Fixing the cracks in disaster mitigation
Glossary
continental drift. The very slow movement of the continents on their underlying tectonic plates. See also tectonic plates and Plate tectonics (Nova: Science in the news, Australian Academy of Science).
frequency. A measure of how frequently a wave goes up and down (oscillates) or the number of waves passing by in a second. A hertz is a unit of frequency – 1 oscillation per second; a kilohertz (kHz) is 1000 hertz – 1000 oscillations per second; a megahertz is 1 million hertz – 1 million oscillations per second. For more information see Sound properties and their perception – pitch and frequency (The Physics Classroom, USA).
intensity. Measures the strength of shaking produced by an earthquake at a certain location. Intensity is determined from the effects on paople and structures. Intensity isusually measured in the Modified Mercalli intensity scale (Association of Bay Area Governments, USA).
magnitude. The severity of an earthquake is determined by its magnitude and intensity. The magnitude of an earthquake is a measure of the amount of energy released by the seismic event that caused it. Its intensity is its capacity to cause damage at a given point on the earth's surface. Thus, there is usually one value for an earthquake's magnitude but many measures of its intensity – depending on factors such as the distance of a given point from the earthquake's hypocentre.
Richter scale. A scale for measuring the magnitude or size of an earthquake. The scale relates to the energy released by an earthquake and is determined from the logarithm of the amplitudes (heights) of the seismic waves recorded at seismograph stations on the Earth's surface. See also magnitude and intensity. For more information see The severity of an earthquake, Measuring the size of an earthquake and Magnitude intensity comparison (United States Geological Survey).
The largest recorded earthquake, measuring 9.5 on the Richter scale, occurred in Chile in 1960. Seismologists have devised several other scales of measuring the magnitude of earthquakes, although the Richter scale remains the main scale used by the media to inform the public about earthquake size. For more information about the earthquake in Chile see Great Chile earthquake of May 22, 1960 (National Geophysical Data Center, USA).
seismic waves. Waves that transmit the energy released from movement of the Earth's crust. Primary waves (P-waves) are longitudinal waves that shake the ground in the direction of the wave. Secondary waves (S-waves) are shear waves that shake the ground perpendicular to the direction of travel. For more information see What is seismology? (Michigan Technological University, USA).
shear strength. The maximum stress a material will bear when it is twisted or otherwise deformed without stretching or compression.
tectonic plates. The Earth's surface is made up of huge tectonic plates that have moved very slowly during geological history, and continue to move, thus changing the position of continent and oceans. The plates are about 100 kilometres thick and move at a rate of about 1-12 centimetres per year. See also continental drift. For more information see Plate tectonics (Nova: Science in the news, Australian Academy of Science).
Posted October 2006.