Article,
Morphology of the subducting slab boundary in the northeastern Japan arc
Physics of the Earth and Planetary Interiors, 102 (1-2): 89--104 (June 1997)
DOI: 10.1016/S0031-9201(96)03258-X
Abstract
The detection and location of the subducting slab boundary is crucial
for a better understanding of the mechanical coupling between the
subducting oceanic plate and the overlying continental plate, and
of the occurrence of large and destructive earthquakes on the main
thrust zone in the forearc region of subduction zones. It is also
important for the understanding of the dehydration of the subducting
slab and the genesis of island arc magmatism and volcanism. In this
study we have estimated the morphology of the upper surface of the
subducting Pacific plate under northeastern (NE) Japan by using 1372
SP converted waves at the slab boundary, and 666 earthquakes beneath
the Pacific Ocean that are well located with sP depth phases. We
found that the Pacific slab beneath NE Japan subducts at a small
dip angle of less than 10 deg for the first descent to about 20 km
depth and has a steeper dip angle of 15-25 deg at 30-50 km depth
and about 30 deg at deeper areas. The slab depth is approximately
50 km under the Pacific coast, 150 km under the Japan Sea coast,
and nearly 100 km beneath the volcanic front. At 60-150 km depths,
earthquakes in the lower plane of the double seismic zone occur in
the central part of the slab, while the upper-plane events occur
within 10-15 km beneath the slab boundary. There is a change in the
slab surface geometry around 38N which corresponds to the landward
extension of an oceanic fracture zone. The strike of the slab is
NNE in the north and becomes NE in the south. The southern part of
the slab has a steeper dip angle than the northern part. We believe
that these features are related to the different patterns of interplate
seismicity and mechanical coupling between the northern and southern
parts of NE Japan. Many great (M 8.0-8.5) interplate earthquakes
have occurred in northern NE Japan during recorded history (830-1995),
and microearthquakes show a wide distribution in the forearc region
from the Japan Trench to the Pacific coast, suggesting strong interplate
seismic coupling in that region. In contrast, only a few M 7.0-7.5
class earthquakes have occurred in southern NE Japan in the same
period, and microearthquakes tend to cluster in many small areas,
suggesting weaker interplate coupling there. The subducted slab may
be segmented by the oceanic fracture zones, and the degree of seismic
coupling changes from segment to segment. The behavior of the segment
in southern NE Japan might be affected by the subducted seamounts
so that the slab subducts at a steeper dip angle. This interpretation
is supported by geomagnetic, topographic and geological studies which
show that many small-sized seamounts were subducted in the past from
the southern part of the Japan Trench, but almost no seamounts were
subducted beneath the northern part.
