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SD-Lecture08-Earthquakes

# SD-Lecture08-Earthquakes - Soil Dynamics Lecture 08...

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Soil Dynamics Lecture 08 Earthquakes

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Definitions. Focus – point of rupture of a fault Focal depth 1. Shallow-focus : 0 to 70 km deep; (constitute about 75% of all earthquakes); 2. Intermediate-focus : 70 to 300 km deep; (constitute about 22% of all earthquakes); 3. Deep-focus : 300 to 700 km deep; (constitute about 3% of all earthquakes). Epicenter Epicentric distance Hypocentric distance L
The effective distance to the causative fault. This distance is the epicentric distance should be to the midpoint of the fault length. Length L of the fault rupture. Length L of the fault rupture. epicentric distance epicentric distance

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In the figure below, the hanging wall is thrust upward and over the footwall. This is called a reverse thrust fault . Other types of faults between these two blocks can be normal , oblique and strike-slip faults.
A sudden displacement of the crust is called a slip . Slips are described according to their movement. For example, when a person stands on either side of the San Andreas fault in mid-California, and looks across the fault, that fault’s movement will towards that person’s right. Therefore, the San Andreas fault is a right-lateral movement . The other three types of movements are left-lateral , normal and reverse . The process where an oceanic plate slides beneath a continental plate is known as a subduction . This is the case for the Pacific plate that is sliding beneath the North America continental plate. The seismic activity common to the states of California, Oregon and Washington ensues from this subduction. The seismic waves radiate out of from the focus. The compression waves (primary or P- waves) travel through the earth’s interior to reach the surface first. These compression waves displace materials directly behind or ahead of their path of travel. The shear waves (secondary or S-waves) displace material at right angles to their line of travel and reach the surface later. These shear waves have horizontal and vertical components since their propagation path may be in any direction from the source. The S-waves travel more slowly than the P-wave, but they transmit more energy. Thus, S-waves cause the bulk of the damage to surface structures.

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The primary cause of seismic waves (vibrations) are, (1) the sudden dislocations and changes within the earth’s crust plates due to their movements against each other, (2) volcanic eruptions, and (3) deep artificially induced explosions. During an earthquake, the sudden changes in the sea floor at depth (large rising and dropping) set a massive wave in the water in motion. As the wave approaches a land mass, the deep sea floor transitions gradually to a shallower floor. Since the wave has a constant mass, this change causes the wave height to increase. Also, its velocity decreases due to the increased friction with the shallower floor.
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