Scattering of elastic waves by a fracture zone containing randomly distributed cracks     

Jun Kawahara  Teruo Yamashita 《Pure and Applied Geophysics》1992,139(1):121-144

We theoretically study the scattering ofP, SV andSH waves by a zonal distribution of cracks, which simulates a fault fracture zone. An investigation is conducted how the geometrical properties of the crack distribution and the frictional characteristics of the crack surface are reflected in the attenuation and dispersion of incident waves, as well as in the amplitudes of the transmitted and reflected waves from the zone. If the crack distribution within the fault zone changes temporally during the preparation process of the expected earthquake, it will be important for earthquake prediction to monitor it, utilizing the scattering-induced wave phenomena.We consider the two-dimensional problem. Aligned cracks with the same length are assumed to be randomly distributed in a zone with a finite width, on which elastic waves are assumed to be incident. The distribution of cracks is assumed to be homogeneous and sparse. The crack surface is assumed to be stress-free, or to undergo viscous friction; the latter case simulates fluid-filled cracks. The opening displacement of the crack is assumed to be negligibly small. The idea of the mean wave formalism is employed in the analysis, and Foldy's approximation is assumed.When the crack surface is stress-free, it is commonly observed for every wave mode (P, SV andSH) that the attenuation coefficientQ ^–1 peaks aroundka1, the phase velocity is almost independent ofk in the rangeka<1 and it increases monotonically withk in the rangeka>1, wherek is the intrinsicS wavenumber anda is the half length of the crack. The effect of the friction is to shift the peak ofQ ^–1 and the corner of the phase velocity curve to the low wavenumber range. The high wavenumber asymptote ofQ ^–1 is proportional tok ^–1 independently of model parameters and the wave modes. If the seismological observation thatQ ^–1 ofS waves has a peak at around 0.5 Hz in the earth's crust is combined with our results, the upper limit of crack size within the crust is estimated about 4 km. The information regarding the transmitted and reflected waves, such as the high wavenumber limit of the amplitude of the transmitted wave etc., allows estimation of the strength of the friction.  相似文献   

Unified approach to amplitude attenuation and coda excitation in the randomly inhomogeneous lithosphere   总被引：2，自引：0，他引：2  

Haruo Sato 《Pure and Applied Geophysics》1990,132(1-2):93-121

A unified model is proposed for explaining the frequency dependent amplitude attenuation and the coda wave excitation on the basis of the single scattering process in the randomly inhomogeneous lithosphere. Adopting Birch's law and a direct proportion between density and wave velocity, we statistically describe the inhomogeneous medium by one random function characterized by the von Karman autocorrelation function. We calculate the amplitude attenuation from the solid angle integral of scattered wave energy on the basis of the Born approxiimation after subtracting the travel-time fluctuation effect caused by slowly varying velocity inhomogeneities. This subtraction is equivalent to neglect energy loss by scattering within a cone around the forward direction. The random inhomogeneity of the von Karman autocorrelation function of order 0.35 with the mean square fractional fluctuation of 7.2×10^–3 1.3×10^–2 and the correlation distance of 2.15.1 km well explains observed backward scattering coefficientg and the ratioQ _P ^–1 /Q _S ^–1 , and observed and partially conjecturedQ _S ^–1 for frequencies between 0.5 Hz and 30 Hz.  相似文献   

Attenuation of aqueous metal transport at two natural acid rock drainage occurrences in the Yukon Territory, Canada     

