# Acoustic Emission/Seismicity at Depth Beneath an Artificial Lake after the 2011 Tohoku Earthquake
## 4 AE Activity before and after the Tohoku Earthquake  
A large number of micro-earthquakes were recorded in western Sendai city from 5 February 2005 to 1 August 2012, a period spanning the time of the Tohoku earthquake (Figure 1). The distribution of hypocenters defines two areas of AE activity, largely corresponding to seismic events before and after the Tohoku earthquake, respectively (Figures 2 and 3). Figure 2: Vertical cross section showing hypocenters of micro-earthquakes before the Tohoku earthquake projected onto the vertical plane defined by line A–B (location in Figure 1b). The dashed line shows the downward projection of the Nagamachi-Rifu fault. The red star represents the source location of the earthquake of magnitude (M) 5.0 occurred on 15 September 1998. The AE events before the Tohoku earthquake occurred at depths between 8 and 14 km on the Nagamachi-Rifu fault. However, after the earthquake, this activity ceased and a new active zone appeared to the west, generating hundreds of events between 11 March 2011 and 1 August 2012 at depths of 8 to 14 km. Activity in the study area did not increase immediately after the Tohoku earthquake, but began a gradual rise on 3 April and totaled 11 events (maximum magnitude 2.3) by 21 April. After 24 April, AE activity increased further, and by 1 August 2012, more than 1550 AE events had occurred. It is known that smaller earthquakes (e.g., aftershocks) occur after a previous large earthquake in the same area of the main shock, and the frequency of aftershocks decreases with the reciprocal of time after the main shock (Omori's law). However, clear mainshock and aftershocks are not identified after the Tohoku earthquake in Figure 4; thus, the typical decay of earthquake swarms is difficult to discern. The relationship between the event activity and the seasonal variations in water level in other years were investigated. However, the clear relationship as seen in Figure 4 was not observed before the Tohoku earthquake and after the studied period, and the seismic activity was very low in 2017. Figure 5 shows the Gutenberg-Richter relation before and after 11 March 2011. The \(b\)-value was 1.0 before 11 March and 0.92 after 11 March, indicating that larger events became more frequent after the Tohoku earthquake. Among the micro-earthquakes after 11 March 2011, we identified events that shared similar waveforms (Figure 6), known as repeating earthquakes or multiplets [17; 18; 19]. By calculating coherences for all possible pairs of events, and using coherence greater than 0.98 as the criterion for similarity, we identified 985 similar events (63% of those observed), which defined 64 multiplets. Travel-time differences of P- and S-waves were determined by cross-correlating the P- and S-components with their master events. The mean and standard deviation of the travel-time differences between P- and S-waves were 0.024 and 0.04 s, respectively. Source locations were not corrected according to travel-time differences because the number of seismic stations was insufficient to properly constrain them. However, the similarity of P- and S-wave travel times suggests that the multiplet sources were close together on adjacent fractures. During the period following the Tohoku earthquake, eight earthquakes larger than M 3.0 were observed at more than 40 stations, making it possible to estimate their source mechanisms by using P-wave polarity distributions. Figure 7a shows the fault plane solution of the largest of these events (M 3.4 on 3 June 2011), for which only P-wave polarities that were clearly detected by visual inspection were used to determine the nodal planes. The other seven of these events had the same mechanism,  
Figure 3: Vertical cross section showing hypocenters of micro-earthquakes after the Tohoku earthquake projected onto the vertical plane defined by line A–B.