Triple seismic zone and the regional
variation of seismicity along the northern
Honshu arc,
Kawakatsu, H., T. Seno
J. Geophys. Res., 88,
4215-4230, 1983.
Abstract
The regional variation of seismicity
along the northern Honshu arc, Japan, is studied
using accurate focal depths and focal mechanism types.
We use focal depths determined from pP-P time intervals reported in
the ISC bulletins.
For submarine earthquakes,
depths are corrected by considering the pP phase reported in
the bulletins as the pwP phase (the reflection from the ocean surface).
Out of more than 600 well-located earthquakes selected
from the ISC bulletins,
we determine
the types of the focal mechanisms of 184 events using P wave first motion data.
Based on historical seismicity of great and large earthquakes,
we divide the zone of thrust type earthquakes
at the plate interface into two regions:
the shallow thrust zone (0-40 km),
where great earthquakes ($M sub s \(ap 8.0$) occur,
and the deep thrust zone (40-60 km),
where large earthquakes ($M sub s \(ap 7.4$)
occur.
The activity of great or large earthquakes shows a variation along the arc;
in some regions, both the shallow and deep thrust zones are active,
and in other regions,
only one of the thrust zones is active.
The seismicity of recent moderate size
earthquakes ($m sub b > 4$) combined with
the focal mechanism type shows a variation
along the arc, which reflects the variation of the activity of great
or large earthquakes.
Where large earthquakes do not occur in the deep thrust
zone, neither thrust type nor down-dip compression/tension type events occur
in and beneath the deep thrust zone.
Where large earthquakes do occur in the deep thrust zone, we find a number of
thrust type earthquakes.
Further, in the latter case, in some regions, the down-dip
compression and tension type events of the double seismic zones extend seaward
just beneath the deep thrust zone and
form a triple-planed structure of seismicity (the
triple seismic zone).
This study confirms the hypothesis of
previous workers (Seno and Pongsawat, 1981)
on
the causal relation between
the strong seismic coupling of two converging plates at the deep thrust
zone
and extension of the double seismic zone;
i.e., the presence or absence of activity within the slab beneath
the deep seismic zone occurs when the deep thrust zone has
a strong or weak coupling, respectively.
Here, the weak coupling could be interpreted as
either aseismic slip or as low stress buildup
since the last large event occurred at the deep thrust zone.
Triple seismic zones are found offshore of Miyagi prefecture,
where the deep thrust zone has been broken recently in 1978, and
offshore of Fukushima prefecture.
We expect a future large earthquake at the deep thrust zone offshore of
Fukushima prefecture because the presence of
the triple seismic zone suggests stress has been accumulating
and 40 years have passed since the deep thrust zone was ruptured
in 1938.