Investigating the foreshock-main shock-aftershock sequence of the Shizuoka-Seibu earthquake (M = 2.8, 16 January 1981), central Japan, we found evidence that suggests high stress conditions for the foreshock period. The source dimension of the main shock was roughly estimated to be about 0.5 × 0.2 km2. The m value of the Ishimoto-lida's formula was 1.87 for foreshocks and 2.37 for aftershocks.
Waveforms of the foreshocks, main shock, and aftershocks were similar to each other. However, the amplitude ratio of the low (0.9 to 0.8 Hz) to high (3.5 to 10 Hz) frequency band was different between the foreshocks and aftershocks. The ratio for the foreshocks was lower than that for the aftershocks, indicating that the foreshocks contain more high-frequency energy than the aftershocks. The Shizuoka-Seibu sequence included a larger number of foreshocks (70) compared to aftershocks (43). This characteristic, in contrast with large earthquakes, was found common to small-sized earthquakes occurring in the neighborhood areas. These observations can be interpreted based on the asperity model.