11 Nature and Spatial Probability of Future Rock Deformation
in Each Domain using Example Locations
Two approaches to considering the probability of future rock deformation in Kyushu have
been developed. Neither of these approaches is mutually exclusive and, ideally, both should
be integrated into a single methodology. The first approach is based on the precept that the
past is the key to the future and that we may therefore use current rates of activity to assess
the probability of exceedance of various strain rates as a proxy for future rock deformation.
Simple calculations relating earthquake energy to scaling of fault slip to earthquake
magnitude, and converting that to strain rate, suggests that an annual strain rate of ≥4
nanostrains/a within a unit area of 5 x 5 km is consistent with rock deformation that may occur
during the equivalent of 1 m of surface displacement (equivalent to a single magnitude 7
earthquake) in a period of 100,000 years. This calculation of course assumes that all strain
calculated for a particular 5 x 5 km area from the various rock deformation datasets will be
converted to a discrete faulting event across a discrete fault every 100,000 years, which is
probably far too simplistic an approach. However, given various assumptions, threshold
“hazardous” strain rates can be defined for specific rock deformation scenarios of concern,
once those scenarios are decided upon. Plots showing the probability of exceedance of
various strain rates for example locations in Kyushu (Figures 11.1 and 11.2) enable
comparisons between locations, and give us some idea of how likely a given example location
is to exceed a certain level of strain. Whether the various strain rates constitute a risk to a
repository depends on the repository design and the manifestation of the rock deformation at
those locations. The deformation may range from concentrated displacement on a single fault
trace to widely distributed, diffuse, fracturing within a 5 x 5km area enclosing the location.
The second approach is to take note of the rapid geological evolution of the Kyushu region
and consider the possibility that future rock deformation may evolve in both space and time,
and that while past and current rates of deformation capture this evolution to some extent, we
can also qualitatively assess what sort of evolving activity may affect the example locations.
Key features or processes we observe to be driving the rapid evolution, and their impacts in
Kyushu are:
(i)
Roll-back at the Nankai Trough, which may induce migration of arc volcanism
southeastward
(ii)
Southwestward migration of the Kyushu/Palau Ridge (and its subducted part) beneath
Kyushu because of the oblique angle of subduction and the orientation of the volcanic
chain comprising the Kyushu/Palau Ridge. We suggest that an active left-lateral shear
zone in southern Kyushu (revealed by GPS and seismicity) occurs due to the
impingement of the Kyushu-Palau Ridge on the subduction margin (see discussion in
Section 9). It has also been suggested by previous studies that uplift in the forearc
above the ridge subduction point is related to Kyushu-Palau Ridge subduction. It is also
possible and localisation of heat flow and Aso-type volcanism in the arc is related to the
position of the Ridge (see Section 7). Thus, we could expect left-lateral shear of
southern Kyushu, uplift in the forearc, and localisation of certain types of volcanism to
migrate also.
(iii)
The northeast limit on extension associated with the Okinawa Trough is adjacent to the
western end of the Beppu Shimabara Graben (BSG) in the vicinity of Nagasaki and
Kumamoto. It is unclear whether this zone of extension continues to develop, but it
could be increasing the likelihood of normal faulting and volcanism in that area.
(iv)
Extension in the Kagoshima Graben and volcanism such as at Sakurajima are
geologically recent phenomena in southern Kyushu. Future evolution and development
of this zone may be expected. Future steepening of subduction at the Ryukyu Trench
as a consequence of slab roll-back will encourage migration of the volcanism in the
Kagoshima Graben toward the east and southeast.
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Figure 11.1 (continued on next page): Cumulative strain rate plots for the 11 example locations. EQ
WGTD AV is equal weighted average. See Figure 6.4 for location of the 11 example locations.
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Figure 11.1 (continued): Cumulative strain rate plots for the 11 example locations. EQ WGTD AV is
equal weighted average. See Figure 6.4 for location of the 11 example locations.
(v)
At present the volcanism and faulting associated with the Kagoshima Graben does not
extend further northeastward than Kirishima volcano, with a marked gap in volcanism in
central Kyushu between Kirishima and Aso volcanic centres. In normal situations we
would expect that the volcanic arc would continue to the north approximately located
above the 100 km depth contour on the subducted Philippine Sea Plate. We are not
aware of any current data to explain this gap in volcanism and thus should consider the
possibility of new volcanoes developing in that area.
