Topic: Earthquake Measurement PSSA: A / S8.D.1.1

Objective:  TLW explain how earthquakes are detected.  TLW describe how to locate the epicenter of an earthquake.  TLW explain how the strength and intensity of an earthquake is measured.

MI #1: Detecting Earthquakes  Seismographs are instruments located at or near the Earth’s surface that record seismic waves.  A seismogram is a tracing of earthquake motion created by a seismograph.

MI #2: Location of Earthquakes  The focus of an earthquake is the point inside the Earth where the earthquake begins.  The epicenter of an earthquake is the point on Earth’s surface directly above the focus.

MI #3: S-P Time Method  The S-P Time method is used to determine the exact location of an earthquake’s epicenter.  The arrival times of S and P are compared to determine the distance of the quake from the seismograph station.  A circle is drawn around a seismograph station with a radius that equals the distance from the quake.  The location where the circles from 3 different stations intersect is the epicenter.

MI #4: Magnitude and Intensity  Magnitude is a measure of the strength of an earthquake.  Intensity is a measure of the degree to which people felt an earthquake and the damage it caused.

MI #5: The Richter Scale  The Richter Scale is used to measure the strength of an earthquake.  Each increase of one unit (4 to 5, 6 to 7, etc.) means the measured ground motion is 10x larger.  Table 1 on p. 232 shows the estimated effects of earthquakes with different magnitudes.

MI #6: Modified Mercalli Intensity Scale  The modified Mercalli Intensity Scale is used to measure the intensity of an earthquake.  It rates earthquake intensity on a scale of I to XII (using Roman numerals)

So What…? Real Life Application  Understanding earthquake measurement is important in understanding the significance of Richter scale measurements.