Three-dimensional shallow velocity structure beneath Taal Volcano,Philippines

Based on its numerous historical explosive eruptions and high potential hazards to nearby population of millions, Taal Volcano is one of the most dangerous “Decade Volcanoes” in the world. To provide better investigation on local seismicity and seismic structure beneath Taal Volcano, we deployed a temporary seismic network consisting of eight stations from March 2008 to March 2010. In the preliminary data processing stage, three periods showing linear time-drifting of internal clock were clearly identified from noise-derived empirical Green’s functions. The time-drifting errors were corrected prior to further data analyses. By using VELEST, 2274 local earthquakes were manually picked and located. Two major earthquake groups are noticed, with one lying beneath the western shore of Taal Lake showing a linear feature, and the other spreading around the eastern flank of Taal Volcano Island at shallower depths. We performed seismic tomography to image the 3D structure beneath Taal Volcano using the LOTOS algorithm. Some interesting features are revealed from the tomographic results, including a solidified magma conduit below the northwestern corner of Taal Volcano Island, indicated by high Vp, Vs, and low Vp/Vs ratio, and a large potential hydrothermal reservoir beneath the center of Taal Volcano Island, suggested by low Vs and high Vp/Vs ratio. Furthermore, combining earthquake distributions and tomographic images, we suggest potential existence of a hydrothermal reservoir beneath the southwestern corner of Taal Lake, and a fluid conduit extending to the northwest. These seismic features have never been proposed in previous studies, implying that new hydrothermal activity might be formed in places away from the historical craters on Taal Volcano Island.     

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     

Notes on the 1965 Taal Volcanic eruptions

Neutron activation analyses were performed on samples of olivine-bearing basalt, of volcanic bomb, of volcanic ashes, of residues of water from the old crater lake, of residues of bubbling waters taken near the cinder cone, and of residue of rainwater collected at Parc, Diliman, Quezon City, all of which were volcanic products of the 1965 Taal eruption. The analysis was performed to determine the approximate ratios of important minor and trace elements in the volcanic products, in order to better understand the geochemical processes involved in the 1965 eruption of the Taal Volcano. The radionuclides of Cu, Fe, Na and Mn were positively identified in all the volcanic samples that were irradiated. Scandium was present only in the ash samples. The aproximate Cu/Na ratios for the volcanic ashes, volcanic bomb, and basalt are similar; they ranged from 0.171 to 0.200. The Cu/Na values obtained for the water samples varied from 0.253 to 2.010. The results of fast neutron activation show that the Si, Al, Fe, Mg, and O percentages of the different ash layers are similar. Based on the present work and upon past analyses, it is suggested that the solidification index for Taal is about 10. The eruptivity of Taal Volcano seems to be related to the rift in the crust, rate of migration of the magmatic volatiles, and pressure of the volatile fraction in the magma reservoir. All these factors are probably operating jointly and effectively, making it possible for the Taal Volcano to possess strongly predictable eruptivity, in the light of its past eruption history and its periodic cycle of eruption. It is suggested that the Taal and the Mayon Volcanoes are related from a regional and structural point of view. This observation is reflected in the striking similarity of their respective periodic cycle of eruptions.     

A distinction technique between volcanic and tectonic depression structures based on the restoration modeling of gravity anomaly: a case study of the Hohi volcanic zone, central Kyushu, Japan

In this study, we propose a numerical modeling technique which restores the gravity anomaly of tectonic origin and identifies the gravity low of caldera origin. The identification is performed just by comparing the restored gravity anomalies with the observed gravity anomalies, thus we do not need detailed geophysical and geological information around the buried caldera. The technique has been successfully applied to distinguish the gravity low originated in the buried Shishimuta caldera from other gravity lows in the Hohi volcanic zone, central Kyushu in Japan.     

