Deep root of andesitic volcanoes: New evidence of magma generation at depth in the Benioff zone

The concept of a time-depth correlation between tectonic earthquakes at depth beneath some volcanoes, and their eruptions, developed by the author since 1962, has been confirmed by new observations and successful prediction of renewed volcanic activity in New Zealand.Regular earthquake migrations are observed along the Benioff zone, and volcanic eruptions are found to be related to these seismic migrations beneath the volcanoes, as follows:

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Therefore, in island arcs and continental margins, volcanic activity is the result of two processes occurring beneath the volcanoes: (1) a “tectonic process”, a migration of strain release along the downgoing lithosphere, of which the earthquakes are the manifestation; (2) a “magmatic process”, a relatively fast vertical ascent of magmatic material from the deep root of the volcano, where the observed shocks may be the starting signal from this level.The rate of migration of tectonic earthquakes increases with depth in the upper mantle.An empirical time relationship between the earthquakes occurring at depth beneath a volcano and its eruptions, has been successfully tested for renewed activity at White Island in New Zealand, over the period 1977–1978.     

Intermediate depth earthquakes and volcanic eruptions in Central America, 1961–1972

Eight Central American volcanoes had large eruptions during the period 1961 to 1972. The distribution of intermediate depth earthquakes which occurred during the same period is marked by eight concentrations. Seven of the eight very active volcanoes are spatially related to the concentrations of intermediate depth earthquakes. The centers of the concentrations are typically a few tens of kilometers seaward of the volcanoes. The earthquakes have focal depths of about 70 to 110 km. Directly below the active volcanoes there is little or no intermediate depth seismic activity. Partially melted areas along the deep seismic zone directly below the active volcanoes might explain this distribution. Spatial-temporal progressions relating specific intermediate depth earth-quakes to specific volcanic eruptions have not been recognized. The development of a concentration of intermediate depth earthquakes spatially related to a quiescent volcano may indicate that the volcano will soon enter a period of renewed activity.     

Volcanisme et séismicité dans l’Archipel des Nouvelles-Hébrides

A violent outburst of the Lopevi volcano in the central New Hebrides occurred on the 10th July, 1960. The eruption was preceded 4 months before by a deep earthquake (h=250 kms, Mag. 7 1/4), the focus of which was just under the volcano. An inventory of all shocks recorded in the Group since 1910 has been made and all informations about volcanic eruptions in this region have been collected. A close correlation appeared between these two phenomena. Each of the large volcanic eruptions recorded between 1910 and 1962 followed a deep focus earthquake of magnitude greater than 7. Moderate eruptions were preceded by earthquakes of magnitude between 5 3/4 and 6 3/4. The time between the tectonic shock and the climactic phase of the volcanic activity appears to be related to the distance between the focus and the volcano (i.e. the focal depth), the type of the volcano and the pattern of its eruption. It is of few months duration for the volcanoes in the Central group: Ambrym, Lopévi, the submarine volcano east of Epi and Karua. The authors tried to find the same correlations for others volcanoes in the world for which they have been able to collect dates of eruptions: Asama-Yama (Japan), Bezymiannyi (Kamtchatka), Paricutin and Izalco (Central America), Vesuve, Stromboli (Italy). Thus volcanic eruptions would appear to have their first origin in the mantle. A systematic survey of all volcanoes and deep regional earthquakes would bring evidence of this correlation and may permit a long term prediction of their eruptions.     

Eruption characteristics and cycles at Pavlof Volcano, Alaska, and their relation to regional earthquake activity

