Page 294 - Caldera Volcanism Analysis, Modelling and Response

A Review on Collapse Caldera Modelling 269

Based on experimental results, the activity of both reverse and normal faults
constitutes a possible solution to the so-called space problem (see previous sections)
during major collapses or resurgences (Acocella et al., 2000). Mori and McKee
(1987) in their seminal paper highlight the presence of outward-dipping (away from
the depression) bounding faults at Rabaul caldera and provided a straightforward
solution to the space problem during caldera collapse. Relocation of earthquake
hypocenters at Rabaul (1971–1992) including seismic data employed by Mori and
McKee (1987) were presented in Saunders (2001). In this paper, the subsurface
distribution of hypocenters draws a complex picture of seismic zones particularly
for depths shallower than 2 km (Figure 14), with a pronounced spread of seismicity
at less than B1,000 m. Saunders (2001) interprets this spread of seismicity as
shallow structures consisting of antithetic normal faults. At greater depth, the
seismic pattern may be a approximated by two arcuate faults delineating an elliptical
ring fault with a ellipticity of about two, whereas at depths less than 2 km the
epicentre pattern displays a more circular appearance.
At Campi Flegrei, a circular caldera geometry was recently invoked by the joint
inversion of seismic and gravity data (Capuano and Achauer, 2003), which is in very
good agreement with the symmetry and extension of the area affected by ground
deformation during recent unrest periods (De Natale et al., 2006; Troise et al.,
2007, 2008). Furthermore, seismic activity clusters around the deeper part of the
ring-fault system, with focal mechanisms in agreement with a slightly inward-
dipping geometry of the system (Troise et al., 2003). Studies of local seismicity
(Troise et al., 2003), volcano-tectonic trends imaged by seismic reflection data
(Bruno, 2004) and magnetic data (Cassano and La Torre, 1987) highlight the
influence of the regional stress regime (NE-SW trending regional faults), both on
the caldera geometry as well as on post-caldera processes.
At Campi Flegrei, the geometry of the eastern caldera wall and the alignment
of several monogenetic volcanoes along regional NE–SW trending regional
faults imaged by seismic reflection data highlights the influence of the regional stress
field, both on caldera formation as well as on post-caldera processes (Bruno, 2004).
Gravity and magnetic anomalies are also aligned along the same strike direction
(Cassano and La Torre, 1987).
At Taupo, seismic reflection data indicate the presence of slightly inward-
dipping (towards the caldera centre) faults marking the southern and northern
caldera boundary (Davy and Caldwell, 1998). These faults border the gravity low
situated in the northern part of Lake Taupo.
High-resolution hypocentres for earthquakes that occurred between 1980 and
2000 in the Long Valley caldera area reveal discrete fault planes in the southern part
of the caldera (Prejean et al., 2002)(Figure 15). One of these most active structures
within the caldera (labelled WSMZ1 in Figure 15) is a near-vertical slightly inward-
dipping fault, which may be related to the caldera ring-fracture system. Seismicity
in this zone does not lie along a simple plane but rather along an up to 1 km wide
fault zone. Focal mechanisms indicate that this zone is composed of a series of many
small faults of varying orientations, which are active over a depth range of 3–9 km.
Other fault zones subparallel to WSMZ1 along the southern resurgent dome dip
towards the caldera centre at a significantly lower angle.