Monday, January 12, 2009

Lancaster quake - how fast do seismic waves travel?

Seismic waves travel pretty fast. There are three physically different types of waves - P, S, and L. The velocity depends on which type of wave and what part of the earth you are in - crust, mantle, core. But my rule of thumb for velocities of seismic waves that travel some distance from the epicenter is about 8 km/second (or 8000 meters/second) for a P wave, and 5 km/second for an S wave. Compare this with common velocities such as driving your car at 60 mph, which is about 100 km/hour, or 2 km/minute, or 0.03 km/second (all back of the envelope calculations).

So look at the NEIC report for the Lancaster quake. Now go to the link for Theoretical P-Wave Travel Times. The figures and table tell you how long it will take the P-waves from this quake to travel different distances. Note that these are essentially spherical wavefronts, since the waves travel out at constant velocities in all directions.

click above for larger image
Now this was a small quake, so the waves won't be large enough to be detectable more than a few hundred kilometers away, but this shows how many minutes it would theoretically take the waves to travel out to various distances: to New York in less than 1 minute, to Chicago in 2 minutes, Miami in 3 minutes, Denver in 5 minutes, and on to San Francisco in 7 minutes. There is another map on the same page for times around the world, and a table with the same information. We'll have to return to this topic at some point for a larger quake to understand the nature of the shadow zone.

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