Geodynamics - Measuring Strain

Lecture 8: Measuring Strain The last lecture explained the basic ideas about strain and introduced the strain tensor. This lecture explores a few different ways to measure strain and explains how strain, like stress, can be represented graphically using Mohr’s circle. 1. Measuring Techniques a. Leveling Leveling uses a level telescope and two measuring sticks to measure the vertical distance between two points. Leveling Telescope (level) Benchmark Figure 8.1 b. Triangulation Triangulation uses a telescope mounted on a protractor to measure the angle between two points. Triangulation Benchmark Protractor � Telescope Figure 8.2 c. Trilateration Trilateration uses a laser mounted on a protractor to measure the distance to two objects and the angle between them. Corner Cubes l2 Trilateration l1 2li = c�ti Laser Figure 8.3 d. Very Long Baseline Interferometry (VLBI) and the Global Positioning System (GPS) VBLI and GPS measure the location of objects on the Earth’s surface by using antennae to read signals from space. In VBLI, this signal comes from quasars. In GPS, the signal comes from a satellite. Quaser VLBI �x= c�t GPS (Global Positioning System) Figure 8.4 Synopsis of Measuring Techniques Leveling Triangulation Trilateration VLBI GPS Angle Distance Yes Yes Yes Yes Yes Yes Yes Height Yes Orientation Yes Yes Yes Yes For a more complete treatment of these techniques with examples, see pages 94-107 in Geodynamics by Turcotte and Schubert. 2. Indirect Measurement of Strain In addition to using the techniques listed above, strain can sometimes be measured indirectly by observing the change in shape of objects that naturally have a certain geometry. For example, a geologist may compare the deformed shape of fossils, pebbles, or veins in an outcrop to their undeformed shape to determine the presence of tension, shear, or compression. Deformed Undeformed Figure 8.5 This picture shows how deformed fossils and pebbles can be used to infer the strain in the host rock. Veins Compression Weak Strong Weak Tension Figure 8.6 This picture shows how veins form waves under compression and boudins under tension.