VIII Damage slight in specially designed structures; considerable in ordinary substantial buildings with partial collapse; great in poorly built structures (fall of chimneys, factory stacks, columns, monuments, walls). IX Damage considerable in specially designed structures. Buildings shifted off foundations. Ground cracked conspicuously. X Some well-built wooden structures destroyed. Most masonry and frame structures destroyed. Ground badly cracked. XI Few, if any (masonry) structures remain standing. Bridges destroyed. Broad fissures in ground. XII Waves seen on ground surfaces. Objects thrown upward into air. Damage total. Vancouver Lions Gate Bridge sways VII VI V IV Prince Rupert ALBERTA BRITISH COLUMBIA Port Alberni Chimneys collapse Water mains break Power out for 10 days Campbell River Chimneys collapse Courtenay Chimneys collapse Post office wall collapses Epicentre Seattle Buildings sway Portland Kilometres 0 300 A B ◆ Figure 10.12 A. Modified Mercalli Intensity map showing intensity of shaking associated with an earthquake (magnitude 7.3) that occurred at 10:15 A.M. on June 23, 1946. The epicentre was in central Vancouver Island, near the communities of Courtenay and Campbell River. B. Damage to a school in Courtenay, British Columbia, resulting from the 1946 earthquake. (Part A © Department of Natural Resources Canada. All rights reserved. Photo B reproduced with the permission of Natural Resources Canada, 2010)
246 Chapter 10 Earthquakes and Earth’s Interior the first magnitude scale by using seismic records to estimate the relative sizes of earthquakes. As shown in Figure 10.13, the Richter scale is based on the amplitude of the largest seismic wave (P, S, or surface wave) recorded on a seismogram. Because seismic waves weaken as the distance between the earthquake epicentre and the seismograph increases (in a manner similar to light), Richter developed a method that accounted for the decrease in wave amplitude with increased distance. Theoretically, as long as the same, or equivalent, instruments were used, monitoring stations at various locations would obtain the same Richter magnitude for every recorded earthquake (Richter selected the Wood- Anderson seismograph as the standard recording device). In practice, however, different recording stations often obtained slightly different Richter magnitudes for the same earthquake—a consequence of the variations in rock types through which the waves travelled. Although the Richter scale has no upper limit, the largest magnitude recorded on a Wood-Anderson seismograph was 8.9. These great shocks released an amount of energy that is roughly equivalent to the detonation of 1 billion tonnes of TNT. Conversely, earthquakes with a Richter magnitude of less than 2.0 are not felt by humans. Earthquakes vary enormously Despite their usefulness in providing seis- mologists with a tool to compare earthquake sever- ity, particularly in regions without seismographs, intensity scales have severe drawbacks. In particular, intensity scales are based on effects of earthquakes (largely destruction) that depend not only on the
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