Whenever ‘The Big One’ Hits, It Should Be No Surprise Saturday’s Earthquake Eerily Similar To The One That Heavily Damaged Kobe

If “The Big One” ever hits the Puget Sound basin, no one can claim we weren’t warned.

Saturday’s magnitude 5.0 earthquake, the biggest here in 30 years, was eerily similar in its geology to the much bigger 7.2 quake that heavily damaged Kobe, Japan, two weeks ago, said Bill Steele, seismology-lab coordinator at the University of Washington.

The 7:11 p.m. tremor buttressed recent warnings from geologists that “it can happen here.” Their concern has been prompted by major quakes in California in 1989 and 1994 and a series of recent scientific discoveries about historical earthquakes in this state.

“I think everyone is spooked,” Steele said of local seismologists.

He explained that Saturday’s quake was a mild version of the type of seismic disaster that officials particularly fear.

It was relatively shallow - the closer a quake is to the Earth’s surface, the more its energy is felt - and it occurred in the state’s most heavily populated area. Moreover, it was possibly triggered by the Seattle Fault that severely shook this area with a shallow 7- to 7.5-magnitude quake 1,100 years ago.

Saturday’s earthquake was different than the deeper earthquakes that hit Puget Sound in 1965 and 1949. Those occurred at the boundary between the North American Plate and the Juan de Fuca Plate far beneath Puget Sound.

The Earth’s surface, including its sea floor, is made of crustal plates that float and move on its hot, molten interior.

Their slow collisions generate earthquakes and volcanoes.

The Juan de Fuca Plate sea floor dives under North America off Washington’s coast, angling down under Puget Sound until it merges near the Cascade Mountains with the planet’s mantle, the hot molten layer of rock between the Earth’s crust and its core.

Saturday’s earthquake, in contrast to those in ‘65 and ‘49, was not 30 miles deep at the plate boundary but about 11 miles deep at a fault in the middle of the continent’s crust. Its epicenter has been located on the eastern tip of Maury Island, across Puget Sound from Federal Way.

Still unclear is what underground

feature triggered Saturday’s earthquake. U.S. Geological Survey (USGS) geologist Brian Atwater has broached the idea that the Seattle Fault, first identified in 1992 as the source of the devastating earthquake 1,100 years ago, could have been a cause.

Atwater suggested the Seattle Fault may not be vertical but instead slope southward and downward from central Seattle, possibly extending as far as Maury Island and Federal Way. Another wedge-shaped block of land would rest atop it, and the joint between these two may have slipped Saturday, causing the earthquake.

Scientists at the University of Washington’s geophysics laboratories will discuss that hypothesis this week.

Steele said Saturday’s earthquake is neither a predictor of more earthquakes in the near future nor a guarantee of fewer earthquakes because pressure was relieved. Puget Sound’s geology is simply too complicated and too unknown to draw either conclusion.

Because the earthquake intensity scale is logarithmic, there was a big difference between Saturday’s magnitude 5.0 earthquake and Kobe’s 7.2. The Japanese quake had more than 100 times the ground motion and more than 1,000 times the energy.

That does not mean Saturday’s tremor was insignificant. In simple energy, it approached that of the 1946 Bikini Atoll atomic-bomb test, according to a table in Bruce Bolt’s text “Earthquakes.”

And there were intriguing similarities between the Seattle jolt and that of its Japanese sister city.

Both cities are about the same distance from offshore trenches that mark the collision boundary between two plates. Both are on inland waters and have large areas of filled tidelands. Both quakes were relatively shallow - about 11 miles deep - and both were thus rare because they were relatively big and yet not on plate boundaries.

While no serious damage appears to have been done, the tremor is another reminder that Western Washington is, in the words of USGS coordinator Craig Weaver, “big-time earthquake country.” It lies on the Pacific Rim, where 80 percent of all the earthquake energy in the world is released each year because of the collision between Pacific Ocean plates and surrounding continents.

While Washington does not have a spectacular fracture such as California’s San Andreas Fault, scientists in the last few years have identified several potential threats:

The most common big earthquakes here in the past have occurred about 30 miles under the Puget Sound basin, where the ocean floor of the Juan de Fuca Plate is being subducted under, or overridden by, the North American continent. Examples are a magnitude 6.2 earthquake under Puget Sound in 1939, the 7.1 earthquake in Seattle that killed eight in 1949, and the 6.5 earthquake that injured 31 in 1965.

Also common, but usually less intense, are shallower earthquakes inside the crust along fault lines: earthquakes similar to Saturday’s tremor. Examples include a magnitude 5.9 earthquake near North Bend in 1945 and a magnitude 5.8 earthquake near Mount St. Helens in 1981. That was in the volcano’s seismic zone but not caused by volcanic activity.

The biggest potential earth quakes in terms of released energy occur off Washington’s coast in the shallow subduction zone where the Juan de Fuca Plate begins to dive under the continental plate. The last such earthquake occurred about 300 years ago, and there have been at least six over the past 7,000 years.

Such earthquakes could have maximum magnitudes estimated from 8 to 9.5. At its worst, such an earthquake could muster energies a thousand times greater than the 1989 Loma Prieta quake, ravaging Washington’s coastal communities and causing significant damage as far away as cities on Puget Sound.

Geologists estimated last Octo ber that the Cascade Mountains have averaged two volcanic eruptions per century over the last 4,000 years. Getting new study is Mount Rainier, which last erupted 150 years ago. About 60 mudslides have rumbled off the mountain over the last 10,000 years, some reaching as far as Auburn and Tacoma.

Earthquake damage is hard to predict. It depends on the intensity of the quake, its location, how shallow it is, the kind of soil a structure sits on, and the type of construction used.

University of Washington engineers said last week that Seattle would be unlikely to suffer damage as heavy as Kobe if a similar-sized earthquake struck here, because of differences in American and Japanese construction.

However, they warned, Seattle is vulnerable because of its dependence on bridges across numerous bottlenecks and its large areas of fill on old tideflats. That fill could liquefy and cause structures to fail.