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

 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.

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

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

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.

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.

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 10^-7 5^-8 km 10^-6 5^-8 Typical event characteristics (from Ide et al., 2007) Type D (dislocation) m L (m) Strain D/L SSE shot term ~0.026 5x10^4 5x10^-7 SSE long term 0.11 7x10^4 1.5x10^-6 Silent EQ 0.18 6x10^4 3x10^-6 ETS(Cascadia) 0.02 1.5x10^5 1.3x10^-7 Slow slip(creep) ~0.1 5000 2x10^-5

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.

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

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.

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.