Earthquake Measurement

Locating Earthquakes Seismologists depend on earthquake-sensing instruments called seismographs. Seismograph: instruments located at or near the surface of the Earth that record seismic waves. Seismographs produce seismograms.

How Are Seismograms Used? To calculate when an earthquake began. Seismologists find the start time of an earthquake by comparing seismograms, and noting the difference in the P waves and S waves. Seismograms are also used to find an earthquakes epicenter. The point on Earth’s surface directly above where the earthquake began, or the focus.

The S-P Method Used to find an earthquake’s epicenter. Steps: Collect several seismograms of the same earthquake from different locations. Place the seismograms on a time- distance graph. Place the seismogram tracing of the first P wave with the P wave distance curve. Then the S wave with the S wave distance curve. Find the distance of each station by reading the horizontal line. Find the epicenter on a map from the intersection of all circles.

Finding the Epicenter

The Richter MAGNITUDE Scale Commonly used in the 20th Century to measure the strength of earthquakes. Measures the magnitude of an earth quake by measuring ground motion recorded by seismographs. Magnitude – The strength of an earthquake

Modified Mercalli INTENSITY Scale Intensity – A measure of the degree to which an earthquake is felt by people and the amount of damage caused by the earthquake (if any). Measures intensity on a roman numeral scale of I to XII. An intensity of I = an earthquake not felt by most people An intensity of XII = total damage of an area.