Abstract. Plinian and subplinian volcanic eruptions can be accompanied by tephra falls which may last hours or days, posing threats to people, buildings and economic activity. Numerous historical examples exist of tephra damage and tephra casualties. The mechanisms and consequences of roof collapse from static tephra load are an important area of tephra damage requiring more research.This paper contributes to this work by estimating the structural vulnerability of buildings to tephra load based on both analytical studies and observed damage. New studies are presented of roof strengths in the area around Mt. Vesuvius in southern Italy and of field surveys undertaken in other European volcanic locations to assess building vulnerability to tephra fall.The results are a proposed set of new European tephra fall roof vulnerability curves in areas potentially threatened by explosive volcanic eruptions along with comments on the human casualty implications of roof collapse under tephra loading. Some mitigation recommendations are provided.
Abstract. Ground defom•ations related to unrest episodes in calderas are generally interpreted, like in other volcanic environments, in terms of increased pressure within a magma chalnber embedded in a continuous, elastic medium. h• this framework, the depth of the pressure source is inferred from the size of the deformed area. This scheme works quite xvell for individual volcanoes (for instance Hawaiian shields) where a good correspondence between inflation events and eruptive episodes has been observed. Ground deformation in calderas, however, show unusual features that are difficult to interpret within such a scheme, because considerable positive deformation (in the order of several meters) and microfracturing (i.e. intense seismicity) may occur without eruptions, altough the estimated depth for the magma source is sometime very shallow (less than 1-2 lon). It has been recently suggested that the contact zones at the border of collapsed calderas, could act as stress-strain discontinuity zones and thus bias modeling results obtained for continuous elastic media. We use both observations and theoretical modeling based on 3-D finite element techniques to show that ground deformations in collapsed calderas are strongly influenced by the caldera structure, giving a new perspective in related geological and geophysical observations in such areas.
We have computed static stress changes caused by several earthquakes that occurred in the Apenninic chain (Italy). Static stress associated with fault slip has been computed using the Okada (1992) formulation. Static Coulomb stress changes associated with the three subevents forming the 1980 Irpinia MS = 6.9 main shock indicate that each subevent was consecutively triggered by stress changes produced by the previous ones. Furthermore, aftershocks of this complex faulting event are well correlated with zones of Coulomb stress increase. The interplay of regional stress and local stress changes due to the main shock produces an aftershock distribution wider than expected and a large variation in focal mechanism. The variation in focal mechanisms is consistent with a low level of background regional stress (less than 2 MPa). Moreover, static stress changes due to the Irpinia earthquake appear to have triggered a moderate‐magnitude (ML ≈ 5) seismic sequence in an adjacent tectonic area (close to the town of Potenza), with a delay of some years. The analysis of a further two seismic sequences in the central Apennines, which occurred in 1979 close to the town of Norcia (ML = 5.9) and in 1984 in the Abruzzo National Park (ML = 5.5), also show a clear correlation between aftershocks and the positive Coulomb stress changes generated by the main shocks. Aftershocks of the 1979 Norcia earthquake cluster at two lateral edges of the main fault, as expected for a moderate‐magnitude main shock in which the local stress change is considerably lower than the regional stress field. The static stress changes due to the 1984 main shock in Abruzzo are likely to have triggered a further main shock four days later (ML = 5.1) at the northern edge of the main fault, where the Coulomb stress change is maximum.
This evidence indicates a strong correlation between the earthquakes in the Apenninic chain, through static stress changes, at several time‐ and space‐scales. Modelling of such effects is useful both for improving our knowledge of the earthquake dynamics, and for a better evaluation of seismic hazard.
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