共查询到20条相似文献，搜索用时 31 毫秒
1.
A. W. Hurst 《Bulletin of Volcanology》1980,43(1):121-129
The temperature of the Crater Lake of the active volcano Ruapelm has been recorded by Temperature Telemetry Buoys, to determine if lake temperature is correlated with volcanic activity. These buoys had to be specially designed to cope with the unfavourable environment of Crater Lake. A buoy contains a thermistor to measure the lake temperature, and a radio transmitter to transmit a short signal every few minutes, the interval between signals being a function of temperature. The temperature records obtained from these buoys show that the temperature near the lake surface can vary considerably within a few hours. Some of these variations appear to be caused by disturbances in convective heat transfer occurring in the lake. The occurrence of these short term temperature variations means that there is no simple relation between Crater Lake temperatures and the volcanic activity of Ruapehu. Some rapid increases in temperature followed volcanic earthquakes, but one of the biggest increases in temperature occurred just before a group of earthquakes upder the lake. 相似文献
2.
Hydrophone measurements of acoustic noise levels in the Crater Lake of Mount Ruapehu, New Zealand were made on 18 January 1991 from an inflatable rubber boat on the lake. The greatest sound pressures were recorded in the 1–10 Hz band, with sound levels generally decreasing about 20 dB per decade from 10 Hz to 80 kHz. The low frequency noise did not have an obvious relationship to the tremor observed at a seismic station within 1 km of the lake. The comparatively low levels of middle and high frequency sound meant that at the time of measurement, direct steam input did not make a significant contribution to the heating of Crater Lake. This is consistent with the earlier conclusion that during the last decade a major part of the heat input of Crater Lake has come from lake water that was heated below the lake and recycled back into the lake. 相似文献
3.
G. R. T. Clacy 《Bulletin of Volcanology》1972,36(1):20-28
Volcanic prediction using slow motion tape recorders suitably situated near a volcano, produce good correlations when frequency analysis techniques are used with total energy from semi-continuous tremors, apparent explosions determined to disregard tectonic events. 相似文献
4.
A. W. Hurst 《Bulletin of Volcanology》1998,60(1):1-9
Virtually all the seismicity within Ruapehu Volcano recorded during a 2-month deployment in early 1994, with 14 broadband
seismographs around the Tongariro National Park volcanoes in the North Island of New Zealand, was associated with the active
vent and occurred within approximately 1 km of Ruapehu Crater Lake. High-frequency volcano-tectonic earthquakes and low-frequency
events (similar to bursts of 2 Hz volcanic tremor) were both found to have sources in this region. The high-frequency events,
which often consisted of a smaller precursor event followed approximately 2 s later by the main event, had sharp onsets and
were locatable using standard techniques. The depth of these events ranged from the surface down to approximately 1500 m below
Crater Lake. The low-frequency events did not have sharp onsets and were located by phase-correlation methods. Nearly all
occurred under a small region on the east side of Crater Lake, at depths from 200 to 1000 m below the surface. This low-frequency
earthquake source region, in which no high-frequency events occurred, may be the steam zone within the actual vent of Ruapehu
Volcano.
Received: 30 June 1996 / Accepted: 16 February 1998 相似文献
5.
Ruapehu is a very active andesitic composite volcano which has erupted five times in the past 10 years. Historical events have included phreatomagmatic eruptions through a hot crater lake and two dome-building episodes. Ski-field facilities, road and rail bridges, alpine huts and portions of a major hydroelectrical power scheme have been damaged or destroyed by these eruptions. Destruction of a rail bridge by a lahar in 1953 caused the loss of 151 lives. Other potential hazards, with Holocene analogues, include Strombolian and sub-Plinian explosive eruptions, lava extrusion from summit or flank vents and collapse of portions of the volcano. The greatest hazards would result from renewed phreatomagmatic activity in Crater Lake or collapse of its weak southeastern wall. Three types of hazard zones can be defined for the phreatomagmatic events: inner zones of extreme risk from ballistic blocks and surges, outer zones of disruption to services from fall deposits and zones of risk from lahars, which consist of tongues down major river valleys. Ruapehu is prone to destructive lahars because of the presence of 107 m3 of hot acid water in Crater Lake and because of the surrounding summit glaciers and ice fields. The greatest risks at Ruapehu are to thousands of skiers on the ski field which crosses a northern lahar path. Three early warning schemes have been established to deal with the lahar problems. Collapse of the southeastern confining wall would release much of the lake into an eastern lahar path causing widespread damage. This is a long-term risk which could only be mitigated by drainage of the lake. 相似文献
6.
