Three-dimensional velocity structure and seismicity of Mt. Etna Volcano, Italy

A new set of three-dimensional velocity models beneath Mt. Etna volcano is derived in the present work. We have used P- and S-wave arrivals from local earthquakes recorded at permanent and temporary seismic networks installed since 1980. A set of 1249 earthquakes recorded at more than four seismic stations was selected for traveltime inversion. The velocity models obtained by using different data selection criteria and parametrization display similar basic features, showing a high P-wave velocity at shallow depth in the SE quadrant, in close connection with a high gravimetric Bouguer anomaly. This area shares a low V_p/V_s ratio. High P-wave velocities and high V_p/V_s ratios are obtained along the central conduits, suggesting the presence of dense, intrusive magmatic bodies extending to a depth of about 20 km. The central intrusive core is surrounded by lower P-wave velocities. The relocated earthquake hypocenters also display the presence of an outward dipping brittle region, away from the central conduits, surrounding a ductile zone spatially related to the high P-wave velocity anomalies located in proximity to the central craters.     

Analysis of long-period events recorded at Mount Etna (Italy) in 1992, and their relationship to eruptive activity

Seismic activity recorded at Mount Etna during 1992 was characterized by long-period (LP) events and tremor with fluctuating amplitudes. These signals were associated with the evolution of the eruptive activity that began on December 14, 1991. Following the occurrence of numerous volcano-tectonic earthquakes at the onset of the eruption, LP events dominated the overall seismicity starting in January, 1992. The LP activity occurred primarily in swarms, which were temporally correlated with episodic collapses of the crater floor in the Northeast Crater. Source depths determined for selected LP events suggest a source region located slightly east of Northeast Crater and extending from the surface to a depth of 2000 m. Based on the characteristic signatures of the time series, four families of LP events are identified. Each family shares common spectral peaks independent of azimuth and distance to the source. These spectral features are used to develop a fluid-filled crack model of the source. We hypothesize that the locus of the LP events represents a segment of the magma feeding system connecting a depressurizing magma body with a dike extending in the SSE direction along the western wall of Valle del Bove, toward the site of the Mount Etna eruption. We surmise that magma withdrawal from the source volume beneath Northeast Crater may have caused repeated collapses of the crater floor. Some collapse events may have produced pressure transients in the subjacent dike which acted as seismic wave sources for LP events.     

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 .     

3-D density model of Mt. Etna Volcano (Southern Italy)

A detailed density model of Mt. Etna and its surrounding areas has been evaluated using a 3-D inversion of the gravimetric data acquired in the 1980's. Several high-density and low-density bodies are found, penetrating from shallow depths as far down as 12 km bsl. A positive correlation (in terms of location, extent, density, and velocity) is established between several anomalies of the density model and features identified in previously published seismic tomographies. A prominent high-density body extending down to 7 km bsl is recognized in the southern part of the Valle del Bove, and interpreted as a solidified magmatic intrusion. On the western boundary of this anomaly, a low-density body is interpreted as a bubble and liquid magma mixture. Outside the central area, three other high-density anomalies are imaged and attributed to the earliest phases of volcanic activity in the area. Several interesting low-density anomalies are also identified and correlated with known fault lines and other structural features of the region.     

Time variant analysis of geomagnetic signals describes the volcanic activity of Mt. Etna between early 2000 and late 2002

Volcanomagnetic anomalies have been mostly observed during strong eruptions. Our aim is to improve the geomagnetic data analysis to evidence the anomalies occurring in a larger time span, especially in the phases preceding the eruptive events. We developed a time variant statistical approach and applied it to the 2000–2002 Etna geomagnetic temporal series. It is based on an algorithm that statistically predicts the geomagnetic field at the station on the volcanic edifice by that recorded at the remote one. In such a way a number of significant changes in the time series (called statistical innovations), marking the local magnetic field change, were detected. The distribution of such statistical innovations accurately describes the Etna volcanic evolution: we note a progressive increase of the innovation occurrence as the eruptive cycles were approaching and only few and weak innovations at times between the various eruptive cycles. The significance of this analysis is further confirmed by the close agreement among the mean square prediction error, strain release and the volcanic activity behavior. On the contrary, the geomagnetic field at a single station or its difference at two stations do not have any clear correlation with other measured physical quantities. The complex pattern of the prediction error was also investigated by a multifractal analysis. We found that the Holder regularity increases with the intensification of the volcanic activity, implying that innovations tend to be less sporadic and correlated during the major volcanic phases.     

