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While researching information on earthquakes and their causes, I stumbled upon a very puzzling yet interesting natural phenomenon. It is called Deep earthquakes. Now because it is an extremely complex and technical concept, it is important that I first go over a few basic facts about earthquakes before venturing into the scientific jargon associated with my topic of interest.
Since the 1965 theory of plate tectonics by John Tuzo Wilson, the causes of earthquakes were attributed mostly to the drifts of continental plates and the different types of contact that these plates have with each other. A very good analogy is that of two cars (each representing a continental plate) very very slowly backing into each other. The initial contact is so soft that no real damage happens. However as the two keep on backing up, the friction keeps on increasing until one of the cars will break! That break is exactly similar to an earthquake (only smaller). And as we all know, the contact area where all the forces are applied is called the Epicenter (also known as the origin of the quake!). Epicenters are usually located along Fault areas. Faults are weak zones of the earth’s crust where the two distinct continental plates move in towards each other.
One very famous and relatively active fault zone is located right here in southern California. Called the San Andreas fault, it is large enough to create a magnitude 8 earthquake on the Richter scale. The Richter scale ranging from 1 to 10 is the premier measure used in comparing earthquakes. Naturally it goes without saying that a magnitude 8 earthquake would be devastating!
Now that we I have reviewed the basics on the Plate tectonic theory, let’s point out that according to this theory, earthquakes should not occur at great depths inside the earth because the mantle (underneath the earth crust) is a lot hotter and a so much softer than the crust. In fact, deeper than 100 km the mantle becomes plastic and therefore there is not rigid enough to cause strong contact as in the car/continental plates analogy.
However, on June 8th 1994, a great earthquake rumbled through the earth’s mantle more than 600 kilometers below Bolivia. It was the largest earthquake ever recorded at such depth and the biggest of any kind in the past 15 years. The tremors were felt as far away as Toronto.
The event was truly spectacular. In fact, since their discovery in 1927, we know that deep earthquakes are as regular as clockwork but their cause has teased geophysicist since then. Indeed given what we know about the mechanics of earthquakes how could Deep Earthquakes occur since no friction is not even existent?
It is only ten years ago that Harry W. Green II and his colleagues, in their University of California laboratory, have started to solve the puzzle concerning deep earthquakes.
First it is important to note that almost 30% of all earthquakes occurs at depths exceeding 70 kilometers and only 8% happen at depths greater than 300 kilometers. At these depths the pressure in the mantle is from 3 to 10 gigapascals, which is thousands and thousands of times more than the sea level pressure. This extreme pressure causes the hot melted mantle rocks to flow (exactly like volcano lava) at a very slow rate and consequently the stress produced is by far less than what is needed to cause earthquakes such as the ones along faults or trenches. Nevertheless, deep earthquakes occur. Because the frequency of earthquakes steadily declines down to about 300 kilometers, most geophysicist believe that the events originating between 70 and 300 kilometers below the surface are due to the plate tectonic and friction mechanism occurring on the earth’s crust and upper mantle. Deep earthquakes ( below 300 km ) however, follow an entirely different pattern, so they must be caused by a separate mechanism.
What, then, could cause such a phenomenon? Researchers started to look for correlations in order to understand what goes on at such depths within the earth. Due to the very extreme heat and pressure conditions present in the mantle it is virtually impossible to get enough solid data. However, years of study have provided a lot of information on the composition of the different layers of the earth and it has been discovered that near the earth surface, rocks contain minerals that exhibit a relatively loose packing of atoms causing them to be somewhat unstable. As the pressure on them increases at greater depth within the mantle, the atoms reorganize giving new minerals. The first such transformation occurs in the mantle at depth of about 400 km. In the reaction, olivine, the most abundant mineral of the upper mantle becomes unstable and changes into a phase having a spinel (cubic) structure that is much denser than the original mineral. Researches have shown that the range -300 to -700 km in the mantle is the only place where deep earthquakes occurs and it is also where the olivine mineral transform into spinel. So a correlation linking these transformations and deep earthquakes was established. Further researches showed that depending on the stress applied, the olivine transforms into spinel by a process called nucleation (growth of spinel crystals on the olivine grain boundaries).
When the temperature is high enough and the stress is very important, the olivine transforms quickly into spinel by nucleation and growth mechanism making it very weak and therefore subject to breaking under high stress by the olivine that has already been transformed into spinel and therefore denser and heavier. But this very important stress required is not known to exist, even in the mantle. Therefore, another factor must also be involved.
Other research conducted at the same time suggested, with no evidence that during the transformation of olivine into spinel, the mineral may have exhibited faulting instability. Harry W Green II and his colleagues decided to reproduce the transformation of olivine at high pressure in their laboratories and looked for possible faulting instability during the process.
As expected, the olivine transforming into spinel by nucleation expressed an important instability. In that case, the stress produced in the mantle would be sufficient to cause the earthquakes that have been recorded. One more time, an experiment was done in laboratories in which stress such as the one present in the mantle has been applied to a mass of olivine during its unstable phase. As in the mantle, the stress applied cause the olivine grains to crack, as if two continental plate were going in contact one to another creating a faults. Such events happening at a much greater scale and with a much bigger quantity of olivine are believed to be the cause of deep earthquakes

These findings confirmed the hypothesis by Harry W. Green II and his colleagues that deep earthquakes were caused by the transformation of the most abundant mineral present in the mantle, olivine, into spinel configuration. But one of the essential aspects originally overlooked by this theory was the sudden faulty instability that is now known to be critical to the actual occurance of the deep earthquake. Nonetheless we can easily observe how important the relation between each of the earth’s layers and its composition is.
Indeed, whether we are dealing with Deep earthquakes or just “regular” earthquakes, the end result is the same : fracture and powerfull seismic waves that travel through the different layers of the earth causing even more fractures and destruction along their paths. But furthermore, we can also observe that even though very different in causes, the same mechanisms occur in both cases. Pressure on and transformation of the tectonic plates and the olivine slowly build up with no real effect until it reaches a breaking point when friction is too strong and neither the plate nor the olivine mineral can substain more. That is when the powerful yet unpreventable break happens.