共查询到20条相似文献，搜索用时 187 毫秒
1.
L. Scharff M. Hort A.J.L. Harris M. Ripepe J.M. Lees R. Seyfried 《Journal of Volcanology and Geothermal Research》2008
We report on the evaluation of in situ measurements of eruption velocities and relative mass flux, collected by Doppler radar, as well as acoustic and infrared data, that were recorded at Stromboli volcano. Doppler radar observations were made alongside thermal, acoustic and seismic measurements, to (1) further investigate the complex waveforms of the SW crater by combining infrasonic, infrared and Doppler radar measurements, (2) establish a relationship between infrared, acoustic and Doppler radar measurements and (3) verify that all instruments observe the same behavior in terms of relative mass eruption rate. We also explore the relationship between kinetic and acoustic energy released during an eruption. Comparing the different methods to each other we are able to show that the mass erupted can be estimated either from the total reflected Doppler radar energy or from infrared observations. However, neither thermal nor reflected energy can provide a value in terms of absolute mass in kg. The erupted masses of different eruptions can only be evaluated relative to each other. Using the combined three data sets we especially focus on the eruption dynamics of the SW crater of Stromboli, namely its fluctuations in eruption strength. These pulses in one eruptive event, so called ’pulsations’ dominate more than 40% of the eruptions of SW crater. Previous models that explain pulsations to be generated by multiple consecutively exploding bubbles are supported by our combined analysis of infrasound, thermal data, particle velocities, and the reflected energy. 相似文献
2.
《Journal of Volcanology and Geothermal Research》2006,149(1-2):160-175
The 5 April 2003 paroxysmal explosion at Stromboli volcano was one of the strongest explosive events of the last century. It occurred while the effusive eruption, begun on 28 December 2002 and finished on 22 July 2003, was still on going and the summit craters of the volcano were obstructed. In this paper, we present a reconstruction of the sequence of events based on thermal and visual images collected from helicopter before, during and immediately after the paroxysm. One month before the blast, ash emission and temperature increase at the bottom of the summit craters were observed. An increasing amount of juvenile components in the emitted ash during March suggested that the magma level within the crater was rising accordingly. Hot degassing vents at the bottom of the summit craters were not persistent, and the craters remained almost entirely obstructed by talus accumulation until the paroxysm occurred. Three minutes before the explosion, we recorded a significant increase in temperature inside Crater 1, accompanied by a thicker gas plume. Thirty-two seconds before the blast, reddish ash was emitted from Crater 1. The paroxysm produced a vulcanian explosion that opened the feeder conduit, obstructed for over three months. The blast was accompanied by a shock wave recorded by the INGV seismic network at 07:13:37 GMT. Explosions with hot material started from Crater 1, and after 15 s propagated to Crater 3, about 100 m away. The velocity of ejecta was ∼80 m s− 1, and increased when the eruptive plumes from both craters merged together during the vulcanian phase. An eruptive column rose 1 km above the top of the volcano, and explosions continued mainly at Crater 3. The paroxysm lasted about 9 min, with bombs up to 4 m wide falling on the village of Ginostra, on the west flank of the island, and destroying two houses. This event signalled the start of the declining phase of the effusive eruption, suggesting that the feeder conduit was returning to its former steady conditions, with open vents and continuous, mild strombolian activity. 相似文献
3.
In situ measurement of volcanic eruption velocities is one of the great challenges left in geophysical volcanology. In this
paper we report on a new radar Doppler technique for monitoring volcanic eruption velocities. In comparison with techniques
employed previously (e.g., photographic methods or acoustic Doppler measurements), this method allows continuous recordings
of volcanic eruptions even during poor visibility. Also, radar Doppler instruments are usually light weight and energy efficient,
which makes them superior to other Doppler techniques based on laser light or sound. The proposed new technique was successfully
tested at Stromboli Volcano in late 1996 during a period of low activity. The recorded data allow a clear distinction between
particles rising from the vent and particles falling back towards the vent. The mean eruption velocity was approximately 10 m/s.
Most of the eruptions recorded by radar were correlated to seismic recordings. The correlation between the magnitude of the
volcanic shocks and the eruption force index defined in the paper may provide new insights into magma transport in the conduit.
Received: 15 May 1998 / Accepted: 15 December 1998 相似文献
4.
