Konrad Chrzastowski
The Cascadia subduction zone is a convergent plate boundary that stretches from the North of
Vancouver Island to Northern California. It consists of a sloping fault that separates the Juan de Fuca and
North American Plates. It stretches from the Queen Charlotte Triple junction in the north to the
Mendocino triple junction in the south
Off the Shore of BC, Washington, Oregon and California the Juan De Fuca Plate is subducting
beneath the North American Plate. The North American Plate is overriding the oceanic plate as it moves
in a southwest direction. The Cascadia Subduction Zone is where these two plates meet. The Oceanic
crust of the Pacific has been sinking underneath North America for approximately 200 Million Years, and
is currently diving at a rate of 36mm/yr. The width of the CSZ varies as it depends on the temperature of
the subducting plate as it heats beneath the continent. As it deepens it loses ability to store mechanical
stress and generates earthquakes. Or in the case of a mega thrust like the Cascadia zone the plates are
locked together and frictional stress builds. When the stress exceeds a critical value a sudden failure
occurs along the fault plane that releases strain energy and radiates seismic waves.
At the contact between the two plates, the area of the subduction zone fault could produce a
magnitude 9.0 earthquake which would be devastating. The last time it ruptured was on January 26,
1700. The compression between these plates results in the overlying continental plate actively
deforming. This causes smaller earthquakes than the looming 9.0 mega thrust that is often talked
about. These are common within the deforming part of the North American Plate; and within the brittle
down-going slab of the Juan de Fuca Plate as the brittle slab bends in its dive beneath the North
American Plate.
The coastal areas of the region appear to be moving backwards, apparently being shoved by the
diving plate. Evidence for this can be seen in GPS data. This is called episodic tremor and slip. There are
slow slip events on the mid portion of the subduction zone as it is stuck, and they are accompanied by
unique non-earthquake tremor-like seismic signals that emanate from the same region during slip back.
Occasionally backslides occur as some energy is released. The backslides are accompanied with tremors.
These occur approximately every 14 months as a slow slip that is equivalent to a 6.5 magnitude
earthquake. While these usually occur in 10 seconds the duration of theses slow slip events is two or
more weeks. ETS events occur along the entire length of the Cascadia subduction zone it turns out. With
a periodicity of 11 months off the coast of northern California and every1 8 months off the coast of
central Oregon
The last great mega thrust to rock the Cascadia subduction zone occurred on January 26, 1700. This
earthquake is particularly useful to geologists as it was the last one to hit the region and can offer clues
as to what happened, how to prepare for the next one and when and where the big one will occur.
Geologist mapping Cascadia in the 80s found multiple lines of evidence for a massive Tsunami caused
from the earthquake to have hit the coast in the past. Evidence consisted of:

Evidence for subsidence

Sand sheets beside bays and river mouths

Subsided soil below mud dated to 1700

Decayed tree stumps associated with short term flooding

Written history of indigenous tribes

Radiocarbon dating of soils and tree rings

Turbidity current records
Many things point to a huge earthquake originating from Cascadia around 1700. There are mud
records in Oregon carbon dated to around 1700. As well as tree ring layers that terminate in the 16991700 year season. Which are associated with known flooding of coastal forests associated with the
tsunami that followed the quake. As well as written records from Indigenous people in America as well
as many written accounts coming all the way from Japan of a huge Tsunami hitting their coast.
The timing of the “Big One” is ambiguous. There is geological evidence that indicates that large
earthquakes have occurred at least seven times in the last 3500 years with an average interval of
approximately 540 years in between these events. There is also evidence of Tsunamis associated with
the large earthquakes. The next Tsunami could reach up to 20m in height if the whole zone were to get
fractured. The last large Tsunami in 1700 had waves of at least 10m in height based on models and
human accounts. There are many accounts from Japan that are dated to the hour which gives us a very
precise time to correlate the 1700 Cascadia earthquake by backtracking using a model. If a tsunami were
to occure the more populated cities along the coast are luckily protected by natural barriers (Vancouver,
Victoria, Seattle) or are inland (Oregon) from a potential Tsunamis danger. However many smaller towns
would be devastated by a large Tsunami and although much preparation is in place for an earthquake it
is estimated that just the damage for Vancouver would be approximately 3.5 Billion dollars. With untold
amounts of damage from the earthquake and tsunami worldwide.