The subsurface structure of the AIRA caldera and its vicinity in Southern Kyushu, Japan

Southern Kyushu, Japan, includes a chain of large and small calderas and active volcanoes, and the greatest part of it is covered with thick pyroclastic ejecta. The regional and local structures of this area are discussed from the standpoint of physical volcanology, with consideration of all available data.The regional structure of this area is examined in the light of gravity and geomagnetic anomalies. Two layers of the earth's uppermost crust are defined by spectrum analysis of the gravity anomalies. These two layers are identical with the two identified by seismicwave velocities. The Bouguer gravity anomalies are relatively high and rather monotonous over outcrops of the Mesozoic basement and the granite, but are relatively low and perturbed over calderas and caldera-like structures. Two low-gravity anomalies in Kagoshima Bay are remarkable. One is circular, with its center on the Aira caldera. The other is elongated between the Satsuma and Oosumi peninsulas. The southern end of the latter anomaly is occupied by the Ata caldera. Discussion of the gravity anomalies of the Aira caldera suggests that the subsurface basement has a funnel shape and is overlain by ‘fallback’. The sub bottom geology of the caldera suggests that it is formed by a few smaller depressions, though the distribution of the overall gravity anomalies is parallel with its shape.The southern part of Kagoshima Bay is characterized by a graben-like topography and low-gravity anomalies and, moreover, by several calderas. The middle part, between the Aira and Ata calderas, may have a graben-like structure. A profile crossing the bay through Sakurajima volcano is modeled on the basis of results from drilling and gravity surveys. The basement has a graben-like structure and is filled with coarse and low-density deposits, and the structure continues northwards to the Aira caldera with a funnel shape.A comparison of this area with the Taupo-Rotorua depression in New Zealand and Lake Toba in Indonesia, leads the authors to the conclusion that such major volcanic depressions may have been formed by amalgamation of a series of caldera-like structures which were formed by multiple violent explosions accompanied by ejection of a tremendous amount of pyroclastic material.     

The September 28–30, 1965 eruption of Taal Volcano,Philippines

A moderately violent phreatomagmatic explosive eruption of Taal Volcano, Philippines, occurred from 28 to 30 September, 1965. The main phreatic explosions, which were preceded by ejection of basaltic spatter, opened a new crater 1.5 km long and 0.3 km wide on the southwest side of Volcano Island in Lake Taal. The eruption covered an area of about 60 square kilometers with a blanket of ash more than 25 cm thick and killed approximately 200 persons. The clouds that formed during the explosive eruption rose to heights of 15 to 20 km and deposited fine ash as far as 80 km west of the vent. At the base of the main explosion column, flat, turbulent clouds spread radially, with hurricane velocity, transporting ash, mud, lapilli and blocks. The horizontally moving, debris-laden clouds, sandblasted trees, coated the blast side of trees and houses with mud, and deposited coarse ejecta with dune-type bedding in a zone roughly 4 km in all directions from the explosion crater.     

Shallow structure beneath the Central Volcanic Complex of Tenerife from new gravity data: Implications for its evolution and recent reactivation

We present a new local Bouguer anomaly map of the Central Volcanic Complex (CVC) of Tenerife, Spain, constructed from the amalgamation of 323 new high precision gravity measurements with existing gravity data from 361 observations. The new anomaly map images the high-density core of the CVC and the pronounced gravity low centred in the Las Cañadas caldera in greater detail than previously available. Mathematical construction of a sub-surface model from the local anomaly data, employing a 3D inversion based on “growing” the sub-surface density distribution via the aggregation of cells, enables mapping of the shallow structure beneath the complex, giving unprecedented insights into the sub-surface architecture. We find the resultant density distribution in agreement with geological and other geophysical data. The modelled sub-surface structure supports a vertical collapse origin of the caldera, and maps the headwall of the ca. 180 ka Icod landslide, which appears to lie buried beneath the Pico Viejo–Pico Teide stratovolcanic complex. The results allow us to put into context the recorded ground deformation and gravity changes at the CVC during its reactivation in spring 2004 in relation to its dominant structural building blocks. For example, the areas undergoing the most significant changes at depth in recent years are underlain by low-density material and are aligned along long-standing structural entities, which have shaped this volcanic ocean island over the past few million years.     

