Earth Science Notes Measuring Earthquakes Measuring Earthquakes Recall

Earth Science Notes Measuring Earthquakes

Measuring Earthquakes • Recall: Seismic Waves show up on a seismograph.

Determining Location of EQ • p-waves travel ≈ 360 km/min and s-waves travel ≈ 210 km/min • Distance between the EQ and the Seismograph Station can be found by looking at the difference in arrival times of the p and s waves.

Determining Location of EQ • Measuring the time between the arrival of the “s” and the “p” waves (at three different locations) allows us to find the epicenter of the EQ.

Determining Location of EQ • Knowing the distance from the EQ for all 3 stations helps us find the epicenter of the EQ. • Using the distance as a radius, draw a circle with each station at the center. • Where all three circles intersect is the epicenter.

Scales of Earthquake Intensity/Magnitude • Mercalli - Based on personal interviews of victims in the quake area. Has XII degrees of intensity. • Measures damage done. Modified Mercalli Scale, based on maps in W. Atkinson, 1989, The Next New Madrid Earthquake, Southern Illinois, University Press.

Scales of Earthquake Intensity/Magnitude • Richter Scale - Based on the magnitude of energy released during a quake as measured by a seismograph. • Measures energy released from EQ. • Each increase by 1 on the scale represents a ten-fold increase in magnitude. – 5 on the RS is 10 X greater than a 4

Scales of Earthquake Intensity/Magnitude • Largest quake ever recorded subject of some debate, but is either Alaskan (1964) at 9. 2 or one in South America (1976) which may have been near 9. 5.

1964 ALASKA EARTHQUAKE

1964 ALASKA EARTHQUAKE

Summary Be able to… • Describe the various methods for assessing earthquakes • Determine the epicenter of an earthquake given the difference in time of wave arrival • Compare the Richter and Mercalli Scales