The effect of collapse structures on ground deformations in calderas

Abstract

Ground deformations related to unrest episodes in calderas are generally interpreted, like in other volcanic environments, in terms of increased pressure within a magma chamber embedded in a continuous, elastic medium. In this framework, the depth of the pressure source is inferred from the size of the deformed area. This scheme works quite well 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 km). 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. Copyright 1997 by the American Geophysical Union.