Pavlof Volcano (55° 25′N, 161° 54′W) exhibits two eruption styles: magmatic eruptions of one-to-two-days duration, and phreatic-phreatomagmatic activity lasting several days to two months. Thirty-four eruptions have occurred in historic times; of these the largest are Volcano Explosivity Index=3. Nine magmatic and 13 phreatomagmatic eruptions occurred between 1973–1983. All the magmatic eruptions occurred in the fall, between Sept. 9–Nov. 20. Four magmatic eruptions occurred during November 11–15, but in four different years. A 3-year-long period of eruptive activity between 1973–1976 bears striking resemblance to a period of activity between 1980–1983. No locatable shallow earthquakes (<50 km) have occurred within 30 km of Pavlof since 1973, which is quite unusual for an active island-arc volcano. Shallow events in the adjacent are segments have focal mechanisms with P-axes perpendicular to the arc (and parallel to plate convergence). Deep earthquakes (> 100 km) are clustered beneath Pavlof and several other volcanoes. Their T-axes show downdip tension within the slab. Deep teleseisms (> 160 km) mostly occurred between 1977–1979 when the volcano was not erupting. Catalogued volcanic activity throughout the Alaska/Aleutian arc shows a weak tendency to increase around the time of great (M > 7.8) earthquakes.     

Seismicity at White Island volcano, New Zealand: a revised classification and inferences about source mechanism

The classification of earthquakes at White Island volcano, New Zealand, has been revised to address problems in existing classification schemes, to better reflect new data and to try to focus more on source processes. Seismicity generated by the direct involvement of magmatic or hydrothermal fluids are referred to as volcanic, and that generated by fault movement in response to stresses caused by those fluids, regional stresses, thermal effects and so on are referred to as volcano-tectonic. Spasmodic bursts form a separate category, as we have insufficient information to classify them as volcanic or volcano-tectonic. Volcanic seismicity is divided into short-duration, long-period volcanic earthquakes, long-duration volcanic earthquakes, and harmonic- and non-harmonic volcanic tremor, while volcano-tectonic seismicity is divided into shallow and deep volcano-tectonic earthquakes. Harmonic volcanic tremor is related to sub-surface intrusive processes, while non-harmonic volcanic tremor originates close to active craters at shallow depth, and usually occurs during eruptive activity. Short-duration, long-period volcanic earthquakes come from a single source close to the active craters, but originate deeper than non-harmonic volcanic tremor, and are not related to eruptive activity. Long-duration volcanic earthquakes often accompany larger discrete eruptions. The waveform of these events consists of an initial low-frequency part from a deep source, and a later cigar-shaped part of mixed frequencies from a shallow crater source.     

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.     

Summit eruptions of Klyuchevskoi Volcano,Kamchatka in the early 21st century, 2003–2013

This paper is concerned with eruptions, seismicity, and deformation on Klyuchevskoi Volcano during the summit eruptions of 2012–2013, with the condition of the central crater during the eruptions, and with the effect that is exerted by the height of the lava in the crater on the start of the eruptions. The recurrence of eruptions in the North Volcanic Cluster (NVC), Kamchatka showed that all the four volcanoes in the cluster (Klyuchevskoi, Tolbachik, Shiveluch, and Bezymyannyi) become active during definite phases that were identified in the 18.6-year lunar cycle. This relationship of the NVC eruptions to the active phases in the 18.6-year lunar cycle, as well as the relationship to the 11-year solar activity, showed that eruptions can be predicted, yielding long-term estimates of activity for the NVC volcanoes. The short-term prediction of volcanic eruptions requires knowledge of seismicity and deformation that occur during the precursory period and during the occurrence of eruptions. Seismic activity during the summit eruptions of 2003–2013 took place in the depth range 20–25 km during repose periods of the volcano and at depths of 0–5 km in the volcanic edifice during the eruption. One notes an almost complete absence of any earthquakes at great depths during the summit eruptions. Volcanic tremor (VT) was recorded from the time that the eruptions began and continued to occur until the end. Geodetic measurements showed that the center of the magma pressure beneath the volcano during the parasitic and summit eruptions of 1979–1989 moved in the 4–17 km depth range, while during the summit eruptions of 2003–2013 the center moved in the 15–20 km range. These changes in the depth of the center of magma pressure may have been related to evacuation from shallow magma chambers.     

