17/02/2015
Ancient trees testify to a quake
These logs are an ancient forest on the Oregon coastline that is popping out of the ocean today. The trees grew about 1000 years ago, but the huge forces in the Earth buried them in sediment and then exposed them again.
Oregon sits above the Cascadia subduction zone, where a small piece of ocean crust call the Juan de Fuca Plate is sinking into the mantle beneath North America. As that plate sinks, it is pushing against the continent and causing it to bend.
This bending drags some parts of a continent beneath the ocean while other parts are pushed upward. These trees sit on land that gradually is pushed upward as the continent bends and stress on the fault builds. 1000 years ago, this land stayed above water long enough for large trees to grow, but then the fault broke.
A large earthquake released the stress and let the land snap back to its original shape. When that happened, these trees sank beneath the ocean, killing them. They were then far enough below the water that they were buried by sediment that entombed them for a millennium.
The last earthquake on this subduction zone took place about 300 years ago, so stress is building again. The continent is bending, pushing the submerged trees back above water and allowing recent storms to expose this ancient forest.
In the 2004 Sumatran earthquake, several islands popped up during the quake (see: http://tmblr.co/Zyv2Js1d1NVcI). These trees sit on land that moves the opposite direction; it rises in-between earthquakes and then sinks rapidly when the fault breaks.
Finding trees like this is a type of paleoseismology. In areas where there are no written records it is difficult to know exactly when earthquakes took place, but it is really helpful to know that there was an earthquake about 1000 years ago. Finding trees like this that died on the day of an earthquake gives an approximate date of the earthquake, letting scientists extend our knowledge of the fault’s behavior back in time that much further.
-JBB
Image credit: Wolfram Burner
https://flic.kr/p/chXVMU
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