Underestimating earthquake scale

October 6, 2016

Nuclear Regulation Authority method may underestimate quake sizes: study

http://mainichi.jp/english/articles/20161006/p2a/00m/0na/012000c

A technique that estimates the scale of earthquakes announced by the Earthquake Research Committee in 2006 may be underestimating the size of earthquakes -- a problem for the Nuclear Regulation Authority, which bases its earthquake resistance plans on the system.

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Professor Kazuki Koketsu presented the results of his evaluation of the 2006 system at a research session of The Seismological Society of Japan on Oct. 5. Koketsu is a professor at the Earthquake Research Institute at The University of Tokyo and the head of the Subcommittee for Evaluation of Strong Ground Motion, part of the Earthquake Research Committee.

Koketsu compared the estimations of the 2006 technique and a 2009 method to the actual observed data from the magnitude 7.3 Kumamoto Earthquake in April.

While the 2009 technique predicted a magnitude of 7.0 to 7.2 for the active fault, the 2006 technique underestimated the possible magnitude as between 6.6 and 6.9. Koketsu concluded that the 2009 technique is more appropriate for estimating the scale of earthquakes.

However, the Nuclear Regulation Authority (NRA) still uses the estimates of the strongest possible tremor made by the 2006 system as the basis for examining earthquake resistance design plans for nuclear reactors.

In response to Koketsu's presentation, a representative of the NRA stated at a press conference held on Oct. 5., "We will begin discussion over whether we should adopt the 2009 system after the Subcommittee for Evaluation of Strong Ground Motion has coordinated its views on the matter."

The 2006 technique bases its estimates on both the estimated length and breadth of active faults. In 2009, the Earthquake Research Committee released a new system based mainly on the length of faults in order to calculate the expected magnitude of quakes on as many active faults as possible in a short amount of time. In Koketsu's study, the 2006 system miscalculated the length and width of the faults involved in the Kumamoto earthquake, leading to the underestimation of the scale.

While both techniques appear side by side in the research committee's manual, the committee's national earthquake scale prediction map for quakes measuring at least lower-6 on the 7-point Japanese intensity scale estimated to occur within the next 30 years along active faults are all calculated using the 2009 system.