Structural controls on the 1998 volcanic unrest at Iwate volcano: Relationship between a shallow, electrically resistive body and the possible ascent route of magmatic fluid

Magnetotelluric (MT) measurements were conducted at Iwate volcano, across the entirety of the mountain, in 1997, 1999, 2003, 2006, and 2007. The survey line was 18 km in length and oriented E–W, comprising 38 measurements sites. Following 2D inversion, we obtained the resistivity structure to a depth of 4 km. The surface resistive layer (~ several hundreds of meters thick) is underlain by extensive highly conductive zones. Based on drilling data, the bottom of the highly conductive zone is interpreted to represent the 200 °C isotherm, below which (i.e., at higher temperatures) conductive clay minerals (smectite) are rare. The high conductivity is therefore mainly attributed to the presence of hydrothermally altered clay. The focus of this study is a resistive body beneath the Onigajo (West-Iwate) caldera at depths of 0.5–3 km. This body appears to have impeded magmatic fluid ascent during the 1998 volcanic unrest, as inferred from geodetic data. Both tectonic and low-frequency earthquakes are sparsely distributed throughout this resistive body. We interpret this resistive body as a zone of old, solidified intrusive magma with temperatures in excess of 200 °C. Given that a similar relationship between a resistive body and subsurface volcanic activity has been suggested for Asama volcano, structural controls on subsurface magmatic fluid movement may be a common phenomenon at shallow levels beneath volcanoes.     

Seismicity at Fuego,Pacaya, Izalco,and San Cristobal Volcanoes,Central America, 1973–1974

Seismic data collected at four volcanoes in Central America during 1973 and 1974 indicate three sources of seismicity: regional earthquakes with hypocentral distances greater than 80 km, earthquakes within 40 km of each volcano, and seismic activity originating at the volcanoes due to eruptive processes. Regional earthquakes generated by the underthrusting and subduction of the Cocos Plate beneath the Caribbean Plate are the most prominent seismic feature in Central America. Earthquakes in the vicinity of the volcanoes occur on faults that appear to be related to volcano formation. Faulting near Fuego and Pacaya volcanoes in Guatemala is more complex due to motion on a major E-W striking transform plate boundary 40 km north of the volcanoes. Volcanic activity produces different kinds of seismic signatures. Shallow tectonic or A-type events originate on nearby faults and occur both singly and in swarms. There are typically from 0 to 6 A-type events per day withb value of about 1.3. At very shallow depths beneath Pacaya, Izalco, and San Cristobal large numbers of low-frequency or B-type events are recorded with predominant frequencies between 2.5 and 4.5 Hz and withb values of 1.7 to 2.9. The relative number of B-type events appears to be related to the eruptive states of the volcanoes; the more active volcanoes have higher levels of seismicity. At Fuego Volcano, however, low-frequency events have unusually long codas and appear to be similar to tremor. High-amplitude volcanic tremor is recorded at Fuego, Pacaya, and San Cristobal during eruptive periods. Large explosion earthquakes at Fuego are well recorded at five stations and yield information on near-surface seismic wave velocities (α=3.0±0.2 km/sec.).     

2017年全球活动火山时空分布以及阿贡火山喷发前后的形变特征

归纳总结2017年度全球81座活火山的活动情况,共计活动1058座次,平均每周记录20座活火山的活动信息。根据火山潜在喷发的危险性和火山活动的强弱程度对上述火山进行分级描述,火山活动主要反映了地球表层的构造活动,其中大角度俯冲带的弧后火山最为强烈,小角度的俯冲带、拉张裂谷和走滑为主的板块边界火山活动较为平静,火山活动频繁的印度尼西亚岛链是受灾最为严重的区域。预计全球火山活动将进一步加剧,印尼岛链受火山灾害威胁的程度依然较大。位于印尼岛链巴厘岛上的阿贡火山自2017年9月开始活动以来,整个喷发过程极具代表性,监测阿贡火山喷发过程可为全球典型火山喷发事件研究提供参考。     

On the focal layer,seismicity and volcanicity of the Kurile-Kamchatka zone

The general relationships of the structure of the Kurile-Kamchatka zone are examined. The chain of volcanoes forms a true arc with the radius of 1884 km. The focal layer of earthquakes along the whole arc extends to depth with an angle of 48° ± 5°, and is a part of the surface of a circular cone. The energy of earthquakes at intervals of depth 0–100 km, 101–300 km, and 301–650 km is on the average distributed equally along the whole arc and in time. The activity of volcanoes of the whole zone in time is also rather evenly distributed. The energy lost from the earth’s interior during the volcanic eruptions is not less than the energy emitted as elastic waves during tectonic earthquakes.     