The Negros de Aras Nuée Ardente deposits: a cataclysmic eruption of Socompa volcano (Andes of Atacama,Chile)

Studies of ERT Satellite photographic documents and of acrial photographs with complementary lield work reveal the presence of recent very large nuée ardente deposits north-west of Socompa Volcano (Andean Cordillera of Atacama, northern Chile). Three zones are distinguishable from the bottom of Socompa Volcano to the front of the nuée ardente deposits: 1) pumice blocks are covered with parallel ridges of debris (lava blocks) from the north-western flank of Socompa Volcano, 2) pumice blocks lie upon small cones and flows from El Negrillar volcanoes located inside the graben of Negros de Aras, 3) pumice flow threads its way between cones and flows from El Negrillar volcanoes and stops more than 40 km away from the base of Socompa Volcano. The calculated thermal energy of this cruption is 7.9 × 10²⁵ ergs, being in the range of of the most important recorded eruptions on earth. The pumice is almost aphyric (rare plagioclase, hypersthene and hornblende phenocrysts) and is of dacite composition lying pertectly on the K₂O-SiO₂ trend of the Socompa Volcano. Trace and major element data of the pumice are similar to those of two dacites from a pre-nuée lava flow and a post-nuée lava dome of Socompa Volcano and support a common magmatic origin with the Socompa Volcano lavas. A relative chronology is proposed.     

A large hydrothermal reservoir beneath Taal Volcano (Philippines) revealed by magnetotelluric resistivity survey: 2D resistivity modeling

Taal Volcano, located in the southwestern part of Luzon Island, Philippines, has frequently experienced catastrophic eruptions from both the Main Crater on Volcano Island and flank eruptions. These eruptions have been magmatic, phreatomagmatic, and hydrothermal, with the latter implying the existence of a large-scale hydrothermal system beneath the volcano. We conducted an electrical resistivity survey using the magnetotelluric method in order to identify the location and geometry of the hydrothermal reservoir and sealing cap rock. Two-dimensional inversion using the observed data indicates four similar resistivity sections. The structure at shallow depths corresponds to volcanic deposits and an aquifer. Below 1 km, the structure features a relatively resistive zone beneath the main crater surrounded by a conductive shell. We interpreted these to be a large hydrothermal reservoir with an impermeable cap rock sealing it. Recent ground deformation detected by GPS measurements suggests that the hydrothermal reservoir is active. The interpreted cap rock thins just beneath the main crater and could easily be destroyed by an imbalance in the hydrothermal system. We conclude that this hydrothermal reservoir plays a significant role in driving catastrophic eruptions that begin with a hydrothermal explosion at the main crater.     

A 3D magnetic structure of Izu-Oshima Volcano and their changes after the eruption in 1986 as estimated from repeated airborne magnetic surveys

A 3D magnetic inversion method using a conjugate gradient method (CG method) was developed for constructing 3D magnetization models of a volcanic edifice and applied to aeromagnetic anomalies of Izu-Oshima Volcano surveyed in 1986 and in 1997. The calculated results of the 1986 data show that the volcanic edifice of Izu-Oshima Volcano has a mean magnetization intensity ranging from 10.4 to 12.1 A/m. The derived 3D magnetic structure shows low magnetization zones beneath the west-northwest of the western caldera rim, beneath the west-southwest of Mt. Mihara and beneath Mt. Shiroishi. These features may be related with demagnetizations, reflecting a high thermal state due to magma activities in the 1986 eruption. The comparison between 3D magnetization models in 1986 and in 1997, indicates meaningful changes beneath the C-craters erupted in 1986, suggesting a recovery process of demagnetizations and a considerable decrease of magnetization intensities in the foot of Mt. Futago, indicative of demagnetizations. A derived magnetization model including Izu-Oshima Volcano and its surrounding sea areas clarifies the submerged volcanic edifices around Izu-Oshima Island, and suggests that the old volcanic edifices of Fudeshima, Gyojyanoiwaya, and Okata Volcanoes have been affected by eastward migrations due to massive intrusions of a dike-like structure inferred at the base of Izu-Oshima Volcano.     

