New study says quakes in excess of 9.0 are possible

Just how strong can an earthquake get?  That is a question that has puzzled researchers, and they were surprised when an undersea quake March 11, 2011, registered a magnitude of 9.0. The earthquake was off the Oshika Peninsula of Tōhoku in Japan.

The Seismological Society of America said that a new study suggests the possibility of quakes 9.0 and even greater along the Ring of Fire that includes western Costa Rica.

Earthquake magnitudes are expressed in base-10 exponential numbers like the velocity of the Starship Enterprise. So every increase in a whole number means a quake 10 times stronger. A 9.0 quake is 10 times stronger than an 8.0.

This study, published online by the Bulletin of the Seismological Society of America, calculates the “probable maximum earthquake magnitude within a time period of interest,” estimating the probable magnitude of subduction zone earthquakes for various time periods, including 250, 500 and 10,000 years, said the society.

The study’s results indicated most of the subduction zones can generate magnitude 8.5 or greater over a 250-year period, M 8.8 or greater over 500 years and M 9.0 or greater over 10,000 years. Said the society in a summary of the study:

“The instrumental and historical earthquake record is brief, complicating any attempt to confirm recurrence rates and estimate with confidence the maximum magnitude of an earthquake in a given period. The authors validated their methodology by comparing their findings to the seismic history of the Cascadia subduction zone, revealed through deposits of marine sediment along the Pacific Northwest coast. While some subduction zones have experienced large events during recent history, the Cascadia subduction zone has remained quiet. Turbidite [sedimentary] and onshore paleoseismic studies have documented a rich seismic history, identifying 40 large events over the past 10,000 years.

Some of the participants in the study work for companies that insure property.

Many of Costa Rica’s major quakes are a product of the Coco tectonic plate pushing under the lighter Caribbean plate. The visible junction of the plates is off the Pacific coast.  The Nicoya earthquake Sept. 5, 2012, had a magnitude of 7.5. The quake did significant damage in the community of Nicoya. An April 22, 1991, quake in Limón was measured at 7.8.

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