(vi)
In the Back-arc domain of northern Kyushu and offshore Japan Sea, large earthquakes
are occurring on reactivated northwest- and northeast-striking faults. There may also be
correlation of these structures to some of the monogenetic volcanism. For example,
stress perturbations that arise at fault interSections may be preferred locations for
future small volume monogenetic events. The backarc province hosts a wide variety of
volcanism that is dominated by processes in the mantle and the mantle wedge rather
than subduction. Little is known of the processes leading to irregular volcanic “flare-ups”
in the Back-arc domain and there is large uncertainty is forecasting future activity.
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Based on the features and processes noted above we discuss current strain rates estimated
for these example locations, and offer some suggestions of possible future rock deformation
evolution at or adjacent to each of the example locations in the various domains defined in the
Kyushu Case Study. To aid with the comparison between locations, Figure 11.2 shows all the
probability of exceedance curves on one plot.
Figure 11.2: Combined equal weighted average cumulative strain rate plot for the 11 example locations,
colour-coded by tectonic domain. See Figure 6.4 for location of the 11 example locations.
11.1
Back-arc domain
Example Location 1. Located on an active fault with a low slip rate of 0.1 mm/a. There is a
general good agreement on strain rates between all 3 datasets: current GPS strain rates are
low-moderate, as are seismicity and active fault strain rates (Figure 11.1). Future deformation
can be expected in the form of sinistral strike-slip surface faulting associated with
earthquakes up to about M 7.5 (see Section 10). Because the fault is reactivating an earlier
structure the fault zone complexity (the process zone) can be expected to be wider than on a
long-lived, well-developed, fault.
Example Location 2. Located about 20 km from a low slip rate (0.1 mm/a) active fault.
Current GPS strain rates are moderate to low, and seismicity strain rates are very low (Figure
11.1). Future deformation is a possibility but it is unlikely the process zone of future faulting on
known active faults in the region (20 km away) will extend to this location. Secondary faulting
or fracturing of suitably oriented bedrock faults and fractures is possible in the event of a large
nearby earthquake.
Example Location 11. Located in southwestern Honshu in the southern part of the
monogenetic Abu volcanic field. In the region there are both northeast- and northwest-striking
faults, although the closest known active fault is more than 20 km to the west. The west
Tottori earthquake of 2000 occurred approximately 200 km to the northeast in a similar
tectonic environment. Current GPS strain rates are moderate, and seismicity strain rates are
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very low (Figure 11.1). Reactivation of bedrock structures in this region as strike-slip faults is
possible considering other active faults in the region. There is a possible structural control on
the location of volcanic edifices of the Abu volcanic field and, at this location, possible future
co-location of volcanism and faulting should be assessed.
11.2
Extensional domain
Example Location 3. Located west of Aso Volcano, within the BSG, and about 12km north of
a prominent normal fault extending westward from Aso Volcano. Thick, young pyroclastic
deposits cover the area, which may have obscured some faulting near to the location. Current
GPS strain rates are high, and seismicity strain rates are very low (Figure 11.1). Further
development of the normal faults can be expected in the region. Proximity to Aso Volcano
suggests that ground deformation associated with future volcanism is also probable.
Example Location 10. Located about 30 km SW of Aso Volcano near the southeast margin
of the BSG, and within 5 km of a moderate slip rate normal fault (~0.9 mm/a). Thick, young
pyroclastic deposits cover the area, which may have obscured some faulting near to the
location. Current GPS and active fault strain rates are high, and seismicity strain rates are
very low (Figure 1). Further development of the normal faults can be expected in the region.
Proximity to Aso Volcano and associated connected edifices suggests that ground
deformation associated with future volcanism is also probable. This location is possibly in the
region where future Aso-type volcanism and related tectonism may concentrate with the
migration of the Kyushu Palau Ridge subduction point interSection with the BSG. If this
process is significant in governing the location and migration of a nexus of volcanism, then
Aso-type volcanism could migrate to a location 10 position, 0.5-1.0 million years from now.
Current strain rates at Location 10 are among the highest observed in the study.
