宽频地震观测中强震前的异常波动信号探究     

张雁滨  蒋骏  吕永清  李胜乐  杨辉  李畅  田卫卫 《国际地震动态》2008,(11)

强震前后,在长周期地震观测仪的观测中常能记录到一些异常的地动信号.其中比较有代表性的如Kizawa 以应变仪和重力仪记录到大地震前的前驱波信号的报道(KizawaT.1972 .2001年在昆仑山西口8.1级地震前的几天,我国新疆地震台网的多个台站也同时观测到了低频波信号.但这类强震前的异常信号的普遍性,以及与强震的关联性一直存在不少疑问.本文报道了我们近期所做的相关探索.  相似文献   

A preparation zone for volcanic explosions beneath Naka-dake crater,Aso volcano,as inferred from magnetotelluric surveys     

Wataru Kanda  Yoshikazu Tanaka  Mitsuru Utsugi  Shinichi Takakura  Takeshi Hashimoto  Hiroyuki Inoue 《Journal of Volcanology and Geothermal Research》2008

The 1st crater of Naka-dake, Aso volcano, is one of the most active craters in Japan, and known to have a characteristic cycle of activity that consists of the formation of a crater lake, drying-up of the lake water, and finally a Strombolian-type eruption. Recent observations indicate an increase in eruptive activity including a decrease in the level of the lake water, mud eruptions, and red hot glows on the crater wall. Temporal variations in the geomagnetic field observed around the craters of Naka-dake also indicate that thermal demagnetization of the subsurface rocks has been occurring in shallow subsurface areas around the 1st crater. Volcanic explosions act to release the energy transferred from magma or volcanic fluids. Measurement of the subsurface electrical resistivity is a promising method in investigating the shallow structure of the volcanic edifices, where energy from various sources accumulates, and in investigating the behaviors of magma and volcanic fluids. We carried out audio-frequency magnetotelluric surveys around the craters of Naka-dake in 2004 and 2005 to determine the detailed electrical structure down to a depth of around 1 km. The main objective of this study is to identify the specific subsurface structure that acts to store energy as a preparation zone for volcanic eruption. Two-dimensional inversions were applied to four profiles across the craters, revealing a strongly conductive zone at several hundred meters depth beneath the 1st crater and surrounding area. In contrast, we found no such remarkable conductor at shallow depths beneath the 4th crater, which has been inactive for 70 years, finding instead a relatively resistive body. The distribution of the rotational invariant of the magnetotelluric impedance tensor is consistent with the inversion results. This unusual shallow structure probably reflects the existence of a supply path of high-temperature volcanic gases to the crater bottom. We propose that the upper part of the conductor identified beneath the 1st crater is mainly composed of hydrothermally altered zone that acts both as a cap to upwelling fluids supplied from deep-level magma and as a floor to infiltrating fluid from the crater lake. The relatively resistive body found beneath the 4th crater represents consolidated magma. These results suggest that the shallow conductor beneath the active crater is closely related to a component of the mechanism that controls volcanic activity within Naka-dake.  相似文献   

Evaluation of the resistance of coastal Pinus thunbergii Parlat. forests to the tsunami fluid force in Japan     

Torita  Hiroyuki  Tanaka  Norio 《Natural Hazards》2019,96(3):1141-1152

Natural Hazards - Coastal forests can protect coastal areas from the effects of tsunamis. However, the coastal forests of Tohoku were destroyed by the Great East Japan Tsunami, which was triggered...  相似文献   

Mineralogical and Geochemical Characteristics of the Utanobori Gold Deposit in Northern Hokkaido,Japan     

Thomas T. Sorulen  Ryohei Takahashi  So Tanaka  Kana Suzuki  Akira Imai  Yasushi Watanabe  Shingo Kikuchi 《Resource Geology》2019,69(4):402-429