Combined discrete and continuous gravity observations at Mount Etna

Systematic investigation of discrete gravity measurements has continued at Mount Etna since 1986. The network now covers an area of 400 km² with about 70 stations 0.5–3 km apart. Mass redistributions occurring at depths ranging between about 8 km below sea level and a few hundred metres below the surface (magma level changes within the shallower parts of the feeding conduits) have been identified from these data. Conventional (discrete) microgravity monitoring on a network of stations furnishes only instantaneous states of the mass distribution at continuously active systems. In order to obtain information on the rate at which the volcanic processes (and thus mass transfers) occur, three stations for continuously recording gravity where installed on Mount Etna in 1998. A 16-month long sequence from one of the continuously running stations (PDN, located 2 km from the active northeast crater at the summit of Etna volcano) is presented. After removing the effects of Earth Tide and tilt, the correlation of the residual gravity sequence with simultaneous recordings of meteorological parameters acquired at the same station was analysed. Once the meteorological effects have also been removed, continuous gravity changes are within 10 μGal of gravity changes measured using conventional microgravity observations at sites very close to the continuous station. This example shows how discrete and continuous gravity observations can be used together at active volcanoes to get a fuller and more accurate picture of the spatial and temporal characteristics of volcanic processes.     

Seismogenic stress field beneath Mt. Etna (South Italy) and possible relationships with volcano-tectonic features

Shallow shear-type seismic activity occurring beneath the Etna volcano during 1990–1995 has been analysed for hypocenter locations, focal mechanisms and stress tensor inversion. The results have been examined jointly with Electronic Distance Measurements and tiltmeter data collected in the same period and reported in the literature. Significant seismicity located in the upper 10 km was found to be confined to the time intervals in which ground deformation data indicated inflation of the volcano edifice (e.g., the periods preceding the December 1991–March 1993 and August 1995–March 1996 eruptive phases). The shocks mostly occurred in a sector approximately centered on the crater area and elongated in the East–West direction. The causative seismogenic stress shows a low-dip East–West orientation of σ₁. In agreement with existing knowledge on relationships between local fault systems and magma uprise processes, the shallow seismicity in question is tentatively explained as being due to lateral compression by magma inside a nearly North–South system. The volcano deflation phase revealed by Electronic Distance Measurements and tilt data during the 1991–1993 major eruption was not accompanied by any significant shear-type shallow event. Below the depth of 10 km, the North–South prevailing orientation of σ₁ reflects the dominant role of the regional stress.     

新疆泥火山群地震前兆异常实时监测与预报的研究

基于网络技术的视频监控服务,实现了对新疆北天山地区3个泥火山点的实时监测,可在线实时查看泥火山活动情况,分析预报人员依据泥火山活动图像可开展地震预测研究。新疆艾其沟泥火山网络视频监控服务系统扑捉到了2次6级地震前火山液面明显的宏观异常变化现象,这说明基于宽带网络技术的网络视频监控服务,可实现互联网用户使用客户端远程软件连接服务器,实现在线实时查看泥火山活动情况的监控画面,并依据泥火山群地震观测网,捕捉泥火山群地震前兆异常。     

High precision tilt observation at Mt. Etna Volcano,Italy

In 2007–2008, we installed on Mt. Etna two deep tilt stations using high resolution, self-leveling instruments. These installations are a result of accurate instrument tests, site selection, drilling and sensor positioning that has allowed detecting variations related to the principal diurnal and semidiurnal tides for first time on Mt. Etna using tilt data.     

