The occurrence of Mt Barca flank eruption in the evolution of the NW periphery of Etna volcano (Italy)

Geological surveys, tephrostratigraphic study, and ⁴⁰Ar/³⁹Ar age determinations have allowed us to chronologically constrain the geological evolution of the lower NW flank of Etna volcano and to reconstruct the eruptive style of the Mt Barca flank eruption. This peripheral sector of the Mt Etna edifice, corresponding to the upper Simeto valley, was invaded by the Ellittico volcano lava flows between 41 and 29 ka ago when the Mt Barca eruption occurred. The vent of this flank eruption is located at about 15 km away from the summit craters, close to the town of Bronte. The Mt Barca eruption was characterized by a vigorous explosive activity that produced pyroclastic deposits dispersed eastward and minor effusive activity with the emission of a 1.1-km-long lava flow. Explosive activity was characterized by a phreatomagmatic phase followed by a magmatic one. The geological setting of this peripheral sector of the volcano favors the interaction between the rising magma and the shallow groundwater hosted in the volcanic pile resting on the impermeable sedimentary basement. This process produced phreatomagmatic activity in the first phase of the eruption, forming a pyroclastic fall deposit made of high-density, poorly vesicular scoria lapilli and lithic clasts. Conversely, during the second phase, a typical strombolian fall deposit formed. In terms of hazard assessment, the possible occurrence of this type of highly explosive flank eruption, at lower elevation in the densely inhabited areas, increases the volcanic risk in the Etnean region and widens the already known hazard scenario.     

Review of Microgravity Observations at Mt. Etna: A Powerful Tool to Monitor and Study Active Volcanoes

Microgravity observations at Mt. Etna have been routinely performed as both discrete (since 1986) and continuous (since 1998) measurements. In addition to describing the methodology for acquiring and reducing gravity data from Mt. Etna, this paper provides a collection of case studies aimed at demonstrating the potential of microgravity to investigate the plumbing system of an active volcano and detect forerunners to paroxysmal volcanic events. For discrete gravity measurements, results from 1994–1996 and 2001 are reported. During the first period, the observed gravity changes are interpreted within the framework of the Strombolian activity which occurred from the summit craters. Gravity changes observed during the first nine months of 2001 are directly related to subsurface mass redistributions which preceded, accompanied and followed the July-August 2001 flank eruption of Mt. Etna. Two continuous gravity records are discussed: a 16-month (October 1998 to February 2000) sequence and a 48-hour (26–28 October, 2002) sequence, both from a station within a few kilometers of the volcano's summit. The 16-month record may be the longest continuous gravity sequence ever acquired at a station very close to the summit zone of an active volcano. By cross analyzing it with contemporaneous discrete observations along a summit profile of stations, both the geometry of a buried source and its time evolution can be investigated. The shorter continuous sequence encompasses the onset of an eruption from a location only 1.5 km from the gravity station. This gravity record is useful for establishing constraints on the characteristics of the intrusive mechanism leading to the eruption. In particular, the observed gravity anomaly indicates that the magma intrusion occurred “passively” within a fracture system opened by external forces.     

A multi-disciplinary study of the 2002–03 Etna eruption: insights into a complex plumbing system

The 2002–03 Mt Etna flank eruption began on 26 October 2002 and finished on 28 January 2003, after three months of continuous explosive activity and discontinuous lava flow output. The eruption involved the opening of eruptive fissures on the NE and S flanks of the volcano, with lava flow output and fire fountaining until 5 November. After this date, the eruption continued exclusively on the S flank, with continuous explosive activity and lava flows active between 13 November and 28 January 2003. Multi-disciplinary data collected during the eruption (petrology, analyses of ash components, gas geochemistry, field surveys, thermal mapping and structural surveys) allowed us to analyse the dynamics of the eruption. The eruption was triggered either by (i) accumulation and eventual ascent of magma from depth or (ii) depressurisation of the edifice due to spreading of the eastern flank of the volcano. The extraordinary explosivity makes the 2002–03 eruption a unique event in the last 300 years, comparable only with La Montagnola 1763 and the 2001 Lower Vents eruptions. A notable feature of the eruption was also the simultaneous effusion of lavas with different composition and emplacement features. Magma erupted from the NE fissure represented the partially degassed magma fraction normally residing within the central conduits and the shallow plumbing system. The magma that erupted from the S fissure was the relatively undegassed, volatile-rich, buoyant fraction which drained the deep feeding system, bypassing the central conduits. This is typical of most Etnean eccentric eruptions. We believe that there is a high probability that Mount Etna has entered a new eruptive phase, with magma being supplied to a deep reservoir independent from the central conduit, that could periodically produce sufficient overpressure to propagate a dyke to the surface and generate further flank eruptions.Editorial responsibility: J. Donnelly-Nolan     

