earthquake epicenter lab

7/24/2019 Earthquake Epicenter Lab

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Finding An Earthquakes Epicenter

Problem:How can we find an earthquakes epicenter?

Introduction:Earthquakes occur because of a sudden release of stored energy. This energy has built up

over long periods of time as a result of tectonic forces within the earth. This suddenmotion causes shock waves (seismic waves) to radiate from their point of origin called

the focusand travel through the earth. It is these seismic waves that can produce ground

motion which people call an earthquake. Seismic waves can travel far and can bedetected by sensitive scientific instruments called seismographs.

What are earthquake (Seismic) Waves?

A seismic wave is simply a means of transferring energy from one spot to another withinthe earth. Although seismologists recognize different types of waves, we are interested in

only two types: P (primary) waves and S (secondary) waves. Within the earth, P waves

can travel through solids and liquids, whereas S waves can only travel through solids.

The speed of an earthquake wave is not constant but varies with many factors. Speedchanges mostly with depth and rock type. P waves travel between 6 and 13 km/sec. Swaves are slower and travel between 3.5 and 7.5 km/sec.

What's a Seismogram?

A highly simplified simulated recording of earthquake waves (a seismogram) can be seen

to the left. Study this sample seismogram and be sure you can identify these parts:

P-waves and the P-wave arrival time

S-waves and the S-wave arrival time

S-P interval (expressed in seconds) S-wave maximum amplitude (measured in mm)


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Western USAWestern USAWestern USAWestern USA

100 200 300 400 500 600 700 800Scale km


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Step 1: Determine the S-P time interval for 3 different stations.Find the time it took between beginning of the P-wave and the beginning of the S-wave.

Station 1: Eureka, CA

________ sec.

Station 2: Elko, NV

________ sec

Station 3: Las Vegas, NV

________ sec


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Step 2: Determining Distance from S-P

Use the S-P graph below and the estimates you made for the S-P time intervals for thethree seismograms to complete the table below. The horizontal grid is in one second

intervals.

Station S-P Interval Distance from the Epicenter

Eureka, CA 50 seconds ____________km

Elko, NV 72 seconds ____________km

Las Vegas, NV 64 seconds ____________km

Step 3: Triangulation of the EpicenterTo find the epicenter, you must use the distance from at least threedifferent stations. To

do this, set a compass at a radius equal to the distance from the epicenter. Use yourcompass to then draw a circle, centered on that station. Do this for all stations. Whenyou have completed the circles for all stations, find the location where those three circles

meet (or are closest to meeting). This is the epicenter.

What city is the epicenter closestto? _____________________________


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Step 4: Determining the Magnitude of the Earthquake

A well-known scale used to compare the strengths of earthquakes involves using therecords (the seismograms) of an earthquake's shock waves. The scale, known as the

Richter Magnitude Scale, was introduced into the science of seismology in 1935 by Dr.

C. F. Richter of the California Institute of Technology in Pasadena. The magnitudeof an

earthquake is an estimate of the total amount of energy released during fault rupture. The

Richter magnitude of an earthquake is a number: about 3 for earthquakes that are strongenough for people to feel and about 8 for the Earth's strongest earthquakes. Although the

Richter scale has no upper nor lower limits, earthquakes greater than 9 in Richtermagnitude are unlikely. The most sensitive seismographs can record nearby earthquakes

with magnitude of about -2 which is the equivalent of stamping your foot on the floor.

The Richter magnitude determination is based on measurements made on seisograms.

Two measurements are needed: the S-P time interval and the Maximum Amplitude of

the Seismic waves. You already know how to measure the S-P interval.

The following illustration shows how to make the measurement of the S wave's

maximum amplitude. The blue horizontal grid lines are spaced at 10 millimeter intervals.In this example the maximum amplitude is about 185 mm.

The Richter Nomogram

Although the relationship between Richter magnitude and the measured amplitude and S-

P interval is complex, a graphical device (a nomogram) can be used to simplify the

process and to estimate magnitude from distance and amplitude.

In the diagram below the dotted line represents the "standard" Richter earthquake. Thisstandard earthquake is 100 km away and produces 1 mm of amplitude on the

seismogram. It is assigned a magnitude of 3. Other earthquakes can then be referenced to

this standard.


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Note that a 100 km-away earthquake of magnitude 4 would produce 10 mm of amplitude

and a magnitude 5 would produce 100 mm of amplitude: 1, 10 and 100 are all powers of10 and this is why the Richter Scale is said to be "exponential." A change of one unit in

magnitude (say from 4 to 5) increases the maximum amplitude by a factor of 10.

Step 4a:Refer back to the epicenter distances on page 4 and enter them into the table

below.

Step 4b: Use the graphs on page 3 to find the amplitude (in mm.)of each S-wave.

Enter the height in mm on the table below.

Step 4c: For each station, use a ruler to draw a line from each amplitude on the

nomogram (on the right) to the corresponding distance (on the left).

Station Distance from epicenter Amplitude (in mm.)

Eureka, CA

Elko, NV

Las Vegas, NV

Step 5: After youve drawn the lines on the nomogram, find the point at which they

intersect (or the closest point). This is the approximate magnitude.

The approximate magnitude of this earthquake was ___________________.


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Asian EarthquakeUsing the steps above, determine the epicenter of this earthquake, as well as its

magnitude.

Epicenter: _______________________________ (closest city)

Magnitude _________________

100 200 300 400 500 600 700 Scale km


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Station 1: Pusan, South Korea

Station 2: Tokyo, Japan

Station 3: Akita, Japan

Time:

________ sec

Amplitude:

________ mm

Time:

________ sec

Amplitude:

________ mm

Time:________ sec

Amplitude:

________ mm


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Mexican Earthquake


Magnitude _________________

100 200 300 400 500 600 700 Scale km


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Station 1: Chihuahua

Station 2: Mazatlan

Station 3: Rosarito

Time:

________ sec

Amplitude:

________ mm

Time:________ sec

Amplitude:

________ mm

Time:________ sec

Amplitude:

________ mm


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Western USA


Magnitude _________________

100 200 300 400 500 600 700 800Scale km


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Station 1: Fresno, CA

Station 2: Las Vegas, NV

Station 3: Phoenix, AZ

Time:

________ sec

Amplitude:

________ mm

Time:

________ sec

Amplitude:

________ mm

Time:________ sec

Amplitude:

________ mm


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earthquake epicenter lab

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