n earthquake happens when two blocks of the earth slip past one another suddenly. The surface where they slip is referred to as fault or fault plane. The location below the surface where the earthquake starts is known as the hypocenter, while the location directly above it on the earth’s surface is called the epicenter. Earthquakes are largely caused by the rupture of geological faults, and to know more about such phenomena one must study geology. Paul Favret has academic qualifications in both geology and geophysics.
The outermost layer of the earth is fragmented into about 15 major slabs that are known as tectonic plates. These slabs form the lithosphere which is comprised of the crust and the upper part of the mantle. Tectonic plates move pretty slowly, relative to one another, ideally a few centimeters per year. However, even this movement can cause a significant amount of deformation at the plate boundaries, thereby resulting in earthquakes.
Observations show that most of earthquakes are linked to tectonic plate boundaries. The theory of plate tectonics can additionally be used in order to provide a simplified explanation of the global distribution of earthquakes, while its certain characteristics can be explained with the usage of a simple elastic rebound theory.
Below the tectonic plates lies the asthenosphere of the earth. It is the mechanically weak and ductile region of the upper mantle of Earth. Asthenosphere behaves like a fluid over really long-time scales. There are several theories that attempt to explain what really drives the movement of tectonic plates. Three of the major forces that have been proposed as the key drivers of tectonic plate movement are:
- Mantle convection currents: Warm mantle currents tend to drive and carry plates of lithosphere along a much like a conveyor belt.
- Ridge push (buoyant upwelling mantle at mid-ocean ridges): Plates newly formed at oceanic ridges are warm. Hence, they have a higher elevation at the oceanic ridge in comparison to colder and denser plate material further away. Gravity causes the higher plate at the ride to push away the lithosphere lying further from the ridge
- Slab pull: The older and colder plates sink at subduction zones. As they cool down, they become denser than the underlying mantle. The cooler, sinking plate pulls the rest of the warmer plate along behind it. A subduction zone basically refers to a collision between two of the tectonic plates of the earth, where one plate sinks into the mantle underneath the other plate.
According to research, slab pull is the major driving force for most plate movement, as the plates with more of their edges being subducted are ones moving faster. Paul Favret mentions that earthquakes take place in subduction zones for a variety of reasons. There are a number of stresses associated with the collision of two plates. These stresses cause deformation in the overriding plate, and therefore shallow earthquakes. Shallow earthquakes also tend to take place on the subducting slab when a locked zone
