Nature of heterogeneity of the upper mantle beneath the northern Philippine Sea as inferred from attenuation and velocity tomography

We investigated the physical properties in the upper mantle beneath the Philippine Sea using a theoretical relation derived by Karato [Mapping water content in the upper mantle. Subduction factory, AGU Monograph, in press]. From the attenuation model of Shito and Shibutani [Phys. Earth Planet. Interact., in press] and the velocity model of Widiyantoro et al. [Earth Planet. Sci. Lett. 173 (1999) 91], observed attenuation and velocity anomalies were evaluated to explain the temperature, water content, and chemical heterogeneities in the target area. The results indicate that the observed anomalies in the shallower regions (50–200 km) may be due to chemical composition effects (e.g., concentration of iron), in addition to the temperature and water content anomalies. In contrast, for the deep upper mantle (300–400 km), the observations can be explained by only the effects of high water content (10–50 times higher than the average mantle). These inferred properties of the mantle are consistent with the tectonic history of the Philippine Sea region, which has had a long history of subduction and active magmatism.     

High-pressure partial melting of garnet pyroxenite: possible mafic lithologies in the source of ocean island basalts

Many ocean island basalts (OIB) that have isotopic ratios indicative of recycled crustal components in their source are silica-undersaturated and unlike silicic liquids produced from partial melting of recycled mid-ocean ridge basalt (MORB). However, experiments on a silica-deficient garnet pyroxenite, MIX1G, at 2.0-2.5 GPa show that some pyroxenite partial melts are strongly silica-undersaturated [M.M. Hirschmann et al., Geology 31 (2003) 481-484]. These low-pressure liquids are plausible parents of alkalic OIB, except that they are too aluminous. We present new partial melting experiments on MIX1G between 3.0 and 7.5 GPa. Partial melts at 5.0 GPa have low SiO₂ (<48 wt%), low Al₂O₃ (<12 wt%) and high CaO (>12 wt%) at moderate MgO (12-16 wt%), and are more similar to primitive OIB compositions than lower-pressure liquids of MIX1G or experimental partial melts of anhydrous or carbonated peridotite. Solidus temperatures at 5.0 and 7.5 GPa are 1625 and 1825°C, respectively, which are less than 50°C cooler than the anhydrous peridotite solidus. The liquidus temperature at 5.0 GPa is 1725°C, indicating a narrow melting interval (∼100°C). These melting relations suggest that OIB magmas can be produced by partial melting of a silica-deficient pyroxenite similar to MIX1G if its melting residue contains significant garnet and lacks olivine. Such silica-deficient pyroxenites could be produced by interaction between recycled subducted oceanic crust and mantle peridotite or could be remnants of ancient oceanic lower crust or delaminated lower continental crust. If such compositions are present in plumes ascending with potential temperatures of 1550°C, they will begin to melt at about 5.0 GPa and produce appropriate partial melts. However, such hot plumes may also generate partial melts of peridotite, which could dilute the pyroxenite-derived partial melts.     

Experimental examination on the heterogeneity parameter C_v of earthquake precursors

Introduction Many earthquake cases indicate that there exists obvious heterogeneity in temporal-spatial distribution of earthquake precursors (FENG, 1983; MEI, et al, 1993). Therefore, it is very crucial for the study of earthquake prediction to describe the heterogeneity in temporal-spatial distribution of earthquake precursors and to understand its physical mechanism. Up to now, a lot of useful researches have been done in this field. Recently, WANG, et al (1999) and CHEN, et al (2000)…     

A formula for gravimetric terrain corrections using powers of topographic height

Journal of Geodesy - ?The application of Stokes' formula to create geoid undulations requires no masses outside the geoid. However, due to the existence of the topography, terrain...     

