The Cascadia Subduction Zone, or CSZ, runs from Northern Vancouver Island in BC, Canada to Cape Mendocino in California. A subduction zone is created when one tectonic plate sinks and slides, or subducts, below another tectonic plate. Usually, this occurs with a dense oceanic plate subducting below a less dense continental plate. In the case of the CSZ, the Juan de Fuca plate is sinking below the North American plate. The Juan de Fuca plate is located in between the Pacific plate and the North American plate, making up the ocean floor along the coastline. The Juan de Fuca plate is being pushed towards the North American plate by a mid-ocean ridge between itself and the Pacific plate. A mid-ocean ridge is where new material in the form of magma is being pushed to the surface, causing the ground on either side to spread. The Juan de Fuca plate is a remnant of a much larger plate that is gradually disappearing as subduction, which is driven primarily by gravity pulling down on the dense subducting plate, overtakes spreading from the ridge. Further south, the ridge has subducted entirely and the Pacific plate has come into contact with the North American plate, creating the San Andreas fault system.
When one plate slides beneath another, it’s not a smooth process. The weight of the overlying plate wedge generates a lot of friction, which will lock the plates together until the frictional forces are overcome. An earthquake occurs when plates overcome friction in a certain location, allowing stress to be released and the plates to slip past one another. These slips can be slow or fast, small or large. Subduction zone earthquakes are located along the line of subduction, where the most severe slipping can occur. Here, the plates are strongly locked by friction, building up stress until there is a large release in the form of a megathrust earthquake. Topography on the subducting plate, sediments and fluids trapped between the plates, and the geology of the upper plate all vary along and across the plate boundary and affect the frictional stress on the plate boundary. All of these factors make predicting earthquakes extremely difficult.
Earthquakes also occur deeper down within the subducting Juan de Fuca plate or shallower within the North American plate. At greater depth along the plate boundary is a transitional zone, where there are distinct episodes of slow slip accompanied by seismic tremor. During a slow slip event, the Juan de Fuca plate will move a few cm past the North American plate over a period of a few weeks. These motions are imperceptible to all but the most sensitive GPS instrumentation. However, they relieve stress between plates just like megathrust events. At still greater depth, the plates move past each other continuously because of the impact of high temperature and pressure on the plate boundary. Processes deep within the subduction zone are also responsible for the beautiful Cascade volcanoes.
Because stress builds up gradually over time, large earthquakes do not occur often. The last megathrust earthquake on the CSZ was in 1700. Based on geologic evidence, earthquakes like this have recurred every 400 to 600 years. This suggests that we are due for a megathrust event in the next hundred years or so. The timing and size of this expected earthquake, however, is impossible to predict with our current understanding. Observations from megathrust earthquakes around the globe suggest that at least some earthquakes had subtle precursors that were not previously recognized because of inadequate data. A better understanding of the mechanisms causing such precursors may eventually improve earthquake forecasts.
One of the goals of Cascadia2021 is to improve our imaging of the plate boundary using techniques analogous to those used to image within the human body. If we know what the interface between the Juan de Fuca plate and the North American plate looks like, we can better understand the forces at play and assess the risk of a large slip event in the near future. The more detailed images we obtain, the better our models of expected ground shaking will be for various earthquake scenarios.
– Kaisa Autumn
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