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.     

Seismohydrological effects related to the NW Bohemia earthquake swarms of 2000 and 2008: Similarities and distinctions

For more than 20 years, seismohydrological investigations have been undertaken at the mineral aquifer system of Bad Brambach (Vogtland, Germany). Two strong swarm earthquake series in 2000–2001 and 2008–2009 at the Nový Kostel epicentre (Czech Republic, 10 km E of BB) have enabled for the first time a comparison of seismological and groundwater hydraulic features in a semi-quantitative way. In spite of their similar spatial distribution in 2001 and 2008, the earthquake foci of each swarm migrated differently through time, horizontally as well as in depth. The seismic energy of the 2008–2009 events was released predominantly within 1 month, in contrast to 2000–2001 when it occurred over 3 months. The main distinctive features of each are seen in the hydraulic pressure anomalies which accompanied the earthquake swarms: number, shape, and progression (duration) of the anomalies. The comprehensive hydraulic data, with high temporal resolution, suggest that fluid triggering dominated not only the earthquake initiating phases. In particular, the long-lasting seismicity of the 2008–2009 swarm can be attributed to a continued triggering of weak earthquakes by over-pressured deep fluids. Here, the remaining static strain was obviously not sufficient to generate strong earthquakes as at the beginning of the earthquake swarm periods. Furthermore, the enduring high fluid pressure in 2009 could also indicate a continuation of the long-term gas flow increase observed at several gas outlets in the Vogtland/NW Bohemia region between 1998 and 2008. However, it is not possible at present to derive a systematic relationship between anomaly occurrence and seismic activity, as generally proposed in the context of earthquake prediction discussion.     

Understanding the collapse–eruption link at Stromboli,Italy: A microanalytical study on the products of the recent Secche di Lazzaro phreatomagmatic activity

The Secche di Lazzaro (SDL) phreatomagmatic activity, with the associated Neostromboli sector collapse, represents the most powerful activity of the last 6 ky at Stromboli. As revealed by its present-day activity, Stromboli is one of the most eruptive volcanoes in Italy, and flank instability, along its NW flank, is a common process. Volcano instability is often dramatised by explosive eruptions, thus it is of crucial importance to understand the linking between volcano collapse and the plumbing system itself. The possible role of pre-eruptive magmatic processes as triggers of explosive eruptions can be mainly preserved by minerals and revealed by petrochemical studies. We studied the juvenile components (scoria and pumice) of the pyroclastic deposits from the SDL phreatomagmatic activity with the aim to understand the eruption–collapse link.     

Characterizing high energy explosive eruptions at Stromboli volcano using multidisciplinary data: An example from the 9 January 2005 explosion

Stromboli is well known for its persistent, normal explosive activity, consisting of intermittent, mild to moderate, Strombolian explosions that typically occur every 10–20 min. All tephras erupted during this activity usually fall back into the crater terrace, and consist of volatile-poor scoriae fed by Highly Porphyritic (HP) magma. More occasionally, large explosions or “paroxysms” eject a greater quantity of tephra, mainly consisting of HP scoriae and pumice clasts of Low Porphyritic (LP) magma, but also including large lithic blocks. In addition to this activity, between 2004 and 2006 high energy explosions, displaying an intermediate eruptive style between that of normal and paroxysmal explosions in terms of column height, duration and tephra dispersal, were observed to occur at a frequency of one to eight events per year. While many volcanological, geochemical and geophysical studies have focused in the last few years on the two end-members of activity, i.e. normal or paroxysmal, a detailed investigation on these intermediate types of events has not been carried out yet. Here we report of a study on the 9 January 2005 explosion, one of the high energy explosions during which the main fountaining phase lasted nearly a minute causing ejection of coarse bombs up to a height of 120 m, and of ash and lapilli to > 200 m. An accompanying ash plume rose up to 500 m at the end of the explosion. We present a multidisciplinary approach that integrates the results from analysis of live-camera images with compositional and textural characterization of the erupted products. Major element composition of glassy groundmass and 3D views of textures in the erupted scoriae support the hypothesis based on volcanological observations that this explosion falls between normal and paroxysmal activity, for which we use the term “intermediate”. By comparing the video-camera images of the 9 January 2005 explosion with volcanological features of other high energy explosions that occurred at Stromboli between June 2004 and October 2006, we find that three additional events can be considered intermediate explosions, suggesting that this type of activity may be fairly common on this volcano. The results of this study, although preliminary given our limited dataset, clearly indicate that the methodology used here can be successfully applied to better define the range of eruptive styles typifying the normal explosive activity, potentially improving our capability of eruption forecasting and assessing volcanic hazard at Stromboli.     