Y.T. John Kwong 《中国地球化学学报》2006,25(B08):229-230

Naturally acidic drainage associated with pyritic black shale has been observed in many locations in the Yukon Territory. While not necessarily linked to known mineral deposits, most of these natural acid rock drainage occurrences show elevated dissolved concentrations of trace elements, especially zinc, nickel, copper, cadmium and arsenic. Based on field observations, microbial investigation, chemical analyses and geochemical modeling, the fate and transport of potentially deleterious elements at two natural acid drainage occurrences with slightly different settings are examined. The Macintosh Creek is a small, acidic stream （pH 2.98-3.40）, 2 km long, located in the Macmillan Pass area of east-central Yukon amidst known sedimentary exhalative massive sulfide mineralization but remains undisturbed by exploration activities. Its trace metal content is apparently derived from groundwater discharges, which gave as much as 5.0, 2.5, 0.7, 0.13 and 0.03 mg/L ofZn, Ni, Cu and As, respectively. Interaction and sorption reactions with algal mats, biofilms and iron oxyhydroxides appear to be the dominant mechanisms attenuating aqueous contaminant transport along the stream. Cryogenic precipitation further consolidates the ferricrete formation and reduces the mobility of the sorbed metals. The tributaries of the Engineering Creek along the Dempster Highway in northern Yukon drain through a series of dolomite, phyllite, argillite, limestone, black shale, sandstone and conglomerate with no known concentration of mineralization. In this area, the water chemistry fully reflects the local geology with acidic streams invariably associated with black shale occurrences. Groundwater seeps in the headwaters area of the km-180 Creek completely enclosed in black shale gave pH 3.0 and as much as 148, 39, 2.9 and 9.1 mg/L of Zn, Ni, Cu and As, respectively. Sorption with iron oxyhydroxide and organic matter appear to dominate the attenuation of contaminant transport along the stream. However, once entered into carbonate-dominated terrains, secondary carbonate minerals exercise additional geochemical control on the local water chemistry as a result of neutralization.  相似文献   

Elastic properties of granulite facies rocks of Mahabalipuram, Tamil Nadu, India     

M. V. M. S. Rao  K. J. Prasanna Lakshmi  L. P. Sarma  K. B. Chary 《Journal of Earth System Science》2006,115(6):673-683

Compressional and shear wave velocities and attenuation measurements have been carried out in some of the borehole samples of acidic, basic and intermediate granulites of Mahabalipuram, Tamil Nadu, India. The results have been obtained at ambient conditions using ‘time-of-flight’ pulse transmission technique at 1.0 MHz frequency. The results show linear relationships between velocity and density, and velocity and attenuation properties of the rocks. The acidic granulites show lower velocities and higher attenuation than the intermediate and basic granulites. The average values of the Poisson’s ratio of acidic, intermediate and basic granulites have been found to be 0.210, 0.241 and 0.279 respectively. The variations in velocities and attenuation in these low porosity crystalline rocks are found to be strongly influenced by their mineral composition. The laboratory velocity data (extrapolated to high pressure) of the present study and the published field velocity data from deep seismic sounding studies indicate that these granulite facies rocks may belong to mid-crustal depths only.  相似文献   

The New Berlin: Memory,Politics, and Place by Karen E. Till     

Chris Steele 《The Professional geographer》2006,58(3):357-359

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Ground motion scaling in the Marmara region, Turkey   总被引：1，自引：0，他引：1  

A. Akinci  L. Malagnini  R. B. Herrmann  R. Gok  M. B. Sørensen 《Geophysical Journal International》2006,166(2):635-651

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Dual simulations of fluid flow and seismic wave propagation in a fractured network: effects of pore pressure on seismic signature   总被引：1，自引：0，他引：1  

S. Vlastos  E. Liu  I. G. Main  M. Schoenberg  C. Narteau  X. Y. Li  B. Maillot 《Geophysical Journal International》2006,166(2):825-838

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Mapping memorials for Edith Cavell on the colonial edge     

Katie Pickles 《New Zealand geographer》2006,62(1):13-24

Abstract:  This paper identifies and explains the pattern of memorialization in New Zealand for English nurse Edith Cavell, arguing that while the influences of empire and colonial mimicry provided a strong overarching framework, local context and parochial expressions were also strong at the colonial edge. In 1915 Cavell was executed by the occupying German forces in Belgium for her resistance work. The general transnational pattern of remembrance that resulted from the huge reaction to her death is explained. The paper then turns to the New Zealand memorialization of Cavell. With reference to monuments for other medical women, and addressing where Cavell does not appear in the landscape, the last section contextualizes Cavell's New Zealand memorialization.  相似文献   

