Near‐surface weakening in Japan after the 2011 Tohoku‐Oki earthquake

The magnitude (MW ) 9.0 Tohoku-Oki earthquake on 11 March 2011 was one of the largest in recent history. Ground motion caused by the seismicity around the time of the main shock was recorded by KiK-net, the strong-motion network that covers most of Japan. By deconvolving waveforms generated by earthquakes that are recorded at the surface and in a borehole at KiK-net station FKSH18, we detect a reduction of shear-wave velocity in the upper 100 m of about 10%, and a subsequent healing that varies logarithmically with time. Using all available borehole and surface records of more than 300 earthquakes that occurred between 1 January 2011 and 26 May 2011, we observe a reduction in the shear-wave velocity of about 5% in the upper few hundred meters after the Tohoku-Oki earthquake throughout northeastern Japan. The area of the velocity reduction is about 1,200 km wide, which is much wider than earlier studies reporting velocity reductions following other larger earthquakes. The reduction of the shear-wave velocity is an indication that the shear modulus, and hence the shear strength, is reduced over a large part of Japan.