Seismologists used to tell people not to worry too much about big quakes triggering smaller ones due to the relative scarcity of records of this happening, but the biggest quake of the year, an 8.6 tremblor in the East Indian Ocean, actually set off smaller quakes well into the following week and beyond according to new data from the University of California, Berkeley as well as the US Geological Survey.
Though the quake hit just outside of Sumatra, the April 11 quake was one of the tenth largest in the last hundred years and actually caused smaller quakes as far away as California, Japan and Indonesia, who all reported 7.0 quakes within days of the Sumatran quake.
Luckily, these 7.0 quakes hit in less densely populated areas, as Roland Burgmann, professor of earth and planetary sciences at UC Berkeley, suggested that these quakes might have been truly disastrous had they hit in urban areas. Burgmann notes that the “right kind of earthquake” could very well trigger a sort of ripple effect of smaller quakes, flying in the face of prior assumptions. That being said, these quakes remain few and far between. The 8.6 tremblor in the East Indian Ocean and its ripple quakes are in fact so newsworthy in the first place because they are the exception, not the rule.
So what is the right kind of quake? Burgmann and colleagues including Fred P. Pollitz, a research seismologist, have conducted research on all of the biggest tremblor quakes in recent memory including the tragic 2011 9.0 Tohoku quake in Japan, and the 2004 9.2 Sumatra-Andaman quake. These earlier quakes have resulted in some smaller quakes in surrounding areas, but only to modest effect. The East Indian Ocean quake is truly spectacular in its effect thanks to its Love waves. This quake was a “strike-skip” quake which generates these waves more effectively than a typical quake. These waves can, travel great distances to affect fault lines far, far away from the initial quake. This was largely theoretical prior to the East Indian Ocean quake which proved that one major quake in part of the world can, in fact, create some very intense quakes in other parts of the world.
For any readers wondering if this quake is only impressive to the layman, take it from Burgmann himself, who insists that it was one of the “weirdest earthquakes” ever seen. Burgmann compares the quake to the 1906 San Francisco Earthquake, but “fifteen times” more energetic and much larger.
Rather than a direct ripple effect creating bigger and then smaller and smaller quakes, these strike-skip quakes are theorized to create a series of quakes of various sizes, many of which being undetectable, as the energy travels under the surface. Finally this chain reaction of smaller reactions eventually creates a larger rupture such as those we saw in California and Japan.
We don’t usually see this because the energy typically dissipates more quickly. It’s not common for a quake to send a wave of energy that becomes less and then more powerful.
Of course, this is only one theory. The truth is that seismologists still don’t know why an 8.6 in the East Indian Ocean caused a 7.0 in California, but the research has turned up some exciting and curious details.