Acoustic signals in Ruapehu Crater Lake, which are now being telemetered via a satellite transmission system, show promise as a possible precursor of increased volcanic activity from Ruapehu. The start of a recent period of rapid heating of Crater Lake was preceded by low-frequency (2 Hz) acoustic signals. These accompanied similar frequency seismic signals, but seemed to be produced independently. Audio-frequency (350–3000 Hz) acoustic noise also showed a very clear peak shortly before the lake temperature started to rise. 相似文献
7.
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. 相似文献
8.
Between January 1966 and December 1973 approximately 100 water samples wert collected from the Crater Lake of Mt Ruapehu. From analyses of the samples, changes in chloride and magnesium concentrations and pH emerged as the most useful indicators for the occurrence of the two major processes associated with eruptive activity: Chloride concentrations vary in response to changes in fumarolic activity, arising from the degassing of magma; rises in magnesium concentrations are due to interaction of lake water with freshly injected, hot andesitic material. Similarly, variations in temperature, pH and the ratio Mg/Cl enable the effects of dilution and evaporation to be considered. The thermal power required to maintain elevated lake temperatures ～200 MW during quiet periods, reaching 1000 MW during active periods, is largely transferred by fumarolic steam. 相似文献
9.
Volcanic earthquakes, and their relationship to eruptions at Ruapehu and Ngauruhoe volcanoes 总被引：1，自引：0，他引：1
J.H. Latter 《Journal of Volcanology and Geothermal Research》1981,9(4):293-309
Crustal earthquakes near Ruapehu and Ngauruhoe fall into two classes, each of which can be subdivided. On the one hand, there are high-frequency events ( 3 Hz) with sharp, well-defined phases, mainly concentrated beneath Ruapehu Crater Lake. Low-frequency events (< 2 Hz), on the other hand, are common at shallower depths under both volcanoes. These are usually emergent multiple events, and are often closely associated with eruptions.The low-frequency events resemble Minakami's B-type and explosion earthquakes, but sometimes occur where no vent exists and rather deeper than his formal definition (< 1 km) permits. More importantly, they lack reliable criteria (wave-form or magnitude differences) to distinguish between his two groups. Whether or not they accompany an eruption (Minakami's definition of explosion earthquake) appears to depend on whether the volcanoes are in a “closed-” or “open-vent” condition. The high-frequency earthquakes are similar in wave-form to Minakami's A-type. However, many at Ruapehu (here designated “roof-rock” earthquakes) originate at shallower depths than the B-type earthquakes, which is contrary to Minakami's definition.Difficulty in applying Minakami's classification rigorously, and the fact that low frequencies may be due to abnormal attenuation of higher frequencies along the path, rather than to their suppression or absence at the source, has led to reclassification of earthquakes near the volcanoes into two broad groups, tectonic and volcanic. The former includes all high-frequency earthquakes, and those discrete events in which dominant low frequencies are due to path effects. The latter includes multiple and emergent events which show evidence of prolonged or repetitive source mechanism. Dominant low frequencies are ascribed to occurrence in heat-weakened material, and high frequencies to instantaneous source mechanisms operating in competent rock. The term volcano-tectonic describes tectonic earthquakes within some arbitrary distance of a volcano.At Ngauruhoe and Ruapehu, volcanic earthquakes accompany explosive, vent-clearing eruptions. Subsequent “open-vent” degassing and ash emission, however, although often powerful and prolonged, usually occurs without earthquakes. Such activity is, however, frequently accompanied by volcanic tremor. At Ruapehu, under “closed-vent” conditions, when lake temperature is low, low-frequency earthquakes up to magnitude ML = 3.4 have occurred without any eruption.Five types of phreatic eruptions are identified at Ruapehu, each having a distinctive seismic pattern. The three most explosive types appear to be generated by a chain reaction process, and all involve flashing of water to steam; the first by failure of the roof, with little precursory seismicity, after a “closed-vent” period, during which lake temperature decreases; the second, after prolonged heating of the lake and much preliminary volcanic tremor, interpreted as due to rising magma; and the third, under “open-vent” conditions in the wake of one of the two preceding types. A fourth probably occurs in wet sediments near the base of the lake, as a result of upward migration of hot gas, and a fifth, aseismic, or accompanied by very weak volcanic tremor, is associated with convective overturn within Crater Lake. 相似文献
10.
