Along a 1,000 kilometer stretch from Northern Vancouver Island to Cape Mendocino, California lies the Cascadia Subduction Zone: the area where the Juan de Fuca Plate slides beneath the North American Plate. The Cascadia Subduction Zone, however, is not your typical subduction zone. Rather, it is characterized by its unique “slow slip events” that occur in the intervals between megathrust earthquakes along the fault.
A subduction zone forms when dense oceanic plate collides with a less dense continental plate sliding beneath it. Subduction zones are often marked by volcanic arcs, oceanic trenches, and earthquakes that are both shallow and deep, deepening as one plate subducts further beneath the other. The Cascadia Subduction Zone has produced a number of earthquakes with magnitudes greater than 8, often referred to as “megathrust” earthquakes. Over time, Cascadia becomes locked by frictional forces, causing strain to slowly build until the rocks slip past each other. When the rocks slip, it causes an intensely powerful earthquake. The transition zone, or ETS zone, denotes the section of the plate boundary beneath the “locked” portion, and slow slip events occur below this zone.
The slow slip phenomenon, also known as episodic tremor and slip (ETS), is an episode of relative tectonic plate movement and high frequency seismic tremors. These episodes occur over regular time intervals, often lasting a few weeks whereas earthquakes occur in seconds or minutes. Since this motion occurs slowly and over a longer period, people nearby do not feel these tremors. During that time, the transition zone slides only a few centimeters to relieve stress on the plate boundary in that particular area. However, while the slow slip in one area relieves stress, it consequently adds stress to the locked parts of the fault. Slow slip motion occurs very slowly, making it difficult to accurately record. Nonetheless, it is possible to measure movement of the surface with GPS instruments during these slow slip events.
This USGS diagram shows the Cascadia subduction zone and the associated hazards. On the bottom right the diagram shows the “locked” the “ETS” zone (sometimes referred to as the transition zone) and the “stable slip zone” which is where the slow slip phenomena occurs.
What does this mean for hazards?
Subduction zone earthquakes tend to be large, which can lead to great destruction. Part of the reason that these subduction zone earthquakes are so large is because of the slow slip phenomenon. The slow slipping that is occurring farther inland releases stress on the slip zone but causes tectonic stress to build up along the shallower locked portion of the fault. As a result of these slow slip events, there is currently 300 years of stress built up since the last megathrust earthquake. Once enough stress is built up from movement along the slip zone, the locked zone will release this accumulated stress and rupture. This rupture can involve 10 to 20 meters of fault slip, producing a large subduction zone earthquake.
These megathrust earthquakes can be extremely destructive. Evidence for the destruction of the 1700 Cascadia megathrust earthquake includes oral histories from indigenous people from the region, records of an “orphan tsunami” hitting Japan’s shore, age dating of coastal trees that were submerged when land moved downward during the earthquake, and underwater landslide records off the coast of the Pacific Northwest.
The Cascadia Subduction Zone is different from other subduction zones around the world because there are very few earthquakes here. This indicates that the locked zone on the Cascadia Subduction Zone is more strongly locked than other subduction zones and that there has been no outlet for the stress build-up since the last megathrust earthquake. This could potentially produce larger earthquakes along the subduction zone, resulting in widespread destruction in the Pacific Northwest. Subduction zone earthquakes vary in magnitude but tend to be large and destructive, so this build-up could mean the difference between a magnitude 8 earthquake and a magnitude 9 earthquake, which has 10x the ground motion.
Large-scale destruction in the Pacific Northwest would be devastating for the inhabitants of the area and since there are not frequent earthquakes in this region, people may be less prepared for earthquake hazards. To combat this, a recent interdisciplinary research study at Lewis and Clark University has created a video game named Cascadia 9.0 that educates its players on disaster preparedness for the inevitable future megathrust earthquake.
Catching slow slip in the act
Since slow slip happens deep below Earth’s surface and consists of low magnitude tremors that cannot be felt by humans, how do we know that they exist? Previous research shows how slow slip events occur on short (days to weeks) and long (months to years) scales, and these events can be recorded by GPS stations at the surface. GPS data shows that during the events there is westward movement of the continental plate, indicating slip, followed by a return to eastward movement like a spring being compressed until the next slow slip episode releases it again. The result is a “sawtooth” pattern of movement measured by the GPS stations.
Understanding slow slip events is important because these events offer an insight into the earthquake cycle that subduction zones experience. If there is a defined relationship between slow slip events and the much larger megathrust earthquake events, then we might be able to better understand their nature. Geophysical instruments like GPS stations, borehole strainmeters, and seismometers are extremely important for monitoring and understanding these slow slip events because the movement along the subduction zone is so minute that they would not be noticed otherwise—and these seemingly small events can have large-scale consequences.