**Hirokazu Moriya**

Graduate School of Engineering, Tohoku University, 6-6-04, Aramaki Aza Aoba, Aoba-ku, Sendai 980-8579, Japan; hirokazu.moriya.e1@tohoku.ac.jp; Tel.: +81-22-795-7996

Received: 20 July 2018; Accepted: 16 August 2018; Published: 20 August 2018

**Abstract:** Acoustic emission (AE)/seismicity activity increased near the city of Sendai, Japan, after the 11 March 2011 Tohoku earthquake in a newly seismically active region near the Nagamachi-Rifu fault, which caused a magnitude 5.0 earthquake in 1998. The source of this activity was around 12 km beneath an artificial lake. At the same time, activity on the Nagamachi-Rifu fault nearly ceased. More than 1550 micro-earthquakes were observed between 11 March 2011 and 1 August 2012, of which 63% exhibited similar waveforms and defined 64 multiplets. It appears that crustal extension of about 2 m during the Tohoku earthquake and additional extension of about 1 m during the following year changed the stress field in this region, thus generating micro-earthquakes and controlling their frequency. However, it has been presumed that crustal movement during the Tohoku earthquake did not affect the direction of principal stress, and that these events induced repeated quasi-static slips at asperities and the resultant micro-earthquakes.

**Keywords:** acoustic emission swarm; 2011 Tohoku earthquake; repeating earthquake; multiplet; crustal movement
