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Earthquake signals of a long-period (about 50 sec), such as the one observed at 2:12 on July 14, had been observed 14 times until July 21 with broadband seismograph. This earthquake vibration synchronizes well with stepped change of NIED's tiltmeter (in the sense that Mt. Oyama is being eleveted) in the Miyake-jima. Refer to EIC seismological note No.84 (Figure 3) for our model relating to this phenomenon.

Broadband seismograph data at NIED's FREESIA stations: TYM, JIZ,SGN were collected for 17 long-period earthquakes. The data were bandpass filtered (0.01-0.1 Hz) and transformed into displacement. They were inverted into 6 components of moment tensor source including isotropic component.

Figure 1 shows velocity records (bandpass filtered 0.01 - 0.1 Hz) at JIZ for 14 long-period earthquakes till July21. Every waveform is almost similar and the source duration is the same regardless of the size. Derived isotropic component and step (read from the graph) of NIED's tiltmeter at the Miyake-jima.

Interpretation and comments :

The total of derived isotropic moment tensor till July 21 becomes 14.4 x10**17 Nm, and is equivalent to volume expansion of 2.9 x10**7 m^3. It is reported that collapse volume estimated from pictures is more than 2 x10**8 m^3. Therefore, further examination is necessary to judge how the long-period earthquakes are related to the summit collapse.