World Stress Map Conference - International Lithosphere Program ...

Friday, October 17 th
Oral Session V: Seismotectonics and hazard
Stress orientations in eastern North America seismic zones
Stephane Mazzotti and John Townend
Geological Survey of Canada
smazzotti@nrcan.gc.ca
Crustal stress orientations in eastern North America are fairly homogeneous over spatial
scales of thousands of kilometers. Borehole measurements constrain the axis of maximum
horizontal compressive stress (SHmax) to lie in the NE-SW quadrant and focal mechanisms
reveal a transition from primarily reverse in Canada to strike-slip in the eastern United States.
Although most earthquakes are compatible with these large-scale stress conditions, some
large earthquakes, notably the 1925 M=6.2 Charlevoix, have focal mechanisms that imply
localized stress perturbations. Better defining the regional stress field and local stress
perturbations is critical for understanding the source of intraplate earthquakes and the
associated hazard. We use a Bayesian inversion technique to estimate the principal stress
orientations from focal mechanisms in eight eastern North America seismic zones. Six of
these zones are associated with, and probably reactivate, Iapetus rift faults. The remaining two
are not associated with known tectonic structures. In four of the eight zones (Montreal, West
Quebec, Eastern Tennessee, and New Madrid), the inverted SHmax direction is parallel to
nearby borehole breakout observations. For the Ottawa and North Appalachian zones, SHmax is
less well resolved due to the small number of available mechanisms but is likewise parallel
with the direction obtained from borehole data within the estimated uncertainties (±30°). The
remaining two seismic zones (Charlevoix and Lower St Lawrence) stand out. In both zones,
regional borehole data indicate a SHmax direction of N050°, parallel to the St. Lawrence valley
and the main Iapetus rift faults. In the Lower St Lawrence seismic zone, SHmax can be
estimated from focal mechanism data with a posteriori uncertainties of ±20° and reveals a
clockwise rotation of ~50° relative to the regional borehole SHmax direction. The Charlevoix
seismic zone encompasses two main earthquake clusters. The NW cluster (underneath the
north shore) yields a SHmax orientation of N054°±12°, parallel to the regional borehole trend,
whereas the SE cluster (underneath the river and south shore) yields an orientation of
N098°±13°, rotated ~45° clockwise relative to the regional trend. The Lower St Lawrence and
Charlevoix rotations are similar to that observed for the aftershock sequence of the 2003
Bardwell earthquake north of the New Madrid seismic zone. The source of these pronounced
rotations remains unknown, but is probably related to the mechanical conditions that produce
the regionally anomalous earthquake concentrations themselves.
102 3 rd World Stress Map Conference, 15.-17. October 2008 in Potsdam