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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 ]
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]
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 and 300 km (43 and 186 mi) are commonly termed "mid-focus" or "intermediate-depth" earthquakes.
Most earthquakes are small, with rupture dimensions less than the depth of the focus so the rupture doesn't break the surface, but in high magnitude, destructive earthquakes, surface breaks are common. [8] Fault ruptures in large earthquakes can extend for more than 100 km (62 mi). [8]
The earthquake struck some 30 kilometres (19 mi) off the coast of Efate. [7] The United States Geological Survey (USGS) placed the earthquake at M w 7.3 with a depth of 57.1 kilometres (35.5 mi). The focal mechanism indicated oblique-normal faulting. Together with the hypocentral depth, this implied faulting within the subducting Australian ...
Deep-focus earthquake, also called a plutonic earthquake, an earthquake with a depth exceeding 70 kilometres (43 mi). [5] Doublet earthquake, an earthquake that consists of at least two or more mainshocks of nearly identical magnitude, separated by a period of time. [6]
The data for an earthquake is plotted using a lower-hemisphere stereographic projection. The azimuth and take-off angle are used to plot the position of an individual seismic record. The take-off angle is the angle from the vertical of a seismic ray as it emerges from the earthquake focus.