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Earthquakes in the Pacific Northwest
 

The Puget Sound region of Washington state was jolted from its morning routine at 10:55 a.m. on Wednesday, February 28th, 2001 by a magnitude 6.8 earthquake, centered just 11 miles northeast of Olympia. In Seattle, Tacoma, and other communities across the region, the ground shook for up to 40 seconds, rattling buildings and sending some occupants into a panic.

This event, named the Nisqually earthquake, was actually slightly larger than the Northridge, California  6.7 magnitude earthquake of January 17, 1994. Yet, based on preliminary reports, it appears that losses in the Nisqually earthquake will total only $2 billion while the Northridge earthquake was the costliest disaster in U.S. history with an economic loss of $40 billion. Likewise, only one death, a heart attack victim, was reported in the Seattle area, while 57 people lost their lives in the Northridge earthquake.

Why was the damage in the Washington so much less than the damage caused by the 1994 Northridge earthquake, when their magnitudes were so similar-- 6.8 vs. 6.7?

Was it because the Seattle area was so much better prepared for earthquakes than the Los Angeles area? Or was it due to differences in the earthquakes themselves?

Major difference #1: Location
Much of the difference in the total damage caused by the Nisqually and the Northridge earthquakes can be attributed to the location of each – and not just the epicenter (a measurement of latitude and longitude), but the depth as well. The Northridge earthquake occurred 11 miles below the earth’s surface, deep for a California earthquake, but considered shallow by comparison with other regions. The Nisqually earthquake occurred 33 miles below the earth’s surface. Obviously, a town 11 miles away from the epicenter of an earthquake would suffer greater losses than a town located 33 miles away. That may seem to be an obvious statement, but earthquake depth is often entirely overlooked in reports of an earthquake's effects.

Major difference #2: Tectonics
The fact that western Washington State can experience such deep earthquakes, while southern California cannot, is the direct result of the tectonic environment found in each region. In the Pacific Northwest the oceanic Juan de Fuca plate subducts beneath, or pushes against the continental North American plate in a northeasterly direction at the rate of 1 to 2 inches per year. This collision of plates creates stresses that cause extension, or the stretching and pulling apart of the earth’s crust within the Juan de Fuca plate, where the Nisqually earthquake occurred.

The fault slip, or fault movement, for the Nisqually earthquake was what geologists call "normal," meaning that the ground above the tilted fault plane moved downward relative to the material beneath the plane moving upward.The fault associated with the 1994 Northridge earthquake is probably part of the Oak Ridge fault system and has been called by geologists a blind thrust fault, or a shallow-dipping reverse fault that terminates before it reaches the surface. In other words, it moves much like an ocean wave that crests but stops abruptly before it breaks. This type of fault, therefore, may produce uplift, but never any clear surface rupture.

Major difference #3: Ground Shaking
While an earthquake has one magnitude and one epicenter, it produces a range of ground shaking levels at sites throughout the region depending on distance from the earthquake, the rock and soil conditions at sites, and variations in the propagation of seismic waves from the earthquake due to complexities in the structure of the Earth's crust. Ground-motion data from seismometers across Washington State was processed by the U.S. Geological Survey to create a shaking intensity map. A comparison of this map with the shaking intensity map for the Northridge earthquake showed that the total area of potentially damaging shaking was fairly similar for the two events, but the area of high-intensity shaking for the Northridge earthquake was essentially absent for the Nisqually event.

This difference is primarily due to the hypocentral depths of the two earthquakes. Someone standing roughly 28 miles away from the epicenter of the Northridge earthquake would have experienced similar ground shaking to someone standing at the very epicenter of the Nisqually earthquake.  The shaking intensity maps for both quakes verify this conclusion. Ground shaking intensities were far greater at the epicenter and nearby areas of the Northridge earthquake because the source of the earthquake was but 11 miles below the earth's surface as opposed to the 33 mile depth of the Nisqually quake.

"Did You Feel It?" reports from the Nisqually earthquake showed that most residents throughout the region experienced "very strong" shaking (characteristics of Intensity rating VII) but no reports of severe or violent shaking (characteristics of Intensity rating VIII or IX), even in those places nearest the epicenter. Reports from the Northridge earthquake showed that residents in the epicentral region (and some areas farther afield) reported severe, even violent, shaking (Intensity VIII or IX), while communities farther away experienced more moderate intensities.

Major difference #4: After Shocks
Another striking difference between these two earthquakes became apparent soon after the Nisqually earthquake struck. Whereas the Northridge earthquake was followed by hundreds of smaller earthquakes, or aftershocks, within the first week, one week after the Nisqually earthquake only two aftershocks had been recorded. The largest of the aftershocks to the Nisqually earthquake was a magnitude 3.4 jolt that struck about 14 hours after the magnitude 6.8 mainshock. The largest Northridge aftershock, by comparison, measured magnitude 5.9, and five other first-week aftershocks exceeded magnitude 5. 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.

Conclusion
As it turns out, the results and damaging affects of each earthquake came as no real surprise to seismologists. Previous large earthquakes in Washington at similar depths in the subducting Juan de Fuca plate, in 1949 and 1965, were also followed by very limited aftershock activity, so the Nisqually aftershock sequence, or lack thereof, was really quite typical. Likewise, the Northridge aftershock sequence was in no way unusual compared to the average activity of southern California aftershock sequences. Thus, the obvious differences between these two major earthquakes reflects the underlying difference in the source of each -- one in the subducting oceanic plate beneath western Washington, the other in the comparatively shallow, brittle crust of southern California.

[Earthquake comparison information provided by
the Southern California Earthquake Center (SCEC)]


Summary of Earthquake Activity in Washington
The Seattle area has experienced eleven earthquakes a magnitude of 5.0 or greater since 1904.The 1995 (M=5.0) and 1996 (M=5.3) events were shallow crustal events.

The three most recent damaging earthquakes in Washington occurred in 1965 (M=6.5), 1949 (M=7.1) and 2001 (M=6.8). All three were roughly 40 miles deep and in the oceanic plate where it lies beneath the continent.

The largest historic earthquake in either Washington or Oregon occurred in 1872 in the Northern Cascades. This "shallow" earthquake had an estimated magnitude of 7.4 and was followed by many aftershocks. It was probably at a depth of ten miles or less within the continental crust.

In 1993, a magnitude 5.6 earthquake in the Willamette Valley caused 28 million in damages.

Although no large earthquakes have happened along the offshore Cascadia Subduction Zone since our historical records began in 1790, similar subduction zones worldwide do produce "great" earthquakes of a magnitude 8.0 or larger. These occur because the oceanic crust "sticks" as it is being pushed beneath the continent rather than sliding smoothly. Over hundreds of years, large stresses build that are released suddenly in great earthquakes. Such earthquakes typically have a minute or more of strong ground shaking and are quickly followed by damaging tsunamis and numerous large aftershocks. However, it does appear from the comparative information above, that the damaging effects of such earthquakes is directly related to the depth at which they occur.

 
 

Plate Tectonics
Understanding Plate Tectonics

Rupture Mechanics
Understanding Rupture Mechanics

Faults
Types of Faults

Magnitude
Understanding Magnitude

Intensity
Understanding Intensity

Epicenters
Locating Epicenters of Earthquakes