Pressure back to pre-disaster levels

Tectonic stress levels off northeastern Japan back to pre-disaster state

February 04, 2015

http://ajw.asahi.com/article/0311disaster/quake_tsunami/AJ201502040069

By SUSUMU YOSHIDA/ Staff Writer

Pressure exerted by tectonic plate movement off Tohoku that triggered the 2011 earthquake and tsunami has returned to pre-disaster levels, seismologists say.

“Large earthquakes might occur more randomly distributed in time than conventionally expected,” said Bogdan Enescu, an associate professor at the University of Tsukuba.

Researchers from the university and Switzerland-based Eidgenoessische Technische Hochschule analyzed seismic data collected by the Japan Meteorological Agency since 1998.

As part of the study, the team calculated b-values, a ratio comparing the occurrence of small earthquakes in a specific area with larger ones.

Because the b-values decline to under 1 when the frequency of large quakes increases, they assumed values of less than 1 are indicative of high plate stress in the areas being studied.

Although readings in the region at the center of the 2011 earthquake hovered around 0.8 to 0.9 from 1998 until 2005, the b-values dropped to around 0.6 to 0.7 in mid-2005. Those figures then surged to 1 or higher after the Great East Japan Earthquake of March 11, 2011.

But the b-values started to decline again around 2013, and returned to around 0.8 by the fall of 2014--almost the same level registered prior to the magnitude-9.0 quake.

“Observing b-values could be useful in improving the accuracy of massive earthquake forecasting,” said Enescu.

The findings were published in the British scientific journal Nature Geoscience on Feb. 3.