1. Eiichiro Araki Japan Agency for Marine-Earth Science and Technology
Filling observational gap of seafloor dynamics in seismic cycles by seafloor borehole observation network
Eiichiro Araki
Japan Agency for Marine-Earth Science and Technology

2.  Need for capturing full spectrum of the “slow earthquake family”
Understanding how subducting plate interface accumulate and release strain…
 Need for capturing full spectrum of the “slow earthquake family”
GPS resolution limit
frequent
infrequent
large area
Our observation limit may define
“ scaling law “ of the slow earthquake family.
We need to be sensitive to potential dynamics in subduction zone.
Infragravity wave
Microseisms
small area
Ide et al., 2007.

3. Recent example of seafloor borehole observatories in the Nankai Trough (IODP NantroSEIZE project)
borehole sensors
Drilling Vessel Chikyu
DONET1 observation network
Tonankai rupture zone.
Pore-fluid pressure measurement.
Seismic / Tilt /Strain
accretionary wedge
Data available from J-SEIS JAMSTEC web.
980m

4. Discovery of shallow “Slow slip events” by seafloor borehole observatories
in the Nankai Trough.

5. Relevance with “Inter-plate coupling” from seafloor GPS/A observation.
Dense borehole array have a potential to resolve
the reality of
along strike inter-plate coupling variation.
Yokota et al., 2016.

6. Advantage to seafloor GPS/A
1) Continuous measurement
GPS/A
Borehole pore-p/strain + seismic
Ide et al., 2007.
Interval GPS/A measurement cannot resolve
characteristic period of the events in behind.

7. Advantage to seafloor GPS/A
2) Higher sensitivity to smaller spatial scale events.
Borehole pore-p/strain sensitive to spatial derivative of dislocation at depth.
Ide et al., 2007.
Strain sensitivity limit
Event scale L GPS/A Strain/pore-p
100km ^ ^-8
km ^ ^-8
Typical event characteristics (from Ide et al., 2007)
Type D (dislocation) m L (m) Strain D/L
SSE shot term ~ x10^ x10^-7
SSE long term x10^ x10^-6
Silent EQ x10^ x10^-6
ETS(Cascadia) x10^ x10^-7
Slow slip(creep) ~ x10^-5

8. Advantage to seafloor GPS/A
Recent GPS/A results suggests
highly segmented nature of plate coupling.
We should resolve at least as small as
～50 km scale patches.
Yokota et al., 2016.

9. Advantage to APG array in the seafloor
Oceanographic “noise” can be extracted in borehole pore-p/strain record
Ocean “Noise” = Ocean tide + etc…
Pore-P
Seafloor
Pressure
Ocean Loading
Effect of ocean loading
1hPa = 1cm seafloor level
Incoherent ocean “noise” observed in APG array in DONET

10. Another important advantage of borehole array
significant amplification of horizontal motion
in soft sediment seafloor.
Seafloor /borehole records of the M6 earthquake (Apr. 1, 2016)
～ 20 km from the epicenter.
Understanding “Co-seismic” rupture process
from nearby observation is very important.
Ide et al., 2007.

11. Seafloor borehole array as a key component in coming SZO program.
would help address questions how subduction margins accumulate and release strain in seismic cycles.
In seismic cycles …. comparative study of subduction zones!
Disadvantages:
High cost (mainly due to long drill ship operation time)
… but with no other better means to capture SSE/ETS!
2) Long-term stability of borehole strain measurement is yet to confirm.
 Targets of our technical challenge in the next 10 years.