Injury Estimation for Foreseen Earthquakes in an Aging Society and Its Application to Sendai City, Japan

Society is aging rapidly in Japan. For disaster mitigation against periodic earthquakes based on asperity theory in seismology, the transition of the social environment should be considered in the intervals between periodic events. This paper focuses on the aging of society as an important expansion factor in human injury. The major objective of this paper is to estimate the effects of aging on injury including serious cases caused by earthquakes. In this paper, three age classes are used as follows. Young age is defined as the generation under 14 years. Old age is defined as 65 years old or over. Middle age is defined as the other group. This classification is often used by the census. Sendai City as the Miyagi prefectural capital is selected because it is one of the most earthquake-prone regions in Japan. The main earthquake to hit the region is the Off Miyagi earthquake with a magnitude about 7.5 Mj. This event has periodically struck Sendai City at average intervals of about 37 years. A few assumptions are made in estimating the injury rate as a result of an Off Miyagi earthquake without a tsunami. Firstly, the normalized injury ratio proposed by the authors can be applied to estimate the injury rate for a foreseen earthquake. Secondly, the injury rate in each age class as a result of an expected Off Miyagi earthquake can be assumed to be the same as in the previous event in 1978. Finally, only the transition of the population structure regarding multiple factors can be considered in the interval between periodic events. In addition, the emergency response load of taking care of the injured elderly is proposed. (1) In the subject region in Sendai City, the ratio of the old-age population divided by the total population has increased three and half times in the 30 years from the previous Off Miyagi Earthquake in 1978. (2) The normalized injury ratio in the old-age class was evaluated as being two and half times that in the middle-age class. (3) The percentage of injured in the subject region in each age class during the Off Miyagi Earthquake in 1978 was reproduced. (4) Under the assumed occurrence of the expected Off Miyagi earthquake in 2005, the number of injured in the subject region in each age class was estimated considering the increase in the population and the progress of the aging of society. The number of injured has increased five times in the past 30 years. The year 2005 was adopted as the assumed occurrence year of the earthquake because th