A method of body-wave inversion is developed in an attempt to extract the information about asperities or barriers in a fault zone. A sequence of point sources, each being characterized with the seismic moment, the onset time and the location, are iteratively derived from observed records at multi-stations, where the two-dimensional extent of the source location is taken into account. A modification is made of the iterative method of Kikuchi and Kanamori on the formulation of inversion procedure to facilitate the computation.Using this method, we analyse long period P waves of the Tokachi-Oki earthquake of 1968 (Mw = 8.2) and obtain several significant subevents with time durations of ～ 10 s. Their spatio-temporal distribution shows that the rupture process consists of three characteristic stages: (A) a stage of introductory rupture, (B) a stage of main rupture and (C) a stage of aftershocks. The main rupture takes place in the form of clustering around a few sites of the fault plane. The largest subevent occurs in the northwestern corner. The stress drop associated with this event is estimated to be ～ 200 bars, one order of magnitude higher than the stress drop averaged over the entire fault plane. The sum of the seismic moments of the individual subevents amounts to 2.3 × 1028 dyn. cm which approximately coincides with the one estimated from the analysis of long-period surface waves. This implies that the source of the Tokachi-Oki earthquake consists of several major subevents with time durations of ～ 10 s in addition to other minor subevents. 相似文献
Elastic moduli of forsterite were measured between 300 and 1,200 K (? 1.6 times the Debye temperature) by the Rectangular Parallelepiped Resonance method. All the moduli decrease regularly with temperature. A summary of the results is as follows:
The total carbon content analysis for Takashima-oki core extracted from Lake Biwa with time resolutions of approximately 150 years
was used to reconstruct millennial-scale climate change in the East Asian monsoon region over the past 300 ka. The total carbon
content of the Takashima-oki core is an indirect proxy for the East Asian summer monsoon, because the total carbon content
reflects primary productivity within the lake, which is controlled by precipitation in the catchment area. Using these data,
we clarify temporal variations in the East Asian summer monsoon prior to MIS 5, which were previously unresolved. The observed
variations show a ∼1,500-year cycle that is also recorded by paleoclimate indices in the North Atlantic region. We propose
that the East Asian summer monsoon underwent abrupt millennium-scale changes during interglacial stages such as MIS 5 and
7. 相似文献
A 3D magnetic inversion method using a conjugate gradient method (CG method) was developed for constructing 3D magnetization models of a volcanic edifice and applied to aeromagnetic anomalies of Izu-Oshima Volcano surveyed in 1986 and in 1997. The calculated results of the 1986 data show that the volcanic edifice of Izu-Oshima Volcano has a mean magnetization intensity ranging from 10.4 to 12.1 A/m. The derived 3D magnetic structure shows low magnetization zones beneath the west-northwest of the western caldera rim, beneath the west-southwest of Mt. Mihara and beneath Mt. Shiroishi. These features may be related with demagnetizations, reflecting a high thermal state due to magma activities in the 1986 eruption. The comparison between 3D magnetization models in 1986 and in 1997, indicates meaningful changes beneath the C-craters erupted in 1986, suggesting a recovery process of demagnetizations and a considerable decrease of magnetization intensities in the foot of Mt. Futago, indicative of demagnetizations. A derived magnetization model including Izu-Oshima Volcano and its surrounding sea areas clarifies the submerged volcanic edifices around Izu-Oshima Island, and suggests that the old volcanic edifices of Fudeshima, Gyojyanoiwaya, and Okata Volcanoes have been affected by eastward migrations due to massive intrusions of a dike-like structure inferred at the base of Izu-Oshima Volcano. 相似文献