Petrology of lavas from the 2004–2005 flank eruption of Mt. Etna,Italy: inferences on the dynamics of magma in the shallow plumbing system

Following the 2001 and 2002–2003 flank eruptions, activity resumed at Mt. Etna on 7 September 2004 and lasted for about 6 months. This paper presents new petrographic, major and trace element, and Sr–Nd isotope data from sequential samples collected during the entire 2004–2005 eruption. The progressive change of lava composition allowed defining three phases that correspond to different processes controlling magma dynamics inside the central volcano conduits. The compositional variability of products erupted up to 24 September is well reproduced by a fractional crystallization model that involves magma already stored at shallow depth since the 2002–2003 eruption. The progressive mixing of this magma with a distinct new one rising within the central conduits is clearly revealed by the composition of the products erupted from 24 September to 15 October. After 15 October, the contribution from the new magma gradually becomes predominant, and the efficiency of the mixing process ensures the emission of homogeneous products up to the end of the eruption. Our results give insights into the complex conditions of magma storage and evolution in the shallow plumbing system of Mt. Etna during a flank eruption. Furthermore, they confirm that the 2004–2005 activity at Etna was triggered by regional movements of the eastern flank of the volcano. They caused the opening of a complex fracture zone extending ESE which drained a magma stored at shallow depth since the 2002–2003 eruption. This process favored the ascent of a different magma in the central conduits, which began to be erupted on 24 September without any significant change in eruptive style, deformation, and seismicity until the end of eruption.     

Ground deformation modeling of flank dynamics prior to the 2002 eruption of Mt. Etna

On 22 September 2002, 1 month before the beginning of the flank eruption on the NE Rift, an M-3.7 earthquake struck the northeastern part of Mt. Etna, on the westernmost part of the Pernicana fault. In order to investigate the ground deformation pattern associated with this event, a multi-disciplinary approach is presented here. Just after the earthquake, specific GPS surveys were carried out on two small sub-networks, aimed at monitoring the eastern part of the Pernicana fault, and some baselines belonging to the northeastern EDM monitoring network of Mt. Etna were measured. The leveling route on the northeastern flank of the volcano was also surveyed. Furthermore, an investigation using SAR interferometry was performed and also the continuous tilt data recorded at a high precision sensor close to the epicenter were analyzed to constrain the coseismic deformation. The results of the geodetic surveys show a ground deformation pattern that affects the entire northeastern flank of the volcano, clearly shaped by the Pernicana fault, but too strong and wide to be related only to an M-3.7 earthquake. Leveling and DInSAR data highlight a local strong subsidence, up to 7 cm, close to the Pernicana fault. Significant displacements, up to 2 cm, were also detected on the upper part of the NE Rift and in the summit craters area, while the displacements decrease at lower altitude, suggesting that the dislocation did not continue further eastward. Three-dimensional GPS data inversions have been attempted in order to model the ground deformation source and its relationship with the volcano plumbing system. The model has also been constrained by vertical displacements measured by the leveling survey and by the deformation map obtained by SAR interferometry.     

Monitoring Quiescent Volcanoes by Diffuse CO2 Degassing: Case Study of Mt. Fuji, Japan

Since the 8th century, more than seventeen eruptions have been recorded for the Mt. Fuji volcano, with the most recent eruption occurring in 1707 (Hoei eruption). For the past 300 years the volcano has been in a quiescent stage and, since the early 1960s, has exhibited neither fumarolic nor thermal activity. However, the number of low-frequency earthquakes with a hypocentral depth of 10–20 km increased significantly beneath the northeastern flank of Mt. Fuji in 2000–2001, suggesting a possible resumption of magmatic activity. In this study, diffuse CO₂ efflux and thermal surveys were carried out in four areas of the volcano in 2001–2002 in order to detect possible signs of the upward movement of deep magma. At all survey points, the CO₂ efflux was below the detection limit with the exception of a few points with biological CO₂ emission, and ground temperatures at a depth of 20–30 cm were below ambient, indicating no surface manifestations of gas or heat emission. Should magma rise into the subsurface, the diffuse CO₂ efflux would be expected to increase, particularly along the tectonically weakened lineation on the Mt. Fuji volcano, allowing for the early detection of pre-eruptive degassing.     