The Removal of Dissolved Metals by Hydroxysulphate Precipitates during Oxidation and Neutralization of Acid Mine Waters, Iberian Pyrite Belt   总被引：4，自引：0，他引：4  

J. Sánchez España  E. López Pamo  E. Santofimia Pastor  J. Reyes Andrés  J. A. Martín Rubí 《Aquatic Geochemistry》2006,12(3):269-298

This study examines the removal of dissolved metals during the oxidation and neutralization of five acid mine drainage (AMD) waters from La Zarza, Lomero, Esperanza, Corta Atalaya and Poderosa mines (Iberian Pyrite Belt, Huelva, Spain). These waters were selected to cover the spectrum of pH (2.2–3.5) and chemical composition (e.g., 319–2,103 mg/L Fe; 2.85–33.3 g/L SO₄⁼) of the IPB mine waters. The experiments were conducted in the laboratory to simulate the geochemical evolution previously recognized in the field. This evolution includes two stages: (1) oxidation of dissolved Fe(II) followed by hydrolysis and precipitation of Fe(III), and (2) progressive pH increase during mixing with fresh waters. Fe(III) precipitates at pH < 3.5 (stages 1 and 2) in the form of schwertmannite, whereas Al precipitates during stage 2 at pH 5.0 in the form of several hydroxysulphates of variable composition (hydrobasaluminite, basaluminite, aluminite). During these stages, trace elements are totally or partially sorbed and/or coprecipitated at different rates depending basically on pH, as well as on the activity of the SO₄⁼ anion (which determines the speciation of metals). The general trend for the metals which are chiefly present as aqueous free cations (Pb²⁺, Zn²⁺, Cu²⁺, Cd²⁺, Mn²⁺, Co²⁺, Ni²⁺) is a progressive sorption at increasing pH. On the other hand, As and V (mainly present as anionic species) are completely scavenged during the oxidation stage at pH < 3.5. In waters with high activities (> 10⁻¹) of the SO ₄⁼ ion, some elements like Al, Zn, Cd, Pb and U can also form anionic bisulphate complexes and be significantly sorbed at pH < 5. The removal rates at pH 7.0 range from around 100% for As, V, Cu and U, and 60–80% for Pb, to less than 20% for Zn, Co, Ni and Mn. These processes of metal removal represent a significant mechanism of natural attenuation in the IPB.  相似文献   

Early Estimation of Seismic Hazard for Strong Earthquakes in Taiwan   总被引：1，自引：0，他引：1  

Wen-Yu Jean  Yu-Wen Chang  Kuo-Liang Wen  Chin-Hsiung Loh 《Natural Hazards》2006,37(1-2):39-53

A shakemap system providing rapid estimates of strong ground shaking could be useful for emergency response providers in a damaging earthquake. A hybrid procedure, which combines site-dependent ground motion prediction models and the limited observations of the Real-Time Digital stream output system (RTD system operated by Central Weather Bureau, CWB), was set up to provide a high-resolution shakemap in a near-real-time manner after damaging earthquakes in Taiwan. One of the main factors that affect the result of ground motion prediction analysis is the existence of site effects. The purpose of this paper is to investigate the local site effects and their influence in the ground shaking and then establish an early estimation procedure of potential hazard for damaging earthquakes. Based on the attenuation law, the site effects of each TSMIP station are discussed in terms of a bias function that is site and intensity-level dependent function. The standard deviation of the site-dependent ground motion prediction model can be significantly reduced. The nonlinear behavior of ground soil is automatically taken into account in the intensity-level dependent bias function. Both the PGA and the spectral acceleration are studied in this study. Based on the RTD data, event correctors are calculated and applied to precisely estimate the shakemap of damaging earthquakes for emergency response.  相似文献