The Utanobori gold deposit is a low‐sulfidation, epithermal vein‐type deposit located in northern Hokkaido, Japan. The deposit is hosted by conglomerate, sandstone, and tuff of the Middle to Late Miocene Esashi Formation. These rocks were hydrothermally altered. Silica sinters and quartz‐adularia veins are common in the deposit. The quartz‐adularia veins either contain a ginguro band, which corresponds to the main gold‐bearing vein (Type 1 Veins), or do not contain a ginguro band but contain minor adularia (Type 2 Veins). Type 1 Veins are divided into three stages with 12–14 substages. Ore minerals identified include electrum, naumannite, chlorargyrite, bromargyrite, an unidentified Fe‐Sb mineral, and an Fe‐(Sb)‐As mineral. These ore minerals formed in the main mineralization stages I (bands I‐b and I‐d) and II (band II‐a). Scanning electron microscopy with cathodoluminescence images show that cathodoluminescence‐dark microcrystalline quartz exhibiting colloform (ghost‐sphere) texture is closely associated with ore minerals in the Type 1 Vein and Type 2 Vein, and the Al and K contents of such quartz are commonly >1000 ppm. This indicates that the ore minerals were crystallized from alkaline, silica‐saturated fluids at temperatures <200°C, which initially deposited amorphous silica that was recrystallized to microcrystalline quartz. The average Au content of electrum is 52.5 at% Au (n = 10), 65.7 at% Au (n = 20), and 55.5 at% Au (n = 5) in bands I‐b, I‐d, and II‐a, respectively, of Type 1 Veins. The δ³⁴S_CDT values of two fine‐grained disseminated pyrites in the altered conglomerate and bedded tuff in the argillic altered zone are ?4.3 and ?4.2‰. Ar‐Ar dating on adularia yielded 13.6 ± 0.06 Ma, 13.6 ± 0.07 Ma, and 13.6 ± 0.06 Ma for the stages I, II, and III of the Type 1 Vein, respectively. K‐Ar ages determined on adularia in the silica sinter and on whole‐rock of glassy rhyolite of the Esashi Formation are 15.0 ± 0.4 Ma and 14.6 ± 0.4 Ma, respectively. These radiometric ages indicate that silica sinter associated with the rhyolitic volcanic rocks formed prior to the main gold mineralization.  相似文献   

A Method to Suppress Isobaric and Polyatomic Interferences for Measurements of Highly Siderophile Elements in Desilicified Geological Samples     

Xiaoyu Zhou  Ryoji Tanaka  Masahiro Yamanaka  Chie Sakaguchi  Eizo Nakamura 《Geostandards and Geoanalytical Research》2019,43(4):611-633

Sample decomposition using inverse aqua regia at elevated temperatures and pressures (e.g., Carius tube or high‐pressure asher) is the most common method used to extract highly siderophile elements (HSEs: Ru, Rh, Pd, Re, Os, Ir, Pt and Au) from geological samples. Recently, it has been recognised that additional HF desilicification is necessary to better recover HSEs, potentially contained within silicate or oxide minerals in mafic samples, which cannot be dissolved solely by inverse aqua regia. However, the abundance of interfering elements tends to increase in the eluent when conventional ion‐exchange purification procedures are applied to desilicified samples. In this study, we developed an improved purification method to determine HSEs in desilicified samples. This method enables the reduction of the ratios of isobaric and polyatomic interferences, relative to the measured intensities of HSE isotope masses, to less than a few hundred parts per million. Furthermore, the total procedural blanks are either comparable to or lower than conventional methods. Thus, this method allows accurate and precise HSE measurements in mafic and ultramafic geological samples, without the need for interference corrections. Moreover, the problem of increased interfering elements, such as Zr for Pd and Cr for Ru, is circumvented for the desilicified samples.  相似文献   

Interhemispheric observations of field line resonance frequencies as a continuous function of ground latitude in the auroral zones     

