[Geography Notes] on Earthquakes – Shallow, Intermediate, and Deep Foci Pdf for Exam

Earthquakes can take place anywhere between the Earth’s surface and about 700 kilometres beneath the surface. With respect to scientific purposes, this earthquake depth range of 0 – 700 km is classified into 3 zones: shallow, deep and intermediate.

Shallow quakes usually are disposed to be more damaging than deeper quakes. Seismic waves from deep quakes travel farther to the surface, depleting energy along the way.

Shallow Earthquake Geology

Shallow earthquakes are between the depth of 0 and 70 km; intermediate earthquakes, 70 – 300 km (43-186miles) deep; and deep earthquakes, 300 – 700 km (186-434 miles) deep. Usually, the word “deep-focus earthquakes” is used for earthquakes deeper than 70 km. All earthquakes deeper than 70 km are within high slabs of lithosphere which are sinking into the Earth’s mantle.

Deep Foci Earthquakes

The proof for deep-focus earthquakes first emerged in 1922 by H.H. Turner of Oxford, England. Earlier on, all earthquakes had been contemplated to have shallow focal depths. The presence of deep-focus earthquakes was validated in 1931 from researches of the seismograms of various earthquakes, which in turn results in the building of travel-time curves for deep and intermediate earthquakes.

How To Determine A Shallow Intermediate And Deep Foci Earthquake

1. A Seismograph: It is so far the most reliable method to determine the focal depth of an earthquake. That being said, the most obvious evidence on a seismogram that a great earthquake has a deep focus is the small height or amplitude of the recorded surface waves and an effortlessly simple attribute of the P and S waves. Although the surface-wave pattern does usually signal that an earthquake is either shallow or may contain some depth, the most appropriate technique method of identifying the focal depth of an earthquake is to read a depth phase recorded on the seismogram.

2. sP Phase: Another seismic wave used to identify the focal depth is the sP phase – an S wave considered as a P wave from the Earth’s surface at a point quite close to the epicenter. This wave is recorded after the pP by about ½ of the pP-P time duration. The depth of an earthquake is identified from the sP phase in a similar way as the pP phase by using the correct travel-time curves or depth tables for sP. If the pP and sP waves are able to be identified on the seismogram, an accurate focal depth can be established.

Difference Between Shallow and Deep Earthquakes

Remember that an earthquake’s destructive force is dependent not only on its strength but also on location, depth and distance from the epicenter.

Quakes can hit upon near the surface or deep within the Earth. Most earthquakes take place at shallow depths, as acclaimed by the U.S. Geological Survey.

That being said, Italy’s quake was recorded to be very shallow, originating between 4 km and 10 km underground. The magnitude measurements also differed a bit between magnitude 6 and 6.2.

In comparison, the 6.8 measurement of the quake in Myanmar was deeper at 84 km, which is regarded to be an intermediate depth. With that, let’s see how shallow differentiate from deep quakes. Refer to the table below: