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                           EARTHQUAKE REPORT

                       NATURAL RESOURCES CANADA
                       GEOLOGICAL SURVEY OF CANADA
                       SIDNEY, B.C.  V8L 4B2
                       Voice:(250) 363-6500
                       Fax:  (250) 363-6565


EARTHQUAKE RESEARCH NOTE


SCIENTISTS MONITOR A PREDICTED EPISODE OF DEEP FAULT SLIP DEEP BENEATH
VANCOUVER ISLAND AND PUGET SOUND

A forecast episode of the phenomena called "Episodic Tremor
and Slip" (ETS) has begun on the Cascadia subduction zone. Seismographs
of both the Geological Survey of Canada (part of Natural Resources Canada) and
the University of Washington seismic networks have been tracking deep non-earthquake
tremor activity which initiated more than a week ago in the region of central
Puget Sound.  The tremors have slowly migrated north and are now in the region
of southern Vancouver Island. At the same time Global Position System (GPS)
data from the Western Canada Deformation Array (WCDA) in Canada and the
Plate Boundary Observatory (PBO) in the United States reveal that GPS
sites along the tremor path are systematically moving in a reversed direction.
Coastal GPS sites normally move slowly inland up to a centimetre per year,
documenting the strain accumulating for the next giant Cascadia earthquake.
However, during an ETS episode they reverse direction and head seaward.
These small systematic surface displacements of a few millimetres accompanied
by the distinct tremor activity mark an ETS episode.  

In this geographic region ETS episodes have been observed to re-occur roughly
every 14 months with a variation in the repeat time of about two months.
This regular behavior allows them to be forecast. ETS events typically last
two to three weeks. The last episode was in September 2005.

Present understanding of ETS suggests slow slip of several centimetres is
happening on the deeper portion of the Cascadia subduction fault at depths
of 25 to 45 kilometres This slip causes a small change in stress which
triggers the tremor activity. Each event adds stress to the upper locked
portion of the subduction thrust fault, bringing it closer to failure.
Further research is required to determine whether these very small stress
increments play a significant role in triggering giant megathrust earthquakes. 

For Additional Information please visit the following NRCan websites:

ETS Events:
http://gsc.nrcan.gc.ca/geodyn/ets_e.php

Megathrust earthquakes: http://earthquakescanada.nrcan.gc.ca/historic_eq/15-19th/1700/1700_e.php

Earthquake Information: http://earthquakescanada.nrcan.gc.ca/index_e.php

Current ETS monitoring: http://tsea.pgc.nrcan.gc.ca:8080/wcda/etsmon_e.php


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