Seismicity Map
Click
here for a figure displaying
the depth distribution for these earthquakes.
Click
here for a figure displaying the magnitude distribution
of these earthquakes.
The above mapsheets show the locations
of all earthquakes that have occurred in the eastern Juan
de Fuca Strait between January 1981 and December 1999. They
are all of magnitude <= 4.0.
Before 1981 the seismic network was too sparse to provide
adequate depth control over this region. Earthquakes in southwestern
British Columbia and the Pacific Northwest occur in three
main source zones; along the subduction margin at the contact
between the Juan de Fuca and North American plates, within
the subducting Juan de Fuca plate, and within the North American
crust. It is events from this latter source zone which are
presented in the maps above. In many respects, it is these
crustal events that may pose the most significant hazard because
they can occur anywhere within the crust, local to major population
centres, and at shallow depths.
For comparison purposes, in this
atlas, the earthquakes have been divided into a shallow suite
(0-5 km and 5-10 km) and the entire suite which encompasses
both North America and Juan de Fuca plate events, the deepest
event being within the Juan de Fuca plate at a depth of 64
km. In general, the events near the surface have poor depth
control with errors ranging up to 10 km. Errors generally
decrease with depth; events around 20 km depth typically have
errors on the order of 1-3 km. The locations shown here are
"raw" locations from the Geological Survey of Canada earthquake
catalogue, each event being located with a set of stations
from the US and Canada which are at uncontrolled azimuths
and distances from the epicentre. No effort has been made
to optimize the locations of these earthquakes with respect
to one another and any interpretations made from this data
set must take this into account.
North America plate earthquakes
are distributed through the crust with the largest concentration
around 20 km and a maximum
depth of approximately 30 km. The deep events are found below
30 km depth and occur within the Juan de Fuca plate, forming
an approximately 10 km thick zone of events dipping to the
east.
Reference citation:
Mulder, T., and Rogers, G., 2000.
Recorded seismicity of the eastern Juan de Fuca Strait, in:
Mosher, D.C. and Johnson, S.Y. (Eds.), Rathwell, G.J., Kung,
R.B., and Rhea, S.B. (Compilers), Neotectonics of the eastern
Juan de Fuca Strait; a digital geological and geophysical
atlas. Geological Survey of Canada Open File Report 3931
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