Explosion quakes at Stromboli (Italy)

This paper reports the results of two seismic experiments aimed at determining the wave field of explosion quakes at Stromboli Island (Mediterranean Sea, Southern Italy). The typical Strombolian activity mostly consists of explosive phenomena causing pyroclastic, materials to be emitted together with jets of volcanic gases from one or more craters. Stromboli is an active volcano characterized by persistent seismic activity consisting of explosion quakes that are seismic events associated with the explosive volcanic phenomena. Explosion quakes are short lived seismic events occurring intermittently whose amplitude tends to decrease with distance from the vent. A distinctive feature of explosion quakes is the presence on seismograms of two, often clearly distinct, seismic phases. The first, low-frequency seismic phase (<2 Hz) is in fact usually followed by a high-frequency seismic phase (>3–4 Hz) after one second or more. The first seismic phase of explosion quakes has been shown to be characterized by a nearly radial linear polarization and by an apparent propagation velocity estimated at 600–800 m/s. The second phase is characterized by a more chaotic motion and a lower apparent propagation velocity of 150–450 m/s. The wavefield associated with the first low-frequency seismic phase appears to be generated by a resonating P-wave seismic source accompanying gas explosion and emission of pyroclastic materials. The wavefield associated with the second high-frequency seismic phase of explosion quakes appears to be mainly composed of scattered and converted waves due to the critical topography of the volcano.     

Swarms of similar long-period earthquakes in the mantle beneath Mauna Loa Volcano

We present analyses of two swarms of long-period (LP) earthquakes at > 30 km depth that accompanied the geodetically observed 2002–2005 Mauna Loa intrusion. The first LP earthquake swarm in 2002 consisted of 31 events that were precursory and preceded the start of Mauna Loa inflation; the second LP swarm of two thousand events occurred from 2004–2005. The rate of LP earthquakes slowed significantly coincident with the occurrence of the December 26, 2004 M_w 9.3 Sumatra earthquake, suggesting that the seismic waves from this great earthquake may have had a dynamic triggering effect on the behavior of Mauna Loa's deep magma system. Using waveform cross correlation and double difference relocation, we find that a large number of earthquakes in each swarm are weakly similar and can be classified into two families. The relocated hypocenters for each family collapse to compact point source regions almost directly beneath the Mauna Loa intrusion. We suggest that the observed waveform characteristics are compatible with each family being associated with the resonance of a single fluid filled vertical crack of fixed geometry, with differences in waveforms between events being produced by slight variations in the trigger mechanism. If these LP earthquakes are part of the primary magma system that fed the 2002–2005 intrusion, as indicated by the spatial and temporal associations between mantle seismicity and surface deformation, then our results raise the possibility that this magma system may be quite focused at these depths as opposed to being a diffuse network. It is likely that only a few locations of Mauna Loa's deep magma system met the geometric and fluid dynamic conditions for generating LP earthquakes that were large enough to be recorded at the surface, and that much of the deep magma transfer associated with the 2002–2005 intrusion occurred aseismically.     

Did the 1999 earthquake swarm on Gakkel Ridge open a volcanic conduit? A detailed teleseismic data analysis

In 1999, a seismic swarm of 237 teleseismically recorded events marked a submarine eruption along the Arctic Gakkel Ridge, later on also analyzed by sonar, bathymetric, hydrothermal, and local seismic studies. We relocated the swarm with the global location algorithm HYPOSAT and analyzed the waveforms of the stations closest to the events by cross-correlation. We find event locations scattered around 85°35^′ N and 85° E at the southern rift wall and inside the rift valley of the Gakkel Ridge. Waveforms of three highly correlating events indicate a volumetric moment tensor component and highly precise referenced double-difference arrival times lead us to believe that they occur at the same geographical position and mark the conduit located further southeast close to a chain of recently imaged volcanic cones. This result is supported by station residual anomalies in the direction of the potential conduit. Seismicity is focused at the crust–mantle boundary at 16–20 km depth, but ascending toward the potential conduit during the beginning of April 1999, indicating an opening of the vent.     