Evidence of multiple strain fields beneath the eastern flank of Mt. Etna volcano (Sicily,Italy) deduced from seismic and geodetic data during 2003–2004

We carried out a study of the seismicity and ground deformation occurring on Mt. Etna volcano after the end of the 2002–2003 eruption and before the onset of the 2004–2005 eruption. Data were recorded by the permanent local seismic network run by Istituto Nazionale di Geofisica e Vulcanologia – Sezione di Catania and by geodetic surveys carried out in July 2003 and July 2004 on the GPS network. Most earthquakes were grouped in two main clusters located in the northeastern and southeastern sectors of the volcano. The areal distribution of seismic energy associated with the recorded earthquakes allowed us to highlight the main seismogenic areas of Mt. Etna. In order to better understand the kinematic processes of the volcano, 3D seismic locations were used to compute fault plane solutions, and a selected dataset was inverted to determine stress and strain tensors. The focal mechanisms in the northeastern sector show clear left-lateral kinematics along an E-W fault plane, consistent with events occurring along the Pernicana Fault system. The fault plane solutions in the southeastern sector show mainly right-lateral kinematics along a NNE and ENE fault plane and left lateral-kinematics along NW fault planes that together suggest roughly E-W oriented compression. Surface ground deformation affecting Mt. Etna measured by GPS surveys highlighted a marked inflation during the same period and exceptionally strong seawards motion of its eastern flank. The 2D geodetic strain tensor distribution was calculated and the results show mainly ENE-WSW extension coupled with WNW-ESE contraction, indicating right-lateral shear along a NW-SE oriented fault plane. The different deformation of the eastern sector of the volcano, as measured by seismicity and ground deformation, must be interpreted by considering the different depths of the two signals. Seismic activity in the southeastern sector of volcano is located between 3 and 8 km b.s.l. and can be associated with a very strong additional E-W compression induced by a pressurizing source just westwards and at the same depth, located by inverting GPS data. Ground deformation, in contrast, is mainly affected by the shallower dynamics of the fast moving eastern flank which produces a shallower opposing E-W extension. The entire dataset shows that two different processes affect the eastern flank at the same time but at different depths; the boundary is clearly located at a depth of 3 km b.s.l. and could represent the décollement surface for the mobile flank.     

Combining relative and absolute gravity measurements to enhance volcano monitoring

To achieve a balance between uncertainty and efficiency in gravity measurements, we have investigated the applicability of combined measurements of absolute and relative gravity as a hybrid method for volcano monitoring. Between 2007 and 2009, three hybrid gravity surveys were conducted at Mt Etna volcano, in June 2007, July 2008, and July 2009. Absolute gravity data were collected with two absolute gravimeters, which represent the state of the art in recent advances in ballistic gravimeter technology: (1) the commercial instrument FG5#238 and (2) the prototype instrument IMGC-02. We carried out several field surveys and confirmed that both the absolute gravimeters can still achieve a 10 μGal or better uncertainty even when they are operated in severe environmental conditions. The use of absolute gravimeters in a field survey of the summit area of Mt Etna is unprecedented. The annual changes of the gravity measured over 2007–2008 and 2008–2009 provide unequivocal evidence that during the 2007–2009 period, two main phenomena of subsurface mass redistribution occurred in distinct sectors of the volcano, accompanying different eruptive episodes. From 2007 to 2008, a gravity change of ?60?μGal was concentrated around the North-East Rift. This coincided with a zone affected by strong extensional tectonics, and hence might have been related to the opening of new voids. Between 2008 and 2009, a North-South elongate feature with a maximum gravity change of +80?μGal was identified in the summit craters area. This is interpreted to indicate recharge of a deep-intermediate magma storage zone, which could have occurred when the 2008–2009 eruption was still ongoing.     

