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Virginia quake is apples to oranges of Seattle's 6.8 quake

Virginia quake is apples to oranges of Seattle's 6.8 quake

Amid the Twittersphere eruption at the quake in Virginia Tuesday came local questions wondering how it compared to notable quakes around the Northwest, especially the 6.8 Nisqually quake that struck 10 years ago.

But the quakes are difficult to compare, because they were two very different events. The magnitude given to quakes deals with the amount of energy released, but doesn't dictate how that energy is spread out.

In pure mathematical numbers, Seattle's 6.8 quake released about 30 times more energy than the 5.8 Virginia quake did. But the Nisqually quake was a deep quake that was centered some 30 miles underground. The depth made the quake felt over a large area, but also helped mitigate some of the energy from reaching the surface. The Virginia quake was centered just below the surface -- as little as 1/2 mile underground -- allowing almost all of the energy to reach the surface near the epicenter.

Shallow quakes typically do more damage near the epicenter since the energy doesn't have far to go, but aren't felt over as wide a distance. Deeper quakes can be felt over greater distances, but don't do as much damage.

To compare, the quake that hit Christchurch, New Zealand in June was a 6.3, but centered just a few miles below the surface (and it was centered right near the town). That city suffered major damage, even though that quake released 7 times less energy than Seattle's quake.

But the East Coast topography makes quakes a bit of a different animal. There, even shallow quakes can be felt over great distances because that area is on one large tectonic plate, as opposed to multiple plates on the West Coast.

"The waves are able to reverberate and travel pretty happily out for miles," said U.S. Geological Survey seismologist Susan Hough.

In other words, a 5.8 surface quake around the Northwest probably wouldn't be felt as far as it did in Virginia.

The Virginia quake rates as one of the strongest to strike the East Coast in recorded history. The USGS says there was a 5.9 quake near Cape Anne in Massachusetts in 1755, a 5.5 quake that struck near New York City in 1884, a 5.9 quake near Blacksburg, Virginia in 1872, and a 7.3 quake near Charleston, SC in 1886.

So certainly, this rates the largest East Coast quake anyone currently alive would have felt.

But in the Northwest, a quake around that size is not exceedingly rare. Aside from the 6.8 Nisqually quake, we had our own 5.8 quake near Satsop on July 3, 1999. That one had a 25 mile depth, however. A 5.3 to strike Duvall on May 2, 1996 was only 5 miles deep, and a 5.0 to strike Maury Island on Jan. 28, 1995 had an 11 mile depth.

Down in Oregon, a pair of quakes registering 5.9 and 6.0 struck the Klamath Falls area on Sept. 20, 1993.

You can see a list of notable Northwest quakes here.