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Figure 9.47.
The seismicity of the Tonga-Kermadec subduction zone. Two separate
surfaces are present south of 15
◦
S, representing a detached remnant of slab and the
actively subducting Pacific plate. There is also a separate cluster of earthquakes in
the northwest. Contour lines are every 100 km in depth. (From Brudzinski and Chen,
A petrologic anomaly accompanying outboard earthquakes beneath Fiji-Tonga:
corresponding evidence from broadband P and S waveforms,
J. Geophys. Res.,
108
,
2299, doi: 10.1029/2002JB002012, 2003. Copyright 2003 American Geophysical
Union. Reprinted by permission of American Geophysical Union.)
Results of a detailed study of the propagation of the short-wave seismic phases
P
n
and S
n
in the region extending from the Tonga Trench to the Fijian islands
are shown in Fig. 9.49. There is a zone of extremely high attenuation (
Q
<
100) in the upper 100 km above the subducted plate beneath the spreading Lau
back-arc basin, which is consistent with the low-velocity zones and partial melt-
ing. The 200-300-km-deep mantle wedge in the back-arc region has
Q
<
200 -
it is possible that these low
Q
values are associated with dehydration reactions
occurring deep within the subducting slab. To the east of the subduction zone,
at these depths
Q
is considerably higher. The subducted plate itself, down to
670 km depth, is characterized by low attenuation, with
Q
>
900. A similar
highly attenuative zone is present in the mantle wedge above the subducting
Pacific plate beneath Japan. The low-
Q
regions are roughly coincident with the
low-velocity zones shown in Fig. 9.48. The high attenuation and low velocities
above these subducting plates are consequences of the dehydration of the sub-
ducting plate, the rise of water into the mantle wedge, the partial melting and
then the accumulation of melt at high levels beneath and within the crust. When
examined in detail, the low-velocity/low-
Q
zones are only rarely connected to
the subducting plate. This is consistent with volatiles from the subducting plates
