Implications for the thermal regime of acoustic noise measurements in Crater Lake,Mount Ruapehu,New Zealand

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.     

Shallow seismicity beneath Ruapehu Crater Lake: results of a 1994 seismometer deployment

 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     

Temperature observations of Crater Lake,Mt Ruapehu,New Zealand,using Temperature Telemetry Bouys

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.     

Seismicity of Ruapehu volcano,New Zealand, 1971–1996: a review

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 M_L>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.     

Earthquake swarms to the west of Mt Ruapehu preceding its 1995 eruption

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 M_L 4.8. Crustal earthquakes of M_L≥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.     

Depositional record of historic lahars in the upper Whangaehu Valley, Mt. Ruapehu, New Zealand: implications for trigger mechanisms, flow dynamics and lahar hazards

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.     

Volcanic hazard assessment for Ruapehu composite volcano,taupo volcanic zone,New Zealand

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.     

Volcanic earthquakes, and their relationship to eruptions at Ruapehu and Ngauruhoe volcanoes

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 M_L = 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.     

The hydrothermal vent system of Mount Ruapehu,New Zealand — a high frequency MT survey of the summit plateau

High frequency magnetotelluric (MT) measurements made on the summit plateau of Mount Ruapehu, some 1 km to the north of the presently active vent beneath Crater Lake, have been used to derive the electrical resistivity structure associated with the volcanic hydrothermal vent system. The entire summit plateau area is underlain at shallow depth by low resistivity which is inferred to be the result of hydrothermal alteration caused by rising volcanic gases mixing with local groundwater. Two areas of localised higher resistivity, one between 200 and 500 m depth beneath the central part of the plateau, and one at a depth of 1000 m below the northern part of the plateau, are interpreted as being the result of hydrothermal alteration at higher temperature forming chlorite dominated alteration products. These regions are believed to represent the locations of further heat pipes within the volcanic system. Both correlate with the locations of eruption centres on Ruapehu active within the last 10 ka.     

Location of zones of anomalously high s-wave attenuation in the upper crust near ruapehu and ngauruhoe volcanoes, New Zealand

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.     

The use of chemical indicators in the surveillance of volcanic activity affecting the Crater Lake on Mt Ruapehu,New Zealand

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.     

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.     

Evolution of a vent-hosted hydrothermal system beneath Ruapehu Crater Lake,New Zealand

A two-year chemical monitoring program of Ruapehu Crater Lake shows that it has evolved considerably since the volcano's more active eruptive periods in the early 1970s. The present pH (20°C) of 0.6 is about one half unit more acid than the baseline values in the 1970s, whereas S/Cl ratios have increased markedly owing in part to declining HCl inputs into the lake, but also to absolute increases in SO₄ levels which now stand at the highest values ever recorded. Increases in K/Mg and Na/Mg ratios over the 20-year period are attributed to hydrothermal reaction processes in the vent which are presently causing dissolution of previously formed alteration phases such as natroalunite. These observations, combined with results of a recent heat budget analysis of the lake, have led to the development of hydrothermal convection model for the upper portion of the vent. Possible vent/lake chemical reaction processes between end member reactants have been modelled with the computer code CHILLER. The results are consistent with the view that variations in lake chemistry, which are initiated by the introduction of fresh magmatic material into the vent, reflect the extent of dissolution reaction progress on the magmatic material and/or its alteration products. The results also provide insights into the role of such vent processes in the formation of high sulfidation-type ore deposits.     

Geochemical and isotopic evidence for seawater contamination of the hydrothermal system of Taal Volcano, Luzon, the Philippines

 The hydrologic structure of Taal Volcano has favored development of an extensive hydrothermal system whose prominent feature is the acidic Main Crater Lake (pH<3) lying in the center of an active vent complex, which is surrounded by a slightly alkaline caldera lake (Lake Taal). This peculiar situation makes Taal prone to frequent, and sometimes catastrophic, hydrovolcanic eruptions. Fumaroles, hot springs, and lake waters were sampled in 1991, 1992, and 1995 in order to develop a geochemical model for the hydrothermal system. The low-temperature fumarole compositions indicate strong interaction of magmatic vapors with the hydrothermal system under relatively oxidizing conditions. The thermal waters consist of highly, moderately, and weakly mineralized solutions, but none of them corresponds to either water–rock equilibrium or rock dissolution. The concentrated discharges have high Na contents (>3500 mg/kg) and low SO₄/Cl ratios (<0.3). The Br/Cl ratio of most samples suggests incorporation of seawater into the hydrothermal system. Water and dissolved sulfate isotopic compositions reveal that the Main Crater Lake and spring discharges are derived from a deep parent fluid (T≈300  °C), which is a mixture of seawater, volcanic water, and Lake Taal water. The volcanic end member is probably produced in the magmatic-hydrothermal environment during absorption of high-temperature gases into groundwater. Boiling and mixing of the parent water give rise to the range of chemical and isotopic characteristics observed in the thermal discharges. Incursion of seawater from the coastal region to the central part of the volcano is supported by the low water levels of the lakes and by the fact that Lake Taal was directly connected to the China sea until the sixteenth century. The depth to the seawater-meteoric water interface is calculated to be 80 and 160 m for the Main Crater Lake and Lake Taal, respectively. Additional data are required to infer the hydrologic structure of Taal. Geochemical surveillance of the Main Crater Lake using the SO₄/Cl, Na/K, or Mg/Cl ratio cannot be applied straightforwardly due to the presence of seawater in the hydrothermal system. Received: 12 February 1997 / Accepted: 26 January 1998     