Anomalie magnetiche e declinazione in quota in Italia con speciale riguardo per la navigazione aerea strumentale

Riassunto Con i procedimenti del Malkin, verificati sperimentalmente dal Ramsayer, viene intrapreso lo studio dell'influsso sulla declinazione in quota di alcune anomalie magnetiche in Italia, per cui esistono rilievi di dettaglio nella componente verticale (Colli Euganei, Stromboli). Per le zone anomale, capaci di influenzare il mantenimento della rotta nella navigazione aerea con la bussola, vengono proposte carte speciali con l'andamento delle isogone a varie quote fondamentali, di cui si presenta un saggio per l'isola di Stromboli. Attraverso un esame delle anomalie magnetiche in Italia, si indicano le zone di probabile e quelle di sicuro pericolo per la navigazione aerea con la bussola finchè non si potrà costruire per esse carte del tipo proposto. Per il rimanente spazio aereo italiano si prospetta l'opportunità che le isogone per i piloti siano costruite sulla base del campo normale.

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Summary Applying the proceedings of Malkin verified by experiments of Ramsayer is studied the influence on the declination in upper levels of some magnetic anomalies in Italy, for which exist detailed reliefs in the vertical component (Colli Euganei, Stromboli). For the anomalous zones capable to influence the maintaining of the route in compass-airnavigation special maps are proposed containing the position of the isogons at various fondamental heights, an instance of which is presented for the isle of Stromboli. An examination of the magnetic anomalies in Italy shows the zones of probable and those of secure danger for compass-airnavigation as long as it will not be possible to prepare maps in the proposed type. For the rest of the Italian air-space the opportunity is prospected to construct the isogons for the pilots on the basis of the normal field.

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Zusammenfassung Nach dem Malkinschen Verfahren wird der Einfluss einiger magnetischer Störgebiete (Colli Euganei, Stromboli) auf die Flugnavigation berechnet. Es wird vorgeschlagen, wo nötig, spezielle Deklinationskarten mit den Isogonen zu verschiedenen Haupthöhen vorzubereiten; ein Beispiel hiefür wird für die Insel Stromboli gegeben. Die Störgebiete, welche eine Gefahr für die Flugnavigation mit dem Magnetkompass bilden können, werden für Italien begrenzt. In dem übrigen Raum in Italien wird als das beste Verfahren für die Flugnavigation vorgeschlagen, die Isogonen nach dem vom Verf. berechneten Normalfeld darzustellen.

     

Long-term observation of volcanic tremor on Stromboli volcano (Italy): A synopsis

The features of seismic activity on Stromboli are discussed and compared in terms of their relationship with the main changes of volcanic activity from 1990 to 1993.We considered a statistical approach for our data analysis. Cluster analysis was used to seek out classes of spectra which might characterize the condition of the volcanic system. The classes we have found provide insights into a scenario which evolves through different phases of volcanic activity, from paroxysms to low activity. We show that episodes of lava effusion and lava fountaining are heralded by variations in the spectral features of tremor after a preparation time. This result highlights the importance of tremor, and reveals that long-term observations are key to examine slow modifications in a volcanic system such as Stromboli, characterized by open conduits, and persistent explosive activity.     

Automatic Recognition of Landslides Based on Neural Network Analysis of Seismic Signals: An Application to the Monitoring of Stromboli Volcano (Southern Italy)

In the last 9 years, the amount and the quality of geophysical and volcanological observations of Stromboli's' activity have undergone a marked increase. This new information highlighted that the landslides on the Sciara del Fuoco flank are tightly linked to the volcanic activity. Actually, at the beginning of the December 28, 2002, effusive eruption, the seismic monitoring network was less dense than now, and therefore it is not known if there was an increase in the landslide rate before the eruption. Despite this, it is known that a big landslide occurred 2 days after the beginning of the eruption which caused a tsunami (December 30, 2002). More recently, the effusive eruption in February 2007 was preceded by an increase in landslides on the Sciara del Fuoco flank, which were recorded by the seismological monitoring system that had been improved after the 2002–2003 crisis. These episodes led us to believe that monitoring the Sciara del Fuoco flank instability is an important topic, and that landslides might be significant short-term precursors of effusive eruptions at the Stromboli volcano. To automatically detect landslide signals, we have developed a specialized neural algorithm. This can distinguish between landslides and the other types of seismic signals usually recorded at the Stromboli volcano (i.e., explosion quakes and volcanic tremor). The discrimination results show an average performance of 98.67 %. According to the experience of the crisis of 2007, to identify changes that can be considered as precursors of effusive eruptions, we set up an automatic decision-making method based on the neural network responses. This method can operate on a continuous data stream. It calculates a landslide percentage index (LPI) that depends on the number of records that are classified by the net as landslides over a given time interval. We tested the method on February 27, 2007, including the beginning of the effusive phase. The index showed an increase as early as at 09:00 UTC on that day and reached its maximum value (100 %) at 12:00, about 40 min before the onset of the eruption. After the beginning of the effusive phase, the index remains high due to the blocks that roll down along the slope from the front of the lava flow. On the basis of these tests, we propose a decision-making method that is able to recognize a trend in the LPI similar to that of 2007 eruption, allowing the identification of precursors of effusive phases at the Stromboli volcano.     

