Thermal, seismic and infrasonic evidences of variable degassing rates at Stromboli volcano

During June 1999, we measured the amplitude and rate (number of events per second) at which gas exited the vent at Stromboli volcano as discrete gas bursts or puffs. This allowed us to identify two styles of gas burst (puffing) activity. The first is characterized by frequent, rapidly rising puffs, the second by less frequent, slowly rising puffs. Each style persisted over 5–40-min-long durations and was associated with a high and low number of strombolian explosions per hour, respectively. Each period was also associated with characteristic delay times between the arrival of the infrasonic and thermal signals during strombolian explosions; the delays were longer during vigorous puffing periods. To explain our observations, we propose a model in which the degassing process cycles between vigorous and weak degassing phases. During vigorous degassing phases, bubble layers ascend the conduit at a frequency of 0.5–1.0 s⁻¹. This high degassing level reflects a gas-rich magma column and leads to an increased rate in the formation of shallow foams and, hence, an increase in puffing and explosive activity, as well as a higher free surface level and/or gas jet velocity. During weak phases, bubble layers ascend the conduit at a reduced frequency of 0.2–0.3 s⁻¹. During such times the magma column is poor in gas. This leads to a decreased rate of foam layer formation and hence a reduction in puffing and explosive activity, as well as a lower free surface level and/or gas jet velocity. Variations in puffing activity can thus be used to track changes in the rate at which the shallow system is supplied by fresh, gas-rich magma. Our observations indicate that the two degassing styles last from 5 to 40 min and that the switch from one to the other occurs over a matter of minutes.     

Palaeomagnetic data on the andesitic cover of the Sesia-Lanzo Zone (Western Alps)

The remanent magnetization of the andesitic cover of the Sesia-Lanzo Zone has been studied from 16 sites along three transversal sections. The remanent magnetization of the rock is stable and it appears to be primary after a conglomerate test. The mean direction of magnetization is consistent for all sites, but for one exception, with satisfactory values of statistical parameters. Therefore no deformation structures postdating the acquisition of remanent magnetization can be inferred from palaeomagnetic data. The mean direction of magnetization (11 sites, 152 specimens) is: D=135.9, I=–2.9, with ₉₅=8.8. No tectonic correction can yet be made.

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Zusammenfassung Die natürliche remanente Magnetisierung der Andesiten, die die Bedeckung der Sesia-Lanzo Zone bilden, ist an 16 Orten gemessen worden, die entlang drei Querdurchschnitten liegen. Das Gestein besitzt eine stabile NRM, die auf Grund eines Konglomeratstest die ursprüngliche sein müßte. Alle Orte, die annehmbare statistische Werte aufweisen, außer einem, haben die gleiche Magnetisierungsrichtung. Die paläomagnetischen Messungen zeigen keine Strukturen aufeinanderfolgender Deformation bei dem Magnetisierungsvorgang. Die durchschnittliche Magnetisierungsrichtung (11 Orte, 152 Gesteinsproben) ist: D = 135,9, I = –2,9, mit ₉₅ = 8,8. Keine tektonische Korrektur wird bisher eingerechnet.

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Résumé L'aimantation rémanente des andésites qui constituent la couverture de la Zone Sesia-Lanzo a été mesurée en 16 endroits disposés le long de trois sections transversales. La roche a une aimantation rémanente stable, qui, d'après le test du conglomérat, devrait être originelle. Tous les endroits qui présentent des valeurs statistiques acceptables ont, sauf un, la même direction d'aimantation. Les mesures paléomagnétiques ne révèlent donc pas de structures de déformation consécutive à l'acquisition de l'aimantation rémanente. La direction moyenne de l'aimantation (11 sites, 152 échantillons) est: D=135.9, I=–2.9 avec ₉₅=8.8. On n'a pas apporté, pour l'instant, de correction tectonique.

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, Sesia-Lanzo, 16- , . , , , . , , , . , . (11 , 152 ) : D=135,9; =–2,9 ₉₅=8,8. .

     

Magma supply, magma discharge and readjustment of the feeding system of mount St. Helens during 1980

