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Sophie Smells a Shaker 

You have undoubtedly seen the YouTube video "My dog Sophie senses the 6.5 earthquake at the Times Standard." Sophie springs into action and rushes to her owner, Jessica Richelderfer, almost six seconds before the serious shaking begins. Sophie's video is widely invoked to support the notion that animals possess supernatural senses.  

The sudden failure of a fault at depth produces two kinds of waves: fast-moving P-waves (for primary, pressure or push-pull) in which the rocking motion is perpendicular to the wave front, and slower moving S-waves (for secondary, sideways or shear) in which the motion is parallel to the wave-front.

This diagram shows the P and S wave-fronts three seconds after the failure (at the depth of the earthquake). When P-waves reach seismometers, it is possible to determine whether their first motions were pushes or pulls. The black and white quadrants of the sphere are based on that information and show that the stress was oriented north-south. The locations of aftershocks provide the strike of the fault (northeast-southwest). The slip was horizontal on a vertical fault, and thus no tsunami was expected.

The seismic energy radiated in different directions is roughly depicted by the thickness of the diagram's circles. The very weak P-waves felt by Sophie arrived in Eureka six seconds before the much more energetic S-waves. Fortuna, equally distant from the focus or hypocenter of the earthquake, experienced stronger P-waves relative to Eureka's, but smaller S-waves (which are typically more damaging). Fortuna, fortunately, was neither in the direction of the fault's strike nor perpendicular to it.

Port-au-Prince's quake was much larger -- magnitude 7.0 -- and half the distance. (Each unit increase in magnitude represents a 10-fold increase in amplitude, 30-fold in energy.) Our own disaster will occur when the Cascadia subduction megathrust fails ("Garlick's Notebook," Oct. 4, 2007) as it did 310 years ago. The consequent tsunami left a layer of sand in our bay and was recorded in Japan.

Don Garlick is a geology professor retired from HSU. He invites any questions relating to North Coast science, and if he cannot answer it he will find an expert who can. E-mail dorsgarlick@yahoo.com. He thanks Lori Dengler and Bob McPherson for their useful advice. 

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About The Author

Don Garlick

Bio:
Don Garlick is a geology professor retired from Humboldt State University. He invites any questions relating to North Coast science, and if he cannot answer it he will find an expert who can. E-mail dorsgarlick@yahoo.com.

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