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 ML 4.8. Crustal earthquakes of ML≥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. 相似文献
11.
Mt. Ruapehu, in the central North Island of New Zealand, is one of the most lahar-prone volcanoes in the world. Since historic
observations began in 1861 AD, more than 50 individual lahars have been recorded in the Whangaehu valley alone, the natural
outlet to the summit Crater Lake. These lahars have been triggered by a variety of mechanisms, including explosive eruptions
that displaced Crater Lake water over the outlet or ejected it onto the snow-clad summit area of the volcano; rain-remobilisation
of tephra deposits on steep slopes; displacement over the outlet as a result of syn-eruptive changes in lake bathymetry; and
lake break-outs from Crater Lake following impoundment of excess water behind temporary barriers of tephra and/or ice emplaced
over the outlet. However, only 9 lahar deposits can be distinguished in the upper Whangaehu valley on sedimentological, stratigraphic,
geomorphic and petrological grounds, and these are skewed towards either the largest or the most recent flows. In some cases
magnitude can be reconstructed from deposit geometry, with the largest lahars producing the highest level terraces, the coarsest
deposits, and crossing drainage divides into normally inactive channels. This under-representation of historic events reflects
the low preservation potential of unconsolidated deposits in a steep alpine environment, and the overprinting and recycling
effect of large magnitude lahars that rework material down to bedrock and effectively reset the stratigraphic record. Development
of magnitude-frequency relationships for Ruapehu lahars therefore requires the identification of lahar deposits in proximal,
medial and distal settings in order to ensure that the full range of events is represented. 相似文献
12.
《Journal of Volcanology and Geothermal Research》1999,88(4):255-278
From 1971 until 1995, the style of seismicity at Ruapehu changed little, reflecting a period of relatively low eruptive activity and consequent long-term stability within the vent system. Volcanic earthquakes and volcanic tremor were both dominated by a frequency of about 2 Hz. Volcanic earthquakes accompanied all phreatic and phreatomagmatic eruptions, but not small hydrothermal eruptions that originated within Crater Lake. Furthermore, more than half of the ML>3 volcanic earthquakes and changes in the reduced displacement of 2 Hz volcanic tremor by as much as a factor of 20 occurred without any accompanying eruptive activity. Three and 7 Hz volcanic tremor were also recorded, although never at lower-elevation seismometers. At times, this tremor was stronger at the summit seismometer than the 2 Hz tremor. Their source regions were independent of the 2 Hz source, and located at shallower depths. Volcano-tectonic earthquakes were generally unrelated to eruptive activity. The seismicity accompanying the 1995–1996 eruptive activity was significantly different from that of the period 1971 to 1995, and included volcanic tremor with a frequency of less than 1 Hz, simultaneous changes in the amplitude of the previously independent 2 Hz and 7 Hz volcanic tremor, and finally a change in the frequency content of volcanic earthquakes and volcanic tremor from 2 Hz to wideband. Path transmission effects play an important role in determining the characteristics of seismograms at Ruapehu. The presence of Crater Lake affects both the style of eruptions and the accompanying seismicity. 相似文献
13.
Many earthquakes within the crust near Ruapehu and Ngauruhoe volcanoes, recorded at epicentral distances less than 20 km on vertical seismometers, show S-waves of lower dominant frequency than the P-waves. A large number also have amplitudes in the S-group less than those of the P-waves. Whereas the reduced amplitude of S-waves relative to that of P-waves can be a source mechanism effect, the corresponding reduction in dominant frequency should be independent of the source radiation pattern. The most plausible reason for such a reduction in dominant S-wave frequency is that the waves have passed through a zone of partially molten rock. The data are therefore interpreted in terms of the presence of magma in restricted zones near the volcanoes.Using ray paths from 232 hypocentres to three permanent seismograph stations, together with paths from three additional earthquakes to one permanent and two temporary stations, an interpretation in three dimensions has been made of the source of the anomalous attenuation at depths between 2 and 10 km below datum (Ruapehu Crater Lake). Wave paths which lie largely at depths shallower than 2 km cannot be used, as almost all such paths show evidence of enhanced S-wave attenuation, and this is attributed to the presence of superficial pyroclastic and unconsolidated laharic material within 2 km of the surface.At Ruapehu, the data suggest the presence of three principal intrusions, one underlying much of the southwest slopes and reaching as far east as Crater Lake, one beneath the eastern side of the Summit Plateau, and one beneath part of the northeast slopes of the volcano. All three are essentially vertical or steeply dipping structures, detectable to a depth of between 7 and 9 km. The first appears to extend to within about 5 km of the surface, whereas the other two have intruded to within 2 or 3 km. Other, less well-defined, and comparatively small bodies exist beneath both the western and eastern slopes of Ruapehu.In the Ngauruhoe area, few earthquakes have occurred and all have been at depths less than 6 km. Therefore, only shallow attenuating areas can be defined. A small area of anomalous S-wave attenuation occurs beneath the northwest slopes of Ngauruhoe, and another, elongated, body appears to coincide with a fault zone west of the volcano. Both of these lie at depths of about 3 km below datum (less than 2 km below surface in one locality).Finally, areas of high attenuation, at depths of 4–5 km below datum, appear to define a narrow east-west zone about 6 km long in the immediate area of Whakapapa village. Other zones exist east of the volcanic axis, defining a line which cuts the axis on the north east slopes of Ruapehu, at a point where a parasite crater formed a few thousand years ago. 相似文献
14.
