Geology Reference
In-Depth Information
earthquake (Fig. 5.1C). Moreover, they suggest
that interseismic deformation in subduction-zone
settings may be characterized by steady rates of
deformation. A map view of the tide-gauge data
(Fig. 5.12A) provides a coherent overview of the
regional deformation. The maximum rates of
uplift occur at 100-200 km from the trench. The
trend of this zone of high rates is nearly parallel
to the trench. Farther from the trench, rates
diminish and even define a zone of subsidence
west of Osaka (Fig. 5.12A).
This pattern of uplift and subsidence can be
interpreted in several ways. One could assume
that, following the post-seismic interval, elastic
strain has been accumulating above a presently
locked fault plane represented by the shallow
part (<30 km) of the subduction zone interface.
If this were the case, then elastic half-space
models suggest that, above the trailing edge of
the locked thrust, the overlying hanging wall
should buckle upwards, whereas the leading
edge of the hanging wall should be flexed
downward toward the trench. If this elastic
strain were released during a subsequent
subduction-zone earthquake, strong subsidence
in the southeastern coastal region and uplift
west of Osaka would be predicted for these
tide-gauge data. If the interseismic deformation
is compared with the coseismic displacement
resulting from the 1946 Nankaido
M
s
=
8.2
earthquake (Fig. 5.12B), a striking spatial corre-
spondence exists between the zones that have
been uplifted most extensively during the
interseismic interval and those that experienced
the greatest coseismic subsidence. These data
lend support to the concept that the pattern
of interseismic strain accumulation is com-
monly mirrored by the coseismic deformation.
An alternative explanation suggests that the
deformation defined with the tide-gauge data
represents a response to a down-dip migration
of slip along the subduction zone following
major earthquakes. If this were the case, then
the observed deformation is more analogous to
a migrating ripple of deformation (Pollitz
et al
.,
1998; Freymueller
et al
., 2000) and would
represent little storage of elastic strain.
Comparison of the inter- and coseismic displace-
ment maps lends some support to this idea, as
Wakayama
150
best-fit relative
sea-level change
100
50
10.4 m
m
0
-50
corrected annual
mean sea level
-100
-150
1950 55
65
70
75
80
1985
60
Takamatsu
150
100
50
8.5 mm
rms residual
0
-50
scaled
oceanographic
correction
-100
-150
55
60
65
70
75
80
1950
1985
150
Uno
100
50
14.6 mm
rapid
rise
0
-50
-100
-150
slower, steady rise
80
1950
55
60
65
70
75
1985
Time (yr)
Fig. 5.11
Relative sea-level change and Japanese
coastal deformation adjacent to the Nankai Trough.
Subtraction of the oceanographic correction (which is
scaled for each site) from the original measurements
yields the corrected annual mean sea level. The line
fitted to the corrected data defines the long-term
tectonic rock uplift and subsidence. The root mean
squared (rms) residual quantifies the deviation of the
annual data points from the long-term fitted line. All
sites show a steady trend in sea-level change between
1955 and 1985. Note that, at Takamatsu and Uno, an
interval of rapid subsidence (seen as a rise in sea level)
is subtly, but clearly, expressed prior to about 1955.
Modified after Savage and Thatcher (1992).
