Geol 150 – lab - Cal State LA

Geol 150 – lab
I. Determining the Epicenter
What is an earthquake?
Earthquakes occur when there is a sudden release of built up energy. Most earthquakes occur along faults when
one side of the Earth’s surface moves rapidly relative to the other side of the fault. This sudden motion releases
energy in the form of “seismic waves” that radiate out from its point of origin – focus. It is these “seismic waves”
that can produce ground motion, which we call an earthquake.
What is a seismic wave?
A seismic wave is the energy released and transferred within the earth. There are many types of waves but we are
interested in P (primary) waves and S (secondary) waves. P waves travel the fastest and can travel through solids
and liquids; whereas S waves can only travel through solids. These seismic waves can be detected by scientific
instruments called seismographs and recorded on seismograms that are located all around the world.
What is a seismogram?
Seismograms tells us P- and S-wave arrival
times, S-P interval in seconds, and the
How is an earthquake’s epicenter located?
In order to locate the epicenter of an earthquake, you need to examine at least three different seismic stations.
1. Measure the S-P time interval (in seconds) from three seismograms
2. Use the S-P Time Interval curve to determine the distance between the stations and the epicenter
3. Take a compass and set it to the distance for each station.
4. Trace the perimeter around each station using your compass (the epicenter is located somewhere along the
5. The epicenter should be at the point where all three epicentral circles intersect.
This method in determining an earthquake’s epicenter is called triangulation.
II. Determining Magnitude
What is magnitude?
The magnitude tells us how strong a particular earthquake was or the estimate of the total amount of energy
released during a fault rupture. A well-known scale to determine the strength of an earthquake is known as the
Richter Scale. The magnitude is given in numbers – 3 for earthquakes strong enough for people to feel and 9 for
the Earth’s strongest earthquakes. Earthquakes greater than 9 in Richter magnitude are unlikely.
The Richter magnitude determination is based on measurements made on seismograms. Two measurements are
1. S-P time interval/Distance of epicenter (you already measured this in section I)
2. Maximum amplitude
Once the S-P time interval/distance of epicenter and amplitude have been plotted, the magnitude can be
determined by using a nomogram.
Steps to determine magnitude:
1. Plot S-P interval (or distance) from each
seismic station.
2. Plot Amplitude from each seismogram
3. Draw a line to connect “Distance” and
4. Where the line meets on the middle scale is
the magnitude (each seismic station should fall
on or very near the same magnitude. If not,
recheck your answers)
For example, an earthquake that is 100 km away with an amplitude of 1 mm will have a 3.0 magnitude.
This lab has been modified from CSULA’s Virtual Earthquake Lab