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Azhar Shahzad

Earthquake Induced Processes

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There are generally four types of earthquake induced processes: 1) Surface rupture, 2) Ground shaking, 3) Ground failure, and 4) Tsunami and Seiches occurrence. Each type has significance to a safe design of structures and is a concern for engineering geologist defining hazard levels.

1. Surface Rupture

It is the actual displacement and cracking of the ground surface along a fault trace. Surface rupture is confined to a narrow zone along an active fault. Rupture may happen rapidly during an earthquake or it may not occur at all. Displacement beneath a building that exceeds 1 or 2 inches can have a catastrophic effect. Some active faults undergo imperceptibly slow movement, termed fault creep. Although a serious problem for structure, the cumulative effect of this long-term displacement, unlike rupture during an earthquake, is not catastrophic.

2. Ground Shaking

It is the actual trembling or jerking motion produced by an earthquake. It causes widespread damage is one of the most difficult seismic effects to quantity and predict. The damage varies with wave length, duration of shaking, nature of underlying materials, and character of structures. For the same earthquake, the effect of ground shaking can be several times greater at sites with thick, water saturated soil than at those on competent bedrock. Buildings have a fundamental period that is roughly equivalent to the number of their stories. A resonance or amplification of ground shaking develops where the building and underlying soil have a similar fundamental period and causes more-extensive damage. Building with differing fundamental periods will sway differently.

3. Ground Failure

This is because of ground acceleration from an earthquake produces landslides, ground cracking, subsidence and differential settlement. Hilly slopes can fail under a dynamic load, the stress from earthquake motion. Liquefaction is another mechanism of ground failure during an earthquake. Ground motion transforms loose water-saturated granular material to a liquid state. This occurs because the cyclic stress of succeeding waves of ground motion causes pore-water pressure to build up. This rapid increase in pore-water pressure nearly reduces the effective stress of soil mass to zero, a state where it has a least resistance to applied stress. Otherwise solid level ground becomes unable to bear the weight of overlaying structures. Building and structural foundations sink into the liquefied materials, causing tilting and related damage. Differential settlement can cause damage to buildings as some of the soil settles more than other parts underlying the building foundation. Liquefied soil may flow on very low slopes. This special type of landslide is called lateral spread failure.

4. Tsunamis and Seiches

These are similar effects that occur in bodies of water. Tsunami is a Japanese term for large ocean waves generated by submarine earthquakes. Rapid displacement of an undersea fault can cause a wave travelling thousands of miles from the epicenter. On the deep ocean, the tsunami height typically is 1 ft. The wave height increases as it reaches shallower water near shore lines. Seiches or earthquake generated standing waves are not quite as destructive as tsunamis. They occur in enclosed or restricted bodies of water and consist of a standing wave that oscillates across the surface. Seiches generally are of low amplitudes (less than 1 ft.)

 

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