# Acoustic Emission/Seismicity at Depth Beneath an Artificial Lake after the 2011 Tohoku Earthquake
## 5 Discussion  
East-west crustal extension during the Tohoku earthquake relaxed the subduction-related horizontal compressional stress in the study area. The extension decreased normal stresses on east- and west-dipping fault planes and, because shear stress on the reverse faults decreased, they were  
Figure 6: Example of a set of waveforms constituting a multiplet. Amplitudes are normalized to the maximum amplitude of each waveform. The vertical axis represents the number of events. Figure 7: (**a**) Waveforms of eight M \(>\) 3 earthquakes after the Tohoku earthquake and fault plane solution for the largest event (M 3.4, 3 June 2011). Open circle and closed circle represent negative and positive P-wave polarities, respectively; (**b**) Fault plane solution for the 1998 M 5.0 earthquake on the Nagamachi-Rifu fault, which was reproduced following the result by [PERSON] et al. (2014). dynamically stabilized against shear slip. Thus, the number of normal faulting events increased after the Tohoku earthquake [7], and earthquakes on the Nagamachi-Rifu fault nearly ceased after 11 March 2011, as horizontal compressional stress on the fault surface decreased. After the Tohoku earthquake, seismic activity increased in a region 6 km west of the source of the 1998 earthquake (Figure 3), apparently on newly activated fractures. Fault plane solutions (Figure 7a) indicate that the newly activated fractures dip either east or west and slipped with left-lateral strike-slip and reverse mechanisms. Seismic activity on these fractures, which were previously under critical conditions for reverse faulting, would be expected to decrease when horizontal compressional stress was relaxed during the Tohoku earthquake and lowered the shear stress on the fault plane. However, seismic activity increased after the earthquake, so we must seek other contributing factors. One possible explanation is a relative increase of principal stress in the north-south direction [16]. This would have created conditions favoring left-lateral strike-slip faulting. Fault plane solutions after the Tohoku earthquake (Figure 7a) include both left-lateral and reverse components, whereas for the 1998 earthquake (Figure 7b), although a left-lateral component was present, reverse faulting was dominant. This result suggests that a relative increase of principal stress in the north-south direction may have also increased the left-lateral component, but the Tohoku earthquake did not dramatically change the contrast between the principal east-west and north-south stresses. This observation implies that the differential stress was greater before the Tohoku earthquake, and that the Tohoku earthquake did not significantly affect the stress field in the study area. The triggering of earthquakes is sensitive to changes in stress when conditions are critical for shear slip. For example, tidal changes are a likely factor in triggering certain seismic events [20] even though the stress change due to earth tides is no greater than about \(10^{3}\) Pa [21]. On the other hand, the water level of Lake Okura changes by about 20 m, which corresponds to a change in overburden pressure of 200 \(\times\) 10\({}^{3}\) Pa. This increase in the overburden stress would increase the pore pressure in the fractures under the lake. AE events can be triggered by increased pore pressure due to fluid diffusion (or pressure diffusion) associated with increases of water level in a lake or reservoir, and it is known that small changes of pore pressure can trigger earthquakes [1; 22]. Earthquakes triggered by this mechanism have been reported at several kilometers depth beneath a reservoir in the Koyna district of India [23]. Here, micro-earthquakes started when the water level of Lake Okura approached its maximum level of 270 masl, and the rate of events peaked at 25/day just after that water level was reached. Although those facts do not establish a correlation, fluctuations of the lake level should not be precluded as a possibility in explaining the swarm.