S. Takasaki  N. Sato  A. Kadokura  H. Yamagishi  H. Kawano  Y. Ebihara  Y.-M. Tanaka 《Polar Science》2008,2(2):73-86

In the austral summer of 2006–2007, the 48th Japanese Antarctic Research Expedition (JARE-48) installed two unmanned low-power magnetometers to form a closely spaced magnetometer network in combination with the permanent sites at Japan's Syowa Station in Antarctica. To identify field line resonances (FLRs), gradient methods are applied to the data from three adjacent sites in Antarctica and data from conjugate points in Antarctica and Iceland. By analyzing the data from the Antarctic and Icelandic sites individually, the structure of FLRs with high coherence is clearly identified. However, by analyzing the data from closely spaced Antarctic sites, it is more difficult to identify the signature of FLRs because of the inclusion of multiple signals related to the local geomagnetic pulsations over a broad frequency range. The frequency and resonance width of FLRs are determined by applying the amplitude phase gradient method (APGM) to the data from Antarctic sites. This yields the eigenfrequency as a continuous function of ground latitudes in the area surrounding Syowa Station. The mass density in the equatorial region at the L of the auroral zones is estimated from the obtained FLR frequency by numerically solving the standing Alfvén wave equation. The mass density thus obtained is consistent with observational results from previous in situ measurements by spacecraft. The results of the present study demonstrate that data from geomagnetic conjugate points are helpful in identifying FLR in cases in which the magnetometers are too close to each other to enable identification. Once FLR is identified, APGM can be applied to the identified FLR, yielding the FLR frequency as a continuous function of ground latitudes. Therefore, the magnetospheric equatorial mass density is readily estimated with high spatial resolution.  相似文献   

Geometry of the Nojima Fault at Nojima-Hirabayashi, Japan – I. A Simple Damage Structure Inferred from Borehole Core Permeability     

David A. Lockner  Hidemi Tanaka  Hisao Ito  Ryuji Ikeda  Kentaro Omura  Hisanobu Naka 《Pure and Applied Geophysics》2009,166(10-11):1649-1667

The 1995 Kobe (Hyogo-ken Nanbu) earthquake, M = 7.2, ruptured the Nojima fault in southwest Japan. We have studied core samples taken from two scientific drillholes that crossed the fault zone SW of the epicentral region on Awaji Island. The shallower hole, drilled by the Geological Survey of Japan (GSJ), was started 75 m to the SE of the surface trace of the Nojima fault and crossed the fault at a depth of 624 m. A deeper hole, drilled by the National Research Institute for Earth Science and Disaster Prevention (NIED) was started 302 m to the SE of the fault and crossed fault strands below a depth of 1140 m. We have measured strength and matrix permeability of core samples taken from these two drillholes. We find a strong correlation between permeability and proximity to the fault zone shear axes. The half-width of the high permeability zone (approximately 15 to 25 m) is in good agreement with the fault zone width inferred from trapped seismic wave analysis and other evidence. The fault zone core or shear axis contains clays with permeabilities of approximately 0.1 to 1 microdarcy at 50 MPa effective confining pressure (10 to 30 microdarcy at in situ pressures). Within a few meters of the fault zone core, the rock is highly fractured but has sustained little net shear. Matrix permeability of this zone is approximately 30 to 60 microdarcy at 50 MPa effective confining pressure (300 to 1000 microdarcy at in situ pressures). Outside this damage zone, matrix permeability drops below 0.01 microdarcy. The clay-rich core material has the lowest strength with a coefficient of friction of approximately 0.55. Shear strength increases with distance from the shear axis. These permeability and strength observations reveal a simple fault zone structure with a relatively weak fine-grained core surrounded by a damage zone of fractured rock. In this case, the damage zone will act as a high-permeability conduit for vertical and horizontal flow in the plane of the fault. The fine-grained core region, however, will impede fluid flow across the fault.  相似文献   