The oxidation state and mineral phase association of Co, Ce, and Pb in hydrogenetic, diagenetic, and hydrothermal marine ferromanganese oxides were characterized by X-ray absorption near-edge structure (XANES) and extended X-ray absorption fine structure (EXAFS) spectroscopy and chemical extraction. Cobalt is trivalent and associated exclusively with the Mn oxide component (vernadite). Cerium is tetravalent in all genetic-type oxides (detection limit for Ce(III) ∼ 5 at. %), including Fe-rich areas (ferrihydrite) of hydrogenetic oxides, and is associated primarily with vernadite. Thus, the extent of a Ce anomaly does not result from variations in redox conditions, but appears to be kinetically controlled, decreasing when the growth rate increases from hydrogenetic to diagenetic to hydrothermal oxides. Lead is divalent and associated with Mn and Fe oxides in variable proportions. According to EXAFS data, Pb is mostly sorbed on edge sites at chain terminations in Fe oxide and at layer edges in Mn oxide (ES complex), and also on interlayer vacancy sites in Mn oxide (TCS complex). Sequential leaching experiments, spectroscopic data, and electrochemical considerations suggest that the geochemical partitioning in favor of the Mn oxide component decreases from Co to Ce to Pb, and depends on their oxidative scavenging by Mn and Fe oxides. 相似文献
We present in this study the effects of short‐term heating on organics in the Tagish Lake meteorite and how the difference in the heating conditions can modify the organic matter (OM) in a way that complicates the interpretation of a parent body's heating extent with common cosmothermometers. The kinetics of short‐term heating and its influence on the organic structure are not well understood, and any study of OM is further complicated by the complex alteration processes of the thermally metamorphosed carbonaceous chondrites—potential analogues of the target asteroid Ryugu of the Hayabusa2 mission—which had experienced posthydration, short‐duration local heating. In an attempt to understand the effects of short‐term heating on chondritic OM, we investigated the change in the OM contents of the experimentally heated Tagish Lake meteorite samples using Raman spectroscopy, scanning transmission X‐ray microscopy utilizing X‐ray absorption near edge structure spectroscopy, and ultraperformance liquid chromatography fluorescence detection and quadrupole time of flight hybrid mass spectrometry. Our experiment suggests that graphitization of OM did not take place despite the samples being heated to 900 °C for 96 h, as the OM maturity trend was influenced by the heating conditions, kinetics, and the nature of the OM precursor, such as the presence of abundant oxygenated moieties. Although both the intensity of the 1s?σ* exciton cannot be used to accurately interpret the peak metamorphic temperature of the experimentally heated Tagish Lake sample, the Raman graphite band widths of the heated products significantly differ from that of chondritic OM modified by long‐term internal heating. 相似文献
A combined ocean bottom seismometer, multichannel seismic reflection and gravity study has been carried out along the spreading
direction of the Knipovich Ridge over a topographic high that defines a segment center. The youngest parts of the crust in
the immediate vicinity of the ridge reveal fractured Oceanic Layer 2 and thermally expanded and possibly serpentinized Oceanic
Layer 3. The mature part of the crust has normal thickness and seismic velocities with no significant crustal thickness and
seismic velocity variations. Mature Oceanic Layer 2 is in addition broken into several rotated fault blocks. Comparison with
a profile acquired ~40 km north of the segment center reveals significant differences. Along this profile, reported earlier,
periods of slower spreading led to generation of thin crust with a high P-wave velocity (Vp), composed of a mixture of gabbro and serpentinized mantle, while periods of faster spreading led to generation of more
normal gabbroic crust. For the profile across the segment center no clear relation exists between spreading rate and crustal
thickness and seismic velocity. In this study we have found that higher magmatism may lead to generation of oceanic crust
with normal thickness even at ultra-slow spreading rates. 相似文献
The ultra-slow, asymmetrically-spreading Knipovich Ridge is the northernmost part of the Mid Atlantic ridge system. In the
autumn of 2002 a combined ocean-bottom seismometer multichannel seismic (OBS/MCS) and gravity survey along the spreading direction
of the Knipovich Ridge was carried out. The main objective of the study was to gain an insight into the crustal structure
and composition of what is assumed to be an amagmatic segment of oceanic crust. P-wave velocity and Vp/Vs models were built
and complemented by a gravity model. The 190 km long transect reveals a much more complex crustal structure than anticipated.
