Large earthquake clusters likely coincidence, study suggests

Scientists find that large earthquakes do not increase the global seismic hazard for other damaging earthquakes far from the mainshock region.

Large earthquakes greater than 8.8 in magnitude are rare events, but the occurrence of three such earthquakes in the past seven years has left scientists debating whether or not large earthquakes can trigger clusters of other damaging earthquakes. At the same time, a March 2011 study by scientists at the U.S. Geological Survey and the University of Texas in El Paso – which examined earthquakes larger than magnitude 7 over the past 30 years – found no increase in the global seismic hazard for large damaging earthquakes after the occurrence of a single large earthquake.

The Richter Magnitude Scale classifies earthquakes based on the amplitude of seismic waves that travel through the ground during an earthquake. The scale is logarithmic, so each incremental increase in a whole number equates to a 10-fold increase in measured amplitude and about a 32-fold increase in the amount of energy released by the earthquake.

Earthquakes greater than 8.0 in magnitude are termed great earthquakes and they can cause extensive damage.

Earthquakes greater than 8.0 in magnitude occur at a frequency of approximately one per year. However, the majority of these great earthquakes range from 8.0 to 8.5 in magnitude.

Only seven earthquakes larger than 8.8 in magnitude have occurred since 1900 and three of those were in the past seven years: the 9.1 magnitude earthquake in Indonesia on December 26, 2004, the 8.8 magnitude earthquake in Chile on February 27, 2010 and, most recently, the 9.0 magnitude earthquake that struck Japan on March 11, 2011.

Image Credit: USGS

Seismologists are currently debating whether the recent cluster of large earthquakes is a statistical fluke or causally related.

Meanwhile, scientists from the U.S. Geological Survey and the University of Texas in El Paso evaluated aftershocks caused by earthquakes greater than magnitude 7 over the past 30 years. These scientists discovered that large earthquakes can trigger smaller aftershocks in areas both near and far from the mainshock region. The scientists also found that large earthquakes can trigger large aftershocks, but only within two to three rupture lengths from the mainshock region. No increase in the global seismic hazard for large damaging earthquakes was observed after the occurrence of a single large earthquake.

In a press release, lead author and geophysicist Tom Parsons states:

Based on the evidence we’ve seen in our research, we don’t think that large, global earthquake clusters are anything more than coincidence.

The paper titled “Absence of remotely triggered large earthquakes beyond the mainshock region” was published online March 27, 2011 in the journal Nature Geoscience.

Thus scientists continue to debate whether large earthquakes (for example, larger than magnitude 8.0) can trigger clusters of other damaging earthquakes. But – when less powerful earthquakes (magnitude 7.0 and above) are included in a 30-year analysis – U.S. Geological Survey and the UT El Paso scientists found no evidence that such a triggering mechanism occurs.

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Deanna Conners