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Locating Earthquakes
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The distance of each station from the earthquake can be found by reading the X- axis. After finding out the distances, a seismologist can locate an earthquake’s epicenter, as shown next.
The S-P Time Method
To find an earthquake’s epicenter:
The first step in this method is to collect several seismograms of the same earthquake from different locations.
Then, the seismograms are placed on a time-distance graph. The seismogram tracing of the first P wave is lined up with the P-wave time-distance curve, and the tracing of the first S wave is lined up with the S-wave curve, as in the picture.
Richter scale
Seismologist Charles Richter created the scale in the 1930s. Richter wanted to compare earthquakes by measuring ground motion recorded by seismograms at seismograph stations.
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How do seismologists know when and where earthquakes begin?
They depend on earthquake-sensing instruments called seismographs.
Seismographs are instruments located at or near the surface of the Earth that record seismic waves. When the waves reach a seismograph, the seismograph creates a seismo-gram.
A seismogram is a tracing of earthquake motion and is created by a seismograph.
Determining Time and Location of Earthquakes
Earthquake-Resistant Buildings
Seismologists find an earthquake’s start time by com paring seismograms and noting the differences in arrival times of P waves and S waves.
Seismologists also use seismograms to find an earthquake’s epicenter.
An epicenter is the point on the Earth’s surface directly above an earthquake’s starting point.
A focus is the point inside the Earth where an earthquake begins.