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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]
The strongest deep-focus earthquake in seismic record was the magnitude 8.3 Okhotsk Sea earthquake that occurred at a depth of 609 km (378 mi) in 2013. [22] The deepest earthquake ever recorded was a small 4.2 earthquake in Vanuatu at a depth of 735.8 km (457.2 mi) in 2004. [23]
The thickness is heavily dependent on location. For oceanic crust, the seismogenic layer thickness can be 0 to 40 kilometres (0 to 25 mi), and for continental crust, it ranges from 0 to 25 kilometres (0 to 16 mi). [2] It is also important to note that at subduction zones, there is one seismogenic layer being pushed on top of another.
As an example, in the magnitude 7.9 Denali earthquake of 2002 in Alaska, the epicenter was at the western end of the rupture, but the greatest damage was about 330 km (210 mi) away at the eastern end. [7] Focal depths of earthquakes occurring in continental crust mostly range from 2 to 20 kilometers (1.2 to 12.4 mi). [8]
The earthquake was the largest in China since the Maduo earthquake in May 2021 and the deadliest since the Jishishan earthquake in December 2023. [ 6 ] [ 7 ] It was caused by normal faulting and originated within the continental crust at 10 km (6.2 mi) depth.
Earthquakes are common along convergent boundaries. A region of high earthquake activity, the Wadati–Benioff zone, generally dips 45° and marks the subducting plate. Earthquakes will occur to a depth of 670 km (416 mi) along the Wadati-Benioff margin. [citation needed] Both compressional and extensional forces act along convergent boundaries.
Seismic velocities within the Moon's approximately 60 km thick crust exhibit an initial low of 100 m/s at the surface, [53] which escalates to 4 km/s at 5 km depth, and then to 6 km/s at 25 km depth where velocities sharply increase to 7 km/s and stabilize, revealing a consistent composition and pressure conditions in deeper layers. [54] [55]
Wadati–Benioff zone earthquakes develop beneath volcanic island arcs and continental margins above active subduction zones. [3] They can be produced by slip along the subduction thrust fault or slip on faults within the downgoing plate, as a result of bending and extension as the plate is pulled into the mantle. [4]