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. 相似文献
5.
Carmelo Ferlito Marco Viccaro Eugenio Nicotra Renato Cristofolini 《Bulletin of Volcanology》2010,72(10):1179-1190
Over the last 2 years, the south east crater (SEC) at Mt. Etna (Italy) has been characterised by the intermittent emission
of lava flows, often accompanied by paroxysmal episodes with violent strombolian activity and/or lava fountains. One of the
most intense and intriguing episodes occurred on November 16, 2006 during strombolian and effusive activity from the very
top of the SEC. At the eastern base of the crater, a violent and short-lasting outburst generated a 300-m-high eruptive curtain
of finely fragmented magma and steam, which collapsed in a few seconds giving rise to a small but significant pyroclastic
flow. The paroxysm was preceded by progressive sliding of the eastern flank of the SEC edifice, which led to the formation
of a large niche. Previously published models explain the outburst as due to rapid vaporisation of water contained in sediments
on contact with hot lava flowing along the flank of the SEC or, alternatively, to gravitational collapse of a destabilised
portion of the eastern flank of the crater. However, several lines of evidence suggest that these models do not adequately
explain the paroxysm, which we propose was associated with the explosive emission of magma during the rapid opening of an
ESE-WNW-oriented fracture at the eastern base of the SEC. Moreover, geochemical data of the products associated with this
episode show that they are the most primitive and gas-rich among those erupted during the entire 2006 period. We suggest that
the paroxysm of November 16, 2006, which occurred at base of the SEC, was a magma outburst due to rapid ground fracturing
related to a drop in confining pressure upon the underlying dyke consequent to the sliding of the eastern flank of the SEC
cone. Specifically, the unloaded shallow portion of the feeding dyke did not restrain the arrival of a new batch of gas-rich
magma, which worked its way out by fracturing the base of the niche at its weakest point. This paroxysm appears significant
because it reveals the potential development of syn-eruptive dynamics acting directly on the uppermost portion of the plumbing
system which is able to generate pyroclastic flows even on basaltic volcanoes such as Mount Etna. 相似文献
6.
The Hekla eruption cloud on 26–27 February 2000 was the first volcanic cloud to be continuously and completely monitored advecting above Iceland, using the C-band weather radar near the Keflavík international airport. Real-time radar observations of the onset, advection, and waning of the eruption cloud were studied using time series of PPI (plan-position indicator) radar images, including VMI normal, Echotop, and Cappi level 2 displays. The reflectivity of the entire volcanic cloud ranges from 0 to >60 dBz. The eruption column above the vent is essentially characterised by VMI normal and Cappi level 2 values, >30 dBz, due to the dominant influence of lapilli and ash (tephra) on the overall reflected signal. The cloud generated by the column was advected downwind to the north-northeast. It is characterised by values between 0 and 30 dBz, and the persistence of these reflections likely result from continuing water condensation and freezing on ash particles. Echotop radar images of the eruption onset document a rapid ascent of the plume head with a mean velocity of ~30 to 50 m s–1, before it reached an altitude of ~11–12 km. The evolution of the reflected cloud was studied from the area change in pixels of its highly reflected portions, >30 dBz, and tied to recorded volcanic tremor amplitudes. The synchronous initial variation of both radar and seismic signals documents the abrupt increase in tephra emission and magma discharge rate from 18:20 to 19:00 UTC on 26 February. From 19:00 the >45 dBz and 30–45 dBz portions of the reflected cloud decrease and disappear at about 7 and 10.5 h, respectively, after the eruption began, indicating the end of the decaying explosive phase. The advection and extent of the reflected eruption cloud were compared with eyewitness accounts of tephra fall onset and the measured mass of tephra deposited on the ground during the first 12 h. Differences in the deposit map and volcanic cloud radar map are due to the fact that the greater part of the deposit originates by fallout off the column margins and from the base of the cloud followed by advection of falling particle in lower level winds.Editorial responsibility: P. Mouginis-Mark 相似文献
7.
8.
