Premonitory tremors before the 1986 eruptions of Izu-Oshima volcano.

   A series of the volcanic eruptions of Izu-Oshima Volcano began on November 15, 1986. The tremors preceding these eruptions were investigated by using the seismic records obtained at Oshima Weather Station, 0.8km away from the Miharayama crater.<br>   The intermittent tremors began to be observed in July, i.e., four months before the first volcanic eruption. The tremors were generated repeatedly with a relatively stable recurrence time. Their duration and amplitude were relatively stable as well. The generation mechanism of these tremors can be considered something like the following. Energy is continuously supplied into the source area; the accumulated energy is released when it exceeds some threshold value; the vibration in the source area which causes the energy release results in the generation of tremors.<br>   Detailed observation showed that the maximum amplitudes of intermittent tremors gradually increased with time. The E-W components of the tremors were predominant in the early stage, while the N-S ones became dominant toward the end of October. The trains of the tremors generally recurred at regular intervals of about 2 hours. But trains of tremors were generated at intervals of about 3 hours from July 19 to 21, and from October 21 to 24.<br>   The generation of tremors stopped on October 24. Tremors began to be observed again after a quiescence of 3 days, and then not intermittently but continuously. The apparent period of the tremors remained almost constant, i.e., 0.6 seconds, whether the tremors were intermittent or continuous.<br>   Some of these characteristics of the tremors may be explained by the source migration and/or by the path effect, but most of them are attributed to the generation mechanism of the tremors.<br>   The total energy of the premonitory volcanic tremors were estimated to be 10¹⁵ erg. It was found that the energy release rates of the source area increased toward the volcanic eruptions.