March 19, 2006
New research by University of Washington scientists indicates sediment buildup in tectonic plate deformations may play a major role in determining the severity of certain subduction zone earthquakes. Subduction zones — areas of offshore ocean floors where two tectonic plates meet, with one sliding beneath the other — are often where powerful earthquakes occur, including quakes in Indonesia in 2004, Alaska in 1964, Chile in 1960 and the Pacific Northwest in 1700.
The UW research may help determine which subduction zones — and which areas within a zone — might produce the most severe movement when they rupture.
As the subducting plate slides beneath the upper plate, stress begins to build where the plates meet and the upper plate can deform to create a large structure — “a forearc basin.” The basin, a bowl-shaped depression, fills with sediment from nearby rivers that empty into the ocean. Over millions of years, the sediment typically accumulates to great depths, from one-half mile to nearly 2 miles, and in rare cases, 3 miles deep, according to Christopher Fuller, a UW doctoral student in Earth and space sciences.
“In many of them, the sediment will stop the deformation of the upper plate,” said Fuller. “The simplest way to think of it is that the increased weight of the sediment stops the deformation from occurring.”
It appears the most severe subduction zone earthquakes occur in areas where such sediment-filled basins are found, but the reasons aren’t exactly clear. Fuller and his colleagues conducted computer simulations of force experienced during plate subduction to determine how sediment buildup influences major earthquakes. They found that the weight of the sediment strengthens the edge of the plate directly above where earthquakes occur. The stronger edge is deformed far less by subduction than nearby areas without such basins, said Fuller, and that increases the likelihood that large earthquakes will occur in regions with basins.
The Cascadia subduction zone off the coasts of Washington, Oregon and northern California contains forearc basins in several areas, according to Fuller. As it moves to the east at 2 inches a year, the Juan de Fuca tectonic plate slides beneath the North American plate that contains the landmass of the Pacific Northwest. In the process, sediment as deep as 1.5 miles is scraped off the top of the Juan de Fuca plate and is deformed into surface depressions on the North American plate, forming the basins where sediment from coastal rivers is deposited. The probability of large earthquakes is greatest in these areas.
Hypotheses for why severe earthquakes are associated with forearc basins include:
- Changing conditions where the plates meet. As the top part of the subducting plate meets more resistance and travels at a slower speed than the bottom part of the plate, greater strain is created in the plate. This is regarded as the least likely scenario.
- Fault strengthening over time. As the seal on a mayonnaise jar lid develops and strengthens when it hasn’t been opened for weeks, slight movement on the fault caused by deformation within the upper plate (like tapping the jar lid) loosens the bonds and then the fault suddenly moves (like twisting the jar lid). Fuller believes this is the most likely hypothesis.
- Fluid between the plates becomes superheated under pressure and, like water boiling in a teakettle, the fluid pushes upward and counteracts the downward pressure from the upper plate, allowing the fault to rupture.
The modeling research, explained in an article by Fuller and colleagues, was published in the February issue of Geology.
The modeling could have implications in figuring out where, within a subduction zone such as Cascadia, great earthquakes are most likely to occur, said Fuller. However, the modeling is not applicable to every subduction zone because each has different characteristics. For instance, forearc basins do not play the same role in the subduction zone off the Indonesian island of Sumatra, where the massive 2004 earthquake triggered tsunamis that killed hundreds of thousands of people.
Source: University of Washington.