When large earthquakes strike in other parts of the world, SCEC is frequently asked, "What would have happened if it had happened in southern California?" alternatively, "How come there was so much damage (in the other place) despite the earthquake's small magnitude?" "Why was the damage in Washington so much less than the damage inflicted by the 1994 Northridge earthquake, although their magnitudes were so similar— 6.8 vs. 6.7?" SCEC was questioned after the recent earthquake in Washington state. This article compares the geological characteristics of the two earthquakes, which account for the majority of the differences in damage.

An earthquake of a magnitude of 6.8 struck the Puget Sound region of Washington state around 10:55 a.m. on Wednesday, February 28. The ground shook for up to 40 seconds in Seattle, Tacoma, and municipalities around the region, rattling buildings and driving some residents into a panic. A large earthquake rocked the area, centered about 18 kilometers (11 miles) northeast of Olympia, causing significant structural damage to the state capitol building.

Soon after the shaking ceased, it became evident that the damage was less than what an earthquake of this size would have caused in a comparably populated area in southern California. The Nisqually earthquake was somewhat larger than the Northridge earthquake, and was named after a river delta near its epicenter at the southern end of Puget Sound. Despite this, the Northridge earthquake, which had a magnitude of 6.7, was the costliest disaster in US history, causing a $40 billion economic loss. According to preliminary reports, the Nisqually earthquake will result in only a $2 billion loss.

In the Seattle area, only one death, a heart attack victim, was reported, but the Northridge earthquake claimed the lives of 57 people. Is it possible that this is due to the fact that the Seattle area is better equipped for earthquakes than the Los Angeles area? Was it due to changes in the earthquakes themselves, or was it something else?

Depth of the Hypocenter

The difference in total damage inflicted by the Nisqually and Northridge earthquakes can be ascribed to the Nisqually earthquake's location — not its epicenter, but its hypocenter depth. The hypocenter of the Northridge earthquake was 18 kilometers (11 miles) deep, which was deep for a California earthquake but shallow in comparison to other places.

The depth of the Nisqually earthquake was 52 kilometers (33 miles), suggesting that no building was shaken more than would be expected at 52 kilometers from a magnitude 6.8 earthquake. That may seem self-evident, but earthquake depth is frequently disregarded in assessments of an earthquake's impact; on the other hand, no one would be astonished if a town 18 kilometers from an earthquake's epicenter sustained higher losses than one 52 kilometers distant.

Rupture Of A Fault

The geological environment found in each place is the direct cause of the fact that western Washington state can experience such deep earthquakes but southern California cannot. The Nisqually earthquake had a "typical" sense of fault slip, which is that the ground above the tilted fault plane moved downward compared to the material beneath it. Extensional tectonic conditions, where the crust is stretched and pulled apart, are common for this sort of motion.

That may seem strange in the Pacific Northwest, where the oceanic Juan de Fuca plate subducts beneath the continental North American plate, but the collision of plates creates the bending stresses needed to cause extension within the Juan de Fuca plate, which is where the Nisqually earthquake occurred.
 

Movment of the earth

The United States Geological Survey used data from seismometers across Washington state to develop a map of shaking intensity (ShakeMap). This map is shown next to a map from the Northridge earthquake in the picture on the right. The total region of potentially destructive shaking (yellow and warmer colors) was very similar for the two disasters, while the area of high-intensity shaking for the Northridge earthquake (shown in red) was almost non-existent for the Nisqually earthquake.

The difference in these maps is mostly due to the hypocentral depths of the two earthquakes being different. Given that the epicenter of the Nisqually earthquake was 52 kilometers beneath the surface, someone standing at its epicenter would be 45 kilometers from the epicenter of the Northridge earthquake. On the shakemaps, each of these places have a similar color (yellow-orange).

The Community Internet Intensity Maps (called "Did You Feel It?") for each earthquake show the same pattern. Even in the areas closest to the epicenter, reports from the Nisqually earthquake reveal a distribution similar to that shown on the intensity map: a vast swath of "very intense" shaking (Intensity VII), but no reports of severe or violent shaking (Intensities VIII or IX). Residents in the epicenter (and some areas further afield) of the Northridge earthquake reported severe, even violent shaking (Intensities VIII or IX), whereas communities further away suffered more moderate intensities. (David Wald will present a public talk on ShakeMap and the Community Internet Intensity Maps on Thursday, March 15, at Caltech.)

This can be taken a step further by examining the ground accelerations reported during each earthquake at areas with similar hypocentral distances. At right is a graph of acceleration time histories. It compares the Nisqually earthquake data of two strong-motion sensors with a Northridge earthquake record made by a comparable device at a similar hypocentral distance. Though it's tough to draw broad conclusions from such a small sample, there are no significant differences in the accelerations experienced by these earthquakes at identical hypocentral distances.

Aftershocks

There was another notable distinction between these two earthquakes that emerged shortly after the Nisqually quake. Unlike the Northridge earthquake, which was followed by hundreds of minor earthquakes known as aftershocks in the first week, just two aftershocks were registered one week after the Nisqually earthquake. So far, the largest of the Nisqually earthquake's aftershocks has been a magnitude 3.4 jolt that happened roughly 14 hours after the magnitude 6.8 mainshock (and this is unlikely to change). By comparison, the greatest Northridge aftershock had a magnitude of 5.9, while five additional aftershocks in the first week had a magnitude of 5. In the graph to the right, you can observe these discrepancies. Because moderate aftershocks can worsen structural damage, the lack of severe aftershocks was a relief.

Because moderate aftershocks can cause further damage to compromised structures, the lack of strong aftershocks had a role in reducing the total loss from the Nisqually earthquake.

Seismologists were not surprised by the lack of Nisqually aftershocks, as it turned out. The Nisqually aftershock sequence, or lack thereof, is quite typical of previous big earthquakes at this depth on the subducting Juan de Fuca slab, which occurred in 1949 and 1965. Similarly, the aftershock sequence at Northridge was not out of the ordinary when compared to the typical activity of aftershock sequences in southern California. The obvious contrast between the two aftershock sequences reflects the fundamental difference in each mainshock's source — one in the subducting oceanic slab under western Washington, the other in the comparatively shallow, brittle crust of southern California.