Map of the northern Tohoku region showing seismic activity. (a) Comparison of seismicity before and after the 2011 off the Pacific coast of Tohoku (Tohoku-oki) earthquake. Blue and red circles represent earthquakes during the 10-year period before and 2 years after the Tohoku-oki earthquake, respectively. Black and the orange triangles indicate active volcanoes and the Moriyoshi-zan volcano, respectively. (b) Enlarged map around the Moriyoshi-zan volcano showing epicenter distribution (circles and crosses) and the location of permanent seismographic stations (blue inverted triangles). Red circles and purple crosses represent earthquakes after the Tohoku-oki earthquake and low-frequency earthquakes, respectively. The box shows the area of Figure 2.