Unified model for intraplate earthquakes - Intraplate Earthquakes - page 286

( 2005 ) found evidence of perturbation of S T by local sources. They found that a regionally
significant strike-slip regime with a northwest-trending S Hmax was rotated at two locations
and overprinted by local extensional perturbations at two others with wavelengths of tens
to hundreds of kilometers. Mazarbaud et al .( 2005 ) related the extensional deviatoric stress
at the Eastern Massif Central ( Table 11.1 ) to an underlying bulging of the crust at the apex
of the hot mantle plume, inferred from seismic tomography.
In a similar study Baroux et al .( 2001 ) determined the local stress regime near Provence
in southeastern France by inverting focal mechanism data. They found evidence for short-
scale (tens of kilometers) variations in the direction of S Hmax close to the major faults in
three locations. Significantly different stress regimes were detected, both compressional
and extensional. The direction of the compressional stress was found to be both parallel
and rotated relative to S T .

11.5.2 Eastern North America

Mazzotti and Townend ( 2010 ) inverted focal mechanisms to determine the local state of
stress in ten seismic zones in central and eastern North America. They compared the
azimuth of the seismically determined S Hmax with S T obtained from relatively shallow
boreholes within 250 km of the seismic zones. For four seismic zones the two azimuths
were essentially parallel. However, a statistically significant clockwise rotation of

°
was found for the Charlevoix, Lower St. Lawrence, and Central Virginia seismic zones, and
to a lesser extent for the North Appalachian seismic zone ( Table 11.1 ) . The stress rotation
occurs over distances of 50-100 km for the Lower St. Lawrence, Central Virginia, and
northern Appalachian seismic zones and 20-40 km for the Charlevoix seismic zone. The
Charlevoix seismic zone consists of two clusters, the northwest cluster with seismically
determined S Hmax parallel to S T , and the southwest cluster where it is rotated clockwise
47

30-50

. The northwest cluster lies beneath the northern shore of the St. Lawrence River, with
the southwest cluster beneath the river. Seismic tomography results show that the 5-10 km
wide aseismic gap between the two clusters is associated with a high seismic velocity body
(Vlahovic et al ., 2003 ) .
The results of stress inversion of thrust focal mechanisms for five post-2007 (M 3.6-4.1)
and three earlier (M 5.0-6.2) earthquakes in the Hudson Bay region of northeastern Canada
show that locally the S Hmax strikes NNW-SSE, in contrast with the regional NNE-SSW
direction ( Table 11.1 ; Steffen et al ., 2012 ) . The authors attribute this counter-clockwise
rotation of S Hmax to the presence of a roughly E-W oriented fault zone and its combined
effect with the regional perturbation due to glacial isostatic adjustment and the regional
stress field S T .

°

11.5.3 Japan

Kawanishi et al . ( 2009 ) examined earthquakes occurring over a 10-year period with M

1.0 and depths 30 km in the Chogochu district of southwest Japan. Using inversions