Crustal structure beneath Aso Caldera,Southwest Japan,as derived from receiver function analysis

Aso Volcano experienced a huge pyroclastic eruption 90 thousand years ago, and formed a large caldera (18 km × 25 km). In order to test the hypothesis of a magma body in the mid and lower crust that has been suggested geophysically and geochemically, we investigated seismic velocity discontinuities and velocity structure beneath Aso Caldera using receiver functions and a genetic algorithm inversion. We confirm the existence of the Moho at depths between 30 km and 35 km and a large velocity anomaly should exist in the deep portion of the crust beneath Aso Caldera, from imaging of receiver functions observed only at stations outside the caldera. As a result of a more detailed examination with GA inversion, a low velocity layer is detected at depths between 10 km and 24 km beneath the western part of the caldera. S-wave velocity of the layer is estimated to be 2.0–2.4 km/s. We estimate that the low velocity layer contains at most 15% melt or 30% aqueous fluid. The layer exists near the Conrad and at the same depths as the swarm of the low frequency earthquakes and a compressional and dilatational deformation source which are expected to be caused by fluid movement beneath the middle-eastern part of the caldera. Fluid contained in the layer might be related with huge pyroclastic eruptions of Aso Volcano.     

Changes in the height of the active Karymskii Volcano, Kamchatka during the period 1971–2007

Systematic measurements of the height of the summit crater rim on the active Karymskii Volcano showed that the variation of that parameter has been greater during its last eruption, lasting, with short intermissions, from January 1, 1996 until now (October 2007) compared with the earlier eruptions. The periodic increases in the height of Karymskii Volcano were due to explosion discharges of unconsolidated pyroclastic material, with most of this falling on the volcano’s cone. The increased seismicity of Karymskii Volcano intensified the slope movement processes, resulting in a comparatively flat area forming periodically on the crater rim; during separate, not very long, periods the height of the volcanic cone was increasing in discrete steps and at a greater rate. The periodic decrease in the height of Karymskii Volcano is due to compaction of pyroclastic material and, to a much greater extent, after violent explosions which expand the crater by removing its nearsummit circumference. The other contributing factor consists in sagging of the magma column due to partial emptying of the peripheral magma chamber, which makes the internal crater slope steeper, hence causes cone collapse and the cone lower. These occurrences are generally similar to the processes of crater and caldera generation described by previous investigators for other volcanoes of the world.     

The deep structure of Kudryavyi Volcano on Iturup Island as inferred from seismic data

This paper reports the results from seismic probing of Kudryavyi Volcano in the Medvezh’ya caldera on Iturup Island, which is part of the Kuril Islands. Velocity structures for the northwestern slope of the volcano and the Medvezh’ya caldera are given. We identify the basic sedimentary complexes and anomalous zones, which we interpret as intermediate chambers. Possible versions of the geological evolution of this feature are presented.     

Non-explosive,constructional evolution of the ice-filled caldera at Volcán Sollipulli,Chile

 A radar and gravity survey of the ice-filled caldera at Volcán Sollipulli, Chile, indicates that the intra-caldera ice has a thickness of up to 650 m in its central part and that the caldera harbours a minimum of 6 km³ of ice. Reconnaissance geological observations show that the volcano has erupted compositions ranging from olivine basalt to dacite and have identified five distinct volcanic units in the caldera walls. Pre- or syn-caldera collapse deposits (the Sharkfin pyroclastic unit) comprise a sequence which evolved from subglacial to subaerial facies. Post-caldera collapse products, which crop out along 17 of the 20 km length of the caldera wall, were erupted almost exclusively along the caldera margins in the presence of a large body of intra-caldera ice. The Alpehué crater, formed by an explosive eruption between 2960 and 2780 a. BP, in the southwest part of the caldera is shown to post date formation of the caldera. Sollipulli lacks voluminous silicic pyroclastic rocks associated with caldera formation and the collapse structure does not appear to be a consequence of a large-magnitude explosive eruption. Instead, lateral magma movement at depth resulting in emptying of the magma chamber may have generated the caldera. The radar and gravity data show that the central part of the caldera floor is flat but, within a few hundred metres of the caldera walls, the floor has a stepped topography with relatively low-density rock bodies beneath the ice in this region. This, coupled with the fact that most of the post-caldera eruptions have taken place along the caldera walls, implies that the caldera has been substantially modified by subglacial marginal eruptions. Sollipulli caldera has evolved from a collapse to a constructional feature with intra-caldera ice playing a major role. The post-caldera eruptions have resulted in an increase in height of the walls and concomitant deepening of the caldera with time. Received: 12 June 1995 / Accepted: 7 December 1995     

Submarine tumuli and inflated tube-fed lava flows on Axial Volcano,Juan de Fuca Ridge