Volcanic risk ranking for Auckland, New Zealand. I: Methodology and hazard investigation

Volcanic eruptions typically produce a number of hazards, and many regions are at risk from more than one volcano or volcanic field. So that detailed risk assessments can be carried out, it is necessary to rank potential volcanic hazards and events in terms of risk. As it is often difficult to make accurate predictions regarding the characteristics of future eruptions, a method for ranking hazards and events has been developed that does not rely on precise values. Risk is calculated individually for each hazard from each source as the product of likelihood, extent and effect, based on the parameters order of magnitude. So that multiple events and outcomes can be considered, risk is further multiplied by the relative probability of the event occurring (probability_e) and the relative importance of the outcome (importance_o). By adding the values obtained, total risk is calculated and a ranking can be carried out.This method was used to rank volcanic hazards and events that may impact the Auckland Region, New Zealand. Auckland is at risk from the Auckland volcanic field, Okataina volcanic centre, Taupo volcano, Tuhua volcano, Tongariro volcanic centre, and Mt. Taranaki volcano. Relative probabilities were determined for each event, with the highest given to Mt. Taranaki. Hazards considered were, for local events: tephra fall, scoria fall and ballistic impacts, lava flow, base surge and associated shock waves, tsunami, volcanic gases and acid rain, earthquakes and ground deformation, mudflows and mudfills, lightning and flooding; and for distal events: tephra fall, pyroclastic flows, poisonous gases and acid rain, mudflows and mudfills, climate variations and earthquakes. Hazards from each source were assigned values for likelihood, with the largest for tephra fall from all sources, earthquakes and ground deformation, lava flows, scoria fall and base surge for an Auckland eruption on land, and earthquakes and ground deformation from an Auckland eruption in the ocean. The largest values for extent were for tephra fall and climate variation from each of the distal centres. However, these parameters do not give a true indication of risk. In a companion paper the effect of each hazard is fully investigated and the risk ranking completed.     

腾冲地区地壳速度结构的有限差分成像

利用流动台网和固定台站的地震观测数据,采用有限差分层析成像方法反演了腾冲及邻近地区的地壳P波速度结构,分析了腾冲火山区的岩浆活动和龙陵七级地震的深部构造成因.研究结果表明,腾冲火山区的地壳结构具有明显的非均匀性,浅表层偏低的速度主要为盆地内部的松散沉积层、新生代火山堆积及断裂附近的流体裂隙和热泉活动所致;5～15 km之间的高速体可能代表了早期火山通道内冷却固结的岩浆侵入体或难挥发的超铁镁质残留体;地壳深部的低速体则反映了熔融或半熔融的岩浆体,推断火山区下方的岩浆活动与龙陵七级地震震源区地壳深部的岩浆侵入来自同一源区--现今壳内岩浆活动的主要区域.龙陵震源区的地壳速度结构横向变化较大,怒江断裂东侧和龙陵断裂西侧为高速特征,介质应变强度较大,为应力积累的主要载体;两断裂之间的低速区向下延伸至下地壳,可能与地壳深部的岩浆侵入有关;龙陵断裂和怒江断裂明显控制了这一区域的岩浆活动,七级地震正是发生在断裂下方的速度边界附近.地壳介质强度的横向变化导致了震源区应力积累的不均一性,深部岩浆的聚集和动力作用是龙陵地区发生强震的主要原因.     