A Warning System for Stromboli Volcano Based on Statistical Analysis

In this paper we describe a warning system based on statistical analysis for the purpose of monitoring ground deformation at the Sciara del Fuoco (Stromboli Volcano, Sicily). After a statistical analysis of ground deformation time-series measured at Stromboli by the monitoring system known as THEODOROS (THEOdolite and Distancemeter Robot Observatory of Stromboli), the paper describes the solution adopted for implementing the warning system. A robust statistical index has been defined in order to evaluate the movements of the area. A fuzzy approach has been proposed to evaluate an AI (Alarm Intensity) index which indicates the level of hazard of the Sciara del Fuoco sliding.     

Contributo al paleomagnetismo in Italia

Riassunto L'A. illustra come, sotto determinate condizioni, i rilevamenti geomagnetici, in grado di fornirci gli assi principali di polarizzazione magnetica delle masse rocciose, consentano di dedurre elementi attendibili sulla direzione del campo geomagnetico nelle varie epoche geologiche. Esempi vengono date per diverse colate laviche dello Stromboli e per alcuni adunamenti magnetitiferi.

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Summary Under suitable conditions, the magnetic polarization of igneous rockmasses and also the directions of their natural remanent magnetization for paleomagnetic purposes are determinable by geomagnetic surveys. Examples are given for different lava flows at Stromboli and for some magnetite ore-bodies.

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Relazione presentata il 12 Aprile 1958 alla Sesta Assemblea Generale della «Società Italiana di Geofisica e Meteorologia» (Genova: 10–12 Aprile 1958).     

The 2008 West Bohemia earthquake swarm in the light of the WEBNET network

A swarm of earthquakes of magnitudes up to M _L = 3.8 stroke the region of West Bohemia/Vogtland (border area between Czechia and Germany) in October 2008. It occurred in the Novy Kostel focal zone, where also all recent earthquake swarms (1985/1986, 1997, and 2000) took place, and was striking by a fast sequence of macroseismically observed earthquakes. We present the basic characteristics of this swarm based on the observations of a local network WEBNET (West Bohemia seismic network), which has been operated in the epicentral area, on the Czech territory. The swarm was recorded by 13 to 23 permanent and mobile WEBNET stations surrounding the swarm epicenters. In addition, a part of the swarm was also recorded by strong-motion accelerometers, which represent the first true accelerograms of the swarm earthquakes in the region. The peak ground acceleration reached 0.65 m/s². A comparison with previous earthquake swarms indicates that the total seismic moments released during the 1985/1986 and 2008 swarms are similar, of about 4E16 Nm, and that they represent the two largest swarms that occurred in the West Bohemia/ Vogtland region since the M _L = 5.0 swarm of 1908. Characteristic features of the 2008 swarm are its short duration (4 weeks) and rapidity and, consequently, the fastest seismic moment release compared to previous swarms. Up to 25,000 events in the magnitude range of 0.5 < M _L < 3.8 were detected using an automatic picker. A total of nine swarm phases can be distinguished in the swarm, five of them exceeding the magnitude level of 2.5. The magnitude–frequency distribution of the complete 2008 swarm activity shows a b value close to 1. The swarm hypocenters fall precisely on the same fault portion of the Novy Kostel focal zone that was activated by the 2000 swarm (M _L ≤ 3.2) in a depth interval from 6 to 11 km and also by the 1985/1986 swarm (M _L ≤ 4.6). The steeply dipping fault planes of the 2000 and 2008 swarms seem to be identical considering the location error of about 100 m. Furthermore, focal mechanisms of the 2008 swarm are identical with those of the 2000 swarm, both matching an average strike of 170° and dip of 80° of the activated fault segment. An overall upward migration of activity is observed with first events at the bottom and last events at the top of the of the activated fault patch. Similarities in the activated fault area and in the seismic moments released during the three largest recent swarms enable to estimate the seismic potential of the focal zone. If the whole segment of the fault plane was activated simultaneously, it would represent an earthquake of M _L ~5. This is in good agreement with the estimates of the maximum magnitudes of earthquakes that occurred in the West Bohemia/Vogtland region in the past.     

