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Subduction (Japan tectonic context (Nanasei Shoto trench (Phillipine se…
Subduction
Japan tectonic context
Japan trench to E
Two subduction zones to South
Nanasei Shoto trench
Phillipine se aplate subducts Asian Plate (NW)
Tzu trenobhonin trench
Pacific subducts phillipine sea plate (Westward)
Fault
plane
Fault plane = plane which:
Aligns with trench
Matches trend of seismic cluster
2 planes on focal mechanism dividing quadrants
JFAST
Drilled through active fault plane
shortly after Tohoku-Oki equake
Drilled through accretionary wedge
Core sampled from megathrust fault plane
from ~ 1km drill hole
Coupled with seismic study etc
Tohoku-Oki
2011
Comprised several earthquakes
Main shock M 9
N of Tokyo & E of Honshu
associated with thrusting / convergence
Several v significant aftershocks ~Mw 7.9
Displacement & Estimated Slip
Relative to Fukue GPS station site
Tohoku Oki March 2011 Mw9
Max displacement = 40 km
Vertical displacement ~ tens cm (positive & negative)
Beach Balls
Compression
nucleates in white quadrant
moves toward coloured quadrant
coloured = compression
based on p-wave arrival polarity
indicates where compression came from
Stress state ∆ after equake
Stress dissipated / relaxed
∴ 3 principal stresses ~roughly equal after earthquake
i.e. differential stress = released
Cloud of seismicity
Earthquakes nucleate in cloud of seismicity (dissipative events)
∴ unclear where
Tohoku-Oki nucleated
plate boundary trace is
Horst
= expression of downgoing plate extension
extended its upper arches
JFAST
core
few cm of rock accommodated all ~ 40 m displacement at fault !!!
Interface = sampled - fault gouge
Stress state @ lower portion frontal prism
Before earthquake
σ1 subhorizontal
i.e. parallel with rupture direction
After earthquake
similar magnitude for all principal stresses
σ1 vertical