Page 409 - Caldera Volcanism Analysis, Modelling and Response

384 Claudia Troise et al.

3. Displacement Shapes and Maximum Vertical to
Horizontal Ratios

As noted in several papers (e.g. De Natale and Pingue, 1993; De Natale et al.,
1997) the shape of vertical ground deformation pattern at Campi Flegrei is
remarkably constant, independent from the total amount of displacement (during
both uplift and subsidence episodes) which share the same shape apart from the
sign. This feature has been interpreted as the strong effect of ring faults, which
represent displacement discontinuities at the caldera borders, and act to stabilize the
location of the pressure source axis (De Natale et al., 1997). Recently, Battaglia
et al. (2006) noted a marked difference in the ratio between horizontal and vertical
displacements associated with the first phase of large uplift (1982–1983) and the
subsequent subsidence episode (1985–1995) respectively, thus demonstrating that
different source depth and geometry are involved and reflect different mechanisms.
In order to understand the nature of the present uplift episode, it is then crucial to
compare it to previous episodes.
It is well known that surface deformations cannot uniquely constrain the source
(De Natale and Pingue, 1996). However, inferences on source properties can be
extracted from geodetic data, provided simple geometries and mechanisms are
preassigned to the source. In volcanic areas, ellipsoidal sources of homogeneous
pressure changes are most often hypothesized, sometimes degenerating into penny-
shaped cracks (Gudmundsson, 2006). It is also well known that using only vertical
displacement data and simple homogeneous pressure sources cannot be uniquely
constrained, whereas the joint use of vertical and horizontal displacements is
generally able to uniquely constrain the depth and mechanism of such predefined
sources. The most powerful way to monitor both vertical and horizontal
displacements at the same time is to make use of GPS data. Although the precise
modeling of displacement sources generally requires a dense geodetic data sampling,
it is possible to use continuous GPS measurements to extract (from the ratio between
horizontal and vertical data) basic information about source geometry and depth,
and to detect possible changes of the ratio which would indicate modifications in the
depth and/or shape of the intrusion, for example, during intrusions.
We have computed, using an FEM program (DIANA), the ratios of maximum
horizontal to maximum vertical surface displacement for different ellipsoidal
sources in a homogeneous elastic medium with ring faults defined as conical
surfaces on which shear stress is null and opening is via normal strain (see Troise
et al., 2003). An axial-symmetric scheme has been used. The dip of the ring faults is
751, and an average size of 50 m for the smallest elements in the central part of the
model has been used. The scheme has a total size of 20 km 20 km to avoid border
effects (Troise et al., 2003). Resulting curves of displacement ratios versus depth for
three kinds of ellipsoidal sources are shown in Figure 8a. Maximum displacement
ratios are strongly constrained on source geometry, and particularly in the case of
shallow depth sources. Monitoring displacement ratios by continuous GPS should
be then be a very effective method to constrain source geometry and detect
intrusive episodes that cause a significant increase in the ratio of maximum