Recent results of Recent results of JAMSTEC'sJAMSTEC's seismic seismic studies in the Nankai studies in the Nankai

seismogenic zoneseismogenic zoneShuichi KodairaShuichi Kodaira

Institute for Research on Earth EvolutionInstitute for Research on Earth EvolutionJapan Agency for MarineJapan Agency for Marine--Earth Science and Technology Earth Science and Technology

(JAMSTEC)(JAMSTEC)

Special thanks to J.-O. Park, A. Nakanishi, T. Hori, A. Ito, G. Fujie, S. Miura, T. Bata, N. Takahashi, Y. Kaneda

Cummins et al. 2000

MegathrustMegathrust earthquakes in the earthquakes in the Nankai TroughNankai Trough

Earthquake cycles and rupture segmentations are well documented in the Nankai seismogenic zone

Baba and Cummins 2005

JAMSTEC Seismic imaging systemsJAMSTEC Seismic imaging systems

R.V Kaiyo 3176 tonsAirgun 200 liter

100 OBSs

R.V Kairei 4628 tonsStreamer Cable 156ch, 5000m

Airgun 200 liter

JAMSTEC active source seismic JAMSTEC active source seismic studies in the Nankai trough since 1997studies in the Nankai trough since 1997

New findings of structural factors New findings of structural factors controlling megacontrolling mega--thrust earthquakesthrust earthquakes

Off Off MurotoMuroto, Nankai earthquake, Nankai earthquakeLarge subducted seamountLarge subducted seamount (Kodaira et al., 2000, Science)(Kodaira et al., 2000, Science)

Kumano basin, Kumano basin, TonankaiTonankai earthquakeearthquakeBranching splay fault Branching splay fault (Park et al(Park et al.,., 2002, Science)2002, Science)

Off Tokai, Off Tokai, Cyclic ridge subduction and high pore pressure zone at Cyclic ridge subduction and high pore pressure zone at the Tokai slow slip regionthe Tokai slow slip region (Kodaira et al(Kodaira et al.,., 2004, Science)2004, Science)

Off Off KiiKii pen., segmentation boundarypen., segmentation boundaryHigh velocity/density patch and ruptured oceanic crustHigh velocity/density patch and ruptured oceanic crust(Kodaira et al(Kodaira et al.,., JGRJGR 2006)2006)

Seamount subduction and the 1946 Seamount subduction and the 1946 Nankai earthquakeNankai earthquake

A subducted seamount is a key structure which prevents lateral rupture propagation

after Baba and Cummins 2005

Onshore-offshore active source seismic profile crossing central Japan

Baba and Cummins 2005

Cyclic ridge subduction in central Japan imaged by onshore-offshore active source seismic study

Poisson’s ratio; after Kamiya and Kobayashi 2004

Integrated active Integrated active –– passive source seismic imagingpassive source seismic imaging

Imaging the segmentation boundaryImaging the segmentation boundary

Remaining question: Why segmentation boundary is always situatedoff the Kii pen. ? Why occasional rupture synchronization, which generates a giant earthquake, occur ?

Baba and Cummins 2005

High velocity / density patchHigh velocity / density patch

Gravity anomaly after Honda and Kono (2004)

The high velocity body corresponds the high gravity anomaly indicating the Shionomisaki plutonic complex

Fractured oceanic crustFractured oceanic crustThe fractured crust formed by re-activation of a pre-existing structural discontinuity, which was formed during the change in spreading direction [Okino et al., 1999] .

Strike slip earthquakes are observed where the fractures are imaged near the trough axis.

High velocity and density dome: a strongly coupled High velocity and density dome: a strongly coupled patch at the deeper partpatch at the deeper partFractured oceanic crust: landward indentation of a Fractured oceanic crust: landward indentation of a weaker zone at the shallow partweaker zone at the shallow part

What is an effect of those structures on the rupture segmentation and occasional synchronization ?

