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An earthquake is generated when a fault moves. When an earthquake is strong enough, faulting initiated at depths may breach the earth’s surface to form a ground rupture. Faulting tends to occur along zones of weakness such as old or pre-existing faults, fractures (or rock breaks not involving slip), or along bedding planes (contacts between sedimentary rock layers and between different types of rocks.
>The Lithosphere breaks when its strength ...
>The Lithosphere breaks when its strength is overcome by the large amount of stress applied.
>Rock failure that involves the slipping of lithosphere blocks past each other is called faulting.
The ground rupture length depends on the magnitude of the earthquake. Earthquakes with magnitude 6.5 or greater are generally strong enough to produce large-scale ground ruptures. There is no clear-cut relationship between ground rupture length and earthquake magnitude but larger earthquakes tend to have ranges of longer ground ruptures.
The width of deformation along the length of the ground rupture also largely depend on the type of faulting.
It may form more complex fault patterns.
>The complexity varies along the length of ground. Thus, the zone of deformation may include subsidiary branches and secondary faults on either side of the narrow main break or rupture.
Aside from ground rupture, faulting causes movement of the ground in many ways depending on the type of faulting involved. It may cause lateral shifting uplift, subsidence, extension or compression. In addition to variation in size and orientation, different faults can accommodate different styles of rock deformation, such as compression and extension. Not all faults intersect Earth's surface, and most earthquakes do no rupture the surface. When a fault does intersect the surface, objects may be offset or the ground may cracked, or raised, or lowered.
These factors include the type of fault movement (reverse, normal, or strike-slip) and the inclination of the fault plane.
Strike-slip faults are vertical (or nearly vertical) fractures where the blocks have mostly moved horizontally. If the block opposite an observer looking across the fault moves to the right, the slip style is termed right lateral; if the block moves to the left, the motion is termed left lateral.
Normal faults generally occur in places where the lithosphere is being stretched. Consequently they are the chief structural components of many sedimentary rift basins (e.g. the North Sea) where they have major significance for hydrocarbon exploration. They can also be found in deltas, at the rear edges of huge gravitation slumps and slides.
A thrust fault is a break in the Earth's crust, across which younger rocks are pushed above older rocks.
Scientists have tried lots of different ways of predicti...
Scientists have tried lots of different ways of predicting earthquakes, but none have been successful. They have a pretty good idea of where an earthquake is most likely to hit, but they still can't tell exactly when it will happen.
However, the probability of a future earthquake can be calculated, based on scientific data. Scientists at the US Geological Society (USGS) estimate that the probability of a major earthquake occurring in the San Francisco Bay area over the next 30 years is 67%.
>It's good to know if earthquakes are probable, so that residents can prepare. It would be better to predict exactly when earthquakes will occur.
Constructing Seismic Hazard Maps
In many regions, seismic expectancy maps or hazard maps are now available for planning purposes. The anticipated intensity of ground shaking is represented by a number called the peak acceleration or the peak velocity.
To avoid weaknesses found in earlier earthquake hazard maps, the following general principles are usually adopted today:
1. The map should take into account not only the size but also the frequency of earthquakes.
2. The broad regionalization pattern should use historical seismicity as a database, including the following factors: major tectonic trends, acceleration attenuation curves, and intensity reports.
3. Regionalization should be defined by means of contour lines with design parameters referred to ordered numbers on neighboring contour lines (this procedure minimizes sensitivity concerning the exact location of boundary lines between separate zones).
4. The map should be simple and not attempt to micro zone the region.
5. The mapped contoured surface should not contain discontinuities, so that the level of hazard progresses gradually and in order across any profile drawn on the map.
Developing engineered structural designs that are able to resist the forces generated by seismic waves can be achieved either by following building codes based on hazard maps or by appropriate methods of analysis. Many countries reserve theoretical structural analyses for the larger, more costly, or critical buildings to be constructed in the most seismically active regions, while simply requiring that ordinary structures conform to local building codes. Economic realities usually determine the goal, not of preventing all damage in all earthquakes but of minimizing damage in moderate, more common earthquakes and ensuring no major collapse at the strongest intensities. An essential part of what goes into engineering decisions on design and into the development and revision of earthquake-resistant design codes is therefore seismological, involving measurement of strong seismic waves, field studies of intensity and damage, and the probability of earthquake occurrence.