Studies on the Causes of Unique Strong Ground Motions Observed during the 1993 Hokkaido-Nansei-Oki Earthquake Sequence

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We carried out a temporal strong motion observation in the southwestern part of Hokkaido after the 1993 Hokkaido-Nansei-Oki earthquake (M7.8). Unusually high peak acceleration (-1.6g) was observed at OTB during the largest aftershock (8/8/1993 ; M6.3). The OTB station is located at the foot of a small cape (Shibino-Misaki) and its site response shows a sharp peak (a factor of about 80) at about 4 Hz. The unique site response was observed at KNK,located on a small, soft sedimentary basin in Kaminokuni, where liquefaction was observed during the main shock : the amplification factors are large (10to20) at low frequencies (-1 Hz), while they are less than l at high frequencies (>10 Hz). In order to clarify the causes of the above unique strong ground motions, we carried out the following geotechnical measurements near the observation sites : P-S logging, microtremore array observations, and geoelectrical soundings. The following results were obtained :(1) The causes of the unique site response at OTB are combined effects of amplification due to shallow sediments overlaying very hard rock and soil-structure interaction.(2) The causes of the unique site response at KNK are combined effects of amplification at low frequencies and deamplification at high frequencies due to very soft surface sediments. A part of the amplification at low frequencies is attributed to the basin-induced surface waves generated at the basin edge.