Phase tensor ellipses (left column) and β (right column) for four representative periods. (a) 0.4 s, (b) 4 s, (c) 40 s, and (d) 400 s. For each period, observed and calculated phase tensor ellipses and β are shown for comparison. The size of each ellipse is normalized by the major axis. The color of each ellipse shows . The two triangles represent Naruko volcano in the east and Mukaimachi caldera in the west.