Hello butterfly,,it have been for a long time since i posted but i have come across a very interesting topic about depth of earthquakes.
At what depth do earthquakes occur? What is the significance of depth?
Earthquakes occur in the crust or upper mantle which ranges from the earth's surface to about 800 kilometers deep about 500 miles.The strength of shaking from an earthquake diminishes with increasing distance from the earthquake's source, so the strength of shaking at the surface from an earthquake that occurs at 500 km deep is considerably less than if the same earthquake had occurred at 20 km depth.Also, the depths of earthquakes gives us important information about the Earth's structure and the tectonic setting where the earthquakes are occurring. The most prominent example of this is in subsections zones, where plates are colliding and one plate is being subjected beneath another. By carefully plotting the location and depth of earthquakes associated with a subsection zone, we can see details of the zone's structure, such as how steeply it is dipping, and if the down-going plate is planar or is bending. These details are important because they give us insight into the mechanics and characteristics of the deformation in the subsection zone.
The deepest earthquakes occur within the core of sub ducting slabs - oceanic plates that descend into the Earth's mantle from convergent plate boundaries where a dense oceanic plate collides with a less dense continental plate and the former sinks beneath the latter. The plate boundary contact between two such plates generate very large, shallow subsection zone earthquakes such as the Sumatra 2004 M 9.1 event, and the 2011 M 9.0 Japan earthquake, and is only active to relatively shallow depths - approximately 60 km. However, because oceanic slabs are relatively cold with respect to the surrounding mantle in deeper subsection zone environments, faults within the core of these slab remain brittle and can generate earthquakes to depths of as much as 700 km e.g., Pacific Plate beneath Japan and Kamchatka, and beneath Tonga.As the slab descends into the mantle, geology changes viscosity characteristics cause the plate to bend and deform, and generates these earthquakes. The trend of such events can be seen in cross-sections of subsection zones, and are known as wadati beniof zones.
Within continents, and along continental plate boundary transform faults such as the San Andrea's, faults are only active in the shallow crust - perhaps to depths of approximately 20 km.Accurately determining the depth of an earthquake is typically more challenging than determining its location, unless there happens to be a seismic station close and above the epicenter. So generally, errors on depth determinations are somewhat greater than on location determinations.
Determining the depth of an earthquake.
The evidence for deep-focus earthquakes was discovered in 1922 by H.H. Turner of Oxford, England. Previously, all earthquakes were considered to have shallow focal depths. The existence of deep-focus earthquakes was confirmed in 1931 from studies of the seismographs of several earthquakes, which in turn led to the construction of travel-time curves for intermediate and deep earthquakes.The most obvious indication on a seismograph is that a large earthquake has a deep focus is the small amplitude, or height, of the recorded surface waves and the uncomplicated character of the P and S waves. Although the surface-wave pattern does generally indicate that an earthquake is either shallow or may have some depth, the most accurate method of determining the focal depth of an earthquake is to read a depth phase recorded on the seismograph. The depth phase is the characteristic phase P wave reflected from the surface of the Earth at a point relatively near the hypo center. At distant seismograph stations, the p P follows the P wave by a time interval that changes slowly with distance but rapidly with depth. This time interval, p minus P is used to compute depth of focus tables. Using the time difference of P-P as read from the seismograph and the distance between the epicenter and the seismograph station, the depth of the earthquake can be determined from published travel-time curves or depth tables.
Another seismic wave used to determine focal depth is the P phase and S wave reflected as a P wave from the Earth's surface at a point near the epicenter. This wave is recorded after the P by about one-half of the P-P time interval. The depth of an earthquake can be determined from the P phase in the same manner as the P phase by using the appropriate travel-time curves or depth tables for P.If the p-P and P waves can be identified on the seismograph, an accurate focal depth can be determined.
Here are some of the links that help me to get the information;