British Columbia 1700
Subduction earthquake off the
west coast of Vancouver Island. Coastal native
village destroyed and native houses on Vancouver
Island may have been damaged.
earthquake occurred about 9 pm on the night of
January 26 in the year 1700. This was almost 100
years before the first European explorers made
contact with native peoples on the west coast.
The time and date of the earthquake are known
because the large tsunami, or tidal wave,
generated by the earthquake swept across the
Pacific Ocean and did considerable damage at
several places along the east coast of Japan.
Japan has a written history dating to that time,
and the size of the tsunami and its arrival time
were well recorded. Using this, and other
historical data, scientists were able to
calculate the time of occurrence, location, and
size of the generating earthquake.
This was one of
the world's great earthquakes with a magnitude
estimated at 9. Oral traditions of the native
peoples of Vancouver Island suggest the tsunami
destroyed a village at Pachena Bay on the west
coast of Vancouver Island, and that the shaking
damaged houses in the Cowichan Lake region of
south central Vancouver Island.
occurred on the Cascadia Fault, the boundary
between the North American and Juan de Fuca
plates. This type of large earthquake is now
known as a megathrust earthquake.
A megathrust fault is the
boundary between a subducting and an overriding
plate. A megathrust earthquake is produced by a
sudden slip along this fault. The world's largest
earthquakes are all megathrust earthquakes.
Megathrust earthquakes have
never been observed in the short (~150 year)
written history of the west coast of Canada, but
there is compelling evidence that they did occur
in prehistorical times. Some of this evidence
marsh or coastal forest soils point to sudden
land subsidence of about 1 metre occuring at
the same time from Vancouver Island to Northern
Changes in tree
ring growth from coastal old-growth also
suggest a sudden, widespread subsidence and
drowning of roots
Sand layers on
top of the buried coastal marshes, driven in
from offshore bars by the wave of the large
tsunami that rushed into the subsided coastal
(landslide) layers on the deep sea floor far
off the coast from underwater landslides, likely
caused by strong seismic shaking
- marine organisms swept into and preserved in
the bottom muds of coastal lakes that are
separted from the ocean by land elevations of
some 5 m high
- large tsunami in Japan with no local Japanese
earthquake. Modelling this tsunami has revealed
that the most recent earthquake for Cascadia was
M ~ 9.0 and occured on January 26, 1700, at
around 9 pm!
The last point
agrees with some legends of first nation people.
One legend describes a severe ground shaking
on a winter night accompanied by huge waves that
destroyed a coastal village. This legend is
likely reporting the effect of the last
seismic structure studies are being used to
estimate the downdip extent of the potential
seismic rupture zone. The landward extent of the
rupture is an important factor for the shaking
hazard at the inland cities of Victoria
and Vancouver. The potential for the next
megathrust earthquake for Cascadia is also being
studied by monitoring the deformation of the
crust using very precise satellite technology
(GPS), repeat levelling, changes in gravity, and
long term tide gauge measurements.