SHRIMP dating of volcanic rocks from Ningwu area and its geological implications

Preciselydatingofvolcanicrocksisfundamentaltotheunderstandingofthegeologicalevolutionofabasin.However,involcanicrocks,particularlyinthecaseofintermediate-maficrocks,thezirconpopulationsarecomplicatedbecauseofthecommonoccurrenceofinheritance(orcores)and/or…     

Reconstruction of the western Pacific warm pool SST since 1644 AD and its relation to precipitation over East China

Based on coral proxies we reconstructed the western Pacific warm pool sea surface temperature (SST) since 1644 AD. High-frequency reconstructions are based on eight high-pass filtered coral series and raw reconstructions are derived from eight unfiltered coral series, respectively. Validation and comparison with other SST/temperature series show that the reconstructed warm pool SST is highly reliable. The leading periods of warm pool SST are ~2.1, ~2.3, ~2.9, ~3.6, ~3.8, and 80.7-year during the last ~360 y...     

赣中盛源盆地及邻区橄榄玄粗岩系列火山岩中黑云母化学成分特征

本文利用电子探针技术对赣中盛源盆地及邻区橄榄玄粗岩系列火山岩中黑云母的类型、化学成分及结晶时的温度和氧逸度条件等进行了研究。黑云母为镁质黑云母;结晶时温度的大致范围为1060℃～1100℃,氧逸度大致范围为10^-7bar-10^9bar。本文研究表明,黑云母的化学成分研究是揭示岩石成因类型的一种有效途径,可为研究岩体成因类型以及成岩时的物理化学条件提供依据,并对成岩具有重要的指示意义。     

安徽及近邻区地震烈度衰减特征研究

利用安徽及近邻区地震烈度资料,采用椭圆衰减模型和长轴可转向与近、远场补点方法,通过多元回归方法进行拟合,建立该地区地震烈度衰减关系。该衰减关系用于安徽地震应急指挥技术系统震害快速评估模块,为地震灾害损失快速评估提供基础资料;也可用于安徽地区地震安全性评价、震害预测等防震减灾领域。     

Echo-resonance and hydraulic perturbations in magma cavities: application to the volcanic tremor of Etna (Italy) in relation to its eruptive activity

A study is presented of spectral features of volcanic tremor recorded at Mount Etna (Sicily, Italy) following the methods of analysis suggested by the resonant scattering formalism of Gaunaurd and Überall (1978, 1979a, 1979b) and the model for hydraulic origin of Seidl et al. (1981). The periods investigated include summit and flank eruptions that occurred between 1984 and 1993. Recordings from a permanent station located near the top of the volcano were used, and the temporal patterns associated with (a) the average spacing ( ) between consecutive spectral peaks in the frequency range 1–6 Hz, (b) the spectral shape and (c) the overall spectral amplitude were analyzed. values are thought to depend on the physical properties of magma, such as its density, which, in turn, is controlled by the degree of gas exsolution. Variations in the spectral shape are tentatively attributed to changes in the geometrical scattering from the boundary of resonant conduits and magma batches. Finally, the overall amplitude at the station should essentially reflect the state of turbulence of magma within the superficial ascending path. A limit in the application of the resonant scattering formalism to the study of volcanic tremor is given by the fact that the fundamental modes and integer harmonics are difficult to identify in the frequency spectra, as tremor sources are likely within cavities of very complex geometry, rather than in spherical or cylindrical chambers, as expected by theory. This study gives evidence of some correlations between the analyzed temporal patterns and the major events in the volcanic activity, related to both lava flow and explosions at the summit vents. In particular, relatively high values of have been attained during the SE crater eruption of 1984, the complex eruptive phases of September–October 1989 and the 1991–1993 flank eruption, suggesting the presence of a relatively dense magma for all of these events. Conversely, very low values have been recorded in coincidence with the December 1985 activity and the paroxysmal explosions at the summit craters of early 1990, which are interpreted here as fed by fluid-vesiculated magma. Appreciable modifications in the spectral shape have been observed in relation to changes of the volcanic activity that probably preceded the opening and disactivation of shallow dykes or magma batches. Finally, the overall amplitude seems to be a sensitive indicator of the state of gas turbulence within the shallow conduits, as is suggested by the high values attained during phases of intense volcanic activity.     