Source mechanism of long-period events at Kusatsu–Shirane Volcano, Japan, inferred from waveform inversion of the effective excitation functions

We investigate the source mechanism of long-period (LP) events observed at Kusatsu–Shirane Volcano, Japan, based on waveform inversions of their effective excitation functions. The effective excitation function, which represents the apparent excitation observed at individual receivers, is estimated by applying an autoregressive filter to the LP waveform. Assuming a point source, we apply this method to seven LP events the waveforms of which are characterized by simple decaying and nearly monochromatic oscillations with frequency in the range 1–3 Hz. The results of the waveform inversions show dominant volumetric change components accompanied by single force components, common to all the events analyzed, and suggesting a repeated activation of a sub-horizontal crack located 300 m beneath the summit crater lakes. Based on these results, we propose a model of the source process of LP seismicity, in which a gradual buildup of steam pressure in a hydrothermal crack in response to magmatic heat causes repeated discharges of steam from the crack. The rapid discharge of fluid causes the collapse of the fluid-filled crack and excites acoustic oscillations of the crack, which produce the characteristic waveforms observed in the LP events. The presence of a single force synchronous with the collapse of the crack is interpreted as the release of gravitational energy that occurs as the slug of steam ejected from the crack ascends toward the surface and is replaced by cooler water flowing downward in a fluid-filled conduit linking the crack and the base of the crater lake.     

Source process of very-long-period events accompanying long-period signals at Cotopaxi Volcano,Ecuador

Renewed seismic activity of Cotopaxi, Ecuador, began in January 2001 with the increased number of long-period (LP) events, followed by a swarm of volcano-tectonic (VT) earthquakes in November 2001. In late June 2002, the activity of very-long-period (VLP) (2 s) events accompanying LP (0.5–1 s) signals began beneath the volcano. The VLP waveform was characterized by an impulsive signature, which was accompanied by the LP signal showing non-harmonic oscillations. We observed temporal changes of both the VLP and LP signals from the beginning until September 2003: The VLP signal gradually disappeared and the LP signal characterized by decaying harmonic oscillations became dominant. Assuming possible source geometries, we applied a waveform inversion method to the observed waveforms of the largest VLP event. Our inversion and particle motion analyses point to volumetric changes of a sub-vertical crack as the VLP source, which is located at a depth of 2–3 km beneath the northeastern flank. The spectral analysis of the decaying harmonic oscillations of LP events shows frequencies between 2.0 and 3.5 Hz, with quality factors significantly above 100. The increased VT activity and deformation data suggest an intrusion of magma beneath the volcano. A release of gases with small magma particles may have repetitively occurred due to the pressurization, which was caused by sustained bubble growth at the magma ceiling. The released particle-laden gases opened a crack above the magma system and triggered the resonance of the crack. We interpret the VLP and LP events as the gas-release process and the resonance of the crack, respectively.     

Cross-correlation analysis of seismic and volcanic data at Mt Etna volcano,Italy

Seismic data from the MVT-SLN sesmic station located 7 km from the summit area of Mt Etna volcano, which has been operating steadily for the last two decades, have been analysed together with the volcanic activity during the same period. Cross-correlation techniques are used to investigate possible relationships between seismic and volcanic data and to evaluate the statistical significance of the results. A number of significant correlations have been identified, showing that there is an evident relation between seismic events and flank eruptions, and a less clear relation with summit activity, which appears more linked to tremor rather than to the low-frequency events. Particularly interesting are the low-frequency events whose rate of occurrence increases, starting from 17 to 108 days, prior to the onset of the flank activity and are candidates for a useful precursor. On the other hand, a tendency towards the increase in both the duration and the occurrence rate of transients in the volcanic tremor was observed before the onset of summit eruptions. As a result of this study different stages in the volcanic activity of Mt Etna, represented by changes in the characteristics of the recorded seismic phenomena, are identified.     

