Intraplate earthquakes induced by reactivation of buried ancient rift system along the eastern margin of the Japan Sea - Intraplate Earthquakes - page 250

1 1

Depth 0 - 4 km Depth 7 - 9.6 km

0 0

0.0 0.5 1.0 0.0 0.5 1.0

R R

68%
95%

σ 1
σ 3

nev=176

nev=62

1

1

Depth 4 - 7 km Depth 9.6 - 13 km

0

0

0.0 0.5 1.0 0.0 0.5 1.0

R R

nev=153 nev=81

Figure 9.8 Results of stress tensor inversion for four depth ranges (Kato et al ., 2011 ) : 0-4 km,
4-7 km, 7-9.6 km, and 9.6-13 km, showing lower-hemisphere equal-area projections of the orienta-
tions of σ 1 (squares) and σ 3 (circles), each with their marginal confidence limits. Black-filled symbols,
optimal solutions. Gray shades, 68% confidence limits. Open contours, 95% confidence limits. In
the top right margins, frequency histograms of the R-values, with 68% and 95% confidence intervals
denoted by gray and open bars, respectively. In the bottom left margins, the number of events is
shown.

survey (MT) conducted after the occurrence of the mainshock (Yoshimura et al ., 2008 )
supports the interpretation of the presence of water ( Figure 9.9 ) .
Given that σ 1 corresponds to the maximum horizontal stress ( σ h max ), the transition of
the stress field with depth may be explained by an increase in magnitude of the minimum
horizontal stress ( σ h min ). One simple candidate for the origin of such an increase is a
hypothetical, upward flexure of the upper crust, with its hinge axis oriented parallel to
the σ 1 axis ( Figure 9.9 ) . In shallow parts, σ h min remains smaller than σ v (vertical stress)
because of extensional stresses associated with the bending ( σ v = σ 2 , σ h min σ 3 ), resulting

in a strike-slip regime. In deeper parts, by contrast, σ h min becomes larger than σ v because
of compressional stresses associated with the bending ( σ h min

=

= σ 2 , σ v = σ 3 ), which

leads to a thrust-faulting regime. At great depths, σ h min σ 2 ) grows very close to σ h max

( =