Sources of dissolved inorganic carbon in rivers from the Changbaishan area,an active volcanic zone in North Eastern China

Major elements and carbon isotopes of dissolved inorganic carbon(DIC)have been measured in the waters of Changbaishan mountain,a volcanic area in northeastern China,between June and September 2016 to decipher the origin of the CO_2 involved in chemical weathering reactions.Spatial variations of major elements ratios measured in water samples can be explained by a change of the chemical composition of the volcanic rocks between the volcanic cone(trachytes)and the basaltic shield as evidenced by the variations in the composition of these rocks.Hence,DIC results from the neutralization of CO_2 by silicate rocks.DIC concentrations vary from 0.3 to 2.5 mmol/L and carbon isotopic compositions of DIC measured in rivers vary from-14.2‰to 3.5‰.At a first order,the DIC transported by rivers is derived from the chemical weathering’s consumption of CO_2 with a magmatic origin,enriched in~(13)C(-5%)and biogenic soil CO_2 with lower isotopic compositions.The highest δ~(13)C values likely result from C isotopes fractionation during CO_2 degassing in rivers.A mass balance based on carbon isotopes suggest that the contribution of magmatic CO_2 varied from less than 20%to more than 70%.Uncertainties in this calculation associated with CO_2 degassing in rivers are difficult to quantify,and the consequence of CO_2 degassing would be an overestimation of the contribution of DIC derived from the neutralization of magmatic CO_2 by silicate rocks.     

Holocene vegetation change in relation to fire and volcanic events in Jilin,Northeastern China

Volatiles erupted from large-scale explosive volcanic activities have a significant impact on climate and environmental changes.As an important ecological factor,the occurrence of fire is affected by vegetation cover,and fire can feed back into both vegetation and climatic change.The causes of fire events are diverse;and can include volcanic eruptions.The amount of charcoal in sediment sequences is related to the frequency and intensity of fire,and hence under good preservation conditions fire history can be reconstructed from fossil charcoal abundance.Until now,little research on the role of fire has been carried out in northeastern China.In this study,through research on charcoal and tephra shards from Gushantun and Hanlongwan,Holocene vegetation change in relation to fire and volcanic events in Jilin,Northeastern China,was investigated.Where tephra shards are present in Gushantun it is associated with low level of both conifers and broadleaved trees,and is also associated with a pronounced charcoal peak.This suggests forest cover was greatly reduced from a fire caused by an eruption of the Tianchi volcano.We also detected one tephra layer in Hanlongwan,which also has the almost same depth with low level forest pollen values and one charcoal peak.This was caused probably by an eruption of the Jinlongdingzi volcano.     

Frequency-specified EOF analysis and its application to Pacific decadal oscillation

A frequency-specified empirical orthogonal function (FSEOF) analysis is proposed in this study. The aim of FSEOF is to specify a prescribed-band of frequency in leading principal components with less information losing at the ends of the data, thus well characterizing the signals of interest. The FSEOF can well capture prescribed variability in leading modes, and has intrinsic merits in resolving frequency-related modes, especially those associated with low frequency oscillations. An application of the FSEOF to the tropical and northern Pacific sea surface temperature shows that this new method can successfully separate Pacific decadal oscillation (PDO) mode from the El Niño-Southern oscillation mode, and clearly detect all regime shifts of PDO in the past century.     

Seismic sources and attenuation properties at the Campi Flegrei volcanic area

Microearthquake digital data collected at Campi Flegrei during the recent (1982–1985) ground uplift episode have been analyzed in order to infer source and medium seismic properties. The main results obtained from these analyses are:

1. Hypocenter distribution and the size of the seismic zone do not change with time and do not depend on the ground uplift rate. Events occurred clustered in time with no simple causal relations between the cluster occurrences and their energy.
2. Anelastic attenuation does not depend strongly on frequency, showing a constant pattern at high frequencies. The observed values of low and high frequency attenuation, due to the short source receiver distances, do not seriously affect the spectral content of signals radiated by the sources.
3. A constant Brune stress drop pattern (～4–5 bars) as a function of seismic moment is observed. This indicates that the manner of fracturing is almost independent on magnitude of earthquakes (hypothesis of self-similarity (Aki, 1967)). Seismic processes in a prefractured medium can explain the observed small stress drop values.
4. Focal mechanisms from moment tensor estimates show that radiation patterns are mostly well interpreted in terms of double couple source models.
5. The scaling of peak ground motion parameters (A _max andV _max vs seismic moment) can be explained by an ω² source model (constant stress drop) multiplied by an exponential function with a small decay parameter, which takes into account the measured attenuation.

