Ground Motion

one of the primary hazards of earthquakes is ground motion caused by the passage of seismic waves through populated areas. The most destructive waves

Collapsed 10-story apartment building in Islamabad, Pakistan, after earthquake October 8, 2005: The building pancaked as one floor fell, thereby causing each lower floor to collapse. (AP images)

How Tectonic Plates Leads Landslides

Damage from ground shaking and landslides in Yingxiu, Sichuan Province, China, from May 12, 2008, magnitude 7.9 earthquake (T. Kusky)

are surface waves, which in severe earthquakes may visibly deform the surface of the earth into moving waves. Ground motion is most typically felt as shaking; it causes the familiar rattling of objects off shelves reported from many minor earthquakes. The amount of destruction associated with given amounts of ground motion depends largely on the design and construction of buildings and infrastructure according to specific codes.

The amount of ground motion associated with an earthquake generally increases with the magnitude of the quake but depends also on the nature of the substratum—loose, unconsolidated soil and fill tends to shake more than solid bedrock. The Loma Prieta, California, earthquake (1989) dramatically illustrated this phenomenon where areas built on solid rock vibrated the least (and saw the least destruction), and areas built on loose clays vibrated the most. Much of the Bay Area is built on loose clays and mud, including the Nimitz freeway, which collapsed during the event. The area that saw the worst destruction associated with ground shaking was the Marina district. Even though this area is located far from the earthquake epicenter, it is built on loose, unconsolidated landfill, which shook severely during the earthquake, causing many buildings to collapse and gas lines to rupture, initiating fires. more than twice as much damage from ground shaking during the Loma Prieta earthquake was reported from areas over loose fill or mud than from areas built over solid bedrock. similar effects were reported from the mexico City earthquake (1985) because the city is built largely on old lake bed deposits.

Additional variation in the severity of ground motion is noted in the way that different types of bedrock transmit seismic waves. Earthquakes that occur in the western united states generally affect a smaller area than those that occur in the central and eastern parts of the country. This is because the bedrock in the west (California, in particular) is generally much softer than the hard igneous and metamorphic bedrock found in the east. Harder, denser rock generally transmits seismic waves better than softer, less dense rock, so earthquakes of given magnitude may be more severe over larger areas in the east than in the west. From the perspective of ground motion intensity, it is fortunate that more large earthquakes occur in the west than in the east.

Ground motions are measured as accelerations, the rate of change of motion. This type of force is the same as accelerating in a car, where the driver feels pushed gently back against the seat while increasing speed. This is a small force compared with another common force measured as an acceleration, gravity. Gravity is equal to 9.8 meters per second squared, or 1xg (this is what one would feel while jumping out of an airplane). People have trouble standing up and buildings begin falling at one-tenth the acceleration of gravity (0.1xg). Large earthquakes can produce accelerations that greatly exceed—even double or triple—the force of gravity. These accelerations are able to uproot large trees and toss them into the air, shoot objects through walls and buildings, and cause almost any structure to collapse.

some of the damage typically associated with ground motion and the passage of seismic waves includes swaying and pancaking of buildings. During an earthquake buildings may sway with a characteristic frequency that depends on their height, size, construction, underlying material, and intensity of the earthquake. This causes heavy objects to move rapidly from side to side inside the buildings and can cause much destruction. The shaking generally increases with height, and in many cases the shaking causes concrete floors at high levels to separate from the walls and corner fastenings, causing the floors to fall progressively or, pancake one another, crushing all in between. With greater shaking the entire structure may collapse.

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The Basic Survival Guide

The Basic Survival Guide

Disasters: Why No ones Really 100 Safe. This is common knowledgethat disaster is everywhere. Its in the streets, its inside your campuses, and it can even be found inside your home. The question is not whether we are safe because no one is really THAT secure anymore but whether we can do something to lessen the odds of ever becoming a victim.

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