A. Montalto 《Bulletin of Volcanology》1996,57(7):483-492
Since the end of the last magmatic eruption (1890), activity of La Fossa (southern Tyrrhenian Sea, Italy) has consisted of fumarolic emissions of fluctuating intensity. Fluids are discharged principally at two fumarolic fields located in the northern rim of the active crater and at the beach sited at its northern foot. Increased thermal, seismic and geochemical activity has been recorded since 1978, when an earthquake of M=5.5 occurred in the region. This paper combines available geophysical and geochemical information in order to develop a tentative interpretation of two episodes of apparent unrest which occurred in 1985 and 1987–1988, enhancing the risk of renewal of the eruptive activity. The 1985 unrest consisted essentially of a sharp build up of the internal pressure in the shallow hydrothermal system, which was induced by the injection of hot gases of magmatic origin. The crater fumaroles displayed significant increases in CO2 and other acid species, but their outlet temperature did not change. Conversely, the 1987–1988 episode was characterized by appreciable modifications at the crater fumaroles, with only secondary effects at the fumarole system of the beach. The sliding of part of the eastern flank of the La Fossa cone into the sea occurred on 20 April 1988, when the region was affected by crustal dilatation producing a seismic sequence of relatively high intensity. Both episodes of unrest were accompanied by increases of local microseismic activity, which affected the nothern sector of the island in 1985, and the southern one in 1988. Finally, a phase of appreciable areal contraction was detected in 1990, probably due to the effect of the cooling and crystallization of magma at relatively shallow depths, accompanying the increased thermal activity at the crater fumaroles. Regional tectonic stress seems to play an important role in the transition of the volcanic system from a phase of relative stability to a phase of apparent unrest, inducing the heating and the expansion of shallow hydrothermal fluids. Available information is insufficient to indicate whether or not the volcano is building towards the renewal of a magmatic eruption, and there is no evidence to hypothesize episodes of significant magma migration. The frequency of measurements of many parameters needs to be increased in order to learn more about the temporal relationships between geochemical and geophysical variations preceding and accompanying periods of increased thermal activity. This will probably be a valid tool for recognizing short-term precursors of a future eruption, reducing the risk of false alarms. 相似文献
9.
On the basis of data collected during a seismological investigation carried out on Mt. Etna (Italy) during a phase of the volcanic activity marked by progressive transition from Strombolian bursts to effusive paroxysms at the SE crater (January 1990), three types of seismic regime have been recognized. They are interpreted to be linked to modifications in the regimes of volcanic gases and vapours within the upper levels of the magma column, as suggested by recent studies on the dynamics of magmatic fluids. Our analysis also reveals that the use of usual seismic parameters, such as the temporal patterns associated with the occurrence of discrete low-frequency events (“spindles”) and the coefficient m of the Ishimoto-Iida law, can furnish information about variations in the behaviour of the system before the irreversible evolution of the volcanic activity toward eruptive paroxysms. 相似文献
10.
Mt. Veniaminof, Alaska Peninsula, is a stratovolcano with a summit ice-filled caldera containing a small intracaldera cone
and active vent. From January 2 to February 21, 2005, Mt. Veniaminof erupted. The eruption was characterized by numerous small
ash emissions (VEI 0 to 1) and accompanied by low-frequency earthquake activity and volcanic tremor. We have performed spectral
analyses of the seismic signals in order to characterize them and to constrain their source. Continuous tremor has durations
of minutes to hours with dominant energy in the band 0.5–4.0 Hz, and spectra characterized by narrow peaks either irregularly
(non-harmonic tremor) or regularly spaced (harmonic tremor). The spectra of non-harmonic tremor resemble those of low-frequency
events recorded simultaneously with surface ash explosions, suggesting that the source mechanisms might be similar or related.
We propose that non-harmonic tremor at Mt. Veniaminof results from the coalescence of gas bubbles while low-frequency events
are related to the disruption of large gas pockets within the conduit. Harmonic tremor, characterized by regular and quasi-sinusoidal
waveforms, has duration of hours. Spectra containing up to five harmonics suggest the presence of a resonating source volume
that vibrates in a longitudinal acoustic mode. An interesting feature of harmonic tremor is that frequency is observed to
change over time; spectral lines move towards higher or lower values while the harmonic nature of the spectra is maintained.
Factors controlling the variable characteristics of harmonic tremor include changes in acoustic velocity at the source and
variations of the effective size of the resonator. 相似文献
11.
