STRANGE RUMBLINGS

Earth-shattering seismic events can occur away from the fault lines between tectonic plates. And there's no easy way to predict when or where they'll hit

- PROF BILL MCGUIRE

When it comes to earthquakes, always expect the unexpected. That’s the message coming from seismologists Prof Éric Calais, of the École Normale Supérieure (ENS) in Paris, and JeanFrançois Ritz, Director of Montpellier’s CNRS Laboratoire Géosciences.

Underpinning their advice is the reality that Earth sometimes shakes in places it shouldn’t. These mysterious events, called intraplate earthquakes, happen far from the active margins of tectonic plates and in places that are otherwise geologically quiet. Gaining a better understanding of them and an explanation for them has become the mission of the French scientists.

UNPREDICTABLE AND DESTRUCTIVE

The rocky plates that make up the brittle, outer carapace of our world are performing a slow dance across the face of the planet, moving at about the same rate that a person’s fingernails grow. While nearly all the geological action worth talking about takes place where the tectonic plates meet, intraplate quakes are different, occurring in the interiors of the plates far from the margins.

There are good, potentially lifesaving, reasons for Calais and Ritz to want to shed more light on them. Intraplate quakes are rare: the number of significant shakes is tiny compared to what happens at the edges of the plates, with Calais noting that only 20 of a magnitude of 6 and above have been recorded since 1974. That’s less than half of one per cent of the number of similarly sized quakes at plate margins over the same time.

Their rarity, and typically long return periods, makes them difficult to predict, yet they’re capable of causing immense destruction in unprepared urban centres that never regarded quakes as a problem.