Geochemistry of gases and waters discharged by the mud volcanoes at Paternò, Mt. Etna (Italy)

 Approximately 20 km south of Mt. Etna craters, at the contact between volcanic and sedimentary formations, three mud volcanoes discharge CO₂-rich gases and Na–Cl brines. The compositions of gas and liquid phases indicate that they are fed by a hydrothermal system for which temperatures of 100–150  °C were estimated by means of both gas and solute geothermometry. The hydrothermal system may be associated with CO₂-rich groundwaters over a large area extending from the central part of Etna to the mud volcanoes. Numerous data on the He, CH₄, CO₂ composition of the gases of the three manifestations, sampled over the past 5 years, indicate clearly that variations are due to separation processes of a CO₂-rich gas phase from the liquid. The effects of these processes have to be taken into account in the interpretation of the monitoring data collected for the geochemical surveillance of Etna volcano. Received: 4 September 1995 / Accepted: 14 February 1996     

The exceptional activity and growth of the Southeast Crater, Mount Etna (Italy), between 1996 and 2001

Between 1971 and 2001, the Southeast Crater was the most productive of the four summit craters of Mount Etna, with activity that can be compared, on a global scale, to the opening phases of the Pu‘u ‘Ō‘ō-Kūpaianaha eruption of Kīlauea volcano, Hawai‘i. The period of highest eruptive rate was between 1996 and 2001, when near-continuous activity occurred in five phases. These were characterized by a wide range of eruptive styles and intensities from quiet, non-explosive lava emission to brief, violent lava-fountaining episodes. Much of the cone growth occurred during these fountaining episodes, totaling 105 events. Many showed complex dynamics such as different eruptive styles at multiple vents, and resulted in the growth of minor edifices on the flanks of the Southeast Crater cone. Small pyroclastic flows were produced during some of the eruptive episodes, when oblique tephra jets showered the steep flanks of the cone with hot bombs and scoriae. Fluctuations in the eruptive style and eruption rates were controlled by a complex interplay between changes in the conduit geometry (including the growth of a shallow magma reservoir under the Southeast Crater), magma supply rates, and flank instability. During this period, volume calculations were made with the aid of GIS and image analysis of video footage obtained by a monitoring telecamera. Between 1996 and 2001, the bulk volume of the cone increased by ~36×10⁶ m³, giving a total (1971–2001) volume of ~72×10⁶ m³. At the same time, the cone gained ~105 m in height, reaching an elevation of about 3,300 m. The total DRE volume of the 1996–2001 products was ~90×10⁶m³. This mostly comprised lava flows (72×10⁶ m³) erupted at the summit and onto the flanks of the cone. These values indicate that the productivity of the Southeast Crater increased fourfold during 1996–2001 with respect to the previous 25 years, coinciding with a general increase in the eruptive output rates and eruption intensity at Etna. This phase of intense summit activity has been followed, since the summer of 2001, by a period of increased structural instability of the volcano, marked by a series of important flank eruptions.     

New formulae for estimating lava flow volumes at Mt. Etna Volcano, Sicily

 A new data set of Etna lava flows erupted since 1868 has been compiled from eight topographic maps of the volcano published at intervals since then. Volumes of 59 flows or groups of flows were measured from topographic difference maps. Most of these volumes are likely to be considerably more accurate than those published previously. We cut the number of flow volumes down to 25 by selecting those examples for which the volume of an individual eruption could be derived with the highest accuracy. This refined data set was searched for high correlations between flow volume and more directly measurable parameters. Only two parameters showed a correlation coefficient of 70% or greater: planimetric flow area A (70%) and duration of the eruption D (79%). If only short duration (<18 days) flows were used, flow length cubed, L³, had a correlation coefficient of 98%. Using combinations of measured parameters, much more significant correlations with volume were found. Dh had a correlation coefficient of 90% (h is the hydrostatic head of magma above the vent), and  , 92% (where W is mean width and E is the degree of topographic enclosure), and a combination of the two , 97%. These latter formulae were used to derive volumes of all eruptions back to 1868 to compare with those from the complete data set. Values determined from the formulae were, on average, lower by 16% (Dh), 7% (, and 19% . Received: 30 November 1998 / Accepted: 20 June 1999     

Major-ion bulk deposition around an active volcano (Mt. Etna, Italy)