Sergio-Raúl Rodríguez Claus Siebe Jean-Christophe Komorowski Michael Abrams 《Journal of Volcanology and Geothermal Research》2002,113(1-2)
The study area is located in the east part of the Trans-Mexican Volcanic Belt, in the Las Cumbres Volcanic Complex (LCVC) which lies between two large stratovolcanoes: Pico de Orizaba (5700 m a.s.l.) to the south, and Cofre de Perote (4200 m a.s.l.) to the NNE. The most conspicuous structure of the LCVC is a 4-km-diameter circular crater with a dacitic dome in the center, which constitutes the remains of a destroyed stratovolcano.The Quetzalapa Pumice (QP) was produced by a plinian eruption that was dated by the 14C method at 20 000 yr. BP. The eruptive sequence consists predominantly of pumice fall deposits and scarce intra-plinian pyroclastic flow deposits, which crop out on the west flank of the LCVC. The absence of post-plinian ignimbrite deposits is striking.The deposits are well sorted, clast-supported with reverse grading at the base, with a medium to high accessory lithics content. The maximum average thickness of the deposit in the proximal areas is about 15 m and has been divided into three members: the Basal Member (BM), 2 m thick with four submembers (BMf1, BMf2, BMf3, and BMafl), the Intermediate Member (IM), 10 m thick with two submembers (IMpf and IMaf), and the Upper Member (UM), 3 m thick with four submembers (UMpl, UMsdf, UMwaf, and UMpls).The predominant component of the fall deposits is a white, highly vesiculated pumice with 71% SiO2 content. Plagioclase is the most abundant mineral followed by 1–3-mm-long biotite phenocrysts. The accessory lithics are lavas mostly of andesitic composition. Their abundance increases toward the uppermost levels of the sequence.We calculate a minimum volume of 8.4 km3 (2.22 km3 dense rock equivalent), for the entire QP deposit. Isopach and isopleth maps show that the IM deposit has an elongated distribution with a NNE–SSW direction, whereas the UM deposit has a circular distribution.We estimate a maximum eruptive column height for the IM of 20 km. Field studies and isopach and isopleth maps indicate that the eruptive column was affected by a strong wind.Previous studies located the QP source in the Las Cumbres crater. However, based on the isopach and isopleth distribution, and the lack of pumice fall deposits inside the Las Cumbres crater, we consider that the QP emission center is located on the west flank of the LCVC, and was buried by its own pumice fall deposits. It coincides with an explosion crater called La Capilla formed during the closing phase of the QP eruption.A ‘pumice fountain’ model is proposed to explain the observed sequence of deposits. According to this model, the material was emitted through a ‘hose-type’ conduit during a monogenetic eruption of rhyolitic composition. This kind of volcanic activity is not extensively reported in the literature. 相似文献
12.
S. Falsaperla E. Privitera B. Chouet P. Dawson 《Journal of Volcanology and Geothermal Research》2002,114(3-4)
Seismic activity recorded at Mount Etna during 1992 was characterized by long-period (LP) events and tremor with fluctuating amplitudes. These signals were associated with the evolution of the eruptive activity that began on December 14, 1991. Following the occurrence of numerous volcano-tectonic earthquakes at the onset of the eruption, LP events dominated the overall seismicity starting in January, 1992. The LP activity occurred primarily in swarms, which were temporally correlated with episodic collapses of the crater floor in the Northeast Crater. Source depths determined for selected LP events suggest a source region located slightly east of Northeast Crater and extending from the surface to a depth of 2000 m. Based on the characteristic signatures of the time series, four families of LP events are identified. Each family shares common spectral peaks independent of azimuth and distance to the source. These spectral features are used to develop a fluid-filled crack model of the source. We hypothesize that the locus of the LP events represents a segment of the magma feeding system connecting a depressurizing magma body with a dike extending in the SSE direction along the western wall of Valle del Bove, toward the site of the Mount Etna eruption. We surmise that magma withdrawal from the source volume beneath Northeast Crater may have caused repeated collapses of the crater floor. Some collapse events may have produced pressure transients in the subjacent dike which acted as seismic wave sources for LP events. 相似文献
13.
