In the next exercise we will determine the epicenter of an actual earthquake and its magnitude by analyzing three seismograms recorded by stations located in Medford, Ca; Winnemucca, Nv.; and Las Vegas, Nv. (all three stations are marked by a red arrow).

Looking at the seismogram from Medford we could calculate the SP interval, observing that the P wave arrived to this station at 8:17:06. The S wave arrived at 8:18:02. The SP interval is 56 seconds (that is the difference between the P and S wave arrivals). The seismogram also shows that the amplitude of the S wave is 5.

The seismogram from Winnemucca shows that the P wave arrived at 8:17:03. The S wave arrived at 8:17:55. The SP interval is then 52 seconds. The amplitude of the S wave is 7.
The seismogram from Las Vegas shows that the waves arrived at 8:17:08. The S waves arrived at 8:17:59. The SP interval is then 51 seconds. The amplitude of the S wave is 4.
To recap the SP interval for Medford was 56 seconds, for Winnemucca 52 seconds, and for Las Vegas was 59 seconds. Using the SP-distance graph we plot the distances corresponding to the SP interval times. If we follow the line for Medford at 56 seconds until we hit the black line, then we find that the distance corresponding to that SP is 550km. For the SP interval of 52 seconds that corresponds to Winnemucca, the distance is 500km. For Las Vegas with a SP interval of 59 seconds, the corresponding distance is 580km.
Looking at the scale of the map, we could see that 2.5 cm equal 200km.
Since I have to use distances that are going to be greater than 200km, I will just extend my scale based on that if 200km = 2.5cm, then 100km =1.25cm.
Using a compass (to draw the circles) I measure the 550km on my scale corresponding to Medford station. This distance is the radius of the circle that has to be drawn having Medford as the center of the circle.
After we draw the three circles using the corresponding distances of the SP intervals as the radius, using each station as the center of those circles. The point where the three circles intersect is called the epicenter of the earthquake, that in this case is located between Fremont and Tracy.

So far we have determined the exact location of the epicenter, and since we know where the earthquake was located we could determine its magnitude:

Station SP interval Distance from the epicenter Amplitude of the S wave
Las Vegas

Based with the information above, we can construct a nomogram and determine the intensity of the earthquake.

Using the nomogram below we connect the distance and the amplitude for each station, the number where the three lines meet is considered the magnitude of the earthquake, in this case 5.6 in the Richter scale.




Brooklyn College - Geology Department