A seafloor lava field was mapped within the summit caldera of Axial Volcano, Juan de Fuca Ridge, using SeaMARC I sidescan sonor and submersible observations. By analogy with similar subaerial features, we infer that several volcanic seafloor features here formed by the process of lava flow inflation. Flow inflation occurs within tube-fed lava flows when lava continues to be supplied to the interior of a flow that has ceased advancing, thus uplifting the flow's rigid surface and creating a suite of characteristic surface structures. Inflated lavas require a feeder lava tube or tube system connected to a remote lava source, and therefore we infer that inflated submarine lava flows contain lava tubes. Inflated flow features identified from sidescan sonar images elsewhere on Axial Volcano and within the axial valley of the southern Juan de Fuca ridge suggest that flow inflation is a widespread submarine volcanic process.     

Gravity and aeromagnetic constraints on the structure of the Woods Mountains volcanic center, southeastern California

 The Woods Mountain volcanic center is a well-exposed, mildly alkaline volcanic center that formed during the Miocene in southeastern California. Detailed geologic mapping and geochemical studies have distinguished three major volcanic phases: precaldera, caldera forming, and postcaldera. Geologic mapping indicates that caldera formation occurred incrementally during eruptions of three large ignimbrites and continued into a period of voluminous intracaldera lava-flow eruptions. Rhyolitic ignimbrites and lava flows within the caldera are associated with large amplitude, circular gravity, and magnetic minima that are among the most prominent gravity and magnetic anomalies in southeastern California. Analysis of a Bouguer gravity anomaly map, reduced-to-the-pole magnetic intensity map, and three-dimensional gravity and magnetic models indicates that there is a single, funnel- to bowl-shaped caldera approximately 4 km thick and approximately 10 km wide at the surface. This model is consistent with other siliceous, pyroclastic-filled calderas on continental crust, except that most siliceous volcanic centers associated with more than one eruption are characterized by more than one caldera. Received: 20 December 1997 / Accepted: 15 October 1998     

Subsurface geometry and structural evolution of La Gomera island based on gravity data

We hereby present a new Bouguer gravity map of the La Gomera island (Canarian Archipelago), which is analysed and interpreted by means of a 3-dimensional inversion, in order to contribute to the knowledge of the structural setting of this volcanic island and its evolutionary history. A land gravity data set covering the whole island of La Gomera is used in combination with offshore measurements to achieve a better determination of the gravity field in areas near the coasts.The study of this map let us to shed some light on the hypothesis established about the volcanism of this island. Moreover, it shows the information that is hidden from a geological surface exploration, modelling deep sections of the crust in La Gomera, which have been unknown until now.A first interpretation of the Bouguer gravity anomaly is achieved from 1) the residual gravity map calculated by removing a regional component and 2) the total horizontal gradient of the gravity. These residual and derivative maps allow us to identify the horizontal location and borders of the shallowest gravity sources. This provides a useful tool to study the structures associated to the latest periods of the volcanism in the area. Moreover, the information so obtained supports the hypothesis about the migration of volcanic activity towards the south of the island.Subsequently, an inversion process is carried out looking for the 3D-modelisation of the sources of the observed gravity field, which provides a comprehensive view of the structures in volcanic environments. The inversion technique used is based on a genetic algorithm (GA) applied upon a prismatic partition of the subsoil volume, and adopting a priori values of density contrast (positive and negative). The main advantage of this method is that let us to model deep and shallow bodies which exhibit very different geometries and density contrasts. So, results indicate that this inversion strategy can be very effective for characterization of volcanic structures, improving the information from previous geologic and volcanologic studies. The inversion model obtained shows correlation between several sources of the gravity field and the volcanic units associated with the growth of La Gomera Island. The main gravity source of this model is associated with the oldest unit, called the Basal Complex. This unit corresponds to the first submarine growth stage and it is modelled as the most important and deepest high density structure. According to previous geological studies, the following edifice (Old Edifice) was also submarine in its initial phases, later being represented by a wide basaltic shield volcano. The original location and morphology of this Old Edifice is deduced from the distribution of positive density contrasts that appears in the model. Moreover, other gravity field sources are identified and associated to several feeding systems of this stage of the volcanism in La Gomera. The shallowest sections of the model let us recognise the distribution of light material inside the Vallehermoso caldera, surrounded by high density structures. This gives us some insight into the internal structure and morphology of the caldera, pointing to a vertical collapse origin followed by erosion and other destructive processes. Finally, other conclusions are obtained from the correlation found between the sources of the gravity field and the migration of the volcanic activity towards the southern area of the island.     

