Earthquakes are the sudden shaking of the ground caused by seismic waves passing through Earth’s rocks. These seismic waves occur when energy stored in the Earth’s crust is released suddenly, typically when large masses of rock pushing against each other suddenly break and slip. Typically, these occur along fault lines; the world’s major fault lines are located along the edges of the tectonic plates.
Approximately 50,000 earthquakes occur annually around the world. Out of these, approximately 100 are large enough to create substantial damage if their centre is located near populated areas. Extremely large earthquakes occur once per year on average. Earthquakes are responsible for the deaths of millions of individuals over Earth’s history.
Stationed around the world are approximately 3000 seismic stations, designed to measure ground motion. The signals received include primary waves, secondary waves, and surface waves.
Primary (P waves) waves are the first kind of body wave; body waves travel through the interior of the earth, arriving before surface waves. P waves can move through solid rock and fluids. They push and pull the rock it moves through in the same manner that sound waves push and pull the air.
Secondary (S waves) arrive next. This body wave is slower than a P wave and can only move through solid rock. S waves move rock particles up and down or side to side, perpendicular to the direction of the wave.
Finally, surface waves arrive, travelling only through the Earth’s crust. These waves cause most of the shaking of the ground and the resulting destruction.
While most people are familiar with the Richter scale for measuring earthquake strength, moment magnitude (Mw) is quickly becoming the most commonly used method to describe the size of an earthquake. Moment magnitude measures earthquakes in terms of how much energy is released. Moment magnitude can be used for events other than earthquakes, allowing for easy comparisons.
At 8:53pm on June 19, 2012, a moment magnitude-4.9 earthquake occurred in the town of Thorpdale in Australia. A second moment magnitude-4.3 earthquake followed the first one on July 20, 2012. Second earthquakes are a common phenomenon as residual stress on a fault is released, producing smaller aftershocks.
However, the article “Earthquakes that Talk to each Other” by Dr. Januka Attanayake, reveals that new research indicates that these earthquakes actually broke two adjacent faults. In fact, the research goes on to say that the first earthquake actually activated the second earthquake through the earthquakes’ own form of communication.
Following the occurrence of the first earthquake, the University of Melbourne set up 13 temporary seismic stations in Thorpdale. Their data collection revealed that the two earthquakes did not occur on the same fault. In fact, the second earthquake was located approximately 7 kilometres northwest of the first one. This all indicates that the second earthquake was not an aftershock but rather a main shock itself.
Coulomb stress transfer is a seismic geological process related to stress changes in surrounding material caused by individual deformation events. Simply put, this means that an earthquake can change the stress conditions in the surrounding Earth’s crust, and either increasing or decreasing the likelihood of another fault failing and causing another earthquake.
Another possible cause is the water trapped under high compression in the crust near the location of the second earthquake following the first event. This water may have made its way into the second fault plane due to the shaking of the rocks after the first quake. This water can have the effect of a lubricant, reducing the frictional strength holding the fault together.
While this information is fascinating, it does not allow us to forecast the occurrence of future earthquakes. All an earthquake forecast can tell us is the probability of an earthquake of a certain size happening in an area within a time frame. That time frame is usually measured in decades.
The concept of two earthquakes communicating with each other lets us know that earthquakes do not occur as isolated events.
Further details: “Earthquakes that Talk to each Other” by Dr. Januka Attanayake.