Self‐centering structural systems with combination of hysteretic and viscous energy dissipations

This paper presents an innovative set of high‐seismic‐resistant structural systems termed Advanced Flag‐Shaped (AFS) systems, where self‐centering elements are used with combinations of various alternative energy dissipation elements (hysteretic, viscous or visco‐elasto‐plastic) in series and/or in parallel. AFS systems is developed using the rationale of combining velocity‐dependent with displacement‐dependent energy dissipation for self‐centering systems, particularly to counteract near‐fault earthquakes. Non‐linear time‐history analyses (NLTHA) on a set of four single‐degree‐of‐freedom (SDOF) systems under a suite of 20 far‐field and 20 near‐fault ground motions are used to compare the seismic performance of AFS systems with the conventional systems. It is shown that AFS systems with a combination in parallel of hysteretic and viscous energy dissipations achieved greater performance in terms of the three performance indices. Furthermore, the use of friction slip in series of viscous energy dissipation is shown to limit the peak response acceleration and induced base‐shear. An extensive parametric analysis is carried out to investigate the influence of two design parameters, λ₁ and λ₂ on the response of SDOF AFS systems with initial periods ranging from 0.2 to 3.0 s and with various strength levels when subjected to far‐field and near‐fault earthquakes. For the design of self‐centering systems with combined hysteretic and viscous energy dissipation (AFS) systems, λ₁ is recommended to be in the range of 0.8–1.6 while λ₂ to be between 0.25 and 0.75 to ensure sufficient self‐centering and energy dissipation capacities, respectively. Copyright © 2010 John Wiley & Sons, Ltd.     

Linear and angular momentum conservation in hydraulic jump in diverging channels

This paper addresses the integral conservation of linear and angular momentum in the steady hydraulic jump in a linearly diverging channel.The flow is considered to be divided into a mainstream that conveys the total liquid discharge, and a roller where no average mass transport occurs. It is assumed that no macroscopic rheological relationship holds, so mass, momentum and angular momentum integral balances are independent relationships. Normal stresses are assumed to be hydrostatic on vertical, cylindrical surfaces. Viscous stresses are assumed to be negligible with respect to turbulent stresses. Assuming that the horizontal velocity distribution in the mainstream is uniform and that the horizontal momentum and angular momentum in the roller are negligible with respect to their mainstream counterparts, an analytical solution is obtained for the free surface profile of the flow. This solution is fundamental for finding the sequent depths and their positions. Consequently, it permits solving for the length of the jump, which is assumed to be equal to the length of the roller. Mainstream and roller thicknesses can also be derived from the present solution. This model may also be theoretically used to derive the average shear stresses exerted by the roller on the mainstream and the power losses per unit weight. This second relationship, which returns the well-known classical expression for total power loss in the jump, demonstrates that the strongly idealized mechanical model proposed here is internally consistent.     

Dissolution/crystallization kinetics recorded in the 2002–2003 lavas of Stromboli (Italy)

On 28 December 2002, new vents opened on the flanks of Stromboli, just below the summit craters, interrupting the persistent activity of the volcano with a 7-month-long effusive eruption. We here report on the plagioclase size distribution (PlgSD) in lava samples collected following the chronology of the 2002–2003 eruption. Data reveal a linear PlgSD similar to that found in samples of normal Stromboli activity, indicating that the switch from Strombolian explosive to effusive activity is not associated with changes in texture. Nevertheless, the crystal size distribution slopes and intercepts exhibit slight sinusoidal temporal variations that are here ascribed to a magma supply mechanism able to induce “resonance” in the crystal size distribution, with an amplitude that depends on the supply rate.     

First study of fault trench stratigraphy at Mt. Etna volcano, Southern Italy: understanding Holocene surface faulting along the Moscarello fault

