Geoscience Reference
In-Depth Information
of focal mechanisms obtained from a dense network of seismic stations, they found the
azimuth of S Hmax in the
35 km wide seismically active belt lying
along the Japan Sea coast. In this seismic belt, where three M
250 km long and
7.0 earthquakes had
occurred earlier, they found that the S Hmax was rotated
20
°
clockwise with respect to the
S T direction in the adjacent
330 km long and 140 km wide area ( Table 11.1 ) . Based on
modeling, the authors attribute the stress rotation to the deformation of an aseismic fault or
a ductile fault zone in the lower crust beneath the seismic belt.
11.5.4 Continental rift zones
Some examples of stress rotation associated with buried rift pillows were described in
Section 11.4b above. They vary from
75
°
counter-clockwise for the Amazonas basin to
50
°
clockwise for the Kutch rift zone ( Table 11.1 ) .
11.5.5 Bardwell, Kentucky, earthquake sequence
With increasing improvements in seismic monitoring, it now has become possible to detect
stress rotations inferred from analysis of smaller earthquakes. The M 4 June 2003 Bardwell,
Kentucky, earthquake and its aftershocks were recorded on a dense network (Horton et al .,
2005 ) . Stress inversion of very accurately determined focal mechanisms reveal that locally
(to
clockwise ( Table 11.1 ) . The responsible LSC, however, was
not identified by Horton et al .( 2005 ) .
60 km) S T is rotated 40
°
11.6 Magnitude of local stress perturbations
In her analysis of the magnitude of various cases of regional secondary stresses with
detectable rotations of S T (i.e., γ> 15
), Zoback ( 1992a ) showed that S L must be greater
than about half the regional horizontal stress difference, i.e., hundreds of megapascals. This
suggests that, if a detectable stress field rotation is associated with a LSC, the magnitude
of associated S L needs to be of the same order, i.e. hundreds of megapascals. Mazzottii
and Townend ( 2010 ) estimated the magnitudes of S L associated with 30
°
rotation of
S T for the Lower St. Lawrence, Charlevoix, and Central Virginia seismic zones. Assuming
that the responsible seismogenic structures were oriented perpendicular to S L , and using
Equation 8 of Zoback ( 1992a ) , they obtained an estimate of
°
to 50
°
160 to 250 MPa for the
differential stress in the horizontal plane at mid-crustal depths of
8 km. This value was
calculated based on an assumed coefficient of friction of μ
0.8, with near-hydrostatic
pore pressures. The estimate of S L reduces to 20 to 40 MPa for μ
=
0.1, or near-lithostatic
pore pressure. However, the assumptions of such anomalous parameters are not compatible
with recent analysis by Hurd and Zoback ( 2012 ) , who suggest normal values of μ (0.6-0.8)
and hydrostatic pore pressures, supporting the premise here that the stresses associated
with LSCs that lead to moderate and large earthquakes are of the order of hundreds of
megapascals.
=
Search WWH ::




Custom Search