Page 368 - Caldera Volcanism Analysis, Modelling and Response

Magma-Chamber Geometry, Fluid Transport, Local Stresses and Rock Behaviour 343

of caldera slip. In the underpressure model, it is also difficult to explain why
collapses, even if rare, are so much more frequent in basaltic edifices than in
stratovolcanoes (Gudmundsson and Nilsen, 2006). Thus, in basaltic edifices such as
those on Hawaii and the Galapagos Islands, slip on existing calderas, often with very
small or no eruptions, is comparatively common.

7.2. Ring-fault structure and slip
One of the main conclusions of this paper is that purely empirical models, such as
the underpressure model, are unlikely to help us forecast whether ring-fault
formation or slip is likely to occur during an unrest period. In the paper, I argue
that to understand how and when a ring fault develops and why an existing ring
fault slips so infrequently, we must know the state of stress in the host volcano. This
implies the knowledge of the properties of the rock layers and structures that
constitute the volcano. Furthermore, to forecast whether a ring fault is likely to
form or slip during a particular unrest period, we must have a rough idea of the
geometry of the associated magma chamber. Ring-fault formation and slip are
mechanical processes that cannot be forecasted solely on the basis of empirical
criteria; to develop viable models to assess the probability of ring-fault formation or
slip, these processes must be understood in mechanical terms.
While the ring-fault structure is likely to be commonly complex in detail
(Figure 4), it is very important when assessing the probability of slip during
unrest periods to know if the ring fault is generally outward or inward dipping
(Figures 5–9). Many authors have proposed that the dip is primarily outward
(Williams et al., 1970; Branney, 1995; Cole et al., 2005), but observations of ring
faults that are subvertical to inward dipping have a long history (Kuno et al., 1964;
Smith et al., 1961; Filson et al., 1973; Aramaki, 1984; Lipman, 1984, 1997, 2000;
Newhall and Dzurisin, 1988; Gudmundsson, 1998a; Geshi et al., 2002; Lavallee
et al., 2006). As indicated above, the well-documented collapse of the Fernandina
caldera in the Galapagos Islands in 1968 occurred on a ring fault dipping about 801
inwards (Simkin and Howard, 1970). Similarly, the collapse of the Miyakejima
caldera in 2000 was primarily on inward-dipping faults (Geshi et al., 2002). And the
inferred small collapse structures associated with the 1600 AD explosive eruption of
the Huaynaputina volcano in Peru was on vertical or steeply inward-dipping faults
(Lavallee et al., 2006). Also, by definition, all funnel-shaped calderas must dip
inwards (Aramaki, 1984; Lipman, 1997; Cole et al., 2005). All ring faults studied in
Iceland are either vertical or dip inwards (Gudmundsson and Nilsen, 2006).
Similarly, ring dykes are commonly vertical or dip steeply inwards (Oftedahl, 1953;
Almond, 1977). Thus, although one cannot rule out possible outward-dipping ring
faults, formed under special stress conditions (reverse faults are occasionally found
in extensional tectonic environments), the general field evidence seems to favour
most ring faults being vertical or inward-dipping normal faults (Gudmundsson,
1998a).
This conclusion is also in agreement with the conceptual and numerical models
presented in this paper. As regards the conceptual models, they indicate that there
would normally be very little friction to stop the vertical subsidence along an