Thursday, February 16, 2012

Tuesday's Quake

You may (or may not) have heard about the earthquake that occurred off the South Oregon coast this past Tuesday night. Not a lot of folks reported feeling it as it was situated roughly 160 miles west of Coos Bay. The 6.0 magnitude earthquake was unusually strong for an area that does have frequent tremors. So the fact that an earthquake occurred at this location is no reason to be alarmed. But was the strength of the quake something that coastal dwellers and Oregonians as a whole need to worry about? The short answer is: No.

Whenever an earthquake occurs under ocean waters, there is a period of time when we are on "pins and needles" awaiting word on if a Tsunami was generated or not. Luckily, no Tsunami was generated from Tuesday nights quake. The main reason was due to its location. The earthquake occurred along a fracture zone known as the Blanco Fracture Zone. This zone divides the very large Pacific Plate and the locally infamous Juan De Fuca Plate. The graphic below visually explains where the quake happened. The overall movement of the Pacific Plate is away from us. The Juan De Fuca Plate is shifting towards us. So when an earthquake occurs in the Blanco Fracture Zone, the overall movement of the Earth's plates is horizontal. This type of fracture zone is called a "Transform Boundary fault". The most famous of all fault lines in the United States is a transform boundary fault line, the San Andreas Fault. That fault is located in California and is the result of the Pacific Plate and North American Plate rubbing against each other.

Quake centered on the Blanco Fracture Zone
The reason no Tsunami was generated from the Blanco quake is because the overall movement of the plates were horizontal, not vertical. Most Tsunami are created as a result of uplift from one plate moving under another. The displacement of water results in Tsunami. So as long as earthquakes occur in the Blanco zone, the Oregon coast should be spared from Tsunami.

The scientific community, in addition to the media, have been warning the population of the United States west coast that the "Big One" is imminent. Many are predicting a 9.0 magnitude quake will strike again, just as it did on January 26th, 1700.  That quake occurred in a region known as the Cascadia Subduction Zone. Guess what? Oregon is a part of that zone. The Cascadia Subduction zone is an area where the Juan De Fuca Plate is literally sub-ducting, or folding underneath the North American Plate which we are anchored to. The 2011 Japan earthquake struck in a subduction zone where the Pacific Plate moves underneath the Okhotsk plate. As we all know, that resulted in a 9.0 earthquake and a deadly Tsunami that rushed over 6 miles inland and crossed the entire length of the Pacific Ocean!
The Cascadia Subduction Zone sits right off the Oregon coast

Subduction zones are infamous for large quakes and Tsunami. Records from Japan that date back to1700 indicate that a Tsunami reached the eastern shores of Japan as a result of the 9.0 earthquake just off of our coast! The Cascadia Subduction Zone is the region that scientists are looking at for the next big quake. Oregon's coast has been outfitted with extensive Tsunami alert systems and warning signs but there is no way to prevent the Tsunami. Let's hope that this doesn't happen in our lifetime.