Earth Science Notes 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 10X 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

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