Earthquakes
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The findings are based on a new earthquake risk assessment method.
The Pacific Ocean's volcanic 'Ring of Fire' could produce more earthquakes of magnitude 9 or greater, say researchers.

Their findings, reported in the Bulletin of the Seismological Society of America, are based on a new way of calculating the probability of an earthquake.

The new research comes in the wake of the 2004 magnitude 9.3 Sumatra-Andaman earthquake which killed over 230,000 people across the Indian Ocean, and the 2011 magnitude 9 Tohoku earthquake which devastated Japan, claiming almost 19,000 lives.

"The strength of both these earthquakes caught many scientists by surprise," says the study's lead author Dr Yufang Rong, a seismologist with insurer FM Global.

"Almost all past methodologies failed to predict the strength of these earthquakes, so we looked at the problem again."

Existing methods of assessing earthquake risk are based on calculating how often earthquakes of a given magnitude happen along a particular fault line.

All these models are however tied to the limited histories available through earthquake records.

A fresh approach

In their new approach, the authors combined the known earthquake history of an existing fault with observations of tectonic plate movements, to estimate the maximum earthquake magnitude likely to occur over a specific time period.

"We can't know the exact, absolute maximum magnitude, however, we can develop a simple methodology to estimate the probable largest magnitude within a specific time frame," says Rong.

"The probabilities for earthquakes of a specific strength follow a very nice distribution curve. For example, across the whole world, if you have a single magnitude 8 earthquake in a year, you can expect 100 magnitude 7 earthquakes, 1000 magnitude 6, and 10,000 magnitude 5 quakes."

While this was known before, the new method is also able to calculate the probability of stronger earthquakes above magnitude 8.

Rong and colleagues applied their methodology to subduction zones along the Pacific Ring of Fire, finding most will experience a magnitude 8.5 earthquake within 250 years, a magnitude 8.8 within 500 years, and a magnitude 9 or greater within 10,000 years.

The authors validated their findings by examining the seismic history in marine sediment deposits across the Cascadia subduction zone along the United States Pacific Northwest coast.

Cascadia has been the seismically quietest Pacific subduction zone in recent history.

However, Rong's palaeoseismic studies identified an extensive seismic history with 40 large earthquakes over the past 10,000 years.

"I applied the ten thousand year history of the Cascadia subduction zone to our results [for the Pacific Ring of Fire] and it worked very well," says Rong.