Gravity evidence for an extinct magma chamber beneath Syrtis Major, Mars: a look at the magmatic plumbing system

Syrtis Major is an ancient basaltic shield volcano on Mars with a basal diameter of 1100 km. The free-air gravity anomaly is 126 mGal at spherical harmonic degree 50 and reaches its maximum amplitude over the 2 km deep topographic caldera. The observed gravity anomaly cannot be explained by flexurally supported surface topography and requires the presence of a buried, high-density load. The geologically most reasonable interpretation of this high-density load is that it represents the magma chamber of Syrtis Major, now solidified and filled at least in part by dense igneous cumulates. Pyroxene is likely to be the dominant cumulate mineral in this system, although olivine may also be present. Gravity models presented here define the structure of the buried load and in essence provide a look at the magmatic plumbing system of this volcano. The preferred model involves a buried load that is approximately 300×600 km across, roughly twice as large as the topographic caldera. Both the buried load and the caldera are elongated in the north-south direction. In the center of the buried load, the minimum thickness is 2.8 km for an olivine-dominated cumulate system or 3.9 km for a pyroxene-dominated system. The best terrestrial analog for this structure is the Bushveld Complex, an igneous cumulate body that is similar in size and thickness to the Syrtis Major structure. Assuming that the mean crustal density is 2600 kg m⁻³ due to impact brecciation, the elastic lithosphere at Syrtis Major was 10-15 km thick at the time when the topographic load was emplaced. This corresponds to a lithospheric thermal gradient of 28-52 K/km and a surface heat flux of 70-130 mW m⁻². Higher resolution gravity data, such as that which is planned for the 2005 Mars Reconnaissance Orbiter, will permit further refinement of the dimensions of this structure.     

Three-dimensional P-wave velocity structure in the upper crust beneath Kuju Volcano, central Kyushu, Japan

 Kuju Volcano lies near Aso Caldera at the center of Kyushu Island, western Japan. After a few hundred years of dormancy, a phreatic explosion accompanied by a small ash eruption occurred on 11 October 1995. This study was undertaken to determine the subsurface seismic velocity structure associated with the active magmatic regime in the Kuju volcanic region. The three-dimensional, upper crustal, P-wave velocity structure beneath Kuju Volcano was determined using methods for the simultaneous inversion of P-wave arrival times from local earthquakes in and around the Kuju volcanic region for velocities and hypocentral parameters. Results reveal two shallower low-velocity anomalies located in the northern and southern parts of Kuju Volcano, consistent with the presence of significant negative Bouguer gravity anomalies. In addition, a high-velocity anomaly is located approximately 5 km northwest of Mt. Kuju, one of the domes in Kuju Volcano. Beneath this high-velocity anomaly, a low-velocity anomaly is present. This velocity structure suggests a magmatic regime that has a lid consisting of cooled solid material overlying a chamber of partially molten material. Received: 23 September 1997 / Accepted: 20 June 1998     

The geoecological impact of eruptions on Karymskii Volcano and Tokareva crater, Eastern Kamchatka: The 1996–2008 observations

This paper considers the geoecological impacts of eruptions on Karymskii Volcano and the Tokareva crater for the 1996–2008 period, which resulted in changes in (a) the relief around these edifices, (b) the discharge and composition of water in the Karymskii River and other streams in the area, and (c) the discharge and composition of gases in thermal springs. It was found that the concentration of CH₄ previously had been abnormally high in free gases that emanate from the new Piipovskii Springs and an explanation is provided of the decrease in their concentration over time. We detected variations in the radon activity, OARn (Bq/m³), in free gases that are released in the Karymskii caldera hydrothermal occurrences; the variations are consistent with those in the eruptive activity of Karymskii Volcano in 2005–2006. We describe permafrost rocks in the Karymskii caldera that favor the generation of a cryolithic zone.