Seismic recordings of subterranean volcanic activity at Ruapehu during 1964

Volcanic vibrations from Ruapehu volcano which is situated in the centre of the North Island of New Zealand have been recorded on high magnification slow motion tape seismographs and drum seismographs at the Chateau Volcanological Observatory since December, 1960. In 1964, volcanic microtremor with a dominant frequency of 2 c/s commenced late in March, and reached a peak seismic power level of 100 KW for short periods in May. During the maximum phase, the tremor completely ceased for up to 20 minutes, and recommenced with an explosion or burst of strong tremor. The sequence and timing were very similar to the sequence of ash discharge-stoppage-explosion-ash discharge observed during the 1945 eruption of Ruapehu. In 1964 it is thought that the eruption of ash clouds was prevented by the Crater Lake, so that visible activity was limited to a few fumaroles, steam rising from the lake, and turbulence in the water. The lake temperature increased from about 25°C (a normal temperature) on 20 March to 50°C on 26 May. The corresponding rates of heat loss from the lake are 200 and 700 MW respectively, and an additional 150 MW or more was required to heat up the lake, giving a total average heat output of 850 MW for about 4 weeks. The corresponding average seismic power was about 3 KW, which is 0.0005 per cent of the thermal power. If this relationship is constant, the peak thermal power over a period of 6 minutes was of the order of 20,000 MW. The explosions initiating the tremor were mostly identical except in amplitude, and their magnitudes ranged up to 2.3, corresponding to 10¹⁴ erg. This was slightly greater than the energy conserved during the preceding stoppage of tremor. The dominant tremor frequency was normally 2.2 c/s and sometimes 1.2 c/s. Individual bands of frequency within the spectrum varied in power, but coherent migrations of frequency bands occurred on a few occasions. Near the end of the maximum phase, the entire spectrum migrated downwards by half an octave and the tremor power decreased to zero. Tremor power and frequency increased again, and a violent period of explosions and tremor of changing frequency occurred. Quite frequently the explosions initiating the tremor were multiple, and a few explosions occurred which were followed by tremor lasting only a matter of seconds. Explosions apparently unrelated to tremor were very rare and minor. The explosions were located by temporary installation of portable slow motion tape recorders around the volcano. The epicentre is very close to Crater Lake, but the depth cannot be determined from the data. Graphs of tremor power against time covering the active period from April to September, and graphs of cumulative energy against time, and frequency spectrum against time, for explosions and tremor are presented, and slow motion tape recordings will be played many times faster than the recording speed so that the tremor and explosion vibrations can be heard as audible noises.     

Electrical resistivity tomography study of Taal volcano hydrothermal system, Philippines

Taal volcano (311?m in altitude) is located in The Philippines (14°N, 121°E) and since 1572 has erupted 33 times, causing more than 2,000 casualties during the most violent eruptions. In March 2010, the shallow structures in areas where present-day surface activity takes place were investigated by DC resistivity surveys. Electrical resistivity tomography (ERT) lines were performed above the two identified hydrothermal areas located on the northern flank of the volcano and in the Main Crater, respectively. Due to rough topography, deep valleys, and dense vegetation, most measurements were collected using a remote method based on a laboratory-made equipment. This allowed retrieval of information down to a depth of 250?m. ERTs results detail the outlines of the two geothermal fields defined by previous self-potential, CO₂ soil degassing, ground temperature, and magnetic mapping (Harada et al. Japan Acad Sci 81:261–266, 2005; Zlotnicki et al. Bull Volcanol 71:29–49, 2009a, Phys Chem Earth 34:294–408, 2009b). Hydrothermal fluids originate mainly from inside the northern part of the Main Crater at a depth greater than the bottom of the Crater Lake, and flow upward to the ground surface. Furthermore, water from the Main Crater Lake infiltrates inside the surrounding geological formations. The hydrothermal fluids, outlined by gas releases and high temperatures, cross the crater rim and interact with the northern geothermal field located outside the Main Crater.     

