Today, after experiencing an earthquake, anyone may go to the USGS earthquake website and describe the effects at the “Did You Feel It?” page.Īlthough Washington and Oregon have not experienced many major earthquakes in the recent past, they are located above the Cascadia subduction zone ( Figure 10.5). Figure 10.4 shows the intensities felt by the 1964 Great Alaskan Earthquake (when data were collected through mail questionnaires). The USGS has attempted to improve the accuracy of MMI shake maps by soliciting data from the public. Delicately suspended objects may swing.” The MMI scale is a subjective gauge compared to the Moment Magnitude scale. For example, near the epicenter of a small earthquake, or at a location far from a large earthquake, the intensity may be described with an MMI of II: “ Felt only by a few persons at rest, especially on the upper floors of buildings. This scale uses the Roman numerals I - XII to describe the effects of the earthquake in a particular location. The amount of shaking and damage is known as the earthquake’s intensity, and it can be measured by the Modified Mercalli Intensity (MMI) scale. The magnitude of an earthquake does not tell us how much damage is done by the waves in a particular area. Notable Earthquakes of the Western States The largest earthquake in US history was the 1964 Alaskan Earthquake, which had an M w of 9.2. Accordingly, an M9.0 quake would have 100 times the amplitude and 1024 times the energy of a M7.0 earthquake. These scales are logarithmic, meaning that a M9.0 earthquake would have 10 times the amplitude, and release 32 times the energy of a M8.0 earthquake. The United States Geological Survey (USGS) describes earthquakes as minor (M3.0 - 3.9), light (M4.0 - 4.9), moderate (M5.0 - 5.9), strong (M6.0 - 6.9), major (M7.0 - 7.9) and great (M8.0 or higher). Although it is possible to have a magnitude smaller than 1, and there is technically no upper limit, this is the range commonly used when reporting earthquakes to the public. Both the Richter scale and the Moment Magnitude scale use the numbers 1 - 10 to measure the amount of energy released. Hanks and Hiroo Kanamori developed the Moment Magnitude scale, abbreviated M w. The Richter scale is somewhat limited, however, because it cannot accurately measure or compare large earthquakes. The first scale used to measure magnitude was the Richter scale, which measures the amplitude of a seismic wave at a defined distance from the earthquake. Magnitude (M) is the measure of the energy released by the earthquake, whereas the intensity is what people actually experience. There are two ways to measure earthquakes: magnitude and intensity. Although the San Francisco earthquake was quite large, most of the damage to the city was actually caused by a fire, which is a common secondary hazard of earthquakes. Looking at the photo, one can visualize how the foreground moved to the left, while the background moved to the right. The fence was once connected, but today it is separated by a 6-meter (20-foot) gap. The Earthquake Trail, which begins at the Bear Valley Visitor Center, follows the trace of the 1906 San Francisco earthquake to a picket fence along the fault ( Figure 10.3). An excellent example of this kind of offset can be found along the San Andreas Fault at Point Reyes National Seashore. The plane defined by the rupture is known as a fault, and the rock layers become offset along it. The rocks then snap apart, releasing energy in the form of seismic waves ( Figure 10.2). According to the elastic rebound theory, rocks can bend elastically up to a point, until they finally break. Earthquakes occur when a critical amount of stress is applied to the crust.
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