Seismic activity preceding the 1983 eruption of Mt. Etna

The March–August, 1983 eruption of Mt. Etna can be considered as one of the most important in the last years.The analysis of seismic activity during the three months immediately before the eruption showed interesting variations of theb coefficient, in the frequency-magnitude relationship, that have been linked to possible changes of the stress field in the Etnean region.The eruption start was also preceded by a strong seismic crisis with epicenters mostly on the southern, eastern and southwestern flanks of the volcano, and characterized by the shallowness of most of the events (h3 km).The data analysis has led to a hypothesis on the eruption occurrence based on a model of dynamic evolution of the stress field acting on Mt. Etna.     

Volcanic Tremor at Mt. Etna,Italy, Preceding and Accompanying the Eruption of July – August, 2001

The July 17 – August 9, 2001 flank eruption of Mt. Etna was preceded and accompanied by remarkable changes in volcanic tremor. Based on the records of stations belonging to the permanent seismic network deployed on the volcano, we analyze amplitude and frequency content of the seismic signal. We find considerable changes in the volcanic tremor which mark the transition to different styles of eruptive activity, e.g., lava fountains, phreatomagmatic activity, Strombolian explosions. In particular, the frequency content of the signal decreases from 5 Hz to 3 Hz at our reference station ETF during episodes of lava fountains, and further decreases at about 2 Hz throughout phases of intense lava emission. The frequency content and the ratios of the signal amplitude allow us to distinguish three seismic sources, i.e., the peripheral dike which fed the eruption, the reservoir which fed the lava fountains, and the central conduit. Based on the analysis of the amplitude decay of the signal, we highlight the migration of the dike from a depth of ca. 5 km to about 1 km between July 10 and 12. After the onset of the effusive phase, the distribution of the amplitude decay at our stations can be interpreted as the overall result of sources located within the first half kilometer from the surface. Although on a qualitative basis, our findings shed some light on the complex feeding system of Mt. Etna, and integrate other volcanological and geophysical studies which tackle the problem of magma replenishment for the July–August, 2001 flank eruption. We conclude that volcanic tremor is fundamental in monitoring Mt. Etna, not only as a marker of the different sources which act within the volcano edifice, but also of the diverse styles of eruptive activity. An erratum to this article is available at .     

Fault-controlled Soil CO2 Degassing and Shallow Magma Bodies: Summit and Lower East Rift of Kilauea Volcano (Hawaii), 1997

Soil CO₂ flux measurements were carried out along traverses across mapped faults and eruptive fissures on the summit and the lower East Rift Zone of Kilauea volcano. Anomalous levels of soil degassing were found for 44 of the tectonic structures and 47 of the eruptive fissures intercepted by the surveyed profiles. This result contrasts with what was recently observed on Mt. Etna, where most of the surveyed faults were associated with anomalous soil degassing. The difference is probably related to the differences in the state of activity at the time when soil gas measurements were made: Kilauea was erupting, whereas Mt. Etna was quiescent although in a pre-eruptive stage. Unlike Mt. Etna, flank degassing on Kilauea is restricted to the tectonic and volcanic structures directly connected to the magma reservoir feeding the ongoing East Rift eruption or in areas of the Lower East Rift where other shallow, likely independent reservoirs are postulated. Anomalous soil degassing was also found in areas without surface evidence of faults, thus suggesting the possibility of previously unknown structures. Received: November 2003, revised: January 2005, accepted: January 2005     

长白山天池火山岩浆系统分析

针对外媒报道长白山天池火山在近2年内有可能喷发的言论,在长白山天池火山区布测了一条长度约为103 km的二维大地电磁测深观测剖面,对火山区深部电性结构进行探测研究.由于研究区内不明来源的电磁干扰非常强,对数据采用了远参考处理、Robust处理、Rhoplus分析、张量阻抗分解和基于一维层状介质电阻率与相位互算方法等先进处理技术,获取到一批在强干扰区质量较为可靠的电磁数据,利用数据计算分析了长白山天池火山区二维构造走向和感应矢量特征,采用NLCG二维反演技术对资料进行了二维反演解释,并将反演结果与前人探测结果进行了对比分析.探测结果表明:在天池火山口下方存在明显的直立型岩浆通道,岩浆通道在下方约5~8 km位置形成关闭;在火山口下方往北方向附近,在埋深位置约7 km深处存在一个明显的低阻异常体,电阻率小于10 Ωm,且与岩浆通道对接,推测其可能是地表浅部发育的岩浆囊;在长白山山门附近C07-C09号测点之间和C04-C05号测点之间,在埋深约7~17 km深处发现近直立型低阻带,低阻带与下方低阻体直接相连,推测低阻带内赋存有活动的岩浆;随着埋深的增加,从天池火山口南部约20 km位置往北方向,在埋深13~30 km之间壳内广泛发育明显的低阻异常体,推测其可能是活动的岩浆囊.反演结果与前人探测结果整体电性特征相似,但又局部不同.     

