基于远场参考的超低频(ULF)地震地磁数据处理——以2003年1月日本茨城县5.1级地震为例

野外观测和室内试验表明,地震孕育和发生过程中可能产生电磁信号。地震电磁现象在防震减灾中具有潜在的应用价值,已成为21世纪国内外地学研究热点之一。由于地震电磁信号较弱,通常混杂于空间场源信号及人文噪音中,不易在观测数据中提取出来。以2003年1月日本茨城县5.1级地震为例,探讨远场参考在超低频(ULF)地震地磁数据处理中的应用,进而检测震前局部磁场异常。在此基础上,讨论远场参考法的优势及可能存在的不足。分析结果表明,2003年1月日本茨城县5.1级地震发生前存在局部磁场异常,验证了远场参考方法的有效性,相关成果可为地震地磁数据处理提供有益参考。     

Influence of submarine fumaroles on the distribution of mercury in the sediment of Kagoshima Bay,Japan

To estimate the influence of mercury emitted from submarine fumaroles, the horizontal and vertical distribution of mercury in sediment of Kagoshima Bay was studied. The fumaroles are located in the northern bay head area, and the sediment samples had been taken from 52 points throughout the bay with a gravity core sampler. The core samples obtained were cut at a thickness of 1–2 cm and used for measurements. The total concentration of mercury in surface sediment in the northern and central areas of the bay was 51–679 μg kg^− 1 (average 199 μg kg^− 1, n = 22) and 23–100 μg kg^− 1 (average 55 μg kg^− 1, n = 30), respectively. The highest value was obtained in the vicinity of the fumaroles. The mercury concentration in sediment near the fumaroles varied with depth, which may reflect the variation in fumarolic activity. A successive extraction method was applied to the speciation of mercury in the sediment. The results showed that sediment taken in the vicinity of submarine fumaroles contained a higher percentage of mercury bound with organic matter.     

Long-term features of drifting and standing non-dipole fields as determined from Holocene palaeomagnetic secular variations

Geomagnetic field motions of Holocene secular variations are investigated using a separation method. The palaeomagnetic secular variations from Britain, North America and Australia have been subjected to maximum-entropy method analyses. Based on the results of spectral analyses, the secular variations are separated by band-pass filters into low-frequency components, generally including the period band 1800-3600 yr, and high-frequency components, generally including the period band 1000-1200 yr. There is an interval, from 4200 to 1700 yr BP, which shows clockwise rotational motions in the low-frequency components of all three sites. Westward drifting of geomagnetic fields may be globally dominant. Swinging or elliptical looping motions constrained to a certain direction were observed in the low-frequency components of the British data. The time duration for the persistence of the swinging motion constrained to a certain direction was 3500 years or so, which could be the lifetime of an oscillating stationary field. The duration of the transitional motion was 1000-1300 years, which may indicate the recurrence time of a stationary field.     

Internal structures and high-velocity frictional properties of a bedding-parallel carbonate fault at Xiaojiaqiao outcrop activated by the 2008 Wenchuan earthquake

This paper reports internal structures of a bedding-parallel fault in Permian limestone at Xiaojiaqiao outcrop that was moved by about 0.5 m during the 2008 M_W7.9 Wenchuan earthquake. The fault is located about 3 km to the south from the middle part of Yingxiu-Beichuan fault, a major fault in the Longmenshan fault system that was moved during the earthquake. The outcrop is also located at Anxian transfer zone between the northern and central segments of Yingxiu-Beichuan fault where fault system is complex. Thus the fault is an example of subsidiary faults activated by Wenchuan earthquake. The fault has a strike of 243° or N63°E and a dip of 38°NW and is nearly optimally oriented for thrust motion, in contrast to high-angle coseismic faults at most places. Surface outcrop and two shallow drilling studies reveal that the fault zone is several centimeters wide at most and that the coseismic slip zone during Wenchuan earthquake is about 1 mm thick. Fault zone contains foliated cataclasite, fault breccia, black gouge and yellowish gouge. Many clasts of foliated cataclasite and black gouge contained in fault breccia indicate multiple slip events along this fault. But fossils on both sides of fault do not indicate clear age difference and overall displacement along this fault should not be large. We also report results from high-velocity friction experiments conducted on yellowish gouge from the fault zone using a rotary shear low to high-velocity frictional testing apparatus. Dry experiments at normal stresses of 0.4 to 1.8 MPa and at slip rates of 0.08 to 1.35 m/s reveal dramatic slip weakening from the peak friction coefficient of around 0.6 to very low steady-state friction coefficient of 0.1-0.2. Slip weakening parameters of this carbonate fault zone are similar to those of clayey fault gouge from Yingxiu-Beichuan fault at Hongkou outcrop and from Pingxi fault zone. Our experimental result will provide a condition for triggering movement of subsidiary faults or off-fault damage during a large earthquake.     