The landslide and cataclysmic eruption of Mount St. Helens on May 18, 1980 triggered a sequence of explosive eruptions over the following five months. The volume of explosive products from each of these eruptions decreased uniformly over this period, and the character for each eruption progressed from a fairly continuous eruption lasting more than eight hours on May 18 to a series of short bursts, some of which were spaced 12 hours apart, on October 16–18. The transition in the character of these eruption sequences can be explained by a difference between the magma supply rate and the magma discharge rate from a shallow reservoir.The magma supply rate (MSR) is the rate with which magma is supplied to the level where disruption due to vesiculation occurs. It is determined by dividing the dense-rock-equivalent volume of eruptive products by the total duration of each eruption sequence. The magma discharge rate (MDR) is the rate with which the disrupted magma is discharged through the vent. It is determined by dividing the volume of erupted products by the duration of each explosive burst. The relative magnitude of these two quantities controls the temporal evolution of an explosive event. When MDRMSR the explosive phase of the eruption lasts for several hours as a single continuous event. When MDR>MSR, an eruption is characterized by a series of short explosive bursts at intervals of several minutes to several days. The MSR of the eruptions of 1980 decreased with time from 5500 m' s⁻¹ on May 18 to 7 m³ s⁻² on October 16–18 and approximately fits an exponential decay. The MDR for the same events remained approximately constant at 2000 m³ s⁻¹. Each explosive event has been followed by an aftershock-like series of earthquakes located beneath the volcano at depth mostly between 7 and 14 km. The seismic energy released during each of these series is proportional to the corresponding volume of erupted magma. Deformation data between June and November, 1980 indicate a subsidence of the volcanic structure which can be modeled by a volume collapse of 0.25 km³ located at 9 km depth.We propose a model in which magma is supplied from depths of 7–14 km through a narrow conduit during each eruption. It erupts to the surface at a uniform rate during each eruption. The deep seismic activity following each eruption is related to a readjustment and volume decrease in the deep feeding system. The decrease of the MSR over time is explained by an increase in the viscosity of a progressively water-depleted magma. The amount of water necessary to explain the observed decrease of the MSR is of the order of 4.6%.     

Oblique rifting in Salina, Lipari and Vulcano islands (Aeolian islands, southern Italy)

A structural analysis carried out on the volcanic products of the islands of Salina, Lipari and Vulcano (Aeolian archipelago) points out that the large-scale tectonic setting is dominated by NW-SE trending right-lateral extensional strike-slip faults and by N-S to NE-SW trending normal faults and fractures. This fault pattern generates pull-apart type structures, developing between different right-hand overlapping fault segments and a characteristic extensional imbricate fan geometry at the tip of the major strike-slip faults. All the structures, representing the surface expression of an active crustal discontinuity which controls the evolutionary history of the magmatism of the three islands, are kinematically compatible with a N100°E extension related to a rifting process affecting southern Italy.     

9th International Symposium on Environmental Biogeochemistry

Reports

9th International Symposium on Environmental Biogeochemistry     

Seismotectonic regionalization of the Red Sea area and its application to seismic risk analysis

A seismological evaluation of the Red Sea margin is presented in this contribution based on the concept of seismotectonic regionalization. The geology and the tectonic structure are critically reviewed to define regions of homogeneous seismicity in the study area, and available seismicity data are implemented to estimate the seismic parameters of the region. The results of the study are applied to evaluate the seismic hazard of an offshore platform site.     

Inelastic response and design criteria of plan-wise asymmetric systems

The study of the torsional response of buildings in the inelastic range of behaviour is of great interest since the ability of structures to resist strong earthquakes mainly relies on their ductility and capacity for energy dissipation. Furthermore, an examination of the performance of structures during past earthquakes demonstrates that plan-asymmetric buildings suffered greater damage due to torsional response. The paper deals with this subject by analysing a model which idealizes a one-storey building with resisting elements oriented along two perpendicular directions. In addition to the parameters of the elastic behaviour, the inelastic system response depends on full yield capacity and plan-wise strength distribution. The influence of the criterion adopted for the design of resisting elements on local ductility demand and damage has been evaluated by parametric analysis. In particular, a comparison has been carried out between systems with equal design levels for all elements and systems with design levels dependent on the element location. For a given elastic behaviour and total capacity, the strength distributions in plan have been defined which minimize ductility demand and structural damage. Finally, based on these findings, responses from models designed according to several seismic codes have been compared.     

Damped resonant appendages to increase inherent damping in buildings

It is demonstrated that the addition of a tuned mass-spring-dashpot system with a relatively small mass and a high damping ratio can be an effective way to increase the inherent damping characteristics of buildings and reduce, thus, their response to earthquake excitations. The demonstration is based on a theoretical formulation and on numerical and experimental studies that confirm this formulation. In the theoretical formulation, it is shown first that, if certain conditions are satisfied, the damping ratios in two of the modes of the system that is formed by a building and an appendage in resonance are approximately equal to the average of the corresponding damping ratios of the building and the appendage. Based on this finding, it is then shown that an attached appendage with a high damping ratio and tuned to the fundamental frequency of a building may increase the damping ratio in the fundamental mode of the building to a value close to half the damping ratio of the appendage. In the numerical study, the response of a ten-storey shear building is analysed under two different earthquake ground motions with and without the proposed resonant appendages. Appendages with damping ratios of 20 and 30 per cent are considered. In this study, it is found that under one of the ground motions the maximum displacement of the building's roof is reduced 30 per cent with the appendage with 20 per cent damping and 39 per cent with the one with 30 per cent damping. Similarly, with these two appendages the building's base shear is reduced 31 and 41 per cent, respectively. In the experimental study, a wooden three-storey structural model is tested in a shaking table with and without an appendage designed and constructed to have a damping ratio of 53-5 per cent. The test is conducted under random and sinusoidal base excitations. In the shaking table test under random excitation, the attached appendage reduces the response of the model 38-6 per cent, while in that under sinusoidal vibration 45-2 per cent.