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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