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Earthquakes - Shallow, Intermediate, and Deep Foci

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Shallow Intermediate and Deep Foci

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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:

Points

Shallow Earthquakes

Deep Earthquakes

Intensity

Shaking is more intense from shallow earthquakes since they hit close to the surface like driving down “a bomb directly under a city.

Deep quakes may be less devastating, they’re generally more widely felt.


Example of Quake

The Italy quake is considered to be the most damaging shallow quake that collapsed three towns, home to medieval buildings built before there were building codes.

Most of the deep quake destruction took place in Myanmar centered in the tourist town of Bagan. About 100 brick pagodas dating back centuries have collapsed.

 “They’re very quiet picturesque, but they don’t stand up against earthquakes very well,”

Level of Destruction

Many buildings were composed of brick or stone, which can fall apart at the time of shaking.

A minimum of four people were killed in the Myanmar quake quiver, which also fatigued ancient Buddhist pagodas.

FAQ (Frequently Asked Questions)

1. What is the Depth Phase in Deep and Shallow Earthquake Geology?

Answer: The depth phase is typically an attribute (characteristic) phase pP-a P wave reflected from the Earth’s surface at a point comparatively near the hypocenter. At distant seismograph stations, the pP goes around with the P wave by a time span that changes gradually with distance but quickly with depth. This time interval, pP-P (pP minus P), is used to calculate the depth-of-focus tables. Taking into account and applying the time difference of pP-P as read from the seismogram and the distance between the epicenter and the seismograph station, the depth of the quake can be identified from the issued travel-time curves or depth tables.

2. What are the Properties of Seismic Waves?

Answer: When a seismic wave comes into being a boundary that isolates rocks of different elastic properties, it experiences reflection and refraction. There is a unique kind of complexity because conversion between the wave types generally also takes place at such a boundary: an incident P or S wave can generate reflected P and S waves and refracted P and S waves. Boundaries between structural layers also trigger scattered and diffracted waves. These additional waves are in part held accountable for the complexities noticed in ground motion during earthquake episodes.


The range of frequency of seismic waves is quite large, from as high as the audible range (higher than 20 hertz) to as low as the frequencies of the free fluctuations of the whole Earth, with the gravest period being 54 minutes. Weakness or depletion of the waves in rock thrust upon high-frequency restricts, and in small to moderate earthquakes the dominant frequencies stretch in surface waves from about 1 to 0.1 hertz.