NumericalNumerical simulationsimulationOriginal algorithm: Hori et al. (2004)Original algorithm: Hori et al. (2004)

includes an overall plate geometry and a variation of a includes an overall plate geometry and a variation of a subduction rate determined by geodetic datasubduction rate determined by geodetic datause frictional parameters (ause frictional parameters (a--b, b, σσ, L) only depending on the , L) only depending on the depth of the plate interface along the strike of the plate.depth of the plate interface along the strike of the plate.

Depth (km)

Well simulates earthquake interval,Well simulates earthquake interval, but does not simulate but does not simulate segmentation and occasional synchronization of the segmentation and occasional synchronization of the rupture zonesrupture zones

Large normal stress at the Large normal stress at the ridge/seamount subducted and the ridge/seamount subducted and the strongly coupled patch (at most strongly coupled patch (at most 200MPa)200MPa)

Little stress Little stress accumulation: steady accumulation: steady state slip (b=0)state slip (b=0)

Beneath the Beneath the ShionomisakiShionomisaki: : strike slip fault systemstrike slip fault system

Large L at the area where ruptures Large L at the area where ruptures were preventedwere prevented

Off Tokai: ridge beneath back Off Tokai: ridge beneath back stop) stop) ShionomisakiShionomisaki: igneous body : igneous body overlaying oceanic crustoverlaying oceanic crust

Numerical simulation including Numerical simulation including realistic structuresrealistic structures

Simulation result Simulation result

Well simulate; Well simulate; segmentation and synchronizationsegmentation and synchronizationrupture size and perturbation of rupture size and perturbation of earthquake cycleearthquake cycle

ConclusionsConclusionsCombined MCS and wideCombined MCS and wide--angle seismic studies angle seismic studies are powerful ways to image subduction are powerful ways to image subduction structures in structures in megathrustmegathrust seismogenic zonesseismogenic zonesThe structures of The structures of subductingsubducting oceanic crust as oceanic crust as well as overriding plate are strongly control well as overriding plate are strongly control rupture patterns in the Nankai seismogenic zonerupture patterns in the Nankai seismogenic zoneNumerical simulation incorporating structural Numerical simulation incorporating structural factors obtained by the seismic studies well factors obtained by the seismic studies well explain the historic rupture patterns and the explain the historic rupture patterns and the occurrence of a giant occurrence of a giant megathrustmegathrust earthquake earthquake along the entire Nankai troughalong the entire Nankai trough

Tokai slow slip and its relation to the structureTokai slow slip and its relation to the structure

Ozawa et al. 2002

Simulation result Simulation result

Future directionFuture directionInvestigate proposed hypothesis; Investigate proposed hypothesis; i.e.,i.e., similar processes may similar processes may happen in several other subduction zones where happen in several other subduction zones where segmentation and/or occasional synchronization of segmentation and/or occasional synchronization of megathrustmegathrust ruptures are observed. ruptures are observed.

Baba et al. 2006

Evidence of Evidence of ““subducted seamount/ridge subducted seamount/ridge –– strong coupling hypothesisstrong coupling hypothesis””

The subducted Paleo Zenisu north ridge under the back stop at present is located exactly at the inter-seismic locked zone.

(after Sagiya 1999)

NumericalNumerical simulationsimulationOriginal algorithm: Hori et al. (2004)Original algorithm: Hori et al. (2004)

includes an overall plate geometry and a variation of a includes an overall plate geometry and a variation of a subduction rate determined by geodetic datasubduction rate determined by geodetic datause frictional parameters (ause frictional parameters (a--b, b, σσ, L) only depending on the , L) only depending on the depth of the plate interface along the strike of the plate.depth of the plate interface along the strike of the plate.

Well simulates earthquake interval,Well simulates earthquake interval, but does not simulate but does not simulate segmentation and occasional synchronization of the segmentation and occasional synchronization of the rupture zonesrupture zones

Hori et al. 2004

Slip history and slow events prior to Slip history and slow events prior to the giant earthquakethe giant earthquake