Fig. 3

Waveforms of the volcanic tremor on November 16, 2008. Top panel shows seismic and tilting data obtained by a broadband seismometer and tiltmeter at station MEA. For seismic data, displacement waveforms obtained by a simple integration of the original velocity seismogram are also displayed. The bottom panel shows the seismogram operated at ONT. The horizontal axis expresses local time in Japan on November 16. A spindle-shaped signal of volcanic tremor in the velocity waveforms at around 1:00 is evident. However, a clear ramp function appears in the horizontal components of the displacement waveforms. During the period, indicated by gray shading, large apparent eastward displacement was observed at ONT. At the same time, relatively small but discernible displacement was also recorded at MEA. It should be noted that the horizontal displacement waveforms closely resemble tilting waveforms recorded by a tiltmeter