsoid. The ellipses must first be adjusted to make them compatible. A method is suggested by which an adjustment ellipse is determined analytically for each of the three sections. Application of these adjustment ellipses makes the three sections compatible, and the strain ellipsoid may be determined. The principal axes of the ellipsoid are derived from the ellipsoid matrix by eigenvector analysis. Examples are given of practical applications of this method.     

Earthquakes in Switzerland and surrounding regions during 2010

This report of the Swiss Seismological Service summarizes the seismic activity in Switzerland and surrounding regions during 2010. During this period, 407 earthquakes and 85 quarry blasts were detected and located in the region under consideration. With a total of only 19 events with M_L ≥ 2.5, the seismic activity in the year 2010 was below the average over the previous 35 years. The two most noteworthy earthquakes were the M_L 3.4 Barrhorn event near Sankt Niklaus (VS) and the M_L 3.0 event of Feldkirch, both of which produced shaking of intensity IV.     

Interaction of magma with sedimentary wall rock and magnetite ore genesis in the Panzhihua mafic intrusion,SW China

In southwestern China, several large magmatic Fe–Ti–V oxide ore deposits are hosted by gabbroic intrusions associated with the Emeishan flood basalts. The Panzhihua gabbroic intrusion, a little deformed sill that contains a large titanomagnetite deposit at its base, concordantly intrudes late-Proterozoic dolostones. Mineralogical and chemical studies of the contact aureole in the footwall dolostones demonstrate that the metamorphism was largely isochemical but released large quantities of CO₂ as the rocks were converted to marble and skarns during intrusion of the gabbroic magma. Petrological modelling of the crystallization of the intrusion, using H₂O-poor Emeishan basalt as parent magma, shows that under normal conditions, Fe–Ti oxides crystallize at a late stage, after the crystallization of abundant olivine, clinopyroxene and plagioclase. In order for titanomagnetite to separate efficiently to form the ore deposit, this mineral must have crystallized earlier and close to the liquidus. We propose that CO₂-rich fluids released during decarbonatization of sedimentary floor rocks passed up through the magma. Redox equilibria calculations show that when magma with the composition of Emeishan basalt is fluxed by a CO₂-rich gas phase, its equilibrium oxygen fugacity (fO₂) increases from the fayalite–magnetite–quartz buffer (FMQ) to FMQ + 1.5. From experimental constraints on magnetite saturation in basaltic magma under controlled fO₂, such an oxidizing event would allow magnetite to crystallize near to the liquidus, leading to the formation of the deposit.     

Spectral analysis and design approach for high force‐to‐volume extrusion damper‐based structural energy dissipation

High force‐to‐volume extrusion damping devices can offer significant energy dissipation directly in structural connections and significantly reduce seismic response. Realistic force levels up to 400 kN have been obtained experimentally validating this overall concept. This paper develops spectral‐based design equations for their application. Response spectra analysis for multiple, probabilistically scaled earthquake suites are used to delineate the response reductions due to added extrusion damping. Representative statistics and damping reduction factors are utilized to characterize the modified response in a form suitable for current performance‐based design methods. Multiple equation regression analysis is used to characterize reduction factors in the constant acceleration, constant velocity, and constant displacement regions of the response spectra. With peak device forces of 10% of structural weight, peak damping reduction factors in the constant displacement region of the spectra are approximately 6.5 ×, 4.0 ×, and 2.8 × for the low, medium, and high suites, respectively. At T = 1 s, these values are approximately 3.6 ×, 1.8 ×, and 1.4 ×, respectively. The maximum systematic bias introduced by using empirical equations to approximate damping reduction factors in design analyses is within the range of +10 to ?20%. The seismic demand spectrum approach is shown to be conservative across a majority of the spectrum, except for large added damping between T = 0.8 and 3.5 s, where it slightly underestimates the demand up to a maximum of approximately 10%. Overall, the analysis shows that these devices have significant potential to reduce seismic response and damage at validated prototype device force levels. Copyright © 2007 John Wiley & Sons, Ltd.     

Articulating China’s Science and Technology: Knowledge Collaboration Networks Within and Beyond the Yangtze River Delta Megalopolis in China

In this paper, we reconsider the defining but often overlooked ‘hinge' function of megalopolises by analyzing how megalopolises have articulated national and international urban systems in the context of a globalizing knowledge economy. Taking the case of China's Yangtze River Delta(YRD) region, we particularly focus on knowledge circulation within and beyond the YRD region by analyzing the pattern and process of knowledge collaboration at different geographical scales during the 2004–2014 period. Results show that the structure of scientific knowledge collaboration as reflected by co-publications has been strongest at the national scale whereas that of technological knowledge collaboration as measured by co-patents has been strongest at the global scale. Despite this difference, the structure of both scientific and technological knowledge collaboration has been functionally polycentric at the megalopolitan scale but become less so at the national and global scales. The ‘globally connected but locally disconnected' pattern of Shanghai's external knowledge collaboration suggests that the gateway role of the YRD megalopolis in promoting knowledge collaboration at different geographical scales will take time before it is fully realized.