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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]
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.
This is why an earthquake's depth is often given. The epicenter of the Northridge earthquake in Los Angeles in 1994 was in the San Fernando Valley, the point where the quake was strongest.
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]
A magnitude 5.5 eastern U.S. earthquake can usually be felt as far as 300 mi (483 km) from its origin, and sometimes causes damage as far away as 25 mi (40 km). The relatively shallow depth of this earthquake also contributed to its widespread effects. [32] [33]
The depth to the hypocenter can be estimated by comparing the sizes of different isoseismal areas. In shallow earthquakes, the lines are close together, while in deep events the lines are spread further apart. [9]
Even if the depth of focus of an earthquake is very deep, it can still have a very short epicentral distance. [3] When measuring the epicentral distance of an earthquake with a small epicentral distance, first measure the reading of the initial motion of P wave, and then confirm the arrival of S wave.