Paleoseismology, the study of past earthquakes based on their geological record in the stratigraphy and landscape, is a successful newly developing field of research. The application of fault trench studies in volcanic environments is one of the youngest branches of paleoseismology. In this paper, we present the results of the first exploratory trenches excavated at Mt. Etna in Sicily, the largest European volcano. Modern surface faulting at Etna is a very well known feature, which poses significant hazard to the local community, both in terms of ground displacement of essential lifelines and ground shacking from frequent damaging earthquakes. However, while the geomorphology and the seismicity of the active fault in the Etna region consistently show very high rates of tectonic activity, the Holocene cumulative throw and slip-rates, along with the nature (coseismic vs. creeping fault slip), dimension and timing of the displacement events, are still poorly constrained. For this purpose, we selected as a sample area the Moscarello fault, one of the most outstanding segment of the Timpe system of active normal faults in the volcano’s lower eastern flank. Displaced landforms and volcanic units at the Fondo Macchia basin, in the central sector of this fault, indicate some hundreds of meters of vertical offset in the last ca. 80 kyr, with a long-term slip-rate substantially higher than 1.5–2.0 mm/yr. According to the historical sources and instrumental observations, the Moscarello fault ruptured four times in the last 150 years during shallow (H < 5 km) and moderate magnitude (M < 4.8) earthquakes. These events were associated with severe damage in a narrow epicentral area (macroseismic intensities up to the IX–X grade of the MSK scale) and extensive surface faulting (end-to-end rupture length up to 6 km, vertical offsets up to 90 cm). This clearly indicates very high modern rates of deformation along this fault. We conducted trench investigations at the Fondo Macchia site, in a point where eyewitnesses observed ca. 20 cm of coseismic vertical displacement after the April 21, 1971, M_s=3.7, earthquake. The excavated sections provided direct stratigraphic evidence for a vertical slip-rate of 1.4–2.7 mm/yr in the last ca. 6 kyr. This should be regarded as a minimum slip-rate for the central section of the fault. We explored a single scarp at a single site, while we know from recent historical observations that several parallel scarps may rupture coseismically at Fondo Macchia. Thus, the relevant deformation rate documented for the modern period might be likely extended back in the past to a time-span of some thousands of years at least. As expected, for such a volcanic environment, the activity rates of the Moscarello fault are also significantly higher than for the Apennines normal faults, typically showing slip-rates lower than 1 mm/yr. The agriculturally reworked trench hangingwall stratigraphy did not allow to recognize individual displacement events. Nevertheless, the sedimentary structures observed in the trench footwall strongly suggest that, as for the last 150–200 years of detailed historical record, fault behavior at Fondo Macchia is governed by coseismic surface displacement rather than fault creep. This research confirms that paleoseismology techniques can be effectively applied also in active volcanic environments, typically characterized by rheology and, consequently, seismicity and fault dynamics very different from those of other tectonic environments in which paleoseismology has been firstly developed and is today extensively applied.     

Vent temperature trends at the Vulcano Fossa fumarole field: the role of permeability

Between 1994 and 2010, we completed 16 thermal surveys of Vulcano’s Fossa fumarole field (Aeolian Islands, Italy). In each survey, between 400 and 1,200 vent temperatures were collected using a thermal infrared thermometer from distances of ～1?m. The results show a general decrease in average vent temperature during 1994–2003, with the average for the entire field falling from ～220°C in 1994 to ～150°C by 2003. However, between 2004 and 2010, we witnessed heating, with the average increasing to ～190°C by 2010. Alongside these annual-scale field-wide trends, we record a spatial re-organisation of the fumarole field, characterised by shut down of vent zones towards the crater floor, matched by rejuvenation of zones located towards the crater rim. Heating may be expected to be associated with deflation because increased amounts of vaporisation will remove volume from the hydrothermal system Gambino and Guglielmino (J Geophys Res 113:B07402, 2008). However, over the 2004–2010 heating period, no ground deformation was observed. Instead, the number of seismic events increased from a typical rate of 37 events per month during 1994–2000 to 195 events per month during 2004–2010. As part of this increase, we noticed a much greater number of high-frequency events associated with rock fracturing. We thus suggest that the heating event of 2004–2010 was the result of changed permeability conditions, rather than change in the heat supply from the deeper magmatic source. Within this scenario, cooling causes shut down of lower sectors and re-establishment of pathways located towards the crater rim, causing fracturing, increased seismicity and heat flow in these regions. This is consistent with the zone of rejuvenation (which lies towards and at the rim) being the most favourable location for fracturing given the stress field of the Fossa cone Sch?pa et al. (J Volcanol Geotherm Res 203:133–145, 2011); it is also the most established zone, having been active at least since the early twentieth century. Our data show the value of deploying multi-disciplinary geophysical campaigns at degassing (fumarolic) hydrothermal systems. This allows more complete and constrained understanding of the true heat loss dynamics of the system. In the case study presented here, it allows us to distinguish true heating from apparent heating phases. While the former are triggered from the bottom-up, i.e. they are driven by increases in heat supply from the magmatic source, the latter are triggered from the top-down, i.e. by changing permeability conditions in the uppermost portion of the system to allow more efficient heat flow over zones predisposed to fracturing.     

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.