High-pressure metamorphism and deep-crustal seismicity: evidence from contemporaneous formation of pseudotachylytes and eclogite facies coronas

The metamorphic complex of the Western Gneiss Region (WGR), Norway, constitutes the root of the Caledonian mountain belt and experienced temperatures of 700–800 °C and pressures in excess of 20 kbar during peak metamorphism. Mafic bodies surrounded by strongly banded felsic gneisses commonly exhibit variable reequilibration to granulite and eclogite facies conditions and locally preserve igneous minerals and textures. The Kråkeneset gabbro, located on the island of Vågsøy in the mixed HP/UHP zone of the western WGR, display evidence for extensive metastability through the entire prograde and retrograde P, T histories. Eclogite constitutes less than a few percent of the total volume of the body and high-pressure assemblages typically form thin coronas around magmatic phases or occur along localized zones of brittle deformation and fluid infiltration. The gabbro displays pseudotachylyte vein networks that define subparallel brittle fault zones, <50 cm wide, transecting the gabbro body. The pseudotachylytes contain μm- to mm-scale amoeboid and dendrite-like textures of garnet and plagioclase with inclusions of the eclogite facies minerals orthopyroxene, omphacite, amphibole, and dolomite, suggesting rapid disequilibrium growth of minerals during high-pressure conditions. Textural and petrological evidence from pseudotachylytes and corona structures show that the growth of these unusual textures occurred shortly after pseudotachylyte crystallization by a process of rapid solid-state alteration of a microcrystalline pseudotachylyte matrix. The pseudotachylyte-lined fault zones are in close spatial association with numerous amphibole±carbonate-filled hydrofractures with conspicuous fracture-parallel alteration zones defined by hydrous eclogite facies assemblages. These eclogite facies hydrofractures testify to the existence of high fluid pressures and to fluid infiltration following brittle failure during high-grade metamorphic conditions. Geothermobarometric estimates (ca. T=650–700 °C, P=20 kbar) and petrological data imply that hydrofracturing, pseudotachylyte crystallization, and the subsequent pseudotachylyte alteration process must have occurred during high-pressure metamorphism. Our observations are suggestive of a deep-crustal earthquake scenario where a high-pressurized fluid phase plays a double role by causing both seismic failure through the embrittlement effect and facilitating eclogitization of the metastable anhydrous gabbro. Metamorphic reaction along hydrofractures and fault planes led to the development of eclogite facies foliation fabrics and illustrate the rheological change from brittle to plastic behavior associated with the gabbro to eclogite transition. The formation of weak deep-crustal shear zones following brittle failure represents an arrested initiation of the physical breakup and metamorphic reequilibration of the Kråkeneset gabbro during its residence deep in the former Caledonian collision zone.     

Comparison between thermal–viscous coupling and frictional sliding

We investigated the similarity between thermal–viscous coupling (TVC) and frictional sliding, proposed by Kameyama and Kaneda [Pure Appl. Geophys. 159 (2002) 2011]. We consider a one-dimensional layer composed of viscous material, which is sandwiched and sheared by two thick elastic layers. The rate of viscous deformation depends on the temperature T_c in the viscous layer as well as shear stress τ. The temperature T_c changes owing to heating by viscous dissipation and conductive cooling. We carried out velocity-stepping tests for the steady-state deformation both numerically and analytically, and compared the temporal evolution of small perturbations with that of the spring-block model with rate- and state-dependent friction (RSF). We found that, as is the case of frictional slip stability, the manner of temporal evolution is classified into four regimes depending on whether it is stable or not and whether it is monotonous or oscillatory with time. By further interpreting TVC in terms of general RSF theory by Ruina [J. Geophys. Res. 88 (1983) 10359], we obtained the relations between the parameters appearing in the phenomenological RSF law and the nondimensional parameters which characterize the nature of TVC. A further improvement of this approach might be important for estimating the actual values of frictional constitutive parameters at the deeper portion of seismogenic faults of interplate or inland earthquakes where a ductile deformation is expected to be significant.     

