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IUGG XXIV General Assembly July 2-13, 2007 Perugia, Italy
(S) - IASPEI - International Association of Seismology and Physics of the Earth's
Interior
JSS004 Oral Presentation 1898
GPS Constraints on Seismic Hazards in Continental Intraplate Regions:
Eastern Canada Example
Dr. Stephane Mazzotti
Geological Survey of Canada Pacific Geoscience Centre IAG
Joseph Henton, John Adams
Compared to the long return periods, the historical and instrumental records of intraplate earthquakes
only provide a relatively short seismicity snapshot. In addition, the lack of a physical understanding of
what controls the earthquake distributions make seismic hazard assessments in continental intraplate
regions particularly challenging. Thus, hazard models are often a reflection of the earthquake records,
based on the assumption that the hazard is higher in the zones of (known) recent high activity. An
alternative model is to consider that the hazard is controlled by large-scale geological structures where
the seismicity migrates on a time scale of hundreds to thousands of years. Eastern Canada provides a
good test ground for this alternative hypothesis. Most of the seismicity is concentrated in patches along
the St. Lawrence and Ottawa valleys, which follow the late Precambrian Iapetus rift and aulacogen
system. All known large earthquakes have occurred along this major paleo-tectonic structure. We use a
combination of continuous and campaign GPS data to try to map crustal strain rates in eastern and
discriminate between the two hazard models (historical vs. geological seismic zones). The GPS data
cover periods of 4-12 years with a spatial density of 100-200 km. To a first order, the horizontal velocity
field shows a systematic south-east motion of ~1 mm/yr, with respect to stable North America, which
decreases eastward and possibly reverts to a slight north-west motion in the far-field. The vertical GPS
velocities show a clear south-east tilt from ~8 mm/yr uplift near Hudson Bay to ~2 mm/yr in New
England. These velocity fields can be attributed to post-glacial rebound from the fading of late Holocene
Laurentide ice sheet. The GPS strain rate field is not as well defined and at the limit of resolution of the
existing dataset. However, preliminary results suggest that crustal strain may concentrate in the areas
of highest historical seismicity along the rift structure of the St. Lawrence valley. A direct strain seismic
moment relationship allows us to convert the GPS strain rate to earthquake recurrence statistics, which
in turn can be used to constrain the seismic hazard models. We discuss the impact of including the GPS
data on the hazard level through the various seismic and geological zones.
Keywords: gps, seismic strain, eastern canada