Shallow Intermediate and Deep Foci
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:
FAQs on Earthquakes - Shallow, Intermediate, and Deep Foci
1. What are shallow, intermediate, and deep-focus earthquakes?
The classification of an earthquake is based on the depth of its focus (or hypocenter), which is the point within the Earth where the earthquake originates. The categories are:
- Shallow-focus earthquakes: Occur at depths of 0 to 70 km below the Earth's surface.
- Intermediate-focus earthquakes: Occur at depths between 70 and 300 km.
- Deep-focus earthquakes: Occur at depths greater than 300 km.
Most earthquakes that we hear about are shallow-focus, as this is where the Earth's tectonic plates are most active.
2. What is the main difference between a shallow-focus and a deep-focus earthquake?
The primary difference is their depth of origin. A shallow-focus earthquake originates close to the Earth's surface (less than 70 km deep), while a deep-focus earthquake originates much deeper (over 300 km). This depth directly impacts the intensity of shaking felt at the surface, as seismic waves from deep-focus earthquakes lose more energy while travelling to the surface.
3. Why are shallow-focus earthquakes often more destructive than deep-focus ones?
Shallow-focus earthquakes are generally more destructive because the seismic waves they release have a much shorter distance to travel to reach the surface. As a result, they retain more of their energy. This leads to more intense ground shaking and greater potential for damage to buildings and infrastructure compared to deep-focus earthquakes of the same magnitude, whose waves weaken over the longer distance.
4. What type of plate boundaries are associated with different earthquake depths?
Different plate boundaries typically produce earthquakes of specific depths:
- Shallow-focus earthquakes can occur at all three types of plate boundaries: divergent (plates moving apart), convergent (plates colliding), and transform (plates sliding past each other).
- Intermediate and deep-focus earthquakes are almost exclusively found at convergent boundaries, specifically within subduction zones. This is where one tectonic plate is forced to bend and sink deep into the Earth's mantle beneath another plate, allowing for brittle failure at great depths.
5. How do scientists determine the depth of an earthquake's focus?
Scientists determine the depth of an earthquake's focus by analysing the arrival times of different seismic waves at seismograph stations. They specifically look at the time difference between the primary wave (P-wave) and a depth phase wave (like the pP wave), which is a P-wave that reflects off the Earth's surface near the epicentre before travelling to the station. This time gap is directly related to the earthquake's depth; a larger gap means a deeper focus.
6. Can you provide some real-world examples of regions known for different focus-depth earthquakes?
Certainly. Different regions are prone to specific types of earthquakes based on their geology:
- Shallow-focus example: The San Andreas Fault in California, USA, is a transform boundary that produces frequent and sometimes powerful shallow-focus earthquakes.
- Intermediate and Deep-focus example: The area around the Tonga Trench in the Pacific Ocean and the Andes mountain range in South America are classic examples of subduction zones that produce a full range of earthquakes, from shallow to very deep.
7. What prevents earthquakes from originating very deep within the Earth's mantle?
Earthquakes are caused by the sudden breaking of rock under stress, a process called brittle failure. Deeper within the Earth's mantle, the temperatures and pressures are extremely high. These conditions cause the rock to become hot and ductile, meaning it behaves more like a thick, plastic-like fluid. Instead of breaking, it deforms or flows when under stress. Therefore, the necessary conditions for rock to fracture and cause an earthquake do not exist at these extreme depths.
