Page 269 - Global Tectonics

254 CHAPTER 9

Figure 9.6 Hypothetical section across the Tonga arc based on the attenuation of seismic waves (redrawn from Oliver
& Isacks, 1967, by permission of the American Geophysical Union. Copyright © 1967 American Geophysical Union).

its discoverer(s), and earthquakes on it extend from (extremely low Q of about 50) was defined in the
near the surface, beneath the forearc region, down to uppermost mantle above the downgoing slab in a
a maximum depth of about 670 km. Figure 9.5 shows region about 300 km wide, stretching between the
a section through the Tonga–Kermadec island arc active island arc (Tonga) and backarc ridge (Lau
system with earthquake foci projected on to a vertical Ridge). This implies that the mantle beneath the
plane parallel to the direction of underthrusting. The backarc basin (Lau basin) is much weaker than else-
foci can be seen to occur at progressively greater where or that the lithosphere is considerably thinner.

depths with increasing distance from the site of The data have important ramifications for the origin
underthrusting at the Tonga Trench. Further informa- of backarc basins and will be considered in more
tion on the nature of the Benioff zone was obtained detail in Section 9.10.
from a study of the body wave amplitudes from deep Detailed investigations of the region above the sub-
earthquakes (Fig. 9.6). Seismic arrivals at the volcanic ducting lithosphere have also been carried out using
islands of the arc, such as Tonga, were found to be seismic tomography (Section 2.1.8). Plate 9.1 (between
of far greater amplitude than those recorded to the pp. 244 and 245) shows a section through the Tonga arc

front or rear of the arc at stations such as Raratonga in which the subducting slab is clearly defined by a
and Fiji. The differences in amplitude are usually region of relatively high P-wave velocity. Above this
described quantitatively in terms of the Q-factor, the there is a region of low velocities, beneath the Lau basin

inverse of the specific attenuation factor, and in (see also Section 9.10), corresponding to the region of
general the higher the Q-factor the stronger the rock. extremely low Q in Fig. 9.7. The lowest velocities occur
High Q travel paths give rise to little attenuation, and beneath the Tonga arc volcanoes.
vice versa. Seismic waves traveling up the length of The earthquake activity associated with the down-
the seismic zone appear to pass through a region of going slab occurs as a result of four distinct processes
high Q (about 1000), while those traveling to lateral (Fig. 9.8). In region “a” earthquakes are generated in
recorders pass through a more normal region of low response to the bending of the lithosphere as it begins

Q (about 150). The Benioff zone thus appears to its descent. Bending, or downward flexure of the litho-
define the top of a high Q zone about 100 km thick. sphere, puts the upper surface of the plate into tension,

The Benioff zone had originally been interpreted as and the normal faulting associated with this stress
a large thrust fault between different crustal prov- regime gives rise to the observed earthquakes, which
inces. The seismic data allowed a new interpretation occur to depths of up to 25 km (Christensen & Ruff,
to be made in terms of a high Q belt of Pacifi c 1988).
lithosphere underthrust into the mantle. This inter- Flexural bending of the lithosphere also gives rise to
pretation was refined by Barazangi & Isacks (1971), the topographic bulge present in the subducting plate

by the use of a local seismometer network in the on the oceanward side of the island arc. This regional
region of the Tonga arc (Fig. 9.7). In addition to the rise of sea bed topography is located between 100–
previous results, a zone of very high attenuation 200 km from the trench axis and has an amplitude of