Tomofumi Kozono Hideki Ueda Taku Ozawa Takehiro Koyaguchi Eisuke Fujita Akihiko Tomiya Yujiro J Suzuki 《Bulletin of Volcanology》2013,75(3):1-13
We present precise geodetic and satellite observation-based estimations of the erupted volume and discharge rate of magma during the 2011 eruptions of Kirishima-Shinmoe-dake volcano, Japan. During these events, the type and intensity of eruption drastically changed within a week, with three major sub-Plinian eruptions on January 26 and 27, and a continuous lava extrusion from January 29 to 31. In response to each eruptive event, borehole-type tiltmeters detected deflation of a magma chamber caused by migration of magma to the surface. These measurements enabled us to estimate the geodetic volume change in the magma chamber caused by each eruptive event. Erupted volumes and discharge rates were constrained during lava extrusion using synthetic aperture radar satellite imaging of lava accumulation inside the summit crater. Combining the geodetic volume change and the volume of lava extrusion enabled the determination of the erupted volume and discharge rate during each sub-Plinian event. These precise estimates provide important information about magma storage conditions in magma chambers and eruption column dynamics, and indicate that the Shinmoe-dake eruptions occurred in a critical state between explosive and effusive eruption. 相似文献
14.
An eruption on the eastern flank of Piton de la Fournaise volcano started on 16 November, 2002 after 10 months of quiescence.
After a relatively constant level of activity during the first 13 days of the eruption, lava discharge, volcanic tremor and
seismicity increased from 29 November to 3 December. Lava effusion suddenly ceased on 3 December while shallow earthquakes
beneath the Dolomieu summit crater were still recorded at a rate of about one per minute. This unusual activity continued
and increased in intensity over the next three weeks, ending with the formation of a pit crater within Dolomieu. Based on
ground deformation, measured by rapid-static and continuous GPS and an extensometer, seismic data, and lava effusion patterns,
the eruptive period is divided into five stages: 1) slow summit inflation and sporadic seismicity; 2) rapid summit inflation
and a short seismic crisis; 3) rapid flank inflation, onset of summit deflation, sporadic seismicity, accompanied by stable
effusion; 4) flank inflation, coupled with summit deflation, intense seismicity, and increased lava effusion; and finally
5) little deflation, intense shallow seismicity, and the end of lava effusion. We propose a model in which the pre-intrusive
inflation of Stage 1 in the months preceding the eruption was caused by a magma body located near sea level. The magma reservoir
was the source of an intrusion rising under the summit during Stage 2. In Stage 3, the magma ponded at a shallow level in
the edifice while the lateral injection of a radial dike reached the surface on the eastern flank of the basaltic volcano,
causing lava effusion. Pressure decrease in the magmatic plumbing system followed, resulting in upward migration of a collapse
front, forming a subterranean column of debris by faulting and stoping. This caused intense shallow seismicity, increase in
discharge of lava and volcanic tremor at the lateral vent in Stage 4 and, eventually the formation of a pit crater in Stage
5. 相似文献
15.
16.
The 1968–73 (and continuing) eruption of Arenal Volcano, Costa Rica, a small 1633 m strato-volcano with long periods of repose, defines an eruptive cycle which is typical of Arenal’s pre-historic eruptions. An intense, short explosive phase (July 29–31, 1968) grades into an effusive phase, and is followed by a block lava flow. The eruptive rocks become increasingly less differentiated with time in a given cycle, ranging from andesite to basaltic andesite. Nuées ardentes are a characteristic of the initial explosions, and are caused by fall-back ejecta on slopes around the main crater — an explosion crater in the 1968 eruption — which coalesce into hot avalanches and descend major drainage channels. Total volume of pyroclastic flows was small, about 1.8 ± 0.5 × 10n m3, in the July 29–31 explosions, and are block and ash flows, with much accidental material. Overpressures, ranging up to perhaps 5 kilobars just prior to major explosions, were estimated from velocities of large ejected blocks, which had velocities of up to 600 m/sec. Total kinetic energy and volume of ejecta of all explosions are an estimated 3 × 1022 ergs and 0.03 km3, respectively. The block lava flow, emitted from Sept., 1968 to 1973 (and continuing) has a volume greater than 0.06 km3, and covers 2.7 km2 at thicknesses ranging from 15 to over 100 m. The total volumes of the explosive and effusive phases for the 1968–73 eruption are about 0.05 km3 and 0.06 km3, respectively. The last eruption of Arenal occurred about 1500 AD. based on radiocarbon dating and archaeological means, and was about twice as voluminous as the current one (0.17 km3 versus 0.09 km3). The total thermal energies for this pre-historic eruption and the current one are 8 × 1023 and 18 × 1023, respectively. The total volume of Arenal’s cone is about 6 km3 from 1633 m (summit) to 500 m, and, estimates of age based on the average rate of cone growth from these two eruptions, suggest an age between 20,000 to 200,000 years. 相似文献
17.