Earthquake-induced landslides in Central America

Central America is a region of high seismic activity and the impact of destructive earthquakes is often aggravated by the triggering of landslides. Data are presented for earthquake-triggered landslides in the region and their characteristics are compared with global relationships between the area of landsliding and earthquake magnitude. We find that the areas affected by landslides are similar to other parts of the world but in certain parts of Central America, the numbers of slides are disproportionate for the size of the earthquakes. We also find that there are important differences between the characteristics of landslides in different parts of the Central American isthmus, soil falls and slides in steep slopes in volcanic soils predominate in Guatemala and El Salvador, whereas extensive translational slides in lateritic soils on large slopes are the principal hazard in Costa Rica and Panama. Methods for assessing landslide hazards, considering both rainfall and earthquakes as triggering mechanisms, developed in Costa Rica appear not to be suitable for direct application in the northern countries of the isthmus, for which modified approaches are required.     

Seismic reliability analysis of earth slopes under short term stability conditions

Different models were developed for evaluating the probabilistic three-dimensional (3-D) stability analysis of earth slopes and embankments under earthquake loading. The 3-D slope stability model assumed is that of a simple cylindrical failure surface. The probabilistic models evaluate the probability of failure under seismic loading considering the randomness of earthquake occurrence, and earthquake induced acceleration and uncertainties stemming from the discrepancies between laboratory-measured and in-situ values of shear strength parameters. The models also takes into consideration the spatial variabilities and correlations of soil properties. The probabilistic analysis and design approach is capable of obtaining the 2-D and 3-D static and dynamic safety factors, the probability of slope failure, the earthquake induced acceleration coefficient, the yield acceleration coefficient, the earthquake induced displacement, and the probability of allowable displacement exceedance taking into account the local site effect. The approach is applied to a well known landslide case: Congress Street Landslide in Chicago. A sensitivity analysis was conducted on the different parameters involved in the models by applying those models to the Congress Street landslide considering different levels of seismic hazard. Also, a sensitivity analysis was carried out to study the sensitivity of computed results to input parameters of undrained shear strength, and corrective factors. A comparison was made between the different models of failure. The parametric study revealed that the hypocentral distance and earthquake magnitude have major influence on the earthquake induced displacement, probability of failure and dynamic 2-D and 3-D safety factors.     

An Overview of Some Significant Eastern Canadian Earthquakes and Their Impacts on the Geological Environment,Buildings and the Public

Numerous moderate to large earthquakes have occurred in eastern Canada. Some ofthese events had significant geological effects such as surface faulting, liquefaction,submarine slumping, rock avalanches, rock falls, landslides, railroad embankmentslides, and one tsunami. Some of these earthquakes caused considerable damage tobuildings with unreinforced masonry elements that were located on thick clay deposits. These events also had strong psychological and social impacts, mainly due to the unpreparedness of the population. To minimize these impacts, programs should be designed to map the land and offshore areas most susceptible to mass movements (Earth Sciences), to define buildings most at risk (Engineering) and to educate the public about mitigation actions (Education, science popularization).     

Using Grey System Theory for Earthquake Forecast

By combining conventional grey correlation analysis,grey clustering method and grey forecasting methods with our multi-goal forecast thoughts and the techniques of grey time series processing,we develop six different grey earthquake forecast models in this paper,Using the record of major earthquakes in Japan from 1872 to 1995,we forecast future earthquakes in Japan.We develop an earthquake forecast model.By using the major earthquakes in Japan from 1872 to 1984,we forecast earthquakes from 1985 to 1995 and check the precision of the grey earthquake models.We find that the grey system theory can be applied to earthquake forecast.We introduce the above analysis methods and give a real example to evaluate and forecast.We also further discuss the problems of how to improve the precision of earthquake forecast and how to strengthen the forecast models in future research.     