These results support the hypothesis of earthquakes generated by simple shear fractures along prefractured structures as a response to changes in the stress field due to the ground deformation.     

Sea-floor spreading in the Bay of Biscay and its relationship to the North Atlantic

An analysis of the magnetic anomaly profiles in the Bay of Biscay provides evidence for the former existence of an E-W trending sea-floor spreading axis in Biscay. Identification of the magnetic anomalies indicates that the opening of the Bay of Biscay took place during the Cretaceous, between Barremian and Maestrichtian times, and involved the formation of a triple-ridge junction with the Mid-Atlantic Ridge between 80 and 73 m.y. ago. The asymmetric distribution of magnetic anomalies in the Bay of Biscay is confirmed. This evidence, together with a proposed Lower Cretaceous development of the Mid-Atlantic Ridge suggests that Biscay evolved as a result of a three-phase rotation of Iberia.     

The Pitcairn hotspot in the South Pacific: distribution and composition of submarine volcanic sequences

Multibeam bathymetry and bottom imaging (Simrad EM12D) studies on an area of about 9500 km² were conducted over the Pitcairn hotspot near 25°10′S, 129° 20′W. In addition, 15 dives with the Nautile submersible enabled us to obtain ground-true observations and to sample volcanic structures on the ancient ocean crust of the Farallon Plate at 3500–4300 m depths. More than 100 submarine volcanoes overprint the ancient crust and are divided according to their size into large (>2000 m in height), intermediate (500–2000 m high) and small (<500 m high) edifices. The interpretation of seafloor backscatter imagery accompanied by submersible observations and sampling enabled us to infer that the total volume of submarine lava erupted during hotspot activity is about 5900 km³ within a radius of about 110 km. The most recent volcanic activities occur on both small and large edifices composed of a great variety of lava flows. These flows vary in composition, following a succession from picritic basalt to alkali basalt, trachybasalt, trachy-andesite and to trachyte. Their large range of SiO₂ (48–62%), Na₂O+K₂O (2–11%), Ba (300–1300 ppm), MgO (1–11%), Nb (19–130 ppm), Ni (4–400 ppm) and rare earth elements suggests that crystal–liquid fractionation from basanite and/or picritic melt sources was a major process. The variation in composition between the least evolved basaltic rocks and the other more evolved silicic lava is marked by a difference in their flow morphology (pillow, giant tubes, tabular to blocky flows). The lava composition and field observation indicate that several magmatic pulses giving rise to cyclic eruptions are responsible for the construction of the edifices. The two larger edifices (>2000 m high) show more extensive eruptive events and a wider range in compositional variability than the smaller (<500 m high) ones. Several (five) submersible transects made along the slope of one of the largest edifices (Bounty) enabled us to observe at least nine successive eruptive cycles progressing from pillow and giant tubular basalt to tabular/blocky trachy-andesite and trachyte flows. Pyroclasts and hyaloclastites are often found with these eruptive sequences. The smaller edifices, forming individualized cones, are built mainly of evolved silicic (SiO₂>53%) flows consisting essentially of alternating sequences of trachy-andesite and trachyte. The distribution and composition of the small edifices suggest that they are the result of sub-crustal forceful magma injection and channeling supplied from reservoirs associated with the large volcanoes.     

Relationships between volcanic patterns and neotectonics in Eastern Anatolia from analysis of satellite images and DEM

The late Neogene to Quaternary volcanism in Eastern Anatolia is related to the Arabia–Eurasia convergence but a clear deformation pattern has not yet been established in this region. We have used the distribution and shape of volcanoes and fault geometry as indicators of the tectonic regime. Volcanic edifices and related faults were analyzed in vertical view using SAR–ERS, Spot images and a Digital Elevation Model (DEM). In several places, adjacent volcanoes that form linear clusters or elongated volcanoes are clearly rooted on vertical tension fractures. These are compatible with horizontal σ₃ striking 90°N, associated with σ₁ horizontal (strike-slip regime) or vertical (extensional regime). We mapped the recent faults that are directly associated to volcanoes. Volcanic vents are related to tail-crack, horsetail or releasing bend structures. In this work, it has been possible to define the ESE-striking, 270-km-long Tutak–Hamur–Çaldiran fault that forms a releasing bend testifying to right-lateral motion. Extension is well documented for few places but no recent fold has been observed. Since 8 Ma, the tectonic system is principally strike-slip. Most of the tension fractures being 2 to 10 km in length, so we infer that they affect only part of the crust. Most strike-slip fault zones are of several tens to a few hundred kilometers long and thus not of lithospheric scale. Therefore, the channels used by the magma to reach the surface are crustal structures.     

