Page 389 - Caldera Volcanism Analysis, Modelling and Response

364 Thomas R. Walter

Deflating elliptical magma reservoir, opening at ring-fault

A) Map view, model setup B) Map view, displacement vectors

Magma chamber 20
(10 MPa
pressure drop)
10

0 Subsidence
Subsidence
Subsidence
5 km
y
Opening -10
ring-fault
x -20
-20 -10 0 10 20 -20 -10 0 10 20

C) Side views on opening ring-fault, amount of opening
1
z z
[in m]
x y 0
0

Dike

Opening -10

-10 0 10 -4 0 4
Magma chamber
Figure 7 De£ation of an elliptical magma chamber causes opening at a circumferential
ring fault.The magma chamber is elongated in the x-direction and vertically £attened (10 km
radius in x,5 km in y, and 2.5 km in z), emplaced at (x, y) coordinates (0, 0) at 10 km depth, and
is subject to a pressure drop of 10 MPa. All other parameters and sub¢gure explanations
(A--C) are the same as in Figure 5.The displacement computed at the ring-fault suggests open-
ing should occur at the short-axis side of the ring-fault. A potential dike intrusion would intrude
as shown by the black arrow in (C).

model, the effect of a tectonic earthquake distant from the ring-fault is considered.
The earthquake is simulated by 5 m uniform slip on a 20 20 km fault, 100 km
from the ring-fault (Figure 8A). A low-angle thrust earthquake is simulated, as if in
a subduction zone. As in the models described above, the amount of opening is
calculated at the ring-fault in order to predict the location of a ring-dike. Map view
shows displacement vectors directed to the west towards the earthquake zone and
slight subsidence due to the reactivated (opened) ring-fault (Figure 8B). The model
predicts maximum opening at two opposite sides of the ring-fracture, with slightly