Understanding earthquakesBy Fernando Fajardo |Cebu Daily News
Did you know that about 50,000 earthquakes large enough to be noticed without the aid of instruments occur annually over the entire Earth?
We are lucky, however ,that of these, about 100 only are of a magnitude to produce substantial damage if their centers are near areas of habitation like what we just experienced last Oct. 15 in Cebu and Bohol.
What is an earthquake?
Encyclopedia Britannica defines an earthquake as any sudden shaking of the ground caused by the passage of seismic waves through the Earth’s rocks. Seismic waves are produced when energy stored in the Earth’s crust is suddenly released, usually when masses of rock straining against one another suddenly fracture and “slip.”
Here’s more from the Britannica article:
“Earthquakes often occur along geologic faults, narrow zones where rock masses move in relation to one another. As a fault rupture progresses along or up the fault, rock masses are flung in opposite directions and thus spring back to a position where there is less strain. At any one point, this movement may take place not at once but rather in irregular steps; these sudden slowings and restartings give rise to the vibrations that propagate as seismic waves. Fault rupture starts at the earthquake focus, a spot that in many cases is close to 5–15 km under the surface and propagates in one or both directions over the fault plane until stopped or slowed at a barrier.
“A separate type of earthquake is associated with volcanic activity and is called a volcanic earthquake. Yet it is likely that even in such cases the disturbance is the result of a sudden slip of rock masses adjacent to the volcano and the consequent release of elastic strain energy. The stored energy, however, may in part be of hydrodynamic origin due to heat provided by magma moving in reservoirs beneath the volcano or to the release of gas under pressure.
“Earthquakes can do significant damage to buildings, bridges, pipelines, railways, embankments, and other structures. The type and extent of damage inflicted are related to the strength of the ground motions and to the behavior of the foundation soils. In the most intensely damaged region, called the meizoseismal area, the effects of a severe earthquake are usually complicated and depend on the topography and the nature of the surface materials. They are often more severe on soft alluvium and unconsolidated sediments than on hard rock. At distances of more than 100 km (60 miles) from the source, the main damage is caused by seismic waves traveling along the surface.
“Earthquakes are frequently associated with reports of distinctive sounds and lights. The sounds are generally low-pitched and have been likened to the noise of an underground train passing through a station. The occurrence of such sounds is consistent with the passage of high-frequency seismic waves through the ground. Occasionally, luminous flashes, streamers, and bright balls have been reported in the night sky during earthquakes. These lights have been attributed to electric induction in the air along the earthquake source.
“Following certain earthquakes, very long-wavelength water waves in oceans or seas sweep inshore. More properly called seismic sea waves or tsunamis (tsunami is a Japanese word for “harbor wave”), they are commonly referred to as tidal waves, although the attractions of the Moon and Sun play no role in their formation. They sometimes come ashore to great heights—tens of meters above mean tide level—and may be extremely destructive.
“The usual immediate cause of a tsunami is sudden displacement in a seabed sufficient to cause the sudden raising or lowering of a large body of water. This deformation may be the fault source of an earthquake, or it may be a submarine landslide arising from an earthquake.
“Usually, a major or even moderate earthquake of shallow focus is followed by aftershocks or many lesser-size earthquakes close to the original source region which usually happens if the fault rupture producing a major earthquake does not relieve all the accumulated strain energy at once. In some cases an earthquake may be followed by 1,000 or more aftershocks a day. Although aftershocks tend to be weaker events relative to the power of the main quake, some aftershocks have caused significant damage.
“Most major earthquakes occur without detectable warning, but some principal earthquakes are preceded by foreshocks. In another common pattern, large numbers of small earthquakes may occur in a region for months without a major earthquake. Although earthquakes cause death and destruction through such secondary effects as landslides, tsunamis, fires, and fault rupture, the greatest losses—both of lives and of property—result from the collapse of man-made structures during the violent shaking of the ground. Accordingly, the most effective way to mitigate the damage of earthquakes from an engineering standpoint is to design and construct structures capable of withstanding strong ground motions.
“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.”
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