Earthquake’s “Lightning”

Thunder is the noise made by the phenomenon of lightning. It was only in the mid 20th Century that we learned why lightning is so noisy. Even Aristotle thought thunder was caused by clouds bumping into one another. We now know that thunder is generated by the supersonic thermal expansion of air, as the electrical charge arcs through the atmosphere.

Like thunder, the earthquake is also a noise; it is so low pitched that it is almost inaudible, but so loud that it can cause buildings to shake themselves to bits. So what is the name of the phenomenon that produces this quaking noise?

We tend to lazily call it the “earthquake,” but that is as wrong as calling lightning “thunder.” We need a distinct word to describe the source of earthquake vibrations, equivalent to lightning being the cause of thunder. We need a word to describe earthquake’s “lightning.”

Like a spontaneously firing crossbow, the Earth’s crust is slowly loaded with strain and then suddenly discharged into fault displacement. Since 2000 we have become better at observing the two halves of the process.

One half concerns the sudden release of strain accumulated during hundreds or thousands of years over a large volume of the crust. We can now observe this strain release from continuous GPS measurements or from inter-ferometric analysis of synthetic aperture radar images.

The second half of the process is the distribution of displacement along the fault, which can now be reconstructed by inverting the full signature of vibrations at each seismic recorder.

Focusing on the earthquake vibrations means that we forget all the other consequences of the regional strain release.

For example, hot springs stopped across the whole of northern Japan following the 2011 Tohoku earthquake because the extensional release of compressional strain diverted the water to fill up all the cracks. In the elastic rebound of prolonged extension, half a cubic kilometer of water was squeezed out of the crust over nine months in the region around the last big extensional fault earthquake in the US in Idaho in 1983.

Sudden strain can cause significant land level changes; the city of Valdivia sunk 8 feet in the 1960 Chile earthquake, while Montague Island off the coast of Alaska rose 30 feet in the 1964 Great Alaska earthquake. Whether your building plot is now below sea level or your dock is high out of the sea, land level changes can themselves be a big source of loss.

Then, there are the tsunamis generated by all the regional changes in seafloor elevation due to earthquakes. In the 2011 Tohoku Japan earthquake, it was the subsequent tsunami contributed almost half the damage and almost all of the casualties.

So, what is the name of earthquake’s “lightning?” “Elastic rebound” describes one half of the process and “fault rupture” the other half. But no word combines the two. A word combining the two would have to mean “the sudden transformation of stored strain into fault displacement.” We could have called the origin of thunder “the sudden discharge of electrical charge between the ground and clouds,” but “lightning” slips more easily off the tongue.

There could be a competition to coin a new word to describe the earthquake generation process. Perhaps “strainburst,” “faultspring,” or, as the underground equivalent of lightning, “darkning.” We are scientifically bereft without a word for earthquake’s “lightning.”


Chief Research Officer, RMS
Robert Muir-Wood works to enhance approaches to natural catastrophe modeling, identify models for new areas of risk, and explore expanded applications for catastrophe modeling. Recently, he has been focusing on identifying the potential locations and consequences of magnitude 9 earthquakes worldwide. In 2012, as part of Mexico's presidency of the G20, he helped promote government usage of catastrophe models for managing national disaster risks. Robert has more than 20 years of experience developing probabilistic catastrophe models. He was lead author for the 2007 IPCC 4th Assessment Report and 2011 IPCC Special Report on Extremes, is a member of the Climate Risk and Insurance Working Group for the Geneva Association, and is vice-chair of the OECD panel on the Financial Consequences of Large Scale Catastrophes. He is the author of six books, as well as numerous papers and articles in scientific and industry publications. He holds a degree in natural sciences and a PhD in Earth sciences, both from Cambridge University.

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