The 1986 Kermadec earthquake and its relation to plate segmentation

To evaluate the tectonic significance of the October 20, 1986 Kermadec earthquake (M _w =7.7), we performed a comprehensive analysis of source parameters using surface waves, body waves, and relocated aftershocks. Amplitude and phase spectra from up to 93 Rayleigh waves were inverted for centroid time, depth, and moment tensor in a two-step algorithm. In some of the inversions, the time function was parameterized to include information from the body-wave time function. The resulting source parameters were stable with respect to variations in the velocity and attenuation models assumed, the parameterization of the time function, and the set of Rayleigh waves included. The surface wave focal mechanism derived (=275°, =61°, =156°) is an oblique-compressional mechanism that is not easy to interpret in terms of subduction tectonics. A seismic moment of 4.5×10²⁰ N-m, a centroid depth of 45±5 km, and a centroid time of 13±3 s were obtained. Directivity was not resolvable from the surface waves. The short source duration is in significant contrast to many large earthquakes.We performed a simultaneous inversion ofP andSH body waves for focal mechanism and time function. The focal mechanism agreed roughly with the surface wave mechanism. Multiple focal mechanisms remain a possibility, but could not be resolved. The body waves indicate a short duration of slip (15 to 20 s), with secondary moment release 60s later. Seismically radiated energy was computed from the body-wave source spectrum. The stress drop computed from the seismic energy is about 30 bars. Sixty aftershocks that occurred within three months of the mainshock were relocated using the method of Joint Hypocentral Determination (JHD). Most of the aftershocks have underthrusting focal mechanisms and appear to represent triggered slip on the main thrust interface. The depth, relatively high stress drop, short duration of slip, and paucity of true aftershocks are consistent with intraplate faulting within the downgoing plate. Although it is not clear on which nodal plane slip occurred, several factors favor the roughly E-W trending plane. The event occurred near a major segmentation in the downgoing plate at depth, near a bend in the trench, and near a right-lateral offset of the volcanic are by 80 km along an E-W direction. Also, all events in the region from 1977 to 1991 with CMT focal mechanisms similar to that of the Mainshock occurred near the mainshock epicenter, rather than forming an elongate zone parallel to the trench as did the aftershock activity. We interpret this event as part of the process of segmentation or tearing of the subducting slab. This segmentation appears to be related to the subduction of the Louisville Ridge, which may act as an obstacle to subduction through its buoyancy.     

Oceanic magnetic anomaly amplitudes: variation with sea-floor spreading rate and possible implications

A qualitative examination of oceanic magnetic anomalies suggests that their amplitudes increase with the spreading rate of the ocean floor. This suggestion has been investigated quantitatively by inversion of the anomalies. A variance matching technique has been used to calculate the average magnetisation of the crust with crustal thickness held constant or to calculate the thickness with the magnetisation fixed. In spite of variations in the computed intensities of magnetisation and thickness when compared to spreading rates the results show an over-all increase in the intensities with the rate of spreading. The mechanisms responsible for this could include greater irregularity in distribution of magnetic polarities within the crust at slow spreading ridges, greater intensity of magnetisation in crust produced at fast spreading due to initial chemical properties or enhanced hydrothermal alteration, and a dependence of crustal thickness on spreading rate.     

Surface area, porosity and water adsorption properties of fine volcanic ash particles

