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46 CHAPTER 4

be illustrated by another recent study. In 1998 an
eruption occurred from Axial Volcano, a basaltic
A
shield volcano located on the Juan de Fuca ridge on
the floor of the Pacific ocean. This is a volcano
formed above a hot spot which happens to be
B located directly beneath a mid-ocean ridge. During
the eruption, detection of earthquakes showed that
a dike propagated laterally 50 km away from the
summit caldera and fed a lava flow (the presence of
which was subsequently detected on the sea floor
by a small scientific submarine). The eruption was
Fig. 4.4 Comparison of the seismic signals from a volcanic associated with 3 m of subsidence of the caldera
earthquake (trace A) and a period of volcanic tremor (trace floor, suggesting that magma withdrawal associ-
B). The earthquake is a discrete event, finished in seconds to
ated with dike propagation and eruption caused
tens of seconds, whereas the tremor continues for as long as
partial collapse of the roof of a magma chamber. A
magma is moving beneath the surface, which can be from
subsequent seismic study was carried out using an
minutes to many tens of hours. (Modified from fig. 3 in
McNutt, S.R. (2000) Seismic monitoring. Encyclopedia of artificial seismic source. This study looked at the
Volcanoes. Academic Press, pp. 1095–1119, copyright velocity structure beneath the volcano and indic-
Elsevier (2002).) ated the presence of a low-velocity zone below the
caldera which was most pronounced at a depth of
2.25–3.5 km but which extended to at least 6 km.
mon precursor to eruptions and some forms of it The resolution of the survey was sufficient to show
are generated by the movement of magma within that the low-velocity zone covered an area 8 × 12 km
the volcanic plumbing system. Thus monitoring of when viewed from above. This is considerably
volcanic tremors provides a method of detecting larger than the size of the caldera itself (3 × 8km).
magma movement beneath the ground before an The volume of the magma chamber was estimated
3
3
eruption starts at the surface. If the place where the as being ∼250 km of which only ∼5–21 km was
tremor starts can be located accurately it should actual melt (Fig. 4.6). This illustrates an important
indicate a boundary of the magma storage region point about magma chambers. It is common to think
from which a new dike is propagating. of a magma chamber as containing only molten

A seismic study carried out during the eruption magma. In reality cooling of the magma within the
of Usu volcano in Japan in 2000 illustrates the use of chamber means that crystallization is occurring
all three of these techniques. Seismic tremor was all the time so the low-velocity area inferred to be
detected originating at depths of 5–6 km beneath a magma chamber will actually contain melt sur-
the volcano prior to eruption. This depth coincides rounded by a “mush” of liquid containing crystals.
with the location of a seismic gap and of a low- The amount of actual melt might be small com-
velocity region. The amplitude of the tremor was pared with the total volume of the magma cham-
strongly correlated with measurements of the rate ber, as appears to be the case at Axial volcano.
at which the ground surface was being uplifted
prior to the eruption (Fig. 4.5a). This combination
DEFORMATION TECHNIQUES
of seismic and ground deformation evidence sug-
gests that a magma chamber was located at a depth Volcanic activity is often associated with deforma-
of 5–6 km beneath the surface and that the tremor, tion of the volcanic edifice. There is a range of
ground deformation and subsequent eruption were geophysical methods which can be used to monitor
caused by the upward movement of magma from this deformation including the leveling, tilt mea-
this depth (Fig. 4.5b). surements, GPS (Global Positioning System) and
The power of seismic techniques in giving insight EDM (Electronic Distance Measurement) techni-
into the location and size of magma chambers can ques which are described in detail in Chapter 11.