Bulk atmospheric deposition of major cations (Na, K, Ca, Mg) and anions (Cl, F, SO₄) were measured at 15 sites around an active volcano, Mount Etna, from 2001 to 2003. Their composition indicates several natural sources, among which deposition of plume-derived volcanogenic gas compounds is prevalent for F, Cl and S. Plume-derived acidic compounds are also responsible for the prevailing acidic composition of the samples collected on the summit of the volcano (pH in the 2.45–5.57 range). Cation species have complex origin, including deposition of plume volcanogenic ash and aerosols and soil-dust wind re-suspension of either volcanic or carbonate sedimentary rocks. Variation of the deposition rates during the March 2001–March 2003 period, coupled with previous measurements from 1997 to 2000 (Appl Geochem 16:985–1000, 2001), were compared with the variation of SO₂ flux, volcanic activity and rainfall. The deposition rate was mainly controlled by rainfall. Commonly, about 0.1–0.9% of HF, HCl and SO₂ emitted by the summit crater's plume were deposited around the volcano. We estimate that ∼2 Gg of volcanogenic sulphur were deposited over the Etnean area during the 2002–2003 flank eruption, at an average rate of ∼24 Mg day⁻¹ which is two orders of magnitude higher than that typical of quiescent degassing phases.     

Intrusive Mechanisms at Mt. Etna Forerunning the July-August 2001 Eruption from Seismic and Ground Deformation Data

In this work we present seismological and ground deformation evidence for the phase preparing the July 18 to August 9, 2001 flank eruption at Etna. The analysis performed, through data from the permanent seismic and ground deformation networks, highlighted a strong relationship between seismic strain release at depth and surface deformation. This joint analysis provided strong constraints on the magma rising mechanisms. We show that in the last ten years, after the 1991–1993 eruption, an overall accumulation of tension has affected the volcano. Then we investigate the months preceding the 2001 eruption. In particular, we analyse the strong seismic swarm on April 20–24, 2001, comprising more than 200 events (M_max = 3.6) with prevalent dextral shear fault mechanisms in the western flank. The swarm showed a ca. NE-SW earthquake alignment which, in agreement with previous cases, can be interpreted as the response of the medium to an intrusive process along the approximately NNW-SSE volcano-genetic trend. These mechanisms, leading to the July 18 to August 9, 2001 flank eruption, are analogous to ones observed some months before the 1991–1993 flank eruption and, more recently, in January 1998 before the February-November 1999 summit eruption.     

Activity of Mount Etna preceding the February 1999 fissure eruption: inferred mechanism from seismological and geochemical data

The February 1999 eruption of Mt. Etna took place through a fissure on the SSE flank of the cone of the summit SE Crater. This event was preceded by continuous activity since 1995, sometimes accompanied by violent outbursts from one or more of the three other summit craters (NE Crater, Voragine or Chasm, and Bocca Nuova), and finally by a series of 20 short-lived eruptions from the SE Crater between September 1998 and January 1999. These phenomena could be accounted for by invoking gradual invasion of a shallow small reservoir by more primitive, basic and gas-rich magma coming from depth. The shallow “chamber” is more likely to be a plexus of dikes, which had developed during the previous years (1995–1997), following variations of the local stress field owing to enhanced magma generation and accumulation at the top of the mantle. Magma injection and mixing is evidenced through geochemistry, whereas the state of stress of the volcanic pile and underlying crust is determined using earthquake distributions and focal mechanisms. The behaviour of the seismic tremor amplitude appears to be a good indicator of the state of unrest of the volcano, although not always directly linked to the relative energy of degassing phenomena.     

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.     

Statistical analysis of earthquake activity at Etna volcano (March 1981 eruption)

Seismic activity that preceded, accompanied, and followed the 17–23 March 1981 Etnean eruption has been statistically analyzed.On the grounds of both time evolution of seismicity and catalogue completeness, three time intervals have been defined (12 February–2 March, 12–17 March, 19–31 March) and for each of these periods both the b coefficient of theGutenberg-Richter's (1956) relationship and the E parameter of the cluster size (Shlien andToksoz, 1970) have been calculated.No significant variations were observed between the first and third periods, while lower values of bothb andE coefficients were found in the second one. These findings might indicate that changes in the seismicity features occur just before the eruption start.Small but fast variations in the stress field acting on the volcano might originate this type of seismic activity, while the importance of the tectonic control on volcanic phenomena seems to be confirmed.