Prediction of eruption of Hawaiian volcanoes

Prediction of Hawaiian volcanic eruptions depends primarily on the interpretation of records of earthquakes and tumescence of the volcano. Recent work byJ. P. Eaton of the U. S. Geological Survey appears to demonstrate the presence of two distinct groups of earthquakes. One group originates at a depth of 40 to 60 km, within the earth’s mantle, and is thought to mark the zone of origin of the magma. The other group is of shallower origin and results from change of shape and size of the volcanic edifice. Earthquakes of the deep group occur from time to time, often in swarms, between eruptions and are not useful in predicting an outbreak. Those of the shallow group may accompany the swelling and splitting open of the volcano preceding eruption, but they may also accompany shrinking of the volcano and sinking of the mountain top that appears to result from withdrawal of magma beneath the volcano without surface eruption. Determining whether the quakes result from swelling of shrinking of the volcano depends largely on measurements of tilting of the ground surface. If the volcano is in a swollen condition and continues to swell, a large number of earthquakes of shallow origin is highly suggestive, if not definitely indicative, of imminent eruption. The place of origin of the quakes indicates, sometimes very closely, the location of the coming eruption. It is not yet possible, however, to predict the time of outbreak except in a rather general manner. Sometimes it can be predicted within a few days. At times there may be an oscillation of ground tilting directly preceding the eruption, suggesting a pulsation of magmatic pressure at depth, but this is not yet certain. There appears to be some indication that summit eruptions of either Mauna Loa or Kilauea are preceded by a less definite earthquake pattern, and are therefore less readily predictable, than flank eruptions. No cycle of activity of any great value in predicting activity has been recognized in Hawaii. Intervals between eruptions of Mauna Loa have ranged from a few months to more than 9 years, and Kilauea has been even more variable. In the case of Mauna Loa there has been a rough alternation between summit and flank eruptions, but with many exceptions to this general sequence. Astronomical and tidal cycles have been studied in relation to both time of outbreak and strength of eruption, but without demonstration of any very definite relationship. Eruptions have occurred in every month of the year, but there is a slight tendency for them to cluster just before and after solstice, particularly winter solstice.     

Seismicity and crustal deformation preceding the January 1996 eruptions at Karymsky Volcanic Center,Kamchatka

Two explosive eruptions occurred on 2 January 1996 at Karymsky Volcanic Center (KVC) in Kamchatka, Russia: the first, dacitic, from the central vent of Karymsky volcano, and the second, several hours later, from Karymskoye lake in the caldera of Akademia Nauk volcano. The main significance of the 1996 volcanic events in KVC was the phreatomagmatic eruption in Karymskoye lake, which was the first eruption in this lake in historical time, and was a basaltic eruption at the acidic volcanic center. The volcanic events were associated with the 1 January Ms 6.7 (Mw 7.1) earthquake that occurred at a distance of about 9–17 km southeast from the volcanoes just before the eruptions. We study the long-term (1972–1995) and short-term (1–2 January 1996) characteristics of crustal deformations and seismicity before the double eruptive event in KVC. The 1972–1995 crustal deformation was homogeneous and characterized by a gradual extension with a steady velocity. The seismic activity in 1972–1995 developed at the depth interval from 0 to 20 km below the Akademia Nauk volcano and spread to the southeast along a regional fault. The seismic activity in January 1996 began with a short sequence of very shallow microearthquakes (M ~0) beneath Karymsky volcano. Then seismic events sharply increased in magnitude (up to mb 4.9) and moved along the regional fault to the southeast, culminating in the Ms 6.7 earthquake. Its aftershocks were located to the southeast and northwest from the main shock, filling the space between the two active volcanoes and the ancient basaltic volcano of Zhupanovsky Vostryaki. The eruption in Karymskoye lake began during the aftershock sequence. We consider that the Ms 6.7 earthquake opened the passageway for basic magma located below Zhupanovsky Vostryaki volcano that fed the eruption in Karymskoye lake.     

