Japanese attitudes toward risk control in seismic design in light of observation, prediction, and actual performance of building structures

Despite recent international efforts toward a risk-conscious approach to advancing seismic design, Japan, a country very prone to earthquakes, remains conservative regarding the consideration of risk-based seismic hazards in developing seismic designs. This article attempts to interpret the reasons. First, the evolution of Japanese seismic design codes and specifications is introduced, noting that the design seismic loads have remained essentially the same since the revisions implemented in 1981. Second, Japanese efforts in providing and renewing seismic hazard maps are briefly introduced and discussed. The maps have been developed using the data recorded by Japanese strong motion networks. Some unique challenges in preparing seismic hazard maps for the territory of Japan are presented, in light of the observations of actual earthquakes in the past three decades. On many occasions, the damage disclosed in modern buildings remained limited even when the recorded ground motions significantly exceeded the design earthquake level. An inherent conservatism is embedded in the performance criteria imposed on structural systems and elements. Third, political-legal and sociocultural aspects are discussed. Centralized operation is commonly exercised in Japan for post-disaster relief efforts, and the general public also values "equality" for such support. The factors naturally encourage cautiousness for the revision of relevant seismic codes and specifications.