Geochemistry of Sulfide Chimneys and Basement Pillow Lavas at the Southern Mariana Trough (12.55°N–12.58°N)

Three submarine hydrothermal sites (Snail, Y and Pika sites) in the southern Mariana area were investigated to clarify the geochemical difference between off-axis and on-axis submarine hydrothermal activities and volcanic rocks. The Snail and Y sites are located on the axis of the spreading ridge, and the Pika site is located on the off-axis region. Low-temperature venting of submarine hydrothermal fluids was observed at the Snail and Y sites. High-temperature black smoker activities were found at the Pika site. For further geochemical study, sulfide chimneys and mounds were collected from each site by manned submersible Shinkai 6500 . Basement short drill cores of volcanic rocks and sulfides were also obtained from the Snail and Pika sites using a multi-coring system. All drilled pillow lavas were tholeiitic andesite with a narrow range of chemical variation. Significant enrichments of Rb, Ba and Th were absent in both on-axis (Y and Snail sites) and off-axis (Pika site) samples, suggesting the least incorporation of subducting sediments to the magmatic system. Concentrations of Au and Ag in the sulfide chimney were within the range of massive sulfides at the mid-ocean ridge rather than typical arc-type massive sulfides. It is found that sulfur isotope compositions of sulfides were different between the on-axis and off-axis samples: on-axis samples had heavy δ³⁴S (+2.9–+8.7‰) and off-axis samples (–0.3 to +3.8‰) were similar to the local magmatic value. Such a regional difference probably results from changes of deep hydrothermal processes during the water–rock interaction rather than differences in tectonic settings.     

Reexamination of the fractionation of total dissolved phosphorus in seawater using a modified UV-irradiation procedure,and its application to samples from Suruga Bay and Antarctic Ocean

Application of a modified UV-irradiation procedure to the fractionation of total dissolved phosphorus (TDP) in seawater was examined. By using a peristaltic pump, a sample solution was passed through a quartz column which was coiled around the three Hg-lamps and thus consistently exposed to a controlled UV-irradiation flux. The UV-flux was much lower than that historically employed to hydrolyze total dissolved organic phosphorus (DOP). Before and after the UV-irradiation, colorimetric measurements of soluble reactive phosphorus (SRP) were made by the method of Murphy and Riley (1962) to determine the liberation rate of orthophosphate from P-compounds in the sample. Experiments using 19 different P-compounds indicated that organic phosphate-esters containing only monomers of phosphate were readily decomposed by a 20-minute UV-irradiation period. Release of SRP from organic polyphosphates was minimal. The P released by this procedure is operationally referred to as UV-P. The procedure described herein was employed for fractionation of TDP in seawater. These studies defined several classes of dissolved P: (1) PO₄–P(inorganic phosphate-P), (2) UV-P (Photodecomposable organic-P, most of which consists of organic monophosphate-P), and (3) Org-Poly-P (organic polyphosphate-P). Vertical profiles of DOP and UV-P observed in the oligotrophic regions of Suruga Bay and Antarctic Ocean indicated that UV-P was a major part of total dissolved phosphorus (TDP) in euphotic layers, where inorganic nutrients were probably limiting the active growth of phytoplankton. It is probable that UV-P can be utilized as a source of nutrient-P, in place of PO₄–P.     

A psychological effect of having a potentially viable sequestration strategy

We are pleased to announce the launch of Carbon Balance and Management, a new online open access journal published by BioMed Central.     

The origin of large or great thrust-type earthquakes along subducting plate boundaries

Seismicity, deformation, state of stress, and abundance of fluids along subducting plate boundaries are reviewed, and the origin of large or great thrust-type earthquakes is discussed based on the recent experimental results on the slip behavior of halite and serpentine gouges.Shallow subducting plate boundaries above 20–25 km in depth are characterized by low seismicity, low tectonic stress, inter-plate decoupling, ductile deformation associated with the formation of metamorphic schistosity (except at very shallow depths), metamorphism suggesting solution processes on massive scale, and presence of abundant H₂O. It is argued that these unique features are due to pressure-solution processes, to high fluid pressure, to low strength and stable behavior of clayey sediments under wet environments, and/or to the deformation of soft, unconsolidated sediments at very shallow depths. The low seismicity in this zone is in marked contrast with major strike-slip faults along which large earthquakes occur at depths shallower than 15–20 km. It is emphasized that these unique features are expected only for restricted regions where there is constant supply of H₂O due to progressive metamorphism or where fluids in the rocks are trapped and cannot escape to the surface.Large or great thrust-type earthquakes in subduction zones initiate at depths of 30–50 km, below the shallow decoupled zone. In this focal depth range, the supply of H₂O during progressive metamorphism perhaps diminishes downwards, the overriding and subducting plates are coupled and stick to each other during much of the inter-seismic period, and the resistance to slip (or shear stress) is presumably high. It is suggested that these earthquakes begin to occur at a depth where the plate-boundary zone becomes fairly dry. Deformation at these depths appears to be predominantly ductile, so that the earthquakes cannot be regarded simply as a brittle phenomenon. (1) Creep instability i.e., instability associated with plastic deformation, and (2) dehydration-induced instability are the most likely mechanisms for initiating the earthquakes, and both have some experimental support. Stick-slip of halite gouge while undergoing ductile deformation primarily by intracrystalline gliding is described and discussed as a supporting evidence for (1). Shear resistance of halite gouge increases with increasing confining pressure in stick-slip regimes. Hence the observed stick-slip may be a semi-brittle phenomenon with respect to the pressure dependence of the shear resistance, although the deformation texture cannot be distinguished from that formed by pressure-insensitive flow. Serpentine gouge exhibits violent stick-slip upon its decomposition under dry, not wet, environments, supporting the mechanism (2) above. Exact mechanisms which lead to the unstable fault motion are poorly understood as yet, but stick-slip of both halite and serpentine gouges is recognized only when the slip-rate dependence of friction is negative i.e., lower friction at faster slip rate, consistent with the theoretical prediction of Rice and Ruina (1983). There is a possibility that the thrust-type earthquakes can be explained essentially within the framework of fault constitutive laws developed by Dieterich (1979) and Ruina (1983).