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Epicentral distance During an earthquake seismic waves propagate spherically out from the hypocenter. Seismic shadowing occurs on the opposite side of the Earth from the earthquake epicenter because the liquid outer core refracts the longitudinal or compressional (P-waves) while it absorbs the transverse or shear waves (S-waves). Outside of the seismic shadow zone both types of wave can be detected but, due to their different paths through the Earth, they arrive at different times. By measuring the time difference on any seismograph as well as the distance on a travel-time graph at which the P-wave and S-wave have the same separation, geologists can calculate the distance to the earthquake's epicenter. This distance is called the epicentral distance, commonly measured in ° (degrees) and denoted as Δ (delta) in seismology. Once epicentral distances have been calculated from at least three seismographic measuring stations, it is a simple matter to find out where the epicenter was located using trilateration.
Shallow-focus and deep-focus earthquakes The majority of tectonic earthquakes originate at the ring of fire in depths not exceeding tens of kilometers. Earthquakes occurring at a depth of less than 70 km are classified as 'shallow-focus' earthquakes, while those with a focal-depth between 70 and 300 km are commonly termed 'mid-focus' or 'intermediate-depth' earthquakes. In subduction zones, where older and colder oceanic crust descends beneath another tectonic plate, deep-focus earthquakes may occur at much greater depths (ranging from 300 up to 700 kilometers).[5] These seismically active areas of subduction are known as Wadati-Benioff zones. Deep-focus earthquakes occur at a depth at which the subducted lithosphere should no longer be brittle, due to the high temperature and pressure. A possible mechanism for the generation of deep-focus earthquakes is faulting caused by olivine undergoing a phase transition into a spinel structure.[6]