ERSC 1F90 Geology and the environment Assignment 3 Locating Earthquake Epicentres Introduction An earthquake involves the sudden release of energy that is stored when rocks undergo deformation. When the “strength” of the rock is exceeded by the forces causing deformation the rock breaks and the energy is released in the form of seismic waves or shock waves that radiate out from the center of energy release (a point below the Earth’s surface that is called the focus of the earthquake). The point on the land surface, immediately above the focus is called the epicentre of the earthquake. Three types of seismic wave are produced by an earthquake: Primary Waves (P-Waves) that travel very quickly through rock. Secondary Waves (S-Waves), slower than P-waves, also travel within the crust. Surface Waves, adulatory waves that roll across the ground surface causing damage to structures. P- and S-waves can be detected by seismographs that record the time of arrival of the various waves types and the amplitude of the waves at the location of the seismograph. Because P-waves travel faster than S-waves they arrive at a seismograph first, followed by the S-waves. The difference in time between the first arrival of each wave type depends on the distance of the seismograph from the earthquake focus; the greater the distance the greater the difference in times of arrival at a location away from the epicentre. The amplitude of both P- and S-waves depends on the magnitude of the earthquake (a measure of the energy released by the Earthquake) and the distance from the focus. For a given distance, as the earthquake magnitude increases the amplitude of the waves increases and for a given earthquake magnitude, the amplitude diminishes with increasing distance from the epicentre. In this assignment you will use the time of arrival of P- and S-waves at three seismograph locations to determine the location of the epicentre of the earthquake and the amplitude of the S- waves (along with the distance) to determine the magnitude of the earthquake. Materials needed: Coloured pencils (red, black, blue) A compass for drawing circles. A copy of the Seismographic Charts (available as PDF files at the Assignment 3 web site) A copy of the Assignment 3 Answer Sheet (also available at the Assignment 3 web site) Instructions: Before beginning the assignment you would be wise to go through the tutorial that is available on the Assignment 3 web page from which you downloaded this document.