Summit CO2 emission rates by the CO2/SO2 ratio method at Kīlauea Volcano,Hawaiʻi,during a period of sustained inflation

The emission rate of carbon dioxide escaping from the summit of Kīlauea Volcano, Hawai?i, proved highly variable, averaging 4900 ± 2000 metric tons per day (t/d) in June–July 2003 during a period of summit inflation. These results were obtained by combining over 90 measurements of COSPEC-derived SO₂ emission rates with synchronous CO₂/SO₂ ratios of the volcanic gas plume along the summit COSPEC traverse. The results are lower than the CO₂ emission rate of 8500 ± 300 t/d measured by the same method in 1995–1999 during a period of long-term summit deflation [Gerlach, T.M., McGee, K.A., Elias, T., Sutton, A.J. and Doukas, M.P., 2002. Carbon dioxide emission rate of Kīlauea Volcano: Implications for primary magma and the summit reservoir. Journal of Geophysical Research-Solid Earth, 107(B9): art. no.-2189.]. Analysis of the data indicates that the emission rates of the present study likely reflect changes in the magma supply rate and residence time in the summit reservoir. It is also likely that emission rates during the inflation period were heavily influenced by SO₂ pulses emitted adjacent to the COSPEC traverse, which biased CO₂/SO₂ ratios towards low values that may be unrepresentative of the global summit gas plume. We conclude that the SO₂ pulses are consequences of summit re-inflation under way since 2003 and that CO₂ emission rates remain comparable to, but more variable than, those measured prior to re-inflation.     

Temporal variations of gas compositions of fumaroles in the Tatun Volcano Group,northern Taiwan

Hydrothermal activity is common in the Tatun Volcano Group of northern Taiwan. Helium isotopic compositions of fumarolic samples show that mantle component occupies more than 60% in the previous study. Along with recent seismic results, a magma reservoir is inferred to have existed beneath the area of Da-you-keng, where fumarolic venting is the most active in Tatun Volcano Group. Progressive increases of HCl concentrations and SO₂/H₂S ratio in fumaroles from Da-you-keng have been observed since August 2004. The HCl concentration changed from almost the detection limit to thousands of ppm, even up to 30,000 ppm. SO₂/H₂S ratios varied from almost 0 to 3; hence SO₂ became the dominated S species in this area. These variations were accompanied by rising temperature of fumaroles in the Tatun Volcano Group, especially in the area of Da-you-keng (from boiling point to 131 °C). Meanwhile, ³He/⁴He ratios showed a decreasing trend but returned to normal values shortly thereafter. We propose two possible processes, 1) new magma supply and 2) recent opening of fractures in local area, to explain these observations. Based on the change of ³He/⁴He ratio and lack of ground deformation, we consider the latter might be more plausible.     

Infrasonic tracking of large bubble bursts and ash venting at Erebus Volcano,Antarctica

Explosive degassing at Erebus Volcano produces infrasound that can be used to locate, characterize, and quantify eruptive activity from multiple vents. We use a three element distributed microphone network to pinpoint eruption sources and track the activity at the prominent vents through time. Eruptive mechanisms for both source types are analyzed in conjunction with the telemetered time-synced video imagery. We identify two commonly active vents corresponding to the large (often > 10-m diameter) bubble bursts at the free surface of a persistent phonolitic lava lake (‘Ray Lake’), and the less frequent ash-rich eruptions from a constricted vent (‘Active Vent’) located ∼ 80 m from the lava lake. During a 3-month study interval from 6 January to 13 April 2006 we identified and mapped more than 350 eruptive sources from the lava lake and 20 sources from the ash vent. Lava lake events are characterized by high-amplitude infrasonic transients that reflect rapid (less than a few s) acceleration and rupture of magma bubble films followed by an explosion of pressurized gases. Precise infrasonic localization of the lava lake events to accuracies of a few m indicates variable bubble source locations across a 40 by 50-m region spanning the lava lake. Spatial variability is corroborated by the video data. In contrast, degassing from the ash vent produces longer-duration (tens of s), lower amplitude transients that reflect diminished impulsivity and an extended degassing duration, features that are corroborated by video. Because infrasound networks can operate continuously in all weather conditions and during both diurnal and seasonal polar darkness, and are easily incorporated into automatic processing, they significantly contribute to the completeness and quantification of eruption catalogues for Erebus.     

冲绳海槽宫古段中央地堑中的火山分布及地质意义

采用中国科学院海洋研究所“科学1号”调查船及国家海洋局“向阳红9号”调查船,于20世纪90年代以来在冲绳海槽宫古段进行海底地形及反射地震调查,所取得的数据,首次详细展示了冲绳海槽宫古段南北长约200km区域内海底火山的形态及其空间分布特征。在本段冲绳海槽发育的中轴火山和岛弧火山,分别位于中央地堑和海槽盆地东部边缘紧靠琉球岛弧的西侧。岛弧火山连续性强,总体上自东北而西南都有分布;中轴火山不如岛弧火山的连续性强,只在崂山段、黄岛段、胶南段、莱西段、城阳段和李沧段分布。中轴火山是在中央地堑之后发育的。中轴火山的发育不仅改变了原来中央地堑的位置,也改变了原来中央地堑的形态。中央地堑的左旋雁行排列,以及其形态的改变都是由于中轴火山发育的结果。中央地堑的右旋展布以及海底火山对中央地堑的改造,标志着冲绳海槽的演化已经在拉张和沉降的基础上进入到一个更高级的演化阶段(海底扩张),在岩浆供应不太充足的条件下,海底火山活动是冲绳海槽低速扩张的一种形式,海底扩张沿中央地堑正在进行。     

Volcanic Debris-avalanche Deposits of the Laoheishan Volcano and Huoshaoshan Volcano in Wudalianchi

Large amounts of volcanic debris-avalanche deposits, which take the shape of hummocks, are distributed around the peripheries of the Laoheishan volcano and Huoshaoshan volcano in Wudalianchi World Geopark. In earlier times, they were called "satellite volcanoes", namely, freestanding volcanoes. This paper points out that these deposits actually came from the collapse of the cones of these two volcanoes. When the lava flow spilled out at the base of the slope of the cones, the slope broke up and collapsed under the action of gravity. Later, ravines were formed on the slope. Caved slope clastics, accompanying lava flow, accumulated at the rims of the volcano cones. Although some accumulations may form very large cones, they are not volcanoes, but deposits of volcanic debris avalanches.     

