This dramatic variability among along-strike segments, including segments of weak plate coupling, is very striking.
New GPS/acoustic seafloor observation also confirmed weak plate coupling in Segment D and/or D′ (Matsumoto et al., 2008).
Narrow mantle plumes impinging on the base of the plate cause locally weak plate segments and a large topography at the lithosphere-asthenosphere boundary.
On the outer rim, shallow VLF after-events will be reactivated earlier than they will be in the center because of weak plate coupling.
Within such a hot and weak plate, extensional strain tends to be delocalized and a broad deformation zone is formed more easily (Buck et al. 1999).
Kawasaki et al. (2001) recently identified several slow earthquakes in Segment B, as shown by green patches in Fig. 3(a), implying its very weak plate coupling.
The two-phase damage theory with a grain-size dependent rheology was developed to explain the formation of weak plate boundaries and track the evolution of deformation (e.g., Bercovici and Ricard 2005; Landuyt et al. 2008; Bercovici 2012).
A large tsunami earthquake in 1896 in a shallow segment B′ (Kanamori, 1972) and slow or aseismic slip events in Segment B (Kawasaki et al., 2001), as shown by green patches in Fig. 3(a), indicate weak plate coupling in Segments B and B′.
The interaction between deep and shallow segments, Segments C and C', is considered essential for causing an M 9 earthquake; their combined slips may have triggered large slips in adjacent along-strike segments (Segments B and D) of relatively weak plate coupling where no large slips can happen if they fail by themselves.
Above the level of the décollement, approximately 3,000 to 4,000 mbsf, the rocks would be expected to have cohesions in the range of 5 to 12 MPa (Figure 7) and be floored by a weaker plate boundary fault.
