Reconstruction of the eruptive history of Usu volcano,Hokkaido, Japan,inferred from petrological correlation between tephras and dome lavas

Usu volcano has erupted nine times since 1663. Most eruptive events started with an explosive eruption, which was followed by the formation of lava domes. However, the ages of several summit lava domes and craters remain uncertain. The petrological features of tephra deposits erupted from 1663 to 1853 are known to change systematically. In this study, we correlated lavas with tephras under the assumption that lava and tephra samples from the same event would have similar petrological features. Although the initial explosive eruption in 1663 was not accompanied by lava effusion, lava dome or cryptodome formation was associated with subsequent explosive eruptions. We inferred the location of the vent associated with each event from the location of the associated lava dome and the pyroclastic flow deposit distribution and found that the position of the active vent within the summit caldera differed for each eruption from the late 17th through the 19th century. Moreover, we identified a previously unrecognized lava dome produced by a late 17th century eruption; this dome was largely destroyed by an explosive eruption in 1822 and was replaced by a new lava dome during a later stage of the 1822 event at nearly the same place as the destroyed dome. This new interpretation of the sequence of events is consistent with historical sketches and documents. Our results show that petrological correlation, together with geological evidence, is useful not only for reconstructing volcanic eruption sequences but also for gaining insight into future potential disasters.     

Seismo-meteo-electromagnetic phenomena observed during a 5-year interval around the 2011 Tohoku earthquake

The purpose of the paper is to try to find ULF electromagnetic precursors to the 2011 Tohoku earthquake (EQ), on the basis of extensive investigation of radiations in vertical component of the magnetic field or with a large ratio of the vertical to full horizontal component. Nighttime records have been analyzed of three Japanese fluxgate magnetometers located in a distance from 300 to 1300 km from the epicenter of the main shock, and the frequency range from 10 to 150 mHz was used for the analysis. We have applied wavelet analyses to improve the detection of pulsed signals. All obtained scalograms have been averaged over the nighttime interval from 01 h to 05 h JST and flattened by means of multiplication by square of frequency. The sequence of spectra thus obtained has been compared with the evolution of seismicity, which has resulted in that the radiation in the vertical component has been detected. It exhibits seasonal variations with winter maxima, but it increases further by approaching the moment of the EQ and decreases after that. This radiation seems to be correlated with atmospheric parameters – air humidity, temperature, vapor pressure and rainfall. So, we consider that this radiation cannot be caused by subsurface sources, but its possible sources can be atmospheric discharges. The evolution of this phenomenon can be explained by a seasonal variation of atmospheric parameters and also its variations under the influence of injection of gas from the focal zone of a forthcoming EQ.     

甘肃公婆泉铜矿田中—晚志留世浅海相火山喷发旋回及火山作用演化

根据甘肃公婆泉铜矿田内４条典型地层剖面,确立了甘肃公婆泉群中－上志留统浅海相火山喷发沉积岩系的标准层和层序,并将其划分为１４个韵律和５个旋回。火山喷发旋回由早→晚,火山岩岩性由安山质凝灰岩、安山岩→英安岩、钠流纹岩→石英粗面岩→安山岩,火山岩中铁族元素的丰度由高→低→最低（在石英粗面岩中）→高,形成一个较完整的演化周期。中晚期火山岩所表现出的岩性（出现碱性岩－石英粗面岩）和铁族元素丰度（出现最低值）的突变与区域北西走向断裂的深切活动有关,为铜、银等金属成矿提供主要的地球化学条件。铜、银等成矿元素主要富集在火山喷发旋回中期的英安岩中,表明火山活动中期是主要的成矿期,英安岩可作为重要的找矿标志。此外,早中期火山旋回主要发育于矿田的中西部,而晚期旋回则主要分布在矿田的东部,并且火山喷发的强度和频率增高,表明晚期火山活动中心已明显向东部迁移,因此矿田的东部仍有较大的找矿潜力     

Earthquake swarms to the west of Mt Ruapehu preceding its 1995 eruption

Some months prior to the 1995 eruption of Mt Ruapehu (New Zealand), a series of shallow earthquake swarms occurred about 15–20 km west of the summit of Ruapehu. Several earthquakes in these swarms were felt, and the largest event was M_L 4.8. Crustal earthquakes of M_L≥3.0 within 20 km of the summit of Ruapehu have been rather uncommon in recent years. Furthermore, the two periods of strongest activity were both just before times when the temperature of Crater Lake showed rapid increases. The second of these rapid heating phases was immediately followed by increases in the Mg²⁺ ion concentration in Crater Lake, indicating that chemical interactions were occurring between fresh magmatic material and the lake water. The coincidence between seismicity and lake changes suggested a link with the following eruption. A 1-D simultaneous inversion to locate the earthquakes more accurately showed that most of the earthquakes fell into three spatial clusters, each cluster having a small horizontal cross-section. The predominant depth was about 10–16 km. The b-value of this swarm was 0.74, quite compatible with ordinary tectonic earthquakes. Each cluster of earthquakes lies close to the normal Raurimu Fault which runs predominantly north–south to the west of Ruapehu, with an east-trending branch splaying off near its northern end (see Fig. 1b). Composite focal mechanisms of events in the two more southern clusters are oblique-normal, while the other cluster to the north has an oblique-reverse mechanism. The two oblique-normal mechanisms suggest that extension has occurred on part of the fault. This stress pattern was also observed in the focal mechanism solutions of events that occurred after the eruption, when a denser network of portable seismographs covered the region. Although we cannot definitely connect the occurrence of these swarms to the eruptions later in 1995, there is a strong suggestion that the seismicity was connected to the process of magma movement, which temperature and chemical changes in Crater Lake suggest was occurring during the first half of 1995.     

