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81.
José Fernández José M. Carrasco John B. Rundle Vicente Araña 《Bulletin of Volcanology》1999,60(7):534-544
In this paper we study the application of different geodetic techniques to volcanic activity monitoring, using theoretical analysis. This methodology is very useful for obtaining an idea of the most appropriate (and efficient) monitoring method, mainly when there are no records of geodetic changes previous to volcanic activity. The analysis takes into account the crustal structure of the area, its geology, and its known volcanic activity to estimate the deformation and gravity changes that might precede eruptions. The deformation model used includes the existing gravity field and vertical changes in the crustal properties. Both factors can have a considerable effect on computed deformation and gravity changes. Topography should be considered when there is a steep slope (greater than 10°). The case study of Teide stratovolcano (Tenerife, Canary Islands), for which no deformation or gravity changes are available, is used as a test. To avoid considering topography, we worked at the lowest level of Las Cañadas and examined a smaller area than the whole caldera or island. Therefore, the results are only a first approach to the most adequate geodetic monitoring system. The methodology can also be applied to active areas where volcanic risk is not associated with a stratovolcano but instead with monogenetic scattered centers, especially when sites must be chosen in terms of detection efficiency or existing facilities. The Canary Islands provide a good example of this type of active volcanic areas, and we apply our model to the island of Lanzarote to evaluate the efficiency of the monitoring system installed at the existing geodynamic station. On this island topography is not important. The results of our study show clearly that the most appropriate geodetic volcano monitoring system is not the same for all different volcanic zones and types, and the particular properties of each volcano/zone must be taken into account when designing each system. 相似文献
82.
《地学前缘(英文版)》2020,11(5):1789-1803
Video cameras are common at volcano observatories,but their utility is often limited during periods of crisis due to the large data volume from continuous acquisition and time requirements for manual analysis.For cameras to serve as effective monitoring tools,video frames must be synthesized into relevant time series signals and further analyzed to classify and characterize observable activity.In this study,we use computer vision and machine learning algorithms to identify periods of volcanic activity and quantify plume rise velocities from video observations.Data were collected at Villarrica Volcano,Chile from two visible band cameras located~17 km from the vent that recorded at 0.1 and 30 frames per second between February and April 2015.Over these two months,Villarrica exhibited a diverse range of eruptive activity,including a paroxysmal eruption on 3 March.Prior to and after the eruption,activity included nighttime incandescence,dark and light emissions,inactivity,and periods of cloud cover.We quantify the color and spatial extent of plume emissions using a blob detection algorithm,whose outputs are fed into a trained artificial neural network that categorizes the observable activity into five classes.Activity shifts from primarily nighttime incandescence to ash emissions following the 3 March paroxysm,which likely relates to the reemergence of the buried lava lake.Time periods exhibiting plume emissions are further analyzed using a row and column projection algorithm that identifies plume onsets and calculates apparent plume horizontal and vertical rise velocities.Plume onsets are episodic,occurring with an average period of~50 s and suggests a puffing style of degassing,which is commonly observed at Villarrica.However,the lack of clear acoustic transients in the accompanying infrasound record suggests puffing may be controlled by atmospheric effects rather than a degassing regime at the vent.Methods presented here offer a generalized toolset for volcano monitors to classify and track emission statistics at a variety of volcanoes to better monitor periods of unrest and ultimately forecast major eruptions. 相似文献
83.
2002和2003年夏季流动地震观测揭示,天池火山口附近存在大量的微震活动和一系列震群活动.地震定位结果表明地震主要发生在火山口附近,以震群形式发生的地震全部集中在天池火山口西南部,东北部地震密集区没有观测到震群活动.在夏季以外的其他季节,天池火山区只有一个固定地震台站(CBS)用于地震监测.利用CBS台不同时间的观测纪录,通过波形相关分析发现其他季节的主要震群活动仍然集中在天池西南部.震群的高精度相对定位揭示震源位置沿北西-南东向分布,倾向西南,倾角约80°. 2003年7月13日的震群发生期间,地震震源位置出现从深到浅的迁移现象,同时震源深度较大的地震在不同台站的地震波初动方向几乎全部向上,表明震源具有明显的膨胀分量.考虑到长白山天池火山2002年以来出现明显的地表形变、地球化学异常和谐频地震等现象,我们认为震群活动可能与5 km深度附近存在岩浆热液活动和岩浆增压有关. 相似文献
84.
