Tatsuya Ishiyama

Geomorphology, kinematic history, and earthquake behavior of the active Kuwana wedge thrust anticline, central Japan

[1] We combine surface mapping of fault and fold scarps that deform late Quaternary alluvial strata with interpretation of a high-resolution seismic reflection profile to develop a kinematic model and determine fault slip rates for an active blind wedge thrust system that underlies Kuwana anticline in central Japan. Surface fold scarps on Kuwana anticline are closely correlated with narrow fold limbs and angular hinges on the seismic profile that suggest at least ∼1.3 km of fault slip completely consumed by folding in the upper 4 km of the crust. The close coincidence and kinematic link between folded terraces and the underlying thrust geometry indicate that Kuwana anticline has accommodated slip at an average rate of 2.2 ± 0.5 mm/yr on a 27°, west dipping thrust fault since early-middle Pleistocene time. In contrast to classical fault bend folds the fault slip budget in the stacked wedge thrusts also indicates that (1) the fault tip propagated upward at a low rate relative to the accrual of fault slip and (2) fault slip is partly absorbed by numerous bedding plane flexural-slip faults above the tips of wedge thrusts. An historic earthquake that occurred on the Kuwana blind thrust system possibly in A.D. 1586 is shown to have produced coseismic surface deformation above the doubly vergent wedge tip. Structural analyses of Kuwana anticline coupled with tectonic geomorphology at 10 3 -10 5 years timescales illustrate the significance of active folds as indicators of slip on underlying blind thrust faults and thus their otherwise inaccessible seismic hazards.

SAHKE seismic‐scatter imaging of subduction beneath Wellington, North Island, New Zealand

Scattering reflectivity analysis of onshore seismic data (Seismic Array HiKurangi Experiment) images slab geometry and crustal structure at a geodetically locked subduction boundary. A broad seismic scattering zone (BSZ) up to 10 km thick above the subduction interface at depths of 16–30 km has seismic properties (Vp, Vp/Vs, and Q) that imply high fluid pressures, probably associated with underplated sediments. The listric strike-slip Wairarapa Fault and other splay faults that sole into the BSZ imply long-term weakness. High rates of intraslab seismicity with low rates above the slab imply stress decoupling across the BSZ. Association of high fluid pressures and long-term weakness with geodetic locking is in contrast to similar observations at zones of slow slip at the northern Hikurangi margin and suggest that either (1) pore fluid pressure or locking behavior may cycle over times longer than two decades or (2) local material properties play a significant role in determining if slow slip or locking occurs.