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In seismology, the depth of focus or focal depth is the depth at which an earthquake occurs. Earthquakes occurring at a depth of less than 70 km (43 mi) are classified as shallow-focus earthquakes, while those with a focal depth between 70 km (43 mi) and 300 km (190 mi) are commonly termed mid-focus or intermediate-depth earthquakes. [1]
This can result in extremely deep earthquakes up to 700 kilometres (430 mi) in depth. [3] The base of this layer represents the downwards change in deformation mechanism from elastic and frictional processes (associated with brittle faulting) to a generally aseismic zone where ductile creep becomes the dominant process.
Seismotectonics is the study of the relationship between the earthquakes, active tectonics and individual faults of a region. It seeks to understand which faults are responsible for seismic activity in an area by analysing a combination of regional tectonics, recent instrumentally recorded events, accounts of historical earthquakes and geomorphological evidence.
In its most general sense, the word earthquake is used to describe any seismic event that generates seismic waves. Earthquakes can occur naturally or be induced by human activities, such as mining, fracking, and nuclear tests. The initial point of rupture is called the hypocenter or focus, while the ground level directly above it is the epicenter.
Hypocenter (Focus) and epicenter of an earthquake. An earthquake's hypocenter or focus is the position where the strain energy stored in the rock is first released, marking the point where the fault begins to rupture. [3] This occurs directly beneath the epicenter, at a distance known as the hypocentral depth or focal depth. [3]
The first, six-level intensity scale was proposed by Egen in 1828 for an earthquake in Rhineland. [4] [5] Robert Mallet coined the term "isoseismal" and produced a map for the 1857 Basilicata earthquake with a three-fold intensity scale and used this and other information to identify the epicentral area (a term he also coined). [6]
During an earthquake, seismic waves propagates in all directions from the hypocenter. Seismic shadowing occurs on the opposite side of the Earth from the earthquake epicenter because the planet's liquid outer core refracts the longitudinal or compressional while it absorbs the transverse or shear waves . Outside the seismic shadow zone, both ...
In 1949, Hugo Benioff introduced a method for determining elastic-rebound strain increments of earthquakes on a particular fault. [5] He determined that the square root of an earthquake's energy is proportional to both the elastic rebound strain increment and the rebound displacement, and developed a way to determine whether a series of earthquakes was generated along a single fault structure.