Volcanic Tremor Array Observation Using Three-Component Seismometers at Iwate Volcano, Japan

Abstract

In order to clarify the source region and mechanism of volcanic tremors which are occurring around Iwate Volcano in northern Honshu, Japan, toe arrays of seismometers were deployed at northern area of the volcano. The triangle shape array which consists of 10 elements is used to estimate incident azimuth of volcanic tremors. The aperture of the array is 300m. The linear array (900m) which consists of 14 elements is used to identify wave types and apparent velocity of volcanic tremor. All elements have a three component 2 Hz sensor and a portable data recorder.

During an array observation period from 23 July until 11 August in 1998, more than 70 volcanic tremors were observed but many events were low signal to noise ratio. Vertical components of four volcanic tremor events which were relatively high signal to noise ratio were analyzed using frequency-wavenumber method to estimate both the incident azimuth and the horizontal apparent velocity. Three of the four events has the predominant frequency of about 2 Hz. At the 2 Hz running frequency-wavenumber power spectra were calculated. As a result, the incident azimuth of volcanic tremors was the south direction and the horizontal apparent velocity was about 1000 to 2000 m/s.

Ohjigokudani, where several steam eruptions were reported to occur previously and steam and gas generate now, is to the south direction of the triangle array. The direction was consistent with the direction of the linear array. Then we also used the linear array to estimate the apparent velocity and identify wave types. The apparent velocity was nearly equal to or higher than one of the Rayleigh wave which was estimated from the seismic noise. We also estimated shallow velocity structure of the volcano to constrain source depth of volcanic tremors. These results show that volcanic tremors occur at shallow part in Ohigokudani at western Iwate Volcano not in Yakushi crater at eastern Iwate Volcano.