**1. Introduction**

Acoustic emission (AE)/seismicity is important for evaluating the stability of geological structures; for instance, the delineation of fracture systems from AE activity in geothermal reservoirs is indispensable for thermal energy extraction [1]. The stability evaluation of fractures during hydraulic stimulation in geothermal reservoirs is necessary to avoid inducing large AE events, where the relationship between large AE events and the pore-pressure distribution in reservoirs is studied [2]. The stability of geological structures is also important from the viewpoint of disaster prevention. The dynamic behavior of geological structures has been studied by many seismologists who have examined repeating earthquakes and slow slip at seismic zones [3–5].

The 11 March 2011 Tohoku earthquake and its many aftershocks brought widespread destruction to eastern Japan [6–8]. This seismic activity occurred on both offshore and inland faults. Although thrust events dominated seismicity before and during the Tohoku earthquake, eastward movement of the shallow crust of the overriding tectonic plate caused extension that relaxed the subduction-related horizontal compressional stress locally, resulting in normal movement on some inland faults after the Tohoku earthquake [9].

Close observations of seismic activity are indispensable for the monitoring of the stability of geological structures. This paper describes a seismic swarm that occurred beneath an artificial lake near the plane of an active fault in Sendai city, Japan, after the Tohoku earthquake. It has long been known that changes in the water level of reservoirs can trigger substantial earthquakes, and it is shown that the seismic activity was sensitive to the small change of tectonic stress caused by the water level of the artificial lake.

#### **2. Study Area**

The study area is in the western part of Sendai city, Miyagi Prefecture, Japan, about 150 km west of the Tohoku earthquake epicenter (Figure 1a). Here, a river terrace is present on both sides of the Hirose River. A double arch dam, built across a tributary to the Hirose River in 1961, created a reservoir named Lake Okura, with a 1.6 km<sup>2</sup> surface area and a storage capacity of 2.8 × 107 m3. The water level is normally maintained between 240 and 270 m above sea level (masl), which corresponds to water depths of 0 to 30 m (Miyagi Prefecture, http://www.pref.miyagi.jp/snd-dam/data/okdamcyosui.htm). The Nagamachi-Rifu fault extends approximately 15 km under Sendai city, striking northeast–southwest and dipping about 45◦ to the northwest [10,11]. Estimates of the stress field made by the multiple inverse method [12] indicate that the direction of minimum principal stress is vertical and the direction of maximum principal stress varies from east–west to northwest–southeast.

**Figure 1.** (**a**) Map of northeastern Honshu, Japan, with inset showing location of study area. The solid star marks the epicenter of the Tohoku earthquake; (**b**) Map showing epicenters of micro-earthquakes from 5 February 2005 to 1 August 2012; (**c**) Vertical cross section showing hypocenters of micro-earthquakes projected onto line A–B. Open stars show the location of the magnitude 5.0 earthquake of 15 September 1998.

At Sendai city, very high seismic intensities (greater than 6 on the Japan Meteorological Agency scale) were recorded during the Tohoku earthquake. GPS (Global Positioning System) data documented crustal movements of more than 5 m at the coast and about 2.5 m in Sendai city. The Nagamachi-Rifu fault is known to be active in the Sendai area [10,13,14]. Seismic activity increased near this fault after the Tohoku earthquake, and more than 1550 events were observed between 11 March 2011 and 1 August 2012. A micro-earthquake is physically the same to as an AE and the term "AE" is generally used for much smaller seismic events with smaller magnitude, and this paper discusses the stability of geological structures using micro-earthquakes in the same manner.
