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USGS CMG InfoBank: Seismograph and Epicenter

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Comment: 07:36 - 08:31 (00:55)
Source: Annenberg/CPB Resources - Earth Revealed - 9. Earthquakes
Keywords: seismograph, earthquake, "S-wave", "P-wave", "primary wave", "secondary wave", epicenter, energy, "Richard Hazlett"

Our transcription: Having a single seismograph record showing the time interval between the arrival of "S" and "P" waves is not adequate for locating the epicenter.
It simply tells one how far away the earthquake occurred.
The direction is still missing from this information.
In order to determine that direction, one examines seismograph records from other stations in the regions as well.
One can pull out a map and easily locate the epicenter by drawing circles around each respective seismic station representing the distance inferred to the source of the earthquake based on the time interval between "S" and "P" waves, and where these circles intersect, bingo, that's your source.
That's where the earthquake energy first reached the Earth's surface.

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