Wide variations were measured in the diffuse CO2 flux through the soils in three selected areas of Mt Etna between August 1989 and March 1993. Degassing of CO2 from the area of Zafferana Etnea-S. Venerina, on the eastern slope of the volcano, has been determined to be more strongly influenced by meteorological parameters than the other areas. The seasonal component found in the data from this area has been excluded using a filtering algorithm based on the best fitting equation calculated from the correlation between CO2 flux values and those of air temperature. The filtered data appear to have variations temporally coincident with those from the other areas, thus suggesting a common and probably deep source of gas. The highest fluxes measured in the two most peripheral areas may correlate well with other geophysical and volcanological anomalous signals that preceded the strong eruption of 1991–1993 and that were interpreted as deep pressure increases. Anomalous decreases in CO2 fluxes accompanied the onset and the evolution of that eruption and have been interpreted as a sign of upward migration of the gas source. The variations of CO2 flux at the 1989 SE fracture have also given interesting information on the timing of the magmatic intrusion that has then fed the 1991–1993 eruption. 相似文献
18.
基于三亚(109.6°E,18.4°N)VHF电离层相干散射雷达观测,分析了我国低纬电离层E区场向不规则结构连续性回波的发生特征.研究结果表明:白天,E区连续性回波的多普勒速度范围为-50至25m/s,多普勒宽度主要分布在20至70m/s;连续性回波的高度大约以1km/h的速度缓慢下降,与偶发E层(Es)底部所在高度(hbEs)有很好的相关性,表明在背景电场影响下,Es经梯度漂移不稳定性产生场向不规则结构,引起E区连续性回波.夜间,E区连续性回波的多普勒速度范围为-50至50m/s,多普勒宽度为20至110m/s,回波在时间-高度-强度图上常呈现多层结构,可能与潮汐引起的多个离子层相关;而E区连续性回波的短暂中断,以及120km以上高E区连续性回波的发生,则可能归因于赤道扩展F极化电场的影响.此外,对E区连续性回波多普勒速度与全天空流星雷达风场观测的比较发现,在100km以下,多普勒速度与子午风场有很好的相关性. 相似文献
19.
The 2000 Hekla eruption took place from February 26 to March 8. Its seismic expressions were a swarm of numerous small earthquakes related to its onset, and low-frequency volcanic tremor that continued throughout the eruption. A swarm of small earthquakes was observed some 80 min before the onset of the eruption, and the size of the events increased with time. Low-frequency volcanic tremor, with a characteristic frequency band of 0.5–1.5 Hz and dominant spectral peak(s) at 0.7–0.9 Hz, became visible at 18:19 GMT on February 26, marking the onset of the eruption. The tremor amplitude rose quickly and was very high in the beginning of the eruption. However, it soon began to decrease after about an hour. In general, the seismic activity related to the 2000 Hekla eruption was very similar to what was observed in the previous eruption in 1991. Based on knowledge gained from seismicity and strain observations from 1991, this was the first time that a Hekla eruption was predicted.Editorial responsibility: J Stix 相似文献
20.
A moderately violent phreatomagmatic explosive eruption of Taal Volcano, Philippines, occurred from 28 to 30 September, 1965. The main phreatic explosions, which were preceded by ejection of basaltic spatter, opened a new crater 1.5 km long and 0.3 km wide on the southwest side of Volcano Island in Lake Taal. The eruption covered an area of about 60 square kilometers with a blanket of ash more than 25 cm thick and killed approximately 200 persons. The clouds that formed during the explosive eruption rose to heights of 15 to 20 km and deposited fine ash as far as 80 km west of the vent. At the base of the main explosion column, flat, turbulent clouds spread radially, with hurricane velocity, transporting ash, mud, lapilli and blocks. The horizontally moving, debris-laden clouds, sandblasted trees, coated the blast side of trees and houses with mud, and deposited coarse ejecta with dune-type bedding in a zone roughly 4 km in all directions from the explosion crater. 相似文献