If Puget Sound Is Falling Down
If Puget Sound Is Falling Down
William Steele, the Seismology Lab Coordinator at the University of Washington Geophysics Program, has a son, Chris, who goes to elementary school. “He comes in sometimes and he loves to do stuff.” It seems he’d recently put a sticker on one of the lab’s monitors, and his father had some trouble accessing the equipment. “What an excuse!” Steele never did get into the program he’d wanted to show me.
December 4th of last year there was a magnitude 5.1 quake in Klamath Falls, Oregon. Aftershocks were felt in Washington State. I had headed out to the UW in search of information on recent earthquake activity in the Puget Sound region.
“Oregon is relatively quiet next to Washington. But this year, we’ve had an enormous amount of activity in Oregon, counter to past patterns.” Klamath Falls couldn’t be noisier, said Steele, ticking off the numbers: September 4th, 5.9; Sept. 20th, 5.9, 5.0, 4.3; Dec. 4th, 5.1; and Christmas Day, 4.0, 3.4.
Most of our local activity in the Puget Sound region is recorded by the UW’s lab equipment. They have an emergency preparation computer program called “Beat the Quake,” hailing from the land of quakes, California, which has suffered through quite a lot of severe earthquake damage lately. That’s the program Steele had trouble running on his computer. Fortunately, the UW’s Seismology Lab has far more emergency preparedness information “so we don’t have to begin from ground zero” in the likely event of an earthquake. Steele is also the Public Information Officer covering quakes through the UW. “We have 135 seismic stations throughout Washington and Oregon, currently operating, and we’re expanding. We really cover a tremendously broad area.”
They locate quakes precisely, then determine the magnitude (quantity of total energy released by the quake), location (area affected by the quake), and epicenter (location on the surface directly above the focus, or place where an earthquake originates.)
They collect data about the geology of the region as well. “It’s critical data. This lab is an educational center for graduate students in geophysics.” They also educate citizens. School groups bring in students, and Steele speaks at civic organizations, encouraging people to take action and make themselves safer from earthquakes.
Of course, the big question everyone asks is, “When?”
“We’re not able to put down a date. It’s more complicated because three types of quakes occur in the Puget Sound region. The most common are deep earthquakes.
“Signals travel through the planet’s crust, sometimes all the way from the other side.” Events from anywhere show up on their helicorder sheets, making an analog, a 24-hour record, of every quake. For example, the Klamath Falls quakes, which are very near California on the Oregon coast.
“We cover the Cascade Range, and have multiple stations on every volcano. We have a good station at Mt. Baker, adequate to cover the region.” Earthquakes around volcanoes are very common.
The lab shares data with California for quakes occurring on the border of California and Oregon. “We’re part of the Washington Regional Seismic Network.” Steele showed me a map of Pacific Northwest Seismicity, 1969-1991. There were huge blue clusters in Puget Sound. What are those, I asked. “Moderate, shallow, and deep quakes. The deep clusters are in the Puget Basin.”
Deep earthquakes, the ones you really tend to write home about, are the largest in magnitude as measured on the Modified Mercalli Intensity Scale. The values usually range from 1.0 (not felt) to 7.0 (extreme damage to buildings and land surfaces). They can go even higher, as they have in recent deep quakes in Alaska.
Here’s what’s happening in Puget Sound: about 300 kilometers or more out from the coast is where the deep quakes are generated. There’s a ridge 500 to 700 kilometers out called the Juan de Fuca Ridge, and new material, new sea floor, is being deposited all the time along it. It pushes the Juan de Fuca plate toward the North American plate underneath the Seattle area. The Juan de Fuca plate moves an average of two inches a year, towards us, lifting the other plate.
A border zone locks it up, an interface between the two plates that stops the oceanic plate, making it subduct beneath us, forcing the ocean plate down into the mantle of the Earth. This boundary is called the Cascadia Subduction Zone, and extends from the middle of Vancouver Island in British Columbia down to Northern California.
The Earth’s mantle lies beneath its brittle crust. It’s semi-solid, due to tremendous heat and pressure. “Our Cascade volcanoes are probably there because of plate subduction beneath us. The push deforms the crust and builds up tremendous stresses. Right now, the coast of Washington is rising. It’s bulging up.” The oceanic plate is “cold rock” and the shock of