核工程选址中的火山调查与评价——以吉林省为例

火山活动是核电厂选址中无法回避的重要课题。吉林省新生代以来火山活动极为强烈,特别是第四纪和现代火山活动更为突出,据吉林省火山分布与喷发活动特点,分析了火山与地震活动、火山与构造活动、火山与深部环境的关系,以及火山类型与性质、火山危险性预测和灾害评估。在上述讨论的基础上,提出了在核电厂选址工作中有关火山调查和评价方法的建议     

Large thrust earthquakes and volcanic eruptions

Forty-eight hours after the occurrence of the May 22, 1960 (M _W =9.5) Chile earthquake, Puyehue volcano initiated its eruptive activity. The closeness in space and time of both phenomena provides us with a unique opportunity to examine the possible causal relationship between the sudden strain change and the mechanism of the eruption. From the slip distribution of the 1960 event (Barrientos andWard, 1990) and a static propagator technique, which allows for variable slip faults in vertically heterogeneous media, I calculate the strain field and its depth dependence in the region beneath the volcano. The presented semi-analytical formalism can be applied to any two-dimensional dipping fault. Calculations show extension at the surface of the order of 40 strain, in agreement with what was observed in triangulation networks in the central valley about 50 km oceanward from the line of volcanoes. The amplitude of the strain field beneath the volcano is uniform up to a depth of 20 km and decreases downward. The sudden extension of the region is thought to be the main factor in facilitating the eruption of the volcano. It is postulated that strain beneath the volcano triggered the eruption of the Puyehue-Cordón Caulle volcanic system because it was in a mature stage of its eruptive cycle and there was lack of eruptive activity in other volcanoes located along the 1960 rupture region in the immediate period following the earthquake.     

Seismic energy releases from volcanoes

Seismic energy release during the precursory, eruptive and declining stages of volcanic activities provides various information about the mechanisms of volcanic eruptions and the temporary developments of their activities. Hitherto the energy release patterns from precursory earthquake swarms were used to predict the eruption times, especially of andesitic or dacitic volcanoes. In this paper the discussion is expanded to quantify the total amount of seismic energy released at the threshold of volcanic eruptions, with reference to the results observed at several volcanoes. The results generally indicate that the cumulative seismic energy release from the precursory earthquake swarms exceed 10¹⁷¹⁸ergs before eruptions at any andesitic or dacitic volvanoes. This allows the seismic efficiency, or the ratio of energy radiated seismically, and the energy required for the volumetric expansion to be estimated by incorporating available deformation data with the seismic data. The dependency of seismic efficiency on the type of volcanic activity, i. e. non-explosive outbreaks, phreatic and magmatic eruptions, dome formation, etc., was evaluated from observations at a few volcanoes that provided a variety of examples.     

Introducing the Volcanic Unrest Index (VUI): a tool to quantify and communicate the intensity of volcanic unrest

Accurately observing and interpreting volcanic unrest phenomena contributes towards better forecasting of volcanic eruptions, thus potentially saving lives. Volcanic unrest is recorded by volcano observatories and may include seismic, geodetic, degassing and/or geothermal phenomena. The multivariate datasets are often complex and can contain a large amount of data in a variety of formats. Low levels of unrest are frequently recorded, causing the distinction between background activity and unrest to be blurred, despite the widespread usage of these terms in unrest literature (including probabilistic eruption-forecasting models) and in Volcanic Alert Level (VAL) systems. Frequencies and intensities of unrest episodes are not easily comparable over time or between volcanoes. Complex unrest information is difficult to communicate simply to civil defence personnel and other non-scientists. The Volcanic Unrest Index (VUI) is presented here to address these issues. The purpose of the VUI is to provide a semi-quantitative rating of unrest intensity relative to each volcano’s past level of unrest and to that of analogous volcanoes. The VUI is calculated using a worksheet of observed phenomena. Ranges for each phenomenon within the worksheet can be customised for individual volcanoes, as demonstrated in the companion paper for Taupo Volcanic Centre, New Zealand (Potter et al. 2015). The VUI can be determined retrospectively for historical episodes of unrest based on qualitative observations, as well as for recent episodes with state-of-the-art monitoring. This enables a long time series of unrest occurrence and intensity to be constructed and easily communicated to end users. The VUI can also assist with VAL decision-making. We present and discuss two approaches to the concept of unrest.     

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.