Variation ofb values before the Etnean eruption of March 1981

A fissural eruption occurred from the northern flank of Mt. Etna on March 17, 1981, and the associated earthquake activity, recorded by the seismic network operating on the volcano, was carefully examined to detect possible variations ofb values, according to the frequency-magnitude relationship, before the start of the eruption. The analysis of 4000B-type earthquakes was carried out according to the maximum likelihood method. A significant increase in theb value was noticed since the beginning of February 1981, followed by a sharp decrease in the days before the eruption started. The observed variation pattern was related to possible changes on the stress field acting on the volcano.     

Resistivity and self-potential changes associated with volcanic activity: The July 8, 2000 Miyake-jima eruption (Japan)

The Miyake-jima volcano abruptly erupted on July 8, 2000 after 17 years of quiet and gave birth to a crater, 1 km in diameter and 250 m deep. This expected unrest was monitored during the years 1995–2000 by electromagnetic methods including DC resistivity measurements and self-potential (SP) surveys. Beneath the 2500 yr old Hatcho-Taira summit caldera audio-magnetotelluric soundings made in 1997–98 identified a conductive medium, 200–500 m thick (within the 50 Ω m isoline) located at a few hundred metres depth. It was associated with the active steady-state hydrothermal system centred close to the 1940 cone and extending southward. A DC resistivity meter set in a Schlumberger array with 600, 1000 and 1400 m long injection lines evidenced strong resistivity changes between September 1999 and July 3, 2000 in the vicinity of the newly formed crater. The apparent resistivity has reached about three times its initial values on the 1400 m long line and has lowered to about 20% on the 600 m line. Just prior to the July 8, 2000 eruption SP mapping made inside the summit Hatcho-Taira caldera revealed negative anomalies where positive ones had occurred during the previous tens of years. The largest negative anomaly, −225 mV in amplitude, mainly took place above the 1940 cone which collapsed in the crater formation. A permanent 1 km long SP line across the caldera suggests accelerating changes during the 3 months preceding the eruption. On a larger scale, the comparison between 1995 and 2000 surveys has shown a global increase of the hydrothermal activity beneath the volcano. Its source could have been 250 m to the south of the crater. These observations suggest that the hydrothermal system was slowly disturbed in the months preceding the eruption while drastic changes have occurred during the 2 weeks before the summit collapse when tectonic and volcanic swarms have appeared.     

Petrologic evidence of a complex plumbing system feeding the July–August 2001 eruption of Mt. Etna,Sicily, Italy

After the major 1991–1993 eruption, Mt. Etna resumed flank activity in July 2001 through a complex system of eruptive fissures cutting the NE and the S flanks of the volcano and feeding effusive activity, fire fountains, Strombolian and minor phreatomagmatic explosions. Throughout the eruption, magmas with different petrography and composition were erupted. The vents higher than 2,600 m a.s.l. (hereafter Upper vents, UV) erupted porphyritic, plagioclase-rich trachybasalt, typical of present-day summit and flank activity. Differently, the vents located at 2,550 and 2,100 m a.s.l. (hereafter Lower vents, LV) produced slightly more primitive trachybasalt dominated by large clinopyroxene, olivine and uncommon minerals for Etna such as amphibole, apatite and orthopyroxene and containing siliceous and cognate xenoliths. Petrologic investigations carried out on samples collected throughout the eruption provided insights into one of the most intriguing aspects of the 2001 activity, namely the coeval occurrence of distinct magmas. We interpret this evidence as the result of a complex plumbing system. It consists in two separate magma storage systems: a shallow one feeding the activity of the UV and a deeper and more complex storage related to the activity of LV. In this deep storage zone, which is thermally and compositionally zoned, the favourable conditions allow the crystallization of amphibole and the occurrence of cognate xenoliths representing wall cumulates. Throughout 2001 eruption, UV and LV magmas remain clearly distinct and ascended following different paths, ruling out the occurrence of mixing processes between them. Furthermore, integrating the 2001 eruption in the framework of summit activity occurring since 1995, we propose that the 2001 magma feeding the vents lower than 2,600 m a.s.l. is a precursor of a refilling event, which reached its peak during the 2002–2003 Etna flank eruption.     