On the Use of Multivariate Lévy-Stable Random Field Models for Geological Heterogeneity

Increasing attention has been paid to the use of non-Gaussian distributions as models of heterogeneity in sedimentary formations in recent years. In particular, the Lévy-stable distribution has been shown to be a useful model of the distribution of the increments of data measured in well logs. Frequently, the width of this distribution follows a power–law type scaling with increment lag, thus suggesting a nonstationary, fractal, multivariate Lévy distribution as a useful random field model. However, in this paper we show that it is very difficult to formulate a multivariate Lévy distribution with any nontrivial spatial correlations that can be sampled from rigorously in large models. Conventional sequential simulation techniques require two properties to hold of a multivariate distribution in order to work: (1) the marginal distributions must be of relatively simple form, and (2) in the uncorrelated limit, the multivariate distribution must factor into a product of independent distributions. At least one of these properties will break down in a multivariate Lévy distribution, depending on how it is formulated. This makes a rigorous derivation of a sequential simulation algorithm impossible. Nonetheless, many of the original observations that spurred the original interest in multivariate Lévy distributions can be reproduced with a conventional normal scoring procedure. Secondly, an approximate formulation of a sequential simulation algorithm can adequately reproduce the Lévy distributions of increments and fractal scaling frequently seen in real data.     

Seismic monitoring of explosions: a method to extract information on the isotropic component of the seismic source

The design of a monitoring system for detecting explosions is a very topical problem, both for routine data processing at seismological observatories as well as for the monitoring of a Comprehensive Test Ban Treaty. In this framework it is desirable to have the possibility to quantify the presence of the isotropic component in the seismic source. For this purpose a method is presented, which is based on waveform inversion for the full moment tensor retrieval. The method inverts either full waveforms or separate seismic phases and returns the mechanism and time history of a point source. Moreover, it allows to redefine the hypocentral depth of the event and, in a simplistic way, to optimize the structural model as well. In order to model strong laterally heterogeneous structures, different pairs of structural models can be used for each source-receiver path. The source is decomposed into a volumetric part (V), representing an explosive or implosive component, and into a deviatoric part, containing both the double couple (DC) and the compensated linear vector dipole (CLVD) components. The method is applied to an area in central Switzerland and to the network of the Swiss Seismological Service. The events of interest include both earthquakes and explosions. Despite some modelling inadequacies of the source-time function, the explosions can be well identified with the inverted isotropic component in the source, as long as the number of stations used for the inversion is larger than three. The results of the inversion are better for large epicenter-station distances of the order of 40–90 km.     

Evaluation of deflections of the vertical on the sphere and the plane: a comparison of FFT techniques

This paper presents a set of efficient formulas to evaluate the deflections of the vertical on the sphere using gridded data. The Vening-Meinesz formula, the topographic indirect effect on the deflections of the vertical as well as the terrain corrections are expressed as both 2D and 1D convolutions on the sphere, and consequently can be evaluated by the 2D and the 1D fast Fourier transform (FFT). When compared with the results obtained from pointwise integration, the use of the 1D FFT gives identical results, and therefore these results were used as control values in this paper. The use of the spherical 2D FFT improves significantly the computational efficiency with little sacrifice of accuracy (0.6^″ rms difference from the 1D FFT results). The planar 2D FFT, which is as efficient as the spherical 2D FFT, gives worse results (1.2^″ rms difference from the 1D FFT results) because of the extra approximations. Received: 27 February 1996; Accepted: 24 January 1997     

Ergodicity and the ‘scale effect’

Solute plume spreading in an aquifer exhibits a ‘scale effect’ if the second spatial concentration moment of a plume has a non-constant time-derivative. Stochastic approaches to modeling this scale effect often rely on the critical assumption that ensemble averages can be equated to spatial averages measured in a single field experiment. This ergodicity assumption should properly be evaluated in a strictly dynamical context, and this is done in the present paper. For the important case of trace plume convection by steady groundwater flow in an isotropic, heterogeneous aquifer, ergodicity does not obtain because of the existence of an invariant function on stream surfaces that is not uniform throughout the aquifer. The implications of this result for stochastic models of solute transport are discussed. © 1997 Elsevier Science Ltd. All rights reserved     

台湾附近地形对台风影响的数值模拟

利用ＴＣＭ－９０资料分析并用数值方法成功地模拟了９０１７号台风经过台湾时发生复杂变化的过程，包括环流场副中心和高度场副中心形成及与其主中心的合并过程，环流中心合并时路径跳跃过程，台湾西侧低压中心异常北移过程，台湾海峡低层急流变化过程以及上下层高度中心与环流中心明显偏离的现象，并详细分析了模拟中每隔３小时的演变过程，给出了台风经过台湾前后的结构及详细的路径变化，这种详细的演变图像不可能从间隔６小时的