Earthquakes are a not uncommon occurrence that is due to movements of different sections of the Earth’s crust. The basic theory, plate tectonics, states that there are many plates — pieces of the planet’s crust — that move about on the liquid magma below. Since they do not all move in the same direction, the plates may collide, with one plate moving under another, shear due to sideways movements, or separate, exposing magma which cools and becomes new crust. When plates get stuck, pressure builds up over time until it finally produces sudden movements called earthquakes. These can be tiny shivers of the Earth which are hardly noticed all the way up to cataclysmic shaking that knocks down buildings.
The purpose of this laboratory activity was to explore the nature of earthquakes — to measure lag times and compute distances, find the epicenter of an earthquake, and measure amplitudes to find magnitude on the Richter scale. In order to do this, we accessed Virtual Earthquake, a program from the California State University-Los Angeles Electronic Desktop. Using this simulation program, students can learn actively about earthquakes rather than merely reading about them.
The epicenter of an earthquake is the point at which the greatest movement occurs, although there may be movement over many miles along the fault line, the line where two or more plates meet. The sudden shifting creates waves that move outward in a circle. There are two waves, the p-wave (primary) and the s-wave (secondary). The p-wave is a compression wave that moves longitudinally — that is, there are changes in pressure in the same direction that the wave is moving, similar to sound waves. S-waves are transverse waves that cause the Earth to move perpendicular to the direction the wave is moving, similar to light waves. P-waves move faster than s-waves, so they arrive first. The lag time between the s- and p-waves is different based on distance from the epicenter, so this can be used in order to locate the epicenter if there are seismographic readings from three or more stations.
A seismograph is a picture or graph that shows the movement of the crust at a particular station. In this activity, seismographs from different simulated stations were used to determine how far away each station was located from the epicenter of the quake. This was done by measuring the s-p lag time and plotting the result on a graph of time versus distance. A seismograph can also be used to determine the magnitude or intensity of an earthquake. Using the computed distances and measured amplitudes, magnitude based on the Richter scale can be easily found. The Richter scale, which is logarithmic like the decibel scale, was developed by Richter and Gutenberg in 1935.
The first portion of the lab consisted of measuring lag times at seismic stations after a simulated explosion. The s-p lag times were then converted into distances. In this activity, the epicenter was known. Data was collected from 5 stations that were placed at random points around the epicenter. It was important to ensure the stations were not clustered on one side and that they were located at varying distances. The following table gives the measured s-p lag times and the computed distances.
The second part of the lab involved triggering a simulated earthquake, finding the epicenter, and determining the magnitude based on three or more stations. Although there was data available for all 5 stations, on 2 stations the amplitude was so large that it was not useful for this task. Therefore, only data from stations 2, 3, and 4 were used. Latitude and longitude for the epicenter were determined using a grid. Then, using amplitudes and distances, the magnitude of the quake on the Richter scale was found. Data from all three stations was used to confirm the magnitude.
The epicenter of the earthquake was found to be 37° 10′ N and 121° 45′ W.
I was familiar with causes of earthquakes and the Richter scale already, but in this laboratory experience I learned much more. I found out what S and P waves are, how they travel, and how their travel times can be useful in finding the distance from an earthquake. I also learned to use simple seismographs to measure amplitudes of the waves and, based on distance and amplitude, I learned to determine the magnitude of an earthquake on the Richter scale. In addition to being an excellent learning experience, I found the activity to be fun and interesting. The next time an earthquake occurs, I will know much more about what the numbers mean!