Looking into a volcanic area: An overview on the 350 m scientific drilling at Colli Albani (Rome,Italy)

A 350 m deep borehole was drilled in the Colli Albani volcanic district (Central Italy) in order to: understand the shallow crust structure beneath the volcanic complex; characterize the rock physical properties especially through in-situ measurements and, afterward, laboratory experiments; assess the local present-day stress field; install a broad-band seismometer at depth. The borehole is located adjacent to the western rim of the Tuscolano–Artemisio caldera, where several phenomena of unrest recently occurred. In 1989–90 a seismic swarm affected this area and a related uplift was recognized. In addition, high gas concentrations (mainly CO₂ and H₂S), in aquifers and soils, caused illnesses and casualties among inhabitants and animals in the past.     

Evidence for gas influence on volcanic seismic signals recorded at Stromboli

The meaning of the large variety of seismic waveforms showing different frequencies associated with volcanic activity is still uncertain. No definitive model for source dynamics has been proposed yet. At present, seismic models explaining the origin of the spectral content of the seismic signal are mainly linked to geometrical features of the volcanic conduit and to resonant effects induced by pressure fluctuations in a fluid-filled conduit. Such models assume the physical system to be in a steady state over a long interval of time. At Stromboli, the seismicity produced by each of the three active vents is not stable and can suddenly change in time. Therefore, the application of stable resonator models appears to be unwarranted.On the basis of infrared image analyses, atmospheric pressure, and seismic wavefield signals, we show that different frequency contents and different explosive styles occur at Stromboli at the same time at different vents. Moreover, we give evidence to support the idea that seismicity and explosivity are both controlled by a variable gas flow regime released during explosions at the top of the magma column. We have recognized two main pressure regimes, controlling the explositivity at Stromboli: one is impulsive, short and possibly produced by a gas accumulation beneath a cap rock; the other has an intermittent regime feature, and lasts longer. We demonstrate how the first one generates a monochromatic low-frequency wavefield, while the second shows a high-frequency spectrum where the frequency content depends on pressure fluctuations and on pressure gradient. We suggest different pressure growth and gas flow in the magma as the common source for both explosive style and seismic wavefield.     

Effect of ground-motion asynchronism on the equivalent acceleration of earth dams

Under seismic loads the deformability of an earth dam may induce several effects, including ground-motion amplification and asynchronism between different points of the dam embankment. The paper analyses the asynchronous effects occurring in two existing earth dams, representing well-documented case histories: the El Infiernillo Dam (Mexico) and the Camastra Dam (Italy). Asynchronous effects are analysed by theoretical predictions of the dam seismic response by adopting an advanced dynamic approach, which takes into account the main features that dam soils exhibit under cyclic loading conditions. For different potentially unstable masses within the dam embankment, equivalent accelerations were computed as the ratio between the resultant of the inertial forces and the weight of the volume V associated to the unstable mass. With the exception of very cortical sliding surfaces – not significant for dam stability – in most of the analysed cases the equivalent seismic coefficients do not exceed the peak acceleration at the dam base.     

Eruption dynamics of the SW crater of Stromboli volcano,Italy — An interdisciplinary approach

We report on the evaluation of in situ measurements of eruption velocities and relative mass flux, collected by Doppler radar, as well as acoustic and infrared data, that were recorded at Stromboli volcano. Doppler radar observations were made alongside thermal, acoustic and seismic measurements, to (1) further investigate the complex waveforms of the SW crater by combining infrasonic, infrared and Doppler radar measurements, (2) establish a relationship between infrared, acoustic and Doppler radar measurements and (3) verify that all instruments observe the same behavior in terms of relative mass eruption rate. We also explore the relationship between kinetic and acoustic energy released during an eruption. Comparing the different methods to each other we are able to show that the mass erupted can be estimated either from the total reflected Doppler radar energy or from infrared observations. However, neither thermal nor reflected energy can provide a value in terms of absolute mass in kg. The erupted masses of different eruptions can only be evaluated relative to each other. Using the combined three data sets we especially focus on the eruption dynamics of the SW crater of Stromboli, namely its fluctuations in eruption strength. These pulses in one eruptive event, so called ’pulsations’ dominate more than 40% of the eruptions of SW crater. Previous models that explain pulsations to be generated by multiple consecutively exploding bubbles are supported by our combined analysis of infrasound, thermal data, particle velocities, and the reflected energy.     