The most common volcanic tremor produced by Ruapehu is a continuous signal with a dominant frequency of about 2 Hz. This signal has a sharply peaked spectrum, and an autocorrelation function with a high degree of coherence, even for lags of over 20 seconds. These characteristics strongly suggest that the cause of this tremor is a single resonator, probably a fluid-filled cavity resonating in an “organ-pipe” mode.The stochastic simulation of such a resonator uses the equation of motion of a Simple Harmonic Oscillator, which applies to an “organ-pipe” fundamental resonance, with either the characteristics of the oscillator, or the forcing function, containing a random element. A “white noise” forcing function, which would be appropriate for excitation of the cavity by a high pressure gas input, gave good agreement with the observed spectra and autocorrelation functions. Another possible model used an oscillator with a damping factor which varied randomly, and was sometimes negative, so oscillations built up, rather than decayed. This also gave a reasonable simulation of Ruapehu tremor.The third excitation model used a Poisson process, in which during each time interval there was a certain probability of applying a fixed impulse to the resonator. It was found that the impulses had to be frequent, i.e. several times a second, to match the characteristics of Ruapehu tremor.It has been suggested that tremor is composed of a succession of low-frequency (“B-type”) earthquakes. The results of this simulation show that at Ruapehu tremor could be produced by a resonator with positive feedback just sustaining oscillation, or by a resonator excited by external impulses. The most promising model for low-frequency earthquakes describes them as the result of a major external disturbance of the resonator. 相似文献
15.
Between 1986 and 1990 the eruptive activity of Erebus volcano was monitored by a video camera with on-screen time code and recorded on video tape. Corresponding seismic and acoustic signals were recorded from a network of 6 geophones and 2 infrasonic microphones. Two hundred Strombolian explosions and three lava flows which were erupted from 7 vents were captured on video. In December 1986 the Strombolian eruptions ejected bombs and ash. In November 1987 large bubble-bursting Strombolian eruptions were observed. The bubbles burst when the bubble walls thinned to ∼ 20 cm. Explosions with bomb flight-times up to 14.5 s were accompanied by seismic signals with our local size estimate, “unified magnitudes” (mu), up to 2.3. Explosions in pools of lava formed by flows in the Inner Crater were comparatively weak. 相似文献
16.
The 1995–1996 eruption of Mt. Ruapehu has provided a number of insights into the geochemical processes operating within the magmatic-hydrothermal system of this volcano. Both pre-eruption degassing of the rising magma and its eventual intrusion into the convective zone of the hydrothermal system beneath the lake were clearly reflected in lake water compositions. The eruptions of September–October 1995 expelled the lake, and provided the first-ever opportunity to characterise gas discharges from this volcano. The fumarolic discharges revealed compositions typical of andesite volcanoes and strong interaction with the enclosing meteoric and hydrothermal system fluids. Some 1.1 MT of SO2 gas was released from the volcano between September 1995 and December 1996, whereas ca. twice this amount (2.2 MT equivalent SO2) was erupted as soluble (i.e. leachable) oxyanions of sulphur. Significantly more sulphur was released from the volcano over this period than can be accounted for from the magma volume actually erupted. The evidence suggests that a sizable component of the evolved sulphur was remobilised from the long-lived hydrothermal system within the volcano during the 1995–1996 activity. 相似文献
17.