85.
In summer 2003, a Chaparral Model 2 microphone was deployed at Shishaldin Volcano, Aleutian Islands, Alaska. The pressure
sensor was co-located with a short-period seismometer on the volcano’s north flank at a distance of 6.62 km from the active
summit vent. The seismo-acoustic data exhibit a correlation between impulsive acoustic signals (1–2 Pa) and long-period (LP,
1–2 Hz) earthquakes. Since it last erupted in 1999, Shishaldin has been characterized by sustained seismicity consisting of
many hundreds to two thousand LP events per day. The activity is accompanied by up to ∼200 m high discrete gas puffs exiting
the small summit vent, but no significant eruptive activity has been confirmed. The acoustic waveforms possess similarity
throughout the data set (July 2003–November 2004) indicating a repetitive source mechanism. The simplicity of the acoustic
waveforms, the impulsive onsets with relatively short (∼10–20 s) gradually decaying codas and the waveform similarities suggest
that the acoustic pulses are generated at the fluid–air interface within an open-vent system. SO2 measurements have revealed a low SO2 flux, suggesting a hydrothermal system with magmatic gases leaking through. This hypothesis is supported by the steady-state
nature of Shishaldin’s volcanic system since 1999. Time delays between the seismic LP and infrasound onsets were acquired
from a representative day of seismo-acoustic data. A simple model was used to estimate source depths. The short seismo-acoustic
delay times have revealed that the seismic and acoustic sources are co-located at a depth of 240±200 m below the crater rim.
This shallow depth is confirmed by resonance of the upper portion of the open conduit, which produces standing waves with
f=0.3 Hz in the acoustic waveform codas. The infrasound data has allowed us to relate Shishaldin’s LP earthquakes to degassing
explosions, created by gas volume ruptures from a fluid–air interface. 相似文献
86.
Temporal Evolution of Magmatism in the Northern Volcanic Zone of the Andes: The Geology and Petrology of Cayambe Volcanic Complex (Ecuador) 总被引:4,自引:1,他引:3
SAMANIEGO PABLO; MARTIN HERVE; MONZIER MICHEL; ROBIN CLAUDE; FORNARI MICHEL; EISSEN JEAN-PHILIPPE; COTTEN JOSEPH 《Journal of Petrology》2005,46(11):2225-2252
In the Northern Volcanic Zone of the Andes, the Cayambe VolcanicComplex consists of: (1) a basal, mostly effusive volcano, theViejo Cayambe, whose lavas (andesites and subordinate dacitesand rhyolites) are typically calc-alkaline; and (2) a younger,essentially dacitic, composite edifice, the Nevado Cayambe,characterized by lavas with adakitic signatures and explosiveeruptive styles. The construction of Viejo Cayambe began >1·1Myr ago and ended at 1·0 Ma. The young and still activeNevado Cayambe grew after a period of quiescence of about 0·6Myr, from 0·4 Ma to Holocene. Its complex history isdivided into at least three large construction phases (Angurealcone, Main Summit cone and Secondary Summit cone) and compriseslarge pyroclastic events, debris avalanches, as well as periodsof dome activity. Geochemical data indicate that fractionalcrystallization and crustal assimilation processes have a limitedrole in the genesis of each suite. On the contrary, field observations,and mineralogical and geochemical data show the increasing importanceof magma mixing during the evolution of the volcanic complex.The adakitic signature of Nevado Cayambe magmas is related topartial melting of a basaltic source, which could be the lowercrust or the subducted slab. However, reliable geophysical andgeochemical evidence indicates that the source of adakitic componentis the subducted slab. Thus, the Viejo Cayambe magmas are inferredto come from a mantle wedge source metasomatized by slab-derivedmelts (adakites), whereas the Nevado Cayambe magmas indicatea greater involvement of adakitic melts in their petrogenesis.This temporal evolution can be related to the presence of thesubducted Carnegie Ridge, modifying the geothermal gradientalong the WadatiBenioff zone and favouring slab partialmelting. KEY WORDS: adakites; 40Ar/39Ar dating; Cayambe volcano; Ecuador; mantle metasomatism; Andes 相似文献
87.
