Page 291 - Caldera Volcanism Analysis, Modelling and Response

266 J. Martı ´ et al.

Figure 12 Tomographic images of P-wave velocity beneath the Rabaul caldera. (a) P-wave
velocity and (b) residual velocity di¡erence after subtracting the regional 1-D velocity from
data shown in (a).The signi¢cant low-velocity region under the centre of the caldera is inter-
preted to be a region of high-temperature magma accumulation (reproduced with permission
from Finlayson et al., 2003).

short-wavelength magnetic anomalies within the central part of the caldera as well
as along the caldera periphery (Aran ˜a et al., 2000).

Mid-crustal reservoirs. Seismic studies at the calderas of Rabaul and Long Valley
have revealed the presence of central magma reservoirs (Finlayson et al., 2003; Steck
and Prothero, 1994). These central reservoirs appear to be located at mid-crustal
levels and exhibit a horizontally elongated shape. Their geometry can be best
approximated by a sill-like body or an oblate spheroid. At Rabaul, the diameter of
an oblate body approximates the diameter of the collapse structure measured at the
ground surface (Figure 12). For the Valles caldera, the combination of teleseismic
tomography (Steck et al., 1998) and Kirchhoff wavefield migration (Aprea et al.,
2002) has succeeded in identifying a rather complex arrangement of sills at different
lithospheric levels, as well as a large prolate ellipsoid body around 12 km beneath
the northwest quadrant of the caldera (Figure 13).
P-wave velocity in each of the three calderas varies significantly both vertically
and horizontally attesting to the complexities of the subsurface. The Rabaul
anomaly exhibits an up to 10% decrease in P-wave velocities for a anomalous