Page 272 - Global Tectonics

SUBDUCTION ZONES 257

which represents the distribution of earthquake types making use of a local array of seismographs, identifi ed
around the Aleutian island arc (Stauder, 1968). The belt two Benioff zones beneath the Japan arc that appear to
of earthquakes to the south of the islands is caused by merge down dip (Fig. 9.12). The arrival times of differ-

normal faulting associated with the flexure of the top ent seismic phases indicate that the upper of these

part of the Pacific Plate, which is underthrusting the zones corresponds to the crustal part of the descending
Bering Sea in a northwesterly direction. The groups of slab, and the lower to the lithospheric mantle (Hasegawa
earthquakes lying under or just to the south of the et al., 1994).
island chain are indicative of thrust faulting. The nodal Subsequently, double seismic zones, at depths
planes dip steeply to the south and gently to the north. between 70 and 200 km, have been documented in
It is probable that the latter planes represent the fault numerous well-studied subduction zones (Peacock,
planes, and that these earthquakes are generated by the 2001), and it seems probable that they are a common

relative movement between the Pacific and Bering Sea feature of subduction zone seismicity. In some cases
lithosphere. The single focal mechanism solution indic- focal mechanism solutions for the upper zone earth-
ative of strike-slip movement is either on a sinistral quakes imply down-dip compression, and those for the
strike-slip fault perpendicular to the island chain, as lower zone earthquakes down-dip tension. This sug-
indicated on the diagram, or alternatively on a dextral gests that unbending of the downgoing plate may be
strike-slip fault paralleling the island chain. In view of important, the plate having suffered a certain amount
the oblique direction of underthrusting in this region, of permanent, plastic deformation during its initial
the latter interpretation is perhaps more likely to be descent (Isacks & Barazangi, 1977). However the double
correct (Section 5.3). seismic zones extend to depths well beyond the region
The earthquakes occurring in the Benioff zone in of unbending of the downgoing plates. It is now thought
zone “c” (Fig. 9.8), at depths greater than the thickness that most of these earthquakes are triggered by meta-
of the lithosphere at the surface, are not generated by morphic reactions involving dehydration; those in the
thrusting at the top of the descending plate, because the upper zone associated with the formation of eclogite
asthenosphere in contact with the plate is too weak to (Kirby et al., 1996), and those in the lower zone with the
support the stresses necessary for extensive faulting. At dehydration of serpentinite (Meade & Jeanloz, 1991). It
these depths earthquakes occur as a result of the inter- is suggested that dehydration reactions generate high
nal deformation of the relatively cold and hence strong pore pressures along pre-existing fault planes in the
descending slab of lithosphere. Hasegawa et al. (1978), subducting oceanic lithosphere, producing earthquakes

Figure 9.12 Distribution of earthquakes beneath the northeastern Japan arc. Shaded line is probably the top of the
descending lithosphere (redrawn from Hasegawa et al., 1978, with permission from Blackwell Publishing).