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LAST NAME (IN CAPS): ____________________________________ FIRST NAME: _____________________

4. EARTHQUAKES 1. DETERMINING THE DISTANCE TO THE EPICENTER

Data required: S-wave & P-wave arrival times from a seismogram (Figure EQ-1); the difference in the arrival times between the first P-wave and first S-wave is then used to determine the distance to the earthquake epicenter by reading off from the chart in figure EQ-3.

Figure EQ-1 Seismogram of an earthquake showing P, S, Surface waves, and the S-P interval.

2. LOCATING THE EPICENTER OF AN EARTHQUAKE o Three seismographs needed to locate an epicenter o Each station registers the arrival of the first P wave and the first S wave for each earthquake. o The difference in P and S wave arrival times is called the S-P interval (P-S interval). o A travel time – distance graph (Figure EQ-3) determines each station’s distance to the epicenter by reading

from a graph. For example, the seismogram shown in Figure EQ-2 has S-P interval of 55 sec that corresponds to a distance to epicenter of 540 km.

o A circle with radius equal to distance to the epicenter (i.e. 540 km) is drawn around the station. o Following a similar procedure, draw circles for other two stations. The point where all three circles

intersect is the epicenter of the earthquake.

Figure EQ-2. Sample seismogram showing P, S, and surface waves. The S-P interval is also given.

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Figure EQ-3. Graph to determine the distance to the epicenter using S-P interval.

3. MEASURING THE MAGNITUDE OF EARTHQUAKES

Required data to determine the Magnitude:

1. A nomogram with Distance, Magnitude, and Amplitude components (Figure EQ-4b).

2. A seismogram, from which you will determine the distance to the epicenter and the length of the highest wave (For example, Figure EQ-4a).

Example:

I) For the seismogram given in Figure EQ-4a, the S-P interval is 24 seconds. On column A (Figure EQ-4b), place a mark at 24 sec.

II) Using a millimeter scaled ruler, measure the amplitude (length) of the largest wave. In the example given on Figure EQ-4a, the amplitude is 23 mm. Mark this value on column C.

III) To determine the Richter magnitude, join the two points you marked in (I) and (II) above, using a straight line. That line crosses the middle vertical line (column B), labeled “Magnitude”. The value which the line crosses over is the Magnitude of the earthquake. In the example given in Figure EQ-4, the Magnitude is 5.

Figure EQ-4. A nomogram to determine the Richter Magnitude

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QUESTIONS

GRAPHING SEISMIC DATA & LOCATING THE EPICENTER OF AN EARTHQUAKE

Q1. Assume that an earthquake occurred this morning. The first P-waves of the earthquake were recorded at a recording station in Houston at 6:15:45 and the first S-waves arrived at the same Houston station at 6:16:30.

I) What is the S-P interval time for the earthquake, in seconds? ______sec.

II) Using Figure EQ-3 determine how far the Houston recording station is located from the earthquake’s

epicenter. _________ Km.

III) Using the distance between Houston and the earthquake epicenter you determined in II, above, would you be able to pinpoint where exactly the earthquake happened? Explain your answer. If you said, no, explain why not and state what additional data or information you would require to determine the location of the earthquake’s epicenter (point on a map)?

Q2. Using the seismogram from the Loma Prieta earthquake (Figure 5), determine the following:

I) What is the S-P interval time? _______sec

II) The highest amplitude of the earthquake is _____________ mm

III) The Richter Magnitude of the earthquake is ________ {show the line in Figure EQ-4b}

Figure EQ-5. A seismogram of Loma Prieta earthquake from Sacramento station.

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Q3. Using just the amplitude, which seismograph station is probably closest to the epicenter? Eureka, Elko, or Las Vegas? Circle your answer.

Q4. Using the seismograms given in figure EQ-6, complete the blank spaces below. First, determine the S-P interval, then using Figure EQ-3, determine the distance to epicenter (in kilometers, km) for each recording station.

S-P interval Distance to Epicenter

Eureka, CA ___________ Sec _____________ Km

Elko, NV ___________ Sec _____________ Km

Las Vegas, NV ___________ Sec _____________ Km

Q5. I) Using a drafting compass draw a circle around each recording station on the location map given in Figure

EQ-7. The radius of each circle is the distance to epicenter you determined in Q4.

II) The location where all three circles overlap (or nearly overlap) is the location where the earthquake occurred, the epicenter. Determine how many kilometers away are the following cities from the epicenter:

Sacramento: ________ km

San Bernardino: ________ km

Salt Lake City: ________ km

Q6. I) How strong was the earthquake, that is, what was the Richter magnitude of the earthquake? To determine the magnitude, complete the table below. First, copy the Distance to Epicenter from Q4. Then use the seismograph recordings in figure EQ-6, to determine the maximum S-wave amplitude for each station.

Distance to Epicenter S-wave Amplitude

Eureka, CA _________ Km ________ mm

Elko, NV _________ Km ________ mm

Las Vegas, NV _________ Km ________ mm

II) Use the distance and amplitude data from I) above to draw a line for each

station on the side nomogram (this is similar to Figure EQ-4b). Ideally, all

3 lines should intersect at a single point. The intersection point gives you

the magnitude. For the data in I), the magnitude is ___________.

Q7. Is there a known fault in the vicinity of the epicenter that is likely to have slipped to create the earthquake?

If so, name the fault.

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Figure EQ-6. Seismograms to use to answer Q3 through Q7. Time is in seconds.

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Figure EQ-7. Location map of the three seismograph stations.