Reconnaissance surveys of near-event seismic activity in the volcanoes of the Cascade Range,Oregon

Surveys of near-event seismic activity were made at two principal locations in the Cascade Range in Oregon during the summers of 1969 and 1970. A tripartite array of ultrasensitive high frequency seismometers was deployed about 7.5 km north of the Mt. Hood summit with one of the 1-km legs oriented broadside to the dormant volcano. Seismometers were emplaced over olivine andesite flows associated with the Pinnacle, one of the parasitic cones formed on the flanks of the strato-volcano. During 16 days of operation on the north slope, 53 near events were recorded, most of which originated within the upper crust and were associated with the north-south trending zone of the Cascade Range. Event magnitudes for these near events range from ?1.7 to +1.8 and determination of b-values in the Gutenberg-Richter relationship was ?0.80, indicating a probable tectonic mechanism for the shocks. During the late summer of 1970, a four-station array was operated at Crater Lake Park about 13 km south-southwest of the caldera rim. In addition, an ultraportable outlier station was operated at two locations north of the caldera that resulted from the collapse of ancient Mt. Mazama some 6,600 years ago. Only a limited number of near events with S-P intervals of 4 sec or less were detected at Crater Lake; a larger number were recorded with S-P intervals longer than 4 sec. Event epicenters for the Crater Lake area are broadly distributed in azimuth, indicating the complex structure underlying the Cascade Range in southern Oregon. Crater Lake is located astride the broad upwarp of crystalline pre-Cenozoic rocks believed to extend northeast from the Klamath Mountains to the Ochoco Uplift of central Oregon. Major regional structural trends are also shown by the north-south trending belts of the Cascade volcanoes, probably related to deep fracture zones, and by the northeast-trending shear zones that exist in the Basin and Range province to the southeast of Crater Lake. Regional gravity and aeromagnetic surveys indicate that Crater Lake lies at the intersection of these zones that probably provided the conduits for the rise of magma that ultimately led to the collapse of Mt. Mazma and the formation of Crater Lake. Epicenters for near events recorded at this juncture do not reflect these linear trends and, indeed, a generally smaller incidence of near-event activity was recorded at Crater Lake than was recorded at Mt. Hood. Magnitudes for Crater Lake events with S-P intervals of 4 sec or more range from +0.25 to +2.19, and an examination of the relationship between cumulative frequency and magnitude for these events yields a b-value in the Gutenberg-Richter relationship of ?1.16, indicating the events at Crater Lake, like those detected at Mt. Hood, are associated with tectonic rather than volcanic sources. Events for which depth determinations were made show these sources to be within the crust, occurring in the upper 10 km of the earth’s crust. The relatively low incidence of small magnitude near events within the Oregon Cascade Range shows the aseismicity of the mountain chain which is consistent with the low incidence of earthquakes of a magnitude of 4.5 or greater detected for the volcanic range. The volcanoes of the Cascade Range in Oregon are dormant, and only small numbers of shocks are now being generated, probably from isostatic adjustments within the crust. The Cascade volcanic range, which once was a seismically active island are chain associated with subduction zones off the northwestern coast of America, has moved into a passive phase in which most seismic activity in western Oregon now occurs along the ridge and fracture zones offshore and within the Willamette Downwarp west of the dormant chain.     

Hydroacoustic noise precursors of the 1990 eruption of Kelut Volcano, Indonesia

The results of a hydroacoustic monitoring experiment in the Kelut Crater lake, Indonesia, prior to its 1990 eruption, are presented, with the benefit of hindsight. Indeed, the underwater noise levels in three widely separated frequency bands, together with the lake water temperature, was radio-transmitted and almost continuously recorded from a period of quiescence of the volcano till the onset of its 10 February 1990, eruption, which destroyed the monitoring buoy. The comparative analysis of the noise variations in the three bands, together with seismic and temperature data, have shed light on the mechanisms underlying the pre-eruptive activity. The three acoustic levels had shown conspicuous, yet distinctive, changes prior to the eruption. Acoustic level in the low-frequency (1–50 Hz) band, which increased one year before the resumption of seismic activity and the lake warming up, is interpreted as the result of boiling at depth. The source of high-acoustic level in the audiometric (500–5000 Hz) range is clearly the bubbling of volcanic gases, occurring as a strong convective column in the middle of the lake. From the variations of this audiometric level, we have estimated that the degassing rate in the lake increased by a hundred-fold during the pre-eruptive period. Variations of ultrasonic (20–100 kHz) frequency acoustic level seem to be related with pressure and thermal changes within the hydrothermal system and its rock matrix beneath the lake. In conclusion, this experiment demonstrates the potential of hydroacoustic monitoring as an early warning system at crater lake volcanoes.     

Video and seismic observations of Strombolian eruptions at Erebus volcano,Antarctica

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” (m_u), up to 2.3. Explosions in pools of lava formed by flows in the Inner Crater were comparatively weak.     

Stochastic simulation of volcanic tremor from Ruapehu

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.