The Active Crater at Rincón de la Vieja volcano, Costa Rica, reaches an elevation of 1750 m and contains a warm, hyper-acidic crater lake that probably formed soon after the eruption of the Rio Blanco tephra deposit approximately 3500 years before present. The Active Crater is buttressed by volcanic ridges and older craters on all sides except the north, which dips steeply toward the Caribbean coastal plains. Acidic, above-ambient-temperature streams are found along the Active Crater's north flank at elevations between 800 and 1000 m. A geochemical survey of thermal and non-thermal waters at Rincón de la Vieja was done in 1989 to determine whether hyper-acidic fluids are leaking from the Active Crater through the north flank, affecting the composition of north-flank streams.Results of the water-chemistry survey reveal that three distinct thermal waters are found on the flanks of Rincón de la Vieja volcano: acid chloride–sulfate (ACS), acid sulfate (AS), and neutral chloride (NC) waters. The most extreme ACS water was collected from the crater lake that fills the Active Crater. Chemical analyses of the lake water reveal a hyper-acidic (pH0) chloride–sulfate brine with elevated concentrations of calcium, magnesium, aluminum, iron, manganese, copper, zinc, fluorine, and boron. The composition of the brine reflects the combined effects of magmatic degassing from a shallow magma body beneath the Active Crater, dissolution of andesitic volcanic rock, and evaporative concentration of dissolved constituents at above-ambient temperatures. Similar cation and anion enrichments are found in the above-ambient-temperature streams draining the north flank of the Active Crater. The pH of north-flank thermal waters range from 3.6 to 4.1 and chloride:sulfate ratios (1.2–1.4) that are a factor of two greater than that of the lake brine (0.60). The waters have an ACS composition that is quite different from the AS and NC thermal waters that occur along the southern flank of Rincón de la Vieja.The distribution of thermal water types at Rincón de la Vieja strongly indicates that formation of the north-flank ACS waters is not due to mixing of shallow, steam-heated AS water with deep-seated NC water. More likely, hyper-acidic brines formed in the Active Crater area are migrating through permeable zones in the volcanic strata that make up the Active Crater's north flank. Dissolution and shallow subsurface alteration of north-flank volcanoclastic material by interaction with acidic lake brine, particularly in the more permeable tephra units, could weaken the already oversteepened north flank of the Active Crater. Sector collapse of the Active Crater, with or without a volcanic eruption, represents a potential threat to human lives, property, and ecosystems at Rincón de la Vieja volcano. 相似文献
18.
19.
Mount Erebus, a large intraplate stratovolcano dominating Ross Island, Antarctica, hosts the world's only active phonolite lava lakes. The main manifestation of activity at Erebus volcano in December 2004 was as the presence of two convecting lava lakes within an inner crater. The long-lived Ray Lake, ~ 1400 m2 in area, was the site of up to 10 small Strombolian eruptions per day. A new but short-lived, ~ 1000–1200 m2 lake formed at Werner vent in December 2004 sourced by lava flowing from a crater formed in 1993 by a phreatic eruption. We measured the radiative heat flux from the two lakes in December 2004 using a compact infrared (IR) imaging camera. Daily thermal IR surveys from the Main Crater rim provide images of the lava lake surface temperatures and identify sites of upwelling and downwelling. The radiative heat outputs calculated for the Ray and Werner Lakes are 30–35 MW and 20 MW, respectively. We estimate that the magma flux needed to sustain the combined heat loss is ~ 250–710 kg s− 1, that the minimum volume of the magma reservoir is 2 km3, and that the radius of the conduit feeding the Ray lake is ~ 2 m. 相似文献
20.
Seismicity in the Jingpohu volcanic area was investigated based on the seismic data recorded by the mobile seismic network consisting of 14 stations equipped with 24-bit broad-band 3- component seismographs around Crater Forest, Results show that there appears certain seismicity in Jingpohu and its adjacent areas with a low activity level and most of the recorded earthquakes are the volcanic-tectonic ones, The results of location indicate a dominant focal depth of 10km - 30kin, most of the earthquakes are smaller than ML2,0, and are concentrated in the area of ＂ Crater Forest＂ and on the Dunhua-Mishan fault which runs through the volcanic area. At station No. 2, which has better observation conditions, two types of events, likely associated to volcanism, were recorded; their waveform characteristics are somewhat similar to that of the long-period volcanic event and the volcanic tremor, but with different feature of frequencies. 相似文献