Our understanding on how ash particles in volcanic plumes react with coexisting gases and aerosols is still rudimentary, despite the importance of these reactions in influencing the chemistry and dynamics of a plume. In this study, six samples of fine ash (<100 m) from different volcanoes were measured for their specific surface area, a_s, porosity and water adsorption properties with the aim to provide insights into the capacity of silicate ash particles to react with gases, including water vapour. To do so, we performed high-resolution nitrogen and water vapour adsorption/desorption experiments at 77 K and 303 K, respectively. The nitrogen data indicated a_s values in the range 1.1–2.1 m²/g, except in one case where a a_s of 10 m²/g was measured. This high value is attributed to incorporation of hydrothermal phases, such as clay minerals, in the ash surface composition. The data also revealed that the ash samples are essentially non-porous, or have a porosity dominated by macropores with widths >500 Å. All the specimens had similar pore size distributions, with a small peak centered around 50 Å. These findings suggest that fine ash particles have relatively undifferentiated surface textures, irrespective of the chemical composition and eruption type. Adsorption isotherms for water vapour revealed that the capacity of the ash samples for water adsorption is systematically larger than predicted from the nitrogen adsorption a_s values. Enhanced reactivity of the ash surface towards water may result from (i) hydration of bulk ash constituents; (ii) hydration of surface compounds; and/or (iii) hydroxylation of the surface of the ash. The later mechanism may lead to irreversible retention of water. Based on these experiments, we predict that volcanic ash is covered by a complete monolayer of water under ambient atmospheric conditions. In addition, capillary condensation within ash pores should allow for deposition of condensed water on to ash particles before water reaches saturation in the plume. The total mass of water vapour retained by 1 g of fine ash at 0.95 relative water vapour pressure is calculated to be ~10^–2 g. Some volcanic implications of this study are discussed.Editorial responsibility: J. Gilbert     

火山岩、白云岩储层基质孔隙度计算方法分析(英文)

火山岩、风化壳白云岩等缝洞储层基质孔隙度计算是测井评价亟需解决的难题之一.本文首先将全直径流纹岩声波实验公式(2005年,李宁)与国内外常用的基质孔隙度计算公式进行了细致的对比;进而就该公式在中基性火山岩、风化壳白云岩等缝洞储层中的适用性进行了深入讨论,并以岩心分析资料为基础,详细给出了该公式与其它公式在计算上述储层基质孔隙度时的误筹分布.误差统计结果表明该公式具有更高的精度.通过中国东部和西部三家油田70口井的实际应用验证,在孔隙度从1.5%到15%范围内,该公式不仅适用于酸性火山岩储层,而且适用于中基性火山岩、风化壳白云岩等缝洞储层基质孔隙度的评价.同时,该公式能够最大限度地减小扩径、岩石蚀变等复杂地质条件对计算结果的影响,实用性更强.     

首都圈地区重力变化及其映震能力

对首都圈流动重力观测成果进行非潮汐信息的提取，得到了首都圈地区１９８４—１９９３年期间的重力动态变化图像，认为重力变化与区域构造活动有一定的相关性。以大于２倍均方差作为异常判断标准，统计了北京小网的段差重力异常与地震的对应关系。结果表明：对于测网内发生的４级左右的地震，有异常出现的段差数占总数的１６．９％，对于距测网１００ｋｍ以内发生的５级以上的地震有异常出现的段差数占总数的３７．５％。西北部测环的段差异常出现率比南部和东北部高。     

Eruption-induced modifications to volcanic seismicity at Ruapehu,New Zealand,and its implications for eruption forecasting

Broadband seismic data collected on Ruapehu volcano, New Zealand, in 1994 and 1998 show that the 1995-1996 eruptions of Ruapehu resulted in a significant change in the frequency content of tremor and volcanic earthquakes at the volcano. The pre-eruption volcanic seismicity was characterized by several independent dominant frequencies, with a 2 Hz spectral peak dominating the strongest tremor and volcanic earthquakes and higher frequencies forming the background signal. The post-eruption volcanic seismicity was dominated by a 0.8-1.4 Hz spectral peak not seen before the eruptions. The 2 Hz and higher frequency signals remained, but were subordinate to the 0.8-1.4 Hz energy. That the dominant frequencies of volcanic tremor and volcanic earthquakes were identical during the individual time periods prior to and following the 1995-1996 eruptions suggests that during each of these time periods the volcanic tremor and earthquakes were generated by the same source process. The overall change in the frequency content, which occurred during the 1995-1996 eruptions and remains as of the time of the writing of this paper, most likely resulted from changes in the volcanic plumbing system and has significant implications for forecasting and real-time assessment of future eruptive activity at Ruapehu.     

地下热泉的形成条件与地震

根据湖南省地下热泉的分布情况,对其形成条件进行了综合分析,研究了地下热泉形成条件与地震的关系,提出了充分利用地下热泉的综合观测,为湖南的地震活动提供前兆信息的建议。