What is a megathrust earthquake?
All recorded earthquakes in British Columbia and in the offshore have
occurred either within the Juan de Fuca or the North American plates.
None have occurred along the junction where the two plates are actually in contact. Thus
most recorded earthquakes represent the response of bedrock to stress accumulation within
the plates.
The apparent inactivity within the zone of contact between the Juan de
Fuca and North American plates suggests that either the two plates are locked together and
are accumulating strain or that the contact zone is well lubricated and the plates are
moving smoothly past each other. Stress accumulation that may occur between two locked
plates can result in a potentially very destructive type of earthquake, called a megathrust
earthquake.
Although no megathrust earthquakes have occurred in B.C.'s recorded
history, scientific evidence is accumulating in favour of this possibility:
 | Events in similar areas: The area where the Juan de Fuca plate is
sliding under the North American plate is strikingly similar to other areas in the world
that have experienced major earthquakes, such as Alaska, Chile and Mexico. |
| Stress building up: During the period before an
earthquake, strain accumulates in the over-riding plate causing it to buckle into three
parallel zones. It is bent down slightly adjacent to the thrust zone, uplifted just behind
that and depressed again further inland.
Detailed geodetic surveys reveal that Vancouver Island
is bulging slightly upwards and contracting in an east-west direction. These strains are
accumulating at the rate of a few millimetres over 10 kilometres per year. The pattern is
consistent with strain build up between the two locked plates.
 |
| Deformation cycle in a subduction related megathrust
zone. (After diagram in GEOS 1988, volume 17, number 3, p. 6) |
|
| Past earthquakes: Although megathrust
earthquakes have not been recorded in British Columbia, past earthquakes may have left a
geological record. After the earthquake, the strain accumulated is released and vertical
deformation results in the over-riding plate that is the reverse of that seen during
stress build up. Thus areas like Vancouver Island, that have risen slightly before the
quake, suddenly subside.
 |
| Geologists studying sediments for evidence of
prehistoric earthquakes. |
Geological studies of prehistoric soils in marsh areas
along the coasts of British Columbia and the state of Washington reveal a pattern of
successive buried peat layers. The peat layers are overlain by marine muds that are
deposited during the subsidence following the earthquake. Sometimes, there is a sand layer
immediately on top of the peat that could have been deposited by a tsunami after the earthquake. The marine sediments grade
back up into new peat layers as stress builds up and the land rises again to form a new
marsh. Investigations have revealed several cycles of subsidence and uplift suggesting
that megathrust quakes may have occurred every few hundred years or so in the past. The
last of these events occurred about 300 years ago and affected the coast from B.C to
California.
|
 |
Buried peaty soil, the remains of a
marsh that subsided 1.5 m during an earthquake 300 years ago. The lighter coloured sand
layer was then deposited by a tsunami that followed the earthquake. |
These phenomena could be important indications that we
may be due for a megathrust quake sometime in the future.
Earthquakes in B.C. Home Page
Previous Page - Where do earthquakes occur?
Next Page - What are the effects of earthquakes?
 |
Earthquakes in British Columbia was
originally produced as Information Circular 1991-6 by the BC Geological Survey of the
B.C. Ministry of Energy, Mines and Petroleum Resources, in cooperation with the B.C. Ministry of
Environment, the B.C. Provincial Emergency Program and with the assistance of the
Geological Survey of Canada. It has been amended and updated in this WWW version. |
Copies are available from:
Publications
Office
BC Geological Survey
Ministry of Energy, Mines and Petroleum Resources
P.O. Box 9333. Stn Prov Gov't
Victoria, B.C.
V8W 9N3
This page was last updated
Wednesday, March 08, 2006 |