The magmatic crust is thinner than the global average of 7.1 ± 1.0 km. The young fractured portion of Oceanic Layer 2 has
low seismic velocities while the older part has normal seismic velocities and is broken into several rotated fault blocks
seen as thickness variations of Layer 2. The youngest part of Oceanic Layer 3 is also dominated by low velocities, indicative
of fracturing, seawater circulation and thermal expansion. The remaining portion of Layer 3 exhibits inverse variations in
thickness and seismic velocity. This is explained by a sequence of periods of faster spreading (estimated to be up to 8 mm/year
from interpretation of magnetic anomalies) when more normal gabbroic crust was being generated and periods of slower spreading
(5.5 mm/year) when amagmatic stretching and serpentinization of the upper mantle occurred, and crust composed of mixed gabbro
and serpentinized mantle was generated. The volumetric changes and upward fluid migration, associated with the process of
serpentinization in this part of the crust, caused disruption to the overlying sedimentary layers. 相似文献
Rare earth element (REE) pattern is a unique geochemical tracer and has been measured for various natural materials. Among these, the REE distribution pattern between bacteria and water exhibits anomalous enrichment in the heavy REE (HREE) part, which can act as a signature of bacteria-related materials in natural samples. In this study, the REE binding site on the cell surface of a Gram-positive bacterium (Bacillus subtilis) responsible for HREE enrichment has been identified using extended X-ray absorption fine structure (EXAFS) coupled with a study of the variation in REE distribution patterns. The EXAFS data showed that the HREEs form complexes with multiple phosphate site (including phosphoester site) with a larger coordination number (CN) at lower REE-bacteria ratios ([REE]/[bac]), while light and middle REEs form complexes to the phosphate site with a lower CN. The fraction coordinated to carboxylate increased for all REEs with increasing [REE]/[bac] ratio. On the other hand, the enrichment of HREE in the REE distribution patterns of the bacteria was less marked with increasing [REE]/[bac] ratio. This result is consistent with the EXAFS data, because the REE pattern of surface complex with multiple phosphate in a reference material exhibits a monotonous increase for heavier REE, while phosphate surface complex with a low CN and a carboxylate site reach a maximum around Sm and Eu. Based on these results, it is clear that the REE are primarily bound to the phosphate site and subsequently to the carboxylate site on the bacterial cell surface.Regarding the pH dependence in the range (3 < pH < 7), both the EXAFS and REE pattern data indicate that the fraction of REE-carboxylate increased as the pH increases. The results above obtained for B. subtilis were also valid for Escherichia coli, a Gram-negative bacterium, showing that similar phosphate and carboxylate sites are also available in the cell walls of E. coli, or other Gram negative bacteria. In all our results, the variation in REE patterns correlated with the binding site indicated by EXAFS, showing that the REE pattern itself reflects the binding site of the REE at the bacterial surface for various parameters (pH and [REE]/[bac] ratio). Thus, the REE patterns can be used to estimate the binding sites for lower [REE]/[bac] ratios where spectroscopic techniques cannot be applied.The average bond length between the REE and oxygen was compared for various REE sorbed on bacteria, showing that the bond length for HREE (Er to Lu) was much shorter than those extrapolated from the trend between La and Dy, because of the selective binding of the HREE as the multiple phosphate surface complexes. Our results are consistent with the selective enrichment of the HREE at the bacterial cell surfaces, considering that chemical species with a shorter bond length are more stable. Thus, it is clear that the HREE enrichment at the bacterial cell surfaces is caused by the formation of the multiple phosphate surface complexes. Based on these results, it is suggested that materials having such phosphate sites such as bacteria and bacteria-related materials can induce anomalous HREE enrichment in natural systems. 相似文献
Greenish veins occurring in brecciated bentonite were found in the Kawasaki bentonite deposit of the Zao region in Miyagi Prefecture, Japan. Their occurrence possibly indicates the interaction of bentonite with Fe-rich hydrothermal solutions. In order to prove the hypothesis and understand the long-term mineralogical and petrographic evolution of bentonite during such interactions, the greenish veins and the surrounding altered bentonite were analyzed using X-ray fluorescence (XRF), scanning electron microscopy (SEM), X-ray diffraction (XRD), electron probe micro-analysis (EPMA), scanning transmission electron microscopy with energy dispersed spectroscopy (STEM-EDS) and micro X-ray absorption near-edge structure (XANES). The greenish veins resulting from hydrothermal solution are composed of mixed-layer minerals consisting of smectite and glauconite (glaucony), pyrite and opal. The occurrences indicate that glaucony and pyrite formed almost simultaneously from hydrothermal solution prior to opal precipitation. The mineral assemblages of the greenish veins and their surroundings indicate that the hydrothermal activity had most likely taken place at a temperature of less than 100 °C and that the pH and Eh conditions of the reacted solution were neutral to alkaline pH and reducing. The unaltered bentonite is composed mainly of Al smectite and opal. These minerals coexist as a mixture within the resolution level of the microprobe analyses. On the other hand, the bentonite in contact with the greenish veins consists of discrete opal grains and dioctahedral Al smectite containing Fe and was altered mineralogically and petrographically by the hydrothermal activity. Both the clay minerals and the opal were formed by dissolution and subsequent precipitation from the interaction of the original bentonite with the hydrothermal solution. 相似文献