徐家围子断陷安达地区营城组火山岩喷发期次及其形成演化规律

针对徐家围子断陷安达地区营城组火山岩具有多期喷发、火山岩机构类型多样、叠置分布和岩性复杂的特点,对火山岩喷发期次及火山岩形成演化特征进行研究,找出火山岩的演化规律。其主要表现为:沙河子组沉积末期,本区处于断-拗转换期,盆地内部处于欠补偿特征;营城组一期火山喷发时,主要受西部的活动基底断裂及次级断裂的控制;营城组二期火山喷发时,火山作用由安达西部向东部扩展;营城组三期火山喷发时,深部岩浆活动由强转弱,本区的基底断裂活动减弱,构造活动也向坳陷期形成环境过渡。     

腾冲地区晚新生代火山活动浅析

腾冲火山喷发活动具有多期多次性,并且各期各次火山岩在空间上存在复杂的重叠关系。本文应用数学地质方法对火山岩地质年龄值和某些重要化学组分进行研究,结合火山岩岩石化学特征、野外产状、风化剥蚀程度等进行分期分类。在此基础上,将该区火山活动划分为５个喷发期。它们组成由基性喷发向中酸性喷发过渡的２个喷发旋回。最后,对该区未来火山喷活动进行了预测。     

中国地壳运动观测网络基准站水平位移向量时间序列

2018-05-05美国夏威夷发生基拉韦厄火山喷发和6.9级地震,直接利用Nevada Geodetic Laboratory（相似文献   

The 1996 subaqueous eruption at Academii Nauk volcano (Kamchatka) and its effects on Karymsky lake

A subaqueous eruption in Karymsky lake in the Academii Nauk caldera dramatically changed its water column structure, water chemistry and biological system in less than 24 h, sending major floodwaves down the discharging river and eruption plumes with ash and gases high into the atmosphere. Prior to the eruption, the lake had a pH of about 7, was dominated by bicarbonate, and well stocked with fish, but turned in early 1996 into a stratified, initially steaming waterbody, dominated by sulfate with high Na and K levels, and devoid of fish. Blockage of the outlet led to rising waterlevels, followed by dam breakage and catastrophic water discharge. The total energy input during the eruption is estimated at about 10¹⁶ J. The stable isotope composition of the lake water remained dominated by the meteoric meltwaters after the eruption.     

Precursory groundwater level changes in the period of activation of the weak intraplate seismic activity on the NE margin of the Bohemian Massif (central Europe) in 2005

tWe analyse continuous measurements of groundwater level in two deep wells VS-3 and V-28 at the experimental hydro-meteorological station situated on the NE margin of the Bohemian Massif, central Europe, characterized by the weak intraplate seismic activity. The aim of our study is to examine the relationships between changes in the groundwater level and earthquake occurrence. Based on the tidal and barometric response of the water level, we estimated selected elastic parameters of the observed aquifers: the shear modulus G, the Skempton ratio B, the drained matrix compressibility β and the undrained compressibility β_u. Using these parameters and assuming the homogeneous poroelastic material, we derived the sensitivity of the wells to the crustal volume strain. During the observation period from November 1998 to December 2005 we detected in the VS-3 well two pre-seismic steps, related to August 10, 2005 (M = 2.4) and October 25, 2005 (M = 3.3) earthquakes. Amplitudes of the recorded precursory changes (+6 cm and +15 cm) are several times higher than the values predicted from the theoretical precursory crustal strain and the strain sensitivity of the well. Therefore, we presume that the observed pre-seismic water level steps can be attributed to heterogeneity of poroelastic material. We consequently propose the hypothesis of the origin of precursory events based on the presumption of a sensitive site, at which the well is situated.     

天池火山千年大喷发的岩浆混合作用与喷发机制初步探讨

根据岩浆演化和地球物理深部探测,天池火山之下存在地壳和地幔双层岩浆房。地幔玄武质岩浆向地壳岩浆房的补给,保持了天池火山逾百万年持续不断的喷发活动。本文从天池火山千年大喷发浮岩中的玄武质粗安岩一粗安岩角砾和条带状岩浆的岩相学、矿物学和岩石化学研究,提出地幔的粗面玄武质岩浆向地壳岩浆房的注入,触发千年大喷发,初步探讨了天池火山千年大喷发的岩浆混合作用与喷发机制。