A mechanism for the March 11 earthquake
Slow-slip stress-loading of fault area may have primed the Great East Japan Earthquake
Earthquake Research Institute
Some major earthquakes are preceded by a series of foreshocks leading up to the mainshock. This was true of the March 11 Great East Japan Earthquake, for which the Japan Meteorological Agency (JMA) released a catalog of about 300 foreshocks over the preceding month, including one of magnitude 7.3.
Even so, the mechanism of the March 11 mainshock and the form of any relationship between the foreshocks and mainshock remained unknown. This is partly because signals of some foreshocks are easily masked by the artificial or natural noises from other quakes and ocean waves, making it difficult to establish a complete catalog only by conventional routine methods.
Working from the JMA catalog, Assistant Professor Aitaro Kato and his colleagues at the University of Tokyo’s Earthquake Research Institute created an extended catalog of 1416 shocks in the Tohoku offshore area from which they established that prior to the mainshock there were two sequences of “slow-slip” towards the epicenter.
Slow slip is the slow movement of plate boundary fault but which does not result in a major tremor. In the case of the Great East Japan Earthquake, both the first and second slow-slip sequences stopped near the mainshock epicenter, suggesting that slow-slip caused stress-loading to the epicenter for the mainshock.
This result provides the first plausible mechanism for the Great East Japan Earthquake and the foreshocks leading up to it. While not all major earthquakes are preceded by foreshocks, closer examination of the seismic records of other large earthquakes for slow-slip behavior may yield further invaluable clues to the processes by which earthquakes occur.
(Public Relations Division: Euan McKay, Azusa Minamizaki)
Department release/press release (Japanese)
Aitaro Kato, Kazushige Obara, Toshihiro Igarashi, Hiroshi Tsuruoka, Shigeki Nakagawa, Naoshi Hirata.
Propagation of Slow Slip Leading Up to the 2011 Mw 9.0 Tohoku-Oki Earthquake
Science 10 February 2012: Vol. 335 no. 6069 pp. 705-708, doi:10.1126/science.1215141