Effects of topography and crustal heterogeneities on the source estimation of LP event at Kilauea volcano

The main goal of this study is to improve the modelling of the source mechanism associated with the generation of long period (LP) signals in volcanic areas. Our intent is to evaluate the effects that detailed structural features of the volcanic models play in the generation of LP signal and the consequent retrieval of LP source characteristics. In particular, effects associated with the presence of topography and crustal heterogeneities are here studied in detail. We focus our study on a LP event observed at Kilauea volcano, Hawaii, in 2001 May. A detailed analysis of this event and its source modelling is accompanied by a set of synthetic tests, which aim to evaluate the effects of topography and the presence of low velocity shallow layers in the source region. The forward problem of Green's function generation is solved numerically following a pseudo-spectral approach, assuming different 3-D models. The inversion is done in the frequency domain and the resulting source mechanism is represented by the sum of two time-dependent terms: a full moment tensor and a single force. Synthetic tests show how characteristic velocity structures, associated with shallow sources, may be partially responsible for the generation of the observed long-lasting ringing waveforms. When applying the inversion technique to Kilauea LP data set, inversions carried out for different crustal models led to very similar source geometries, indicating a subhorizontal cracks. On the other hand, the source time function and its duration are significantly different for different models. These results support the indication of a strong influence of crustal layering on the generation of the LP signal, while the assumption of homogeneous velocity model may bring to misleading results.     

Time dependent piezomagnetic fields in viscoelastic medium

We investigated time dependent piezomagnetic fields due to volcanic sources embedded in a viscoelastic, homogeneous half-space. Especially in volcanic areas, the presence of inhomogeneous materials and high temperatures produce a lower effective viscosity of the Earth's crust that calls for considering anelastic properties of the medium. Piezomagnetic properties are carried by grains of titano-magnetite, which occupy only a small fraction of ordinary rock volume and are supposed to be elastic, while the non-magnetic surrounding matrix is assumed to be viscoelastic. From all the possible rheological models, we investigated two cases in which the bulk modulus is purely elastic and the shear modulus relaxes as: (i) a Maxwell solid and (ii) a standard linear solid (SLS). We applied the Correspondence Principle to the analytical elastic solutions for pressurized spherical sources and dislocation sources in order to determine the time dependent piezomagnetic fields in a viscoelastic medium. The piezomagnetic field completely vanishes after the relaxation process for a Maxwell rheology, whereas it is found to decrease over time and reach some finite offset value for a SLS rheology. These different behaviours provide helpful hints in understanding the temporal evolution of piezomagnetic anomalies in volcanic regions.     

三维数值模拟研究俯冲区火山的时空活动性

俯冲板块的深部脱水使得上覆地幔含水, 从而降低含水地幔的熔点, 导致上覆地幔部分熔融。 部分熔融的地幔柱一旦喷发到地表就是俯冲带火山, 也形成新的地壳。 相对于周围的地幔来讲, 具有较小密度和黏度的部分熔融地幔的时空活动性就控制着俯冲带火山的时空分布特征。 本文主要回顾近年来运用三维热力学岩石力学模型数值模拟研究与板片脱水相关的俯冲带火山活动的时空分布特性。 结果表明, 部分熔融地幔的有效黏度和密度是影响俯冲板片之上的三维地幔柱横向分布特征的主要因素。 高黏度的部分熔融地幔(10²⁰~10²¹ Pa·s )易于形成近平行于海沟的、 长波长(70~100 km)的、 薄的波状地幔柱; 低黏度(10¹⁸~10¹⁹ Pa·s )的熔融地幔易于形成平行于海沟的, 短波长(30~50 km)的蘑菇状地幔柱和垂直于海沟的山脊状地幔柱。 当部分熔融地幔和周围地幔的密度相差小于50 kg/m³时, 在俯冲板片之上只能形成长波长低幅度(宽50~100 km, 高10~15 km)的地幔山丘。 岩浆产率随着时间的变化反映了火山活动的生命周期性。 板块俯冲速度会影响地幔柱形成的深度和范围大小。 高效率熔融提取会增加新地壳增长总量。 低的板块俯冲速度和低的熔融提取效率会增加上地壳(花岗岩质)和中地壳(英安岩质)化学成分的比例。 数值模拟结果可以很好地解释如日本东北、 新西兰、 南阿拉斯加俯冲区火山的横向分布特征。     

新疆阿什库勒火山群野外地质科学考察

2011年5月4日至5月30日,由中国地震局地质研究所和新疆维吾尔自治区地震局组成的科考队,完成了2010年度地震行业专项“新疆于田7.3级地震与阿什库勒火山综合科学考察”的野外综合科学考察。火山地质组通过对阿什库勒火山群的野外地质、地貌实地考察,初步查明了阿什库勒盆地新生代火山类型、数量、结构参数和火山活动历史,并且对该地区存在的一些有争议的问题, 如阿什火山1951年5月27日喷发事件的报道、大黑山火山的喷发方式、高台山火山的存在与否等问题提供了野外证据。