抄録
The 2011 Great East Japan Earthquake (Mw 9.0) caused extensive ground damage to the Tokyo Bay area of Japan, which is located more than 300 km from the quake's epicenter. This ground damage occurred not on Holocene sediments but on reclaimed land. In particular, a long crack was found in Hanamigawa-ryokuchi Park in Chiba City, Chiba. The crack was more than 170 m long with a throw of up to 70 cm. Cracks due to the earthquake were described and measured. Additionally, the distribution of sand and water boils due to liquefaction was investigated by examining images from Google Earth and geological columnar section data from Information Bank Chiba. Reclaimed land was identified by analyzing historical aerial photographs. Consequently, it was discovered that the large crack followed the embankment of land reclaimed in 1968. A more recently created embankment is situated approximately 50 m from the outside of the older one. The existence of embankment piles and discontinuity in the sedimentary facies on both sides of the piles were factors in the formation of the crack; similar cases have been previously reported, such as in Iwamoto et al. (2014). In other words, the occurrence of cracks is related to reclamation history, depending on both the boundaries of the work area and its age. These factors should be examined to institute preventive measures on reclaimed land in the future.
