2003年十勝沖地震による周期数秒から十数秒の長周期地震動と石油タンクの被害

Abstract

The 2003 Tokachi-oki, Japan, earthquake generated sloshing of liquid in large oil storage tanks with capacities over 1, 000m3 in Hokkaido and caused damage to many of them. The maximum sloshing wave heights of about 3m were observed at two tanks with natural periods of the fundamental mode of sloshing (Ts) of 7s and 5.9s in the Tomakomai western port area and at a tank (Ts=5s) in the city of Ishikari. Damage occurred in 190 large tanks in the southern part of Hokkaido. In and around the city of Tomakomai, 170 large tanks have been damaged and this corresponds to a high damage ratio of 58%. Serious damage occurred in nine tanks and eight of them concentrated in the Tomakomai western port area, where fire broke out from two tanks and floating roofs sank into the oil in seven tanks. One of the two fires was the ring fire that lasted for about 7 hours, while the other extended to the open top fire and it lasted for about 44 hours. It was long-period strong ground motion with periods from several seconds to around 10s that generated sloshing in the large tanks. In the Yufutsu plain embracing the Tomakomai area and the Ishikari plain, the shaking with a period of 7s was stronger than that in the plains in the eastern part of Hokkaido even though the latter were closer to the source region. In the plains of Yufutsu and Ishikari, the long-period strong ground motion was not confined to the Tomakomai area but distributed widely over the plains with the strongest motion observed in the area along the north-to-south axis connecting the center of the Yufutsu plain with the eastern part of the Ishikari plain. This long-period strong ground motion is presumed to be excited by the sediments reaching a thickness of 2 to 3km inside the plains. Around the Tomakomai western port, the long-lasting long-period shaking was observed with velocity responses with a damping factor of 1% keeping over 1.5m/s in a period range between 3.7 and 8.3s and with the peak value of 2.8m/s at a period of 4.8s. This peak value is about 2.5 times as large as the ground motion level that corresponds to a code established for large oil storage tanks by the Japanese fire laws. In the Tomakomai western port area, no recordings for the last three decades are larger than the ground motion level at periods of several seconds observed during the 2003 Tokachi-oki earthquake. Comparisons between the observed maximum sloshing wave heights and the observed ground motions indicate that the maximum sloshing wave heights scale with the velocity responses at Ts's. From the sloshing response analyses for the tanks in the Tomakomai area based on the observed ground motions, we found that the sloshing with the maximum wave heights of 3m and 1.3m occurred in the seriously damaged tanks with capacities of 30, 000 to 40, 000m3 (Ts-7 to 8s) and about 110, 000m3 (Ts-12s), respectively.