2013年南沙群岛海域温跃层的季节变化及形成机理     

田永青  黄洪辉  巩秀玉  余少梅  靖春生  高璐 《海洋学报》2017,39(12):20-31

利用调查数据及遥感数据揭示了2013年南沙群岛海域温跃层的季节变化特征,温跃层上界深度平均值春、夏、冬季基本一致,介于45~47 m之间,秋季最大,达60 m;温跃层厚度平均值夏、秋、冬季基本一致,介于85~87 m之间,春季相对较小,为78 m。温跃层强度平均值春、夏、秋、冬季几乎一致,介于0.13~0.15℃/m之间。调查海域温跃层上界深度季节变化的形成机理为:春季西深东浅的原因是西部受净热通量较小、大风速、负的风应力旋度以及中南半岛东部外海的中尺度暖涡和反气旋环流共同作用,东部近岸海域净热通量高值、风速相对较小及风应力旋度引起的Ekman抽吸效应共同控制;夏季深度分布较均匀的原因是10°N以北风致涡动混合强但受Ekman抽吸影响,10°N以南风致涡动混合弱但风应力旋度为负值;秋季深度较其他季节平均加深15 m的原因是南沙群岛海域被暖涡占据,暖涡引起的反气旋式环流使得温跃层上界深度被海水辐聚下压;冬季正的风应力旋度产生的Ekman抽吸和冷涡引起的气旋式环流共同作用,使得温跃层上界深度较秋季平均抬升15 m。  相似文献   

风沙流和净风场中瞬时水平风速廓线特征比较   总被引：1，自引：1，他引：0  

高咏晴  亢力强  张军杰  邹学勇  张春来  程宏 《中国沙漠》2017,37(1):48-56

为进一步认识风沙流和净风场中瞬时水平风速廓线特征的异同,在风洞中分别对风沙流和净风场中瞬时水平风速廓线进行了测量,风速采集时间间隔缩短至0.01s,分析了风沙流和净风场中瞬时水平风速、瞬时摩阻风速和瞬时空气动力学粗糙度的变化特征。结果表明：相同来流条件下,风沙流中水平风速脉动强度高于净风场,风沙流中瞬时摩阻风速、瞬时空气动力学粗糙度以及它们的脉动幅度均大于净风场;风沙流和净风场中瞬时摩阻风速概率密度分布均可以表示为正态分布,但其正态分布的特征值却存在一定差别;净风场中瞬时空气动力学粗糙度的概率密度分布表现出单调递减分布,而风沙流中瞬时空气动力学粗糙度的概率密度分布呈现出单峰分布。因此,在相同主流风速下风沙流和净风场中瞬时水平风速廓线特征有明显差别。  相似文献   

山东西部中生代青山群火山岩的地球化学特征及其岩石成因     

张永清  凌文黎  张军波  段瑞春  任邦方  杨红梅 《地质学报》2017,91(12):2697-2709

以山东西部西董地区中生代青山群火山岩为研究对象,在进行系统的元素-同位素分析的基础上,探讨其岩浆源区和岩石成因,同时丰富该区域岩石地层的地球化学特征。西董青山群火山岩Si O2含量变化于52.8%~59.8%之间,主要为钙碱性系列岩石,岩石组合为玄武质粗面安山岩-玄武质安山岩-粗面安山岩,Mg O、TFe2O3、Mn O、Ti O2、Ca O与Si O2具有明显的负相关关系,Al2O3和Na2O与Si O2具有正相关关系;微量元素富集Rb、Ba、Th、U和K等大离子亲石元素,亏损Nb、Ta和Ti等高场强元素,稀土元素总量∑REE变化范围为100×10-6~187×10-6,(La/Yb)N比值范围为8.2~11.1,无明显的Eu异常(δEu=0.9~1.02);显示富集且均一的Sr-Nd同位素组成((87Sr/86Sr)t=0.7047~0.7050,εNd(t)=-16.5~-14.2)。西董青山群火山岩的Nd、Sr同位素组成与Si O2缺乏明显负相关关系,Nb/Ta与La/Nb之间不具有负相关关系以及Nb与Th之间具有正相关关系,指示岩浆在上升和演化过程中地壳混染作用对青山群火山岩岩浆化学组成的影响并不显著;火山岩Mg O含量明显高于玄武岩部分熔融形成的熔体,说明青山群中基性火山岩岩浆的形成过程可能与大陆地壳和地幔岩系之间的相互作用有关。综合分析表明,西董中生代青山群火山岩是大陆下地壳物质经拆沉作用于地幔环境中发生部分熔融,与上地幔岩石发生交代作用后进入地表形成。  相似文献