Small earthquake patterns point to the ‘big one’

Small earthquake patterns point to the ‘big one’

A new way of determining where, when and how big an earthquake will be has been unearthed by Victoria University and the Institute of Geological and Nuclear Sciences (GNS).

Victoria University Emeritus Professor Frank Evison, and GNS Geophysical Statistician Dr David Rhoades, have found that lists of earthquakes, large and small, published regularly by seismological observatories around the world provide a key to better understanding how large earthquakes are generated.

The discovery calls for a radical departure from the idea that has dominated thinking about earthquakes for the past century - that large earthquakes occur in more or less regular cycles.

“Long before each large earthquake, a pattern of smaller earthquakes shows approximately where and when the large earthquake will occur, and how big it will be,” says Professor Evison, School of Earth Sciences.

He likens this process to the stages of conception, gestation and birth with the diagnostic pattern of small earthquakes serving as a pregnancy test.

“Because the Earth’s crust is in a state known as ‘deterministic chaos’, conception can be achieved by a very small earthquake,” Professor Evison says.

“Immediately after conception, a remarkable jump occurs in the number and size of small earthquakes.

“A long gestation period follows, roughly 15 years before the birth of a magnitude 7 earthquake, and 40 years for magnitude 8.”

Professor Evison says this process has occurred widely in California, Greece, Italy, Japan, New Zealand, and a number of other countries. Recent examples include the disastrous earthquakes at Loma Prieta and Northridge (California), Kobe (Japan), Assisi (Italy) and Izmit (Turkey).

Evison and Rhoades have begun the difficult problem of applying this discovery to future earthquakes. With the help of colleagues in Greece and Japan, the pair is carrying out tests in those countries, as well as in New Zealand.

“The goal is to produce annual earthquake forecasts based on the expected locations, times and magnitudes of such earthquakes that, at the time, are in the process of being generated. This should allow a substantial reduction in earthquake risk by focussing the community’s readiness, rationalising the response, and minimising the cost of recovery,” says Professor Evison.

A full account of the discovery by Evison and Rhoades will be published in an upcoming issue of the international journal Pure and Applied Geophysics.

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