In the past, earthquakes were believed to occur because the ground would shake in a very strong manner. After reviewing ground surface displacements that occurred during the 1906 San Francisco earthquake, Henry Reid, Professor of Geology at Johns Hopkins University, determined that earthquakes had an “elastic rebound” because of previously stored stress along a fault line.
Imagine that you’re stretching out a rubber band. Suddenly that rubber band snaps. The stored elastic energy has to go somewhere because it has been suddenly released. This is the principle that Reid proposes through his elastic rebound theory of earthquakes. It is a way for the stored energy to be released through a sudden action.
What Makes an Elastic Rebound Possible During an Earthquake?
During an earthquake, the tectonic plates of the Earth are moving relative to each other. This creates a “strain energy” that builds up along the edges of the fault planes of both tectonic plates. This energy is stored in the rocks along the fault. Fault planes are rarely smooth, so the rocks can store a tremendous amount of energy that is produced by the strain of tectonic movement. This energy can be stored even when it is restricted with a fault interlock.
At some point, the shearing stress that has been produced by the movement of the tectonic plates must be released. This is because there is a finite storage potential in the rocks along the edge of the fault line or the stress builds up along the length of a fault with enough distance that it puts excess pressure on the energy storage potential that is present. When that energy is released, an earthquake is the result, and the shaking ground is the elastic rebound effect of the energy be released and the storage potential reset.
What Does This Mean for the Study of Earthquakes?
There are three factors that must be determined when looking at the elastic rebound of an earthquake.
- You must know what the strength or energy storage potential is for the rocks that are along a tectonic plate fault.
- You must know the exact length of the fault.
- You must know the speed that the tectonic plates are slipping past each other.
When you have information about these three factors, it becomes possible to calculate the time it will take for the strain energy to build up along a fault. This would make it feasible to not only calculate the amount of time it takes for an earthquake to occur, but what its magnitude will likely be when it happens.
This calculation is possible because as tectonic movements occur, the edges of the crust blocks are restrained by fault friction. This causes the ground to bend and the strain energy to begin building. This build-up process will continue to occur until the strain energy storage potential has been completely used up. Then the edges will attempt to restore themselves and this is what causes a rupture and the earthquake itself.
Earthquakes Are Always Building Up Strain Energy
The severity of an earthquake is determined by the amount of strain energy that can be stored. Greater storage capacity will likely cause a more severe earthquake when the elastic rebound occurs, but earthquakes along that fault will be very few. A lower storage capacity will result in earthquakes that are generally less severe, but there will also be more overall earthquakes along that fault.
Not every earthquake is caused by this form of tectonic movement. Volcanoes can induce changes to tectonic movements by the injection or withdrawal of magma, which creates a pressure change along the rock around where the stress points and strain energy is stored. Long-period earthquakes are also a possibility around volcanoes because of consistent movements of magma throughout an earthquake system.
The earthquakes experienced in California would be considered elastic rebound quakes in most circumstances. The expected earthquake in the Seattle region that was promoted by Kathryn Schulz in The New Yorker would also be an elastic rebound quake, but one with a greater storage capacity. Many areas in the Ring of Fire and by volcanoes experience magma-movement related quakes that may or may not put pressure on the major faults.
This means the best solution is to be prepared for an earthquake to occur. Using Reid’s elastic rebound theory, it becomes possible to look at what could be the worst-case scenario for a region. That information allows families to plan and protect themselves in the unlikely event that such an earthquake would occur.