Is this true or false: The earthquakes that cause the most damage usually have a shallow focus?

True. Earthquakes occur when stresses in the earth reach a level greater than the strength of the rock, causing the rocks on opposite sides of the fault to suddenly and violently slip past one another. Stresses acting perpendicular to the fault push the rocks on either side of the fault together. The strength of a fault is related to the size of these stresses and the coefficient of friction of the material forming the fault. At the same time, other stresses act parallel to the fault plane to move the rocks past each other. When enough stress accumulates to overcome the strength of the fault, an earthquake occurs as the rocks snap back toward equilibrium, and release the stored energy in the form of seismic waves, which shake the surrounding rocks. H. F. Ried first developed this hypothesis of how earthquakes occur, called the elastic rebound theory, following the (Great) 1906 San Fransisco earthquake.





Whenever a noteworthy earthquake occurs newspapers, television broadcasts, and scientists quote many terms and quantities not included in the average person's vocabulary. For example, the rupture surface is the portion of the fault which slips when the earthquake occurs. The earthquake rupture begins at one point on the rupture surface called the focus or hypocenter specified with a latitude, a longitude, and a depth. The epicenter is the point on the earth's surface above the hypocenter, specified with only a latitude and longitude. The rupture progresses from the hypocenter along the rupture surface at a finite speed until, for some reason, it stops. The total time of shaking caused by an earthquake is related to the length of time needed for the rupture to progress along the entire rupture surface. A rupture may stop because all of the accumulated stress is released, because it reaches a stronger section of the fault, or because it reaches the end of the fault. What physical conditions allow a rupture to begin, and what causes it to stop are important questions earthquake researchers are now attempting to answer. Generally, but not in all cases, earthquake rupture begins at some point many kilometers deep within the lithosphere and progresses updip along the fault plane over the entire rupture surface.





If the earthquake occurs underneath the sea and continues almost to the sea floor, then the earthquake may create a tsunami. Tsunamis are popularly and erroneously called 'tidal waves,' but have nothing to do with tides or weather. People often wonder whether earthquakes cause faults or faults cause earthquakes. In laboratory experiments we can measure the mechanical strength of rock samples by continuously increasing stress on the sample until it breaks. Many people think of the break as an earthquake; however, usually this is quite unlike real-world earthquakes, which occur on preexisting faults. The Earth's lithosphere is riddled with faults, joints, buried ancient weathering surfaces, and other zones of weakness. It is much easier to slip on an existing zone of weakness, even if it is not optimally oriented, than to create a new fault. Therefore, on the physical Earth, faults always exist before we measure earthquakes occurring along them. cientists classify faults into strike-slip or dip-slip types according to the motion along the fault. Strike-slip faults are approximately vertically dipping with horizontal displacement along the strike of the fault. When looking across the fault, if objects on the opposite side move to your right the fault is a right-lateral strike-slip fault, and if objects on the opposite side move to your left the fault is a left-lateral strike-slip fault. Dip-slip faults are inclined at some angle to the surface, and displacement is primarily normal to strike direction, along the dip of the fault. Geologists give more descriptive names to dip-slip faults depending on the direction of the displacement. For example, on a normal fault the foot wall moves up with respect to the hanging wall. Normal faulting occurs in response to lithospheric extension with the fault plane dipping away from the uplifted rocks. Reverse faulting occurs in response to lithospheric shortening, or compression with the fault plane dipping beneath the uplifted rocks. On a reverse fault the foot wall moves down with respect to the hanging wall. When a reverse fault has a small dip angle, geologists call them thrust faults. Blind thrust faults occur at some depth but do not extend to the surface, forcing the layers of rock above the fault to bend instead of break. Seismologists classify earthquakes according to the motion on the fault and fault type as strike- slip and dip-slip earthquakes (see the previous question for the definitions). Earthquakes that are a combination of strike-slip and dip-slip movements, are called oblique-slip earthquakes. Earthquakes may also be classified in terms of the origin of the stresses that produce them; e. g., volcanic earthquakes are caused by stresses associated with a volcanic eruption. The answer depends on the size of the earthquake. Over the entire Earth, the number of earthquakes with magnitudes of 8 and greater is less than one each year. However, each year there are about 10 earthquakes of magnitude 7 or greater and 100 earthquakes of magnitude 6 or greater. No matter where they occur, these earthquakes are all powerful enough to be recorded by all or mostly all of the world's sensitive seismograph stations, such as UTIG's station HKT.





Each year there are probably also about 1,000 earthquakes of magnitude 5 or more, 10,000 of magnitude 4 or more, etc. However, a seismograph station must be very close to the epicenter to recorded smaller earthquakes; thus, many small earthquakes occurring in remote areas of the Earth are never recorded, located, or catalogued.Is this true or false: The earthquakes that cause the most damage usually have a shallow focus?
True.Is this true or false: The earthquakes that cause the most damage usually have a shallow focus?
True. With shallow focus earthquakes, there is generally less rock for the seismic waves to travel through to get to the surface (more groundshaking because there would be less wave dispersment than with deeper focus earthquakes). The type of faulting %26amp; the type of rock material will also determine the amount of groundshaking.
locus.





and its true. but shallow is on the order of 5 - 10km deep. there arent too many less than 3 km deep.





the most damage occurs from the site soil type. Clay being the worst and stone being the best.





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