Oil tank fire at a refinery in Niigata fllowing the 1964 Niigata earthquake
*Unauthorized reproduction prohibited (Photo provided by courtesy of the Ministry of Defense)

Sloshing damage in oil tanks, which results from long-period (several seconds to more than ten seconds) strong ground motions, occurred not only during the 2003 Tokachi-oki earthquake, but also during the 1964 Niigata earthquake at a refinery in Niigata, and during the 1983 Japan Sea earthquake, at a power plant in Akita. We focused early on long-period strong ground motions and have pursued research in that area for many years.

Significant regional variation can be seen in how long-period strong ground motions occur. For instance, during the 2003 Tokachi-oki earthquake, the large-amplitude long-period ground motions observed in Tomakomai were not observed in Kushiro. Using seismograms amassed at meteorological observatories in various regions, we have analyzed the characteristics of long-period strong ground motions in each region. As a result, we now know that, for example, when an earthquake occurs in the eastern margin of the Japan Sea, an approximate 10-second period of shaking will be particularly amplified in Niigata. Using these results, we can predict the maximum level of long-period strong ground motions for future earthquakes in each area where oil tanks are located. These research results were adopted in revising the seismic design intensity for oil tanks in the Fire Service Law following the 2003 Tokachi-oki earthquake. The seismic design intensity was increased for regions in which large long-period strong ground motions are predicted, such as Niigata.

Long-period strong ground motions obeserved in Tomakomai during the 2003 Tokachi-oki earthquake (graund velocity waveform)

Analysis examples for the amplification characteristics of long-period strong ground motions