X Some well-built wooden structures destroyed; most masonry and frame structures destroyed with foundations. Rails bent. 6.5 - > 7.0 XI Few, if any (masonry) structures remain standing. Bridges destroyed. Rails bent greatly. XII Damage total. Lines of sight and level are distorted. Objects thrown into the air. TABLE 9.6 A comparison of the Richter Scale of earthquake magnitude to the Modified Mercalli Intensity Scale. Graphic by Stan Celestian
Table 9.7 is a magnitude/frequency chart that estimates the worldwide occurrence of earthquakes. The relationship between the magnitude of the earthquake to the frequency is inversely proportional, that is, as the magnitude of the earthquake increases the number of those events decrease. Table 9.8 in an example of this relationship for earthquakes in the United States between the years 2000 and 2012. TABLE 9.8 The number and magnitude of earthquakes with magnitude greater than 3.0, occurring in the United States. Source: Magnitude Average Annually 8 and higher 1 ¹ 7 - 7.9 15 ¹ 6 - 6.9 134 ² 5 - 5.9 1319 ² 4 - 4.9 13,000 (estimated) 3 - 3.9 130,000 (estimated) 2 - 2.9 1,300,000 (estimated) TABLE 9.7 Frequency and Magnitude Earthquake Chart Superscript 1 refers to data since 1900, superscript 2 refers to data obtained since 1990. Information Source USGS, Graphic by Stan Celestian MAGNITUDE / FREQUENCY
The damage created directly by an earthquake can be devastating to individuals, families, communities, and countries. Structural damage created by the shaking of the ground is dependent on three factors: 1. The intensity of the shaking 2. The duration of the shaking 3. The strength of the structure The intensity of the shaking is directly related to the size of the earthquake, the distance the structure is from the epicenter, and the type of substrate upon which the structure is located. The size of the earthquake, or its magnitude, has been previously described. In terms of the physical shaking of the ground, it is the result of the L and R waves, which is only a part of the total energy released by an earthquake. As a general rule, the amount of ground shaking increases 10 times for each single digit increase in the magnitude. For example, the total energy increase from a 5 M L to a 6 M L is 32X. The amount of ground shaking is 10X. From a 5 M L earthquake to a 7 M L it would be 100X (10 x 10). Along with the obvious intensity of the shaking, the duration of the shaking also plays a major role in the damage an earthquake creates. The Loma Prieta earthquake in 1989 created ground shaking that lasted less than 15 seconds. The Great San Francisco Earthquake shook the ground for about 42 seconds. The 1964 Good Friday Earthquake in Alaska lasted up to 4 minutes in some areas. Experiencing a severe shaking of the ground for 15 seconds would be frightening, imagine the ground shaking for a full 4 minutes!
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