1. Go to the web site www.sciencecourseware.com/VirtualEarthquake (or Google “vi

Question

1. Go to the web site www.sciencecourseware.com/VirtualEarthquake (or Google “virtual earthquake”). If Google takes you to a different site, do NOT select Earthquake, which is their “new revised version”, because it does not work with some operating systems. At the bottom of the page, choose Execute Virtual Earthquake, be sure to choose “San Francisco area” earthquake, and follow the directions for locating an earthquake and determining its magnitude. At the end, print your certificate of completion along with its associated table showing your calculations. You will not be turning in the certificate itself, but you will use the information in that table for online submission of this homework.

2. The second page of this PDF file shows the seismogram recorded for an earthquake at a seismic station, along with a nomogram for determining its Richter magnitude. The steps are: 1) Identify the P and S arrivals, and measure the time between P and S. This tells us the distance between the seismic station and epicenter. For the example shown, the time between P and S is 24 seconds, so the earthquake epicenter is 210 km away. 2) Measure the amplitude of the largest peak or trough. For these data which don’t include surface waves, this will always be the first or second S-wave peak or trough. For the example shown, the amplitude is 23 mm. 3) Put a point on the left scale bar indicating the time between P and S arrivals. Put a point on the right scale bar indicating maximum amplitude. Draw a line between the two points, and read the earthquake magnitude on the center scale. For the example shown, the quake’s magnitude is 5.

a) For the example shown, suppose the amplitude were only a tenth as big, or 2.2 mm. How big an earthquake would produce such a small-amplitude record? Magnitude: ________

b) For the example shown, suppose the earthquake were only magnitude 3. How big an amplitude would be recorded? _________ mm. Note that both #2 and #3 are examples of the fact that the Richter scale is logarithmic, with a factor of 10 variation in seismic wave amplitudes representing only a difference of 1 in Richter magnitude.

c) The bottom two plots on p. 2 of this PDF file are imaginary seismograms for the same earthquake, recorded at Sandy and the University of Utah seismic stations. For the Sandy seismogram (shown at lower left), label the P-wave and S-wave arrivals. The time between P and S arrivals is __________ seconds. The amplitude of the largest peak or trough is _________ mm. Plot these values on the P-S scale and amplitude scale, then draw a line connecting them. The earthquake is _____ km from the Sandy station and is magnitude _____________.

d) For the University of Utah seismogram (shown at lower right), label the P-wave and S-wave arrivals. The time between P and S arrivals is __________ seconds. The amplitude of the largest peak or trough is _________ mm. Plot these values on the P-S scale and amplitude scale, then draw a line connecting them. The earthquake is _____ km from the University of Utah station and is magnitude _____________.

Amplitude-23 mm +--?? 10 20 S-P= 24 sec 500 400 300 50 100 40 6 30 5 200 10 100-1-10 60 8 3 2 0.5 0.2 0.1 20 2 5 Amplitude (mm) Magnitude Distance (km)S- P (sec) Sandy Univ. of Utah 1 sec

Explanation / Answer

A) with amplitude 2. 2, maximum magnitude 6 can be recorded for far away epicenter

B) with magnitude 3, maximum 100mm amplitude will be recorded.

C) at sandy, suppose the p wave recorded at 0 second and S wave recorded 4. 33 second hence S-P difference is 4. 33 second and amplitude is 50mm hence by plotting on the given distance-magnitude-amplitude graph,

The epicenter is at ~38km( see the left part of first line at 4. 33 second) and

Then draw the line connecting 38km and 50mm we get magnitude = 4. 1

D) University of Utah, here suppose p wave arrive at 0 second then s wave arrive at 8 second hence we get P-S wave difference is 8 second and the amplitude is 16mm

Put this values in distance-magnitude-amplitude graph we get

Epicenter at ~70km and amplitude is 16mm

Draw line we get magnitude = 4. 1

E) Yes, magnitude is same

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