A hybrid distinct element–boundary element approach for seismic analysis of cracked concrete gravity dam–reservoir systems

This paper proposes a new algorithm for modeling the nonlinear seismic behavior of fractured concrete gravity dams considering dam–reservoir interaction effects. In this algorithm, the cracked concrete gravity dam is modeled by distinct element (DE) method, which has been widely used for the analysis of blocky media. Dynamic response of the reservoir is obtained using boundary element (BE) method. Formulation and various computational aspects of the proposed staggered hybrid approach are thoroughly discussed. To the authors' knowledge, this is the first study of a hybrid DE–BE approach for seismic analysis of cracked gravity dam–reservoir systems. The validity of the algorithm is discussed by developing a two-dimensional computer code and comparing results obtained from the proposed hybrid DE–BE approach with those reported in the literature. For this purpose, a few problems of seismic excitations in frequency- and time-domains, are presented using the proposed approach. Present results agree well with the results from other numerical methods. Furthermore, the cracked Koyna Dam is analyzed, including dam–reservoir interaction effects with focus on the nonlinear behavior due to its top profile crack. Results of the present study are compared to available results in the literature in which the dam–reservoir interaction were simplified by added masses. It is shown that the nonlinear analysis that includes dam–reservoir interaction gives downstream sliding and rocking response patterns that are somehow different from that of the case when the dam–reservoir interaction is approximated employing added masses.     

Continuous monitoring of volcanic eruption dynamics: a review of various techniques and new results from a frequency-modulated radar Doppler system

 In situ measurement of volcanic eruption velocities is one of the great challenges left in geophysical volcanology. In this paper we report on a new radar Doppler technique for monitoring volcanic eruption velocities. In comparison with techniques employed previously (e.g., photographic methods or acoustic Doppler measurements), this method allows continuous recordings of volcanic eruptions even during poor visibility. Also, radar Doppler instruments are usually light weight and energy efficient, which makes them superior to other Doppler techniques based on laser light or sound. The proposed new technique was successfully tested at Stromboli Volcano in late 1996 during a period of low activity. The recorded data allow a clear distinction between particles rising from the vent and particles falling back towards the vent. The mean eruption velocity was approximately 10 m/s. Most of the eruptions recorded by radar were correlated to seismic recordings. The correlation between the magnitude of the volcanic shocks and the eruption force index defined in the paper may provide new insights into magma transport in the conduit. Received: 15 May 1998 / Accepted: 15 December 1998     

Analysis of infrasonic and seismic events related to the 1998 Vulcanian eruption at Sakurajima

We present results from a detailed analysis of seismic and infrasonic data recorded over a four day period prior to the Vulcanian eruptive event at Sakurajima volcano on May 19, 1998. Nearly one hundred seismic and infrasonic events were recorded on at least one of the nine seismic–infrasonic stations located within 3 km of the crater. Four unique seismic event types are recognized based on the spectral features of seismograms, including weak seismic tremor characterized by a 5–6 Hz peak mode that later shifted to 4–5 Hz. Long-period events are characterized by a short-duration, wide spectral band signal with an emergent, high-frequency onset followed by a wave coda lasting 15–20 s and a fundamental mode of 4.2–4.4 Hz. Values of Q for long-period events range between 10 and 22 suggesting that a gas-rich fluid was involved. Explosive events are the third seismic type, characterized by a narrow spectral band signal with an impulsive high-frequency onset followed by a 20–30 second wave coda and a peak mode of 4.0–4.4 Hz. Volcano-tectonic earthquakes are the fourth seismic type. Prior to May 19, 1998, only the tremor and explosion seismic events are found to have an infrasonic component. Like seismic tremor, infrasonic tremor is typically observed as a weak background signal. Explosive infrasonic events were recorded 10–15 s after the explosive seismic events and with audible explosions prior to May 19. On May 19, high-frequency impulsive infrasonic events occurred sporadically and as swarms within hours of the eruption. These infrasonic events are observed to be coincident with swarms of long-period seismic events. Video coverage during the seismic–infrasonic experiment recorded intermittent releases of gases and ash during times when seismic and acoustic events were recorded. The sequence of seismic and infrasonic events is interpreted as representing a gas-rich fluid moving through a series of cracks and conduits beneath the active summit crater.