David A. Clague Alice S. Davis James L. Bischoff Jacqueline E. Dixon Renee Geyer 《Bulletin of Volcanology》2000,61(7):437-449
Glassy bubble-wall fragments, morphologically similar to littoral limu o Pele, have been found in volcanic sands erupted
on Lō'ihi Seamount and along the submarine east rift zone of Kīlauea Volcano. The limu o Pele fragments are undegassed with
respect to H2O and S and formed by mild steam explosions. Angular glass sand fragments apparently form at similar, and greater, depths
by cooling-contraction granulation. The limu o Pele fragments from Lō'ihi Seamount are dominantly tholeiitic basalt containing
6.25–7.25% MgO. None of the limu o Pele samples from Lō'ihi Seamount contains less than 5.57% MgO, suggesting that higher
viscosity magmas do not form lava bubbles. The dissolved CO2 and H2O contents of 7 of the limu o Pele fragments indicate eruption at 1200±300 m depth (120±30 bar). These pressures exceed that
generally thought to limit steam explosions. We conclude that hydrovolcanic eruptions are possible, with appropriate pre-mixing
conditions, at pressures as great as 120 bar.
Received: 22 December 1998 / Accepted: 16 July 1999 相似文献
88.
Mount Rainier is one of the most seismically active volcanoes in the Cascade Range, with an average of one to two high-frequency
volcano-tectonic (or VT) earthquakes occurring directly beneath the summit in a given month. Despite this level of seismicity,
little is known about its cause. The VT earthquakes occur at a steady rate in several clusters below the inferred base of
the Quaternary volcanic edifice. More than half of 18 focal mechanisms determined for these events are normal, and most stress
axes deviate significantly from the regional stress field. We argue that these characteristics are most consistent with earthquakes
in response to processes associated with circulation of fluids and magmatic gases within and below the base of the edifice.Circulation
of these fluids and gases has weakened rock and reduced effective stress to the point that gravity-induced brittle fracture,
due to the weight of the overlying edifice, can occur. Results from seismic tomography and rock, water, and gas geochemistry
studies support this interpretation. We combine constraints from these studies into a model for the magmatic system that includes
a large volume of hot rock (temperatures greater than the brittle–ductile transition) with small pockets of melt and/or hot
fluids at depths of 8–18 km below the summit. We infer that fluids and heat from this volume reach the edifice via a narrow
conduit, resulting in fumarolic activity at the summit, hydrothermal alteration of the edifice, and seismicity.
Received: 10 February 1999 / Accepted: 26 June 1999 相似文献
89.
长白山天池火山公元1199—1200年大喷发历史记载的发现及其意义 总被引:22,自引:2,他引:20
长白山天池火山是世界著名的火山之一,是我国规模最大、最具有潜在喷发危险的一座近代活动火山。目前国内外许多火山学者为了研究天池火山最近一次大喷发年代问题,做了大量工作,并取得了较为精确的^14C年代资料,但地直未取得有关这次大喷发历史记载的证实。本文通过再次收集和查阅上千万字的文史古籍资料进行整理、筛选、考证和分析,首次发现了白头山天池火山于公元1199 ̄1200年间一次特大喷发的历史记载。这对于长 相似文献
90.