Salt Lake City Office of Emergency Preparedness Guidebook General Information About Earthquakes
Shaking and vibration at the surface of the earth resulting from underground movement along a fault plane or from volcanic activity. Large earthquakes hardly ever occur alone. When one earthquake happens, we usually see another at a nearby location. To talk about this phenomenon, seismologists coined three terms: “foreshock,” “mainshock,” and “aftershock.” In any cluster of earthquakes, the one with the largest magnitude is called the mainshock; anything before it is called a foreshock and anything after it is called an aftershock. The fault that moves in the mainshock experiences a great redistribution of the stress on it during the mainshock and it is that disrupted surface that produces most of the aftershocks. Sometimes the change in stress in the mainshock is great enough to trigger aftershocks on nearby faults. However, the stress change dies off quickly with distance from the fault so we rarely see aftershocks more than a few kilometers from the main fault. As a rule of thumb, we say that aftershocks are other earthquakes triggered at a distance from the mainshock fault no greater than the length of that fault.
Most costliest and intense earthquakes in US history Earthquakes pose one of the greatest risks for casualties and costly damage in the United States. During the 1989 World Series, as more than 62,000 fans filled Candlestick Park, a magnitude 7.1 earthquake struck about 60 miles south of San Francisco. The effects of the 20-second quake caused as much as $10 billion in damage. Sixty-two people died. March 1964, a magnitude 9.2 earthquake near Anchorage took 125 lives and caused about $311 million in property losses. 30 blocks of dwellings and commercial buildings were damaged or destroyed in the downtown area of Anchorage. Landslides caused heavy damage and an area of 130 acres broke the ground into blocks that were collapsed and tilted at all angles. In 1811 and 1812, the central Mississippi Valley was struck by three of the most powerful earthquakes in U.S. history. Consider what the impact would be if these events happened today in this region that has more earthquakes than any area east of the Rocky Mountains.
The magnitude of an earthquake is a measure of the amount of energy released. Each earthquake has a unique magnitude assigned to it. This is based on the amplitude of seismic waves measured at a number of seismograph sites, after being corrected for distance from the earthquake. Magnitude estimates often change by up to 0.2 units, as additional data are •included in the estimate. M=1 to 3: Recorded on local seismographs, but generally not felt M=3 to 4: Often felt, no damage M=5: Felt widely, slight damage near epicenter M=6: Damage to poorly constructed buildings and other structures within 10's km M=7: "Major" earthquake, causes serious damage up to ~100 km (recent Taiwan, Turkey, Kobe, Japan, and California earthquakes). M=8: "Great" earthquake, great destruction, loss of life over several 100 km (1906 San Francisco earthquake) –M=9: Rare great earthquake, major damage over a large region over 1000 km.
Emergency officials get fast information about any sudden Earth movements in the Wasatch fault from University of Utah seismologists who man a monitoring system of sensors, computers and telecommunications. University officials said the key feature of the system is the ShakeMap -- a rapidly generated computer map. Within five minutes of an earthquake, the map will give an overview of the location, severity and extent of ground shakes. The data will be similar to the Doppler radar image of a weather disturbance. Information from the real-time quake system will be sent to emergency managers so they quickly can direct personnel and equipment to appropriate areas.
If the earthquake were to occur on a central part of the Wasatch fault, Utah should expect damage to buildings to exceed $4.5 billion in Davis, Salt Lake, Utah and Weber counties. This may only represent 20% of the total economic loss. Un-reinforced masonry buildings (for example, brick homes built before 1960) are particularly vulnerable to ground shaking and are expected to account for 75% of the building losses. Surface faulting and ground failures due to shaking during a large earthquake will cause major disruption of lifelines (utilities, water, sewer), transportation systems (highways, bridges, airports, railways), and communication systems.
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