Environmental science Professor Robert Butler discusses earthquakes and tsunamis

By The Beacon | March 23, 2011 9:00pm

(The Beacon)

By Hannah Gray, News editor -- gray12@up.edu

Q: What causes earthquakes?

A: The two-word answer is plate tectonics, which is motion between lithospheric plates. On their boundaries, forces build up and get released.

This particular plate boundary that released its stored energy on March 11 was the boundary where the Pacific Plate dives beneath Japan. The plates are converging, moving towards each other, and the Pacific Plate bends down and dives down into the Japan trench.

That's a line of deep ocean floor, off the east coast of Japan, and the Pacific plate is a slab which is about 100 kilometers thick, and it dives down into the deeper mantle below Japan.

Q: What causes tsunamis?

A: To produce a tsunami, an earthquake has to move ocean water. Most tsunamis are produced by great, shallow earthquakes. And when we mean shallow, we mean 10 kilometers.

This particular earthquake actually initiated at about 24 kilometers depth, and then it ruptured a big patch of the plate boundary. It changed the shape of the ocean floor, and produced a big mound of ocean water.

Q: What does "9.0" mean?

A: There a bunch of different magnitude scales for earthquakes. The most effective scale to use is a moment magnitude scale.

That actually uses information such as the displacement, the opposite sides of the fault – how far did one side of the fault move with respect to the other side of the fault – and it also uses the area of the fault that got moved.

Q: Explain the risk and what possible scenarios could occur in the Portland area and at UP.

A: The Pacific Northwest has three kinds of earthquakes. One kind is what would be considered to be a deep earthquake.

Another kind is crustal earthquakes. These are on faults, where the North American crust is broken. There is large crustal fault called the Portland Hills Fault. It's immediately across the Willamette River from UP. We know the fault is capable of certainly having magnitude 6.0 earthquakes, maybe even magnitude 7.0 earthquakes.

But we don't know very well, at least, when the last earthquake occurred on that fault. That means it makes it difficult to assess the risk which is posed by that crustal fault.

There is another fault called the East Bank Fault which is basically mapped to go along parallel to the East Bank of the Willamette River, and it runs under the UP campus.

That fault is really hard to evaluate. We know it's there, but we don't have a very good idea about what risk it does or does not pose.

The third kind of earthquake is what people around here in the Pacific Northwest region call "The Big One." This is a great earthquake occurring on the Cascadia Subduction Zone.

We know the last one of these great Cascadia earthquakes, which was probably a magnitude 9, occurred on Feb. 26, 1700 at about 9 p.m. That one is parallel to this great earthquake that just occurred in Japan. It's the same kind of plate boundary.

The 1700 earthquake produced a tsunami which kind of did the mirror image of what the Japan earthquake did – that is, the Cascadia 1700 earthquake created a tsunami that arrived in Japan and caused damage.

- Butler will present "March 11, 2011 Magnitude 9.0 Earthquake and Tsunami in Northern Japan: Comparisons with Past and Future Earthquakes and Tsunamis in Cascadia" today at 4:30 p.m. in Buckley Center, room 163.

-Hannah Gray