Response spectrum predictions for potential near-field and far-field earthquakes affecting Hong Kong: rock sites

To realistically assess the seismic risk relating to built infrastructures in Hong Kong and in the neighbouring coastal cities of southern Guangdong province, it is necessary to predict ground shaking induced by different earthquake scenarios with good accuracy. A companion paper has described the modelling of the spatial and temporal distribution of the diffused seismic activities in the region, based on the newly-developed ‘Expanding Circular Disc’ (ECD) method. Representative Magnitude–Distance (M–R) combinations for both near-field and far-field earthquakes (in relation to Hong Kong) have been derived using the ECD method. The present paper describes the modelling of the response spectrum on rock sites associated with the predicted M–R combinations, using the Component Attenuation Model (CAM) that was also developed recently by the authors, based on stochastic simulations of the seismological model. The significant effects of soil resonance on the response spectrum are described in a separate publication.The accuracy of CAM in modelling ground motion properties on rock sites has been tested here by comparisons with (i) strong motions recorded in Taiwan and South China from the 1999 ‘Chi-Chi’ earthquake in Taiwan (M=7.6), (ii) motions recorded in South China from another earthquake occurring in the southern Taiwan Strait in the same year (M=5.1), and (iii) historical seismic intensity data obtained within South China. The overall capability of CAM in modelling both near-field and far-field attenuation has been shown to be unmatched by existing empirical models. Results of the comparison studies confirm the accuracy of CAM, particularly within an epicentral distance of 300–400 km.This study shows that the developed serviceability response spectra (i.e. at short return periods) are controlled mainly by the earthquake recurrence behaviour of major distant seismic sources. In contrast, the ultimate response spectra (i.e. at long return periods) relate to events with magnitudes close to the maximum credible earthquake (MCE) limit, the effect of which may also be represented by the Characteristic Response Spectrum (CRS). Both types of earthquake scenario can be significantly affected by the regional crustal properties. The proposed response spectrum envelopes have been compared with previously developed recommendations, and a critical review has been conducted. The intrinsic advantages of the ECD–CAM modelling approach have been highlighted, emphasising its directness and transparency when compared with the more complex process required to implement traditional Probabilistic Seismic Hazard Assessment (PSHA).     

基于三重震相波形非线性反演的俯冲带410-km间断面起伏研究

利用NECESSArray宽频带地震台阵记录的P波三重震相波形资料,采用遗传算法,对千岛俯冲板块内部及附近410-km间断面的结构进行了非线性反演.其中,选取了发生在俯冲带的发震时刻为2009年10月10日21时24分(GMT时间)震级为Mw5.9的地震;其三重震相的射线回折点处射线路径的方向与俯冲板块的走向大致一致,...     

Earthquake-induced landslides: 1980–1997

A database of earthquake-induced landslides has been compiled which extends the work of Keefer (Keefer DK. Landslides caused by earthquakes. Bulletin of the Geological Society of America 1984;95:406–421) who covered the period 1811–1980 to 1997. A total of 36 earthquakes world-wide are included, the new database having about the same number of earthquakes as reported by Keefer. Correlations evolving from the new database are compared with those of Keefer. Generally the results are very similar, though the presence of extreme outliers in some of the correlations emphasises the need to be aware of special cases, particularly those involving quick clay landslides. Seismological features, including multiple earthquakes and simultaneous arrival of different phases of seismic waves, also influence the outliers. The correlations between earthquake magnitude and total landslide area, however, differ somewhat from Keefer's. For the intermediate magnitude range 5.3–7.0, a modified correlation is suggested. The scatter of the data from which the correlations are derived is greater than found by Keefer. This is ascribed to the different geographic locations of the earthquakes in the two data sets.     

完全库仑破裂应力变化与云南龙陵震群序列的应力触发

计算和研究了1976年云南龙陵Ms73、Ms74双主震产生的完全库仑破裂应力变化及其对后续震群序列的动态和静态应力触发作用.结果显示,龙陵第2主震受到了第1主震的动态和静态库仑破裂应力的触发作用.龙陵双主震的13个后续强余震中,发生在龙陵三角形块体内部的强余震,90%都受到了第1或第2主震的动态库仑破裂应力触发或静态库仑破裂应力触发作用,发生在三角形块体外部的强余震,2/3受到了第2主震的动态库仑破裂应力触发作用.从触发作用的强度和范围上看,第2主震的动态和静态库仑破裂应力触发作用都大于第1主震.就近场而言,受到龙陵双主震的动态和静态库仑破裂应力触发作用的余震数目相当,但动态库仑破裂应力的触发作用范围大于静态库仑破裂应力.