An earthquake is the shaking that results from stored energy in the Earth that is suddenly released. When an earthquake occurs, seismic waves travel through the earth and can be measured by a seismograph (see the background section of the Earthquake Fingerprints activity for more information).
Most earthquakes (particularly the largest ones) occur at the boundary between 2 tectonic plates. Briefly, the Earth’s crust is composed of several large plates that are in slow but constant motion across the surface of the planet (see the background section of the Plate Patterns activity for more information). The boundary between two plates is the fault. There are many types of faults that push past one another in different ways – strike-slip (where the plates slide past each other like in the demo), normal and thrust (where one plate slides up relative to the other.
I demonstrate this concept with 2 Plexiglas sheets laid side by side on the table. I tape the junction with scotch tape. The 2 pieces of Plexiglas represent 2 of Earth’s tectonic plates; the gap between them represents the fault. The scotch tape represents the friction between the plates – all the soil and rock at the junction that prevents the plates from sliding past one another freely. Then slowly I push the plates past each other in opposite directions, one pushing away from me, one pulling towards me. A bunch of energy is stored and then suddenly, the tape fails and the plates jump apart in a sudden release of energy. Put a couple of Monopoly houses on each piece of Plexiglas and the point is driven home.
The activity that follows has students conducting research on historically significant earthquakes around the world using the USGS Earthquake Archive. Each student is assigned a time frame to research and picks 10 earthquakes in that time frame to record information about. There are around 36 time frames available. The following day, they share details about one of the earthquakes they researched and plot each of the 10 earthquakes on a large world map in the classroom.
What is revealed is a very uneven distribution of earthquakes around the world. Almost all the earthquakes cluster along plate boundaries where plates are colliding (convergent boundaries) or sliding past one another (transform boundaries) although earthquakes occur at divergent boundaries as well.
An alternative way to achieve the same result is to track and plot the locations of earthquakes around the world day by day for several weeks/months until a pattern emerges. The USGS earthquake center provides a daily list of earthquake occurrences on their website with latitude, longitude, and magnitude information.