Example Location 4. Located in northeast Kyushu at the eastern margin of the BSG. The
location is on the flanks of a volcano but north of the current extent of normal faulting
associated with the BSG. Geodetic strain rates are high and very similar to other locations in
the BSG (Figure 11.1). Seismicity strain rates are very low (Figure 11.1). Future hazard may
diminish at this location as the shallowly subducting Shikoku Basin crust encroaches on this
region and shuts down any remaining extensional deformation, over the next 1-2 Ma.
11.3
Forearc domain
Example Location 5. Located in the Forearc domain immediately east of the boundary with
the Extensional domain and about 15 km east of Aso Volcano. The location is between active
traces of low slip rate faults (~0.1 mm/a), and bedrock geology in the area has many steep
faults oriented parallel to the domain boundary between the Extensional and Forearc domains.
Current geodetic strain rates are high and seismicity strain rates are low (Figure 11.1). Future
deformation at this location might occur as a result of reactivation of bedrock structures that
evolve the current unlinked fault strands into a more continuous feature. If trench roll-back at
the Nankai Trough is significant process then it is likely the southeastern margin of the
Extensional domain will also migrate into the current Forearc domain. Deformation at the
location could therefore be expected to switch to normal faulting and increase in rate. The
location is likely to be heavily inundated by ash fall in the event of a major eruption from Aso
Volcano.
Example Location 6. Located near the southeast coast of Kyushu approximately 15 km north
of the nearest known fault at the margin of the Miyazaki Plain. Current GPS strain rates are
high and seismicity strain rates are low (Figure 11.1). However, it is possible that the GPS
strain rates are artificially high here due to choice of subduction interface coupling models
(see discussion in Section 9). Coastal uplift rates of about 0.2 mm/a have been noted nearby
and rates up to 0.7 mm/a further south along the Miyazaki coast. If these rates extend inland
to this location this could result in hundreds of metres of uplift over long time periods, and
represent a significant rock deformation.
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Example Location 7. Located in an area about 30 km distant from the nearest active fault
and about 30 km from the BSG. The location is above the 100 km contour on the subducting
oceanic plate but in an area of no current or past volcanism. There is strong northeast striking
bedrock structure in the area. Current GPS strains are high and seismicity strain rates are low
(Figure 11.1). This location is near to the junction of high left-lateral shear strain cross-cutting
Kyushu (as seen from the GPS velocities: see Section 9) and the southeast boundary of the
BSG. Given that this location is adjacent to the interSection of the left-lateral shear zone with
the BSG, we cannot rule out migration of future rock deformation into this region. Obtaining
improved mantle structure data may indicate reasons for the gap in arc volcanism. This would
be a key future requirement if a location such as this were to be considered as a potential
repository site.
11.4
Southern Arc domain
Example Location 8. Located about 15 km to the east of faults marking the eastern
boundary of the Kagoshima Graben. The location is on the southern end of one of the
northeast striking, low slip rate (~0.1 mm/a), faults of the region, and is just to the south of the
left-lateral shear zone cutting across southern Kyushu (from GPS). Current GPS strain rates
are high, active fault strain rates are moderate, and seismicity strain rates are low (Figure
11.1). Future rock deformation may be characterised by rare surface rupture of the local
active fault, or speculatively by significantly increased rates of deformation if the Kagoshima
Graben is migrating with the roll-back of the subducted plate, and by southward migration of
the left-lateral shear zone cross-cutting southern Kyushu which may be driven by subduction
of the Kyushu-Palau Ridge.
Example Location 9. Located about 15 km west of the western margin of the Kagoshima
Graben. The location is within the former volcanic arc (before it migrated eastward to the
Kagoshima Graben area) (Yamaji et al., 2003). The location is about 10 km south of northeast
striking normal cutting across the old arc volcanoes and is just to the south of the left-lateral
shear zone cutting across southern Kyushu (from GPS). Current GPS strain rates are high,
but slightly less than location 8, and seismicity strain rates are very low, also less than
location 8 (Figure 11.1). Future rock deformation at this location might be less likely than at
location 8 because it appears to be in the trailing position of the locus of volcanism in the
contemporary Kagoshima Graben. The characteristics and possibility of migration of the
faulting located about 10 km to north of the location would be important to understand in
further investigations at this locality. In particular, southward migration of the left-lateral shear
zone cross-cutting southern Kyushu which may be driven by subduction of the Kyushu-Palau
Ridge could be important in future rock deformation at this location.
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