Same-Size Temblors Can Pack Different Punches

The Feb. 28 earthquake in Washington state demonstrated again to scientists just how the quake menace in the Northwest often differs from that in California.

Six times since 1939, quakes ranging from magnitude 5.8 to 7.1 have shaken western Washington within 40 miles of last month’s epicenter near the state capital of Olympia. Yet none of caused nearly the damage or deaths of several quakes during the same period in California.

Although Washington has done a better job lately of retrofitting its buildings, that was not the primary reason for the comparatively minimal damage from last month’s quake, seismology experts said. The main factor, they said, was the depth of the temblor--named the Nisqually quake after the Puget Sound delta under which it occurred.

All six Washington quakes have occurred on a subducting slab of the Juan de Fuca tectonic plate, according to reports in recent weeks by scientists at the University of Washington, the U.S. Geological Survey and the Earthquake Engineering Research Institute.

A subducting slab results from an oceanic plate, over millions of years, “diving”--at about the speed your fingernails grow--under a continental plate. In Washington state, the 60-mile-thick Juan de Fuca plate is diving under the North American continental plate, which runs 30 to 90 miles under the surface.

Periodically, this process generates earthquakes along its upper edge and eventually, 100 miles inland, volcanic eruptions in the Cascade range.

The quakes generally occur in the top three miles of crust of the subducting slab, according to Seth Stein, a subduction expert and geophysicist at Northwestern University.

But because the subduction zone is so deep, much of the energy they generate is dissipated underground before it can do major damage at the Earth’s surface.

Farther off the coast, the plate boundary called the Cascadia Subduction Zone is capable of generating quakes ranging as high as magnitude 9. Those would be much more dangerous for Washington state, but are not as common as the deep quakes that have occurred throughout the last 60 years.

California--with the exception of Humboldt and Del Norte counties on the North Coast, and the inland volcanoes, Mt. Shasta and Lassen Peak--is not affected by the subduction zone.

Most quakes in California are unrelated to subduction, although they may be on plate boundaries, and they happen at far shallower levels than the Nisqually quake,

which originated 33 miles below the surface.

The California quakes, either on horizontal, strike-slip faults like the San Andreas or thrust faults as in Northridge, are more sharply felt on the surface. Their magnitudes may not be any higher than those of the Washington quakes, but they are closer to where people live.

The 1994 Northridge quake took place on a sloping plane three to 12 miles beneath the surface, on a “blind” thrust fault. This meant its surface shaking exceeded Nisqually’s by about 10 times, according to ground motion recordings.

The result is depicted in the maps on this page: They show about the same territorial extent for the Northridge quake and last month’s Washington state temblor. But everywhere on the surface, the Nisqually quake was less violent.

“The Nisqually earthquake caused [only] moderate ground motion throughout the Puget Sound region,” note the authors of the seismology section of the earthquake institute report, Ken Creager and Robert Crosson of the University of Washington and Thomas Pratt and Craig Weaver of the USGS.

“Of the 31 [seismographic] stations for which preliminary data are available, only 13 show peak ground accelerations greater than 10% of gravity and only two stations record values greater than 25% of gravity,” they note.

But some Northridge stations exceeded 100%.

The scientists recognize that “ground motions vary widely from site to site, due in part to the large differences in geologicconditions throughout the Puget Sound,” and do not follow simple patterns.

But this was the case in Northridge too. In both places, scientists frequently are urging the placement of additional recording stations to get a more complete picture. Still, it is clear that Northridge, a magnitude 6.7 quake, was far more violent on the surface than Nisqually, a magnitude 6.8.

Another feature of the Washington state quakes is the paucity of aftershocks, a sharp contrast with most big California quakes.

In the first days following the Nisqually quake, only four small ones were recorded, the largest a mild magnitude 3.4.

A report by the Southern California Earthquake Center notes that “the largest Northridge aftershock . . . measured magnitude 5.9, and five other first-week aftershocks exceeded magnitude 5.”

In short, the biggest Northridge aftershocks were about 100 times the power of the biggest Nisqually aftershock.

Observing that, in all, hundreds of aftershocks were felt in the first week after

Northridge (and thousands have occurred since then), the report says, “Moderate aftershocks are capable of causing further damage to weakened structures, so the lack of significant aftershocks was a factor in mitigating the total loss caused by the Nisqually earthquake.”

The dearth of Nisqually aftershocks came as no surprise to seismologists, since they had observed the same phenomenon in earlier Washington earthquakes.

Not all quakes in Washington are deep ones, however.

Seattle and the Puget Sound have an important surface fault, the Seattle fault, but it has not been as active in the past century as the quakes in the subducting slab of the Juan de Fuca plate.

There also could be a mammoth quake, bigger than anything California’s San Andreas fault may be capable of, on the boundary of the Pacific and Juan de Fuca tectonic plates off the coast of Washington, Oregon and far northern California.

Such a quake, known as a megathrust, could foster a tsunami that would wreak terrible damage. However, that kind of quake is not likely to happen as soon or as often as the deep quakes of the Feb. 28 type, scientists said.

The last such megathrust quake is believed to have occurred in 1700. A tsunami recorded in Japan that year was probably related to it.

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Quake Comparison

The Northridge quake and Washington state’s Nisqually quake were felt over about the same amount of territory. But because Nisqually was centered far deeper than Northridge, its surface shaking was not as great.

Subduction Zone Earthquakes

* The Pacific Northwest is vulnerable to quakes in the subduction zone--the Feb. 28 quake near Seattle was an intraslab quake, which occurred in the descending oceanic plate at a depth of about 33 miles.

* Many large earthquakes occur in subduction zones--where an oceanic plate moves beneath a continental plate. Intraslab quakes occur at greater depths than thrust quakes and San Andreas strike-slip temblors.

Source: U.S. Geological Survey