Upwards migration of seismic focii: A forerunner of the 1989 eruption of Mt Etna (Italy)

Data from a portable array of three-component digital stations, run at Mt Etna from 1988 to early 1990, highlight the seismic behaviour of the volcano before the 1989 eruption, one of the most significant in terms of energy of the last two decades. After a two-year period of weak and discotinuous seismicity, the depth of the seismically active volumes was observed to become shallower a few months before the volcanic event. The overall migration of the events, inferred by hypocentral locations and decreases of S-P time differences at two stations, agrees with other geophysical forerunners and allows further insights into the changes in the stress field leading to the eruption.     

Coseismic Damage at an Archaeological Site in Sicily,Italy: Evidence of Roman Age Earthquake Surface Faulting

Archaeoseismology can provide a useful chronological tool for constraining earthquakes and documenting significant evidence that would otherwise be lost. In this paper, we report a case of surface faulting on ancient man-made structures belonging to the archaeological site of Santa Venera al Pozzo situated along the eastern flank of Mt. Etna volcano in eastern Sicily (southern Italy), which is affected by well-developed tectonic faults. Geological surveys highlight a set of fractures affecting the archaeological ruins, suggesting the occurrence of a capable fault zone across the area. An integrated geophysical survey was carried out in order to identify the main subsurface tectonic discontinuity ascribable to the fault zone. The information derived from different geophysical techniques, such as electrical resistivity tomography, seismic refraction tomography, ground-penetrating radar, and magnetic surveys allowed us to infer that the fractures observed at the surface could have been produced by coseismic rupture. They are conceivably linked to a strong earthquake that probably occurred in the Roman period, around mid-end of the third-century AD; time constraints are inferred through the dating of buildings of the archaeological site.     

Review of seismological studies at Mount Etna

This paper reports the present state of seismological research at Mt. Etna.A schematic classification of the earthquakes that occur on the volcano is proposed, based on both seismogram and spectrum features.We have made both focal solutions and estimates of earthquake source parameters (stress drop values between 2 and 20 bars and small source dimensions).The crust of Etna thus appears as an extremely heterogeneous medium that does not permit great stress accumulation. The coexistence of an extensional regime with an older and deeper compressive one seems confirmed at depths greater than about 7 km.Eruptive and seismic phenomena occur mainly along the principal structural trends of the volcano, but often the directions of the eruptive fractures and the earthquake concentration during the same eruption do not coincide.Tectonics seem to play an important role in controlling seismo-volcanic behaviour.     

Elastic model for the gravity and elevation changes before the 2001 eruption of Etna volcano

For 5 months before the 2001 Mt. Etna eruption, a progressive gravity decrease was measured along a profile of stations on the southern slope of the volcano. Between January and July 2001, the amplitude of the change reached 80 μGal, while the wavelength of the anomaly was of the order of 15 km. Elevation changes observed through GPS measurements during a period encompassing the 5-month gravity decrease, remained within 4–6 cm over the entire volcano and within 2–4 cm in the zone covered by the microgravity profile. We review both gravity and elevation changes by a model assuming the formation of new cracks, uniformly distributed in a rectangular prism. The inversion problem was formulated following a global optimization approach based on the use of Genetic Algorithms. Although it is possible to explain the observed gravity changes by means of the proposed analytical formulation, the results show that calculated elevation changes are significantly higher than those observed. Two alternative hypotheses are proposed to account for this apparent discrepancy: (1) that the assumptions behind the analytical formulation, used to invert the data, are fallacious at Etna, and thus, numerical models should be utilized; (2) that a second process, enabling a considerable mass decrease to occur without deformation, acted together with the formation of new cracks in the source volume.