Avalanche Tremors Detected by Seismometers

Abstract

Tremors due to avalanches were investigated for the purpose of discriminating them from natural earthquakes and understanding the nature of avalanches. In January, 1986, we have obtained many seismometric records of avalanches, small and large, which occurred near the seismic station NUJ, Nou Town, Niigata Prefecture.
Among them, the powder-snow avalanche of January 26 at Shimamichi was the largest one that provided us with the direct information about the ground motion caused by a great snow stream nearby. Prominent phases on the avalanche seismogram are considered to be generated either by great changes in gradient of the slope or by the topographic asperities on the way. Based upon this interpretation, we have estimated the velocity of the snow stream and its impact on the ground surface. The final velocity is 45m/s, which is comparable with the fluid model of avalanche proposed by VOELLMY (1955). The force exerted on the ground is 1.2×10¹³dyn with a duration time of 0.033sec for the first prominent impulsive motion.
Besides the above event, we have identified 22 avalanche events which occurred from January 22 to January 26. Time series of these events showed forerunning activities of small avalanches before a large one.
Another typical avalanche occurred on February 6, 1984, about 4km away from the NUJ station. The waveform of this event resembles that of a teleseismic event with an S-P time being about 30sec. However, we are able to discriminate the records of avalanches from those of earthquakes since the former have the ambiguous first motion and the unusually short duration of coda waves.