Abstract
Many boulders were thrown out of their former sockets by the Western Nagano Prefecture, Japan, earthquake of 1984 ( M jma = 6.8 ). The anomalous high accelerations of 4–16 g were estimated from the displacement of thrown-out boulders, assuming that the seismic waves had a frequency range of 5–10 Hz. Almost all of the thrown-out boulders were found on the mountain-tops, ridges and saddles. The topographic amplifications of seismic waves were estimated using five aftershocks recorded on the mountain-top and at the foot. Average amplitude ratios (mountain-top: foot) of seismic waves are 2–7 in the frequency range concerned. The high acceleration area defined by the distribution of thrown-out boulders is very small (1× 3 km) compared with the length (12 km) of the assumed main fault. Many cracks were also found in this limited small area, which is characterized by extremely low activity of aftershocks and relatively large dislocation.
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