Earthquakes teach us about the interior workings of Earth. Without them, we would not know about the core or any layering deeper than a couple kilometers.
Researchers from the Los Alamos National Laboratory, MIT and Earthquake Research Institute Tokyo have now proposed a new triggering mechanism for earthquakes. Triggered earthquakes happen due to a certain event that may include volcanic activity, other earthquakes or drilling activity (and more). These cause a previously stable or metastable fault to activate. Really, this is just seismologists lingo for „something happened, now the Earth shakes over here too“.
This new triggering mechanism now suggests that the seismic wave sent out by an earthquake may trigger another earthquake along the way. Essentially, causing cascading earthquakes. Kevin Chao from MIT states:
When a surface wave from an earthquake some way off passes through another fault region, it changes the balance between the frictional properties that keep the surfaces locked together, the elasticity that allows the crust to withstand strain, and the stress state that can cause it to fail.
During their analysis, the researchers discovered another amazing feature. The travelling surface wave sped up. The explanation for this is simple but genius. On the left, you can see the interpretation image (Fig. 4) from the paper.
(A) shows the neutral state of the plate. There is a shallow forearc and some stress transfer at the plate interface, nothing spectacular really.
In (B) the earthquakes are triggered. They’re normal faulting earthquakes, which means the plates involved cause an extensive motion.
This is when (C) follows. The extensive motion of the normal faulting causes compression further inland. The researchers postulate a closing of small cracks within the rock. This makes it easier for the seismic wave to progress, therefore, speeding up the propagation. Amazing really, such a small change such a large tail of consequences.
Andrew A. Delorey, Kevin Chao, Kazushige Obara and Paul A. Johnson. Cascading elastic perturbation in Japan due to the 2012 Mw 8.6 Indian Ocean earthquake. DOI: 10.1126/sciadv.1500468