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5 SUMMARY AND RECOMMENDATIONS In the past few years, significant advances have been made in tsunami research (Hwang and Lee, l979). There is considerably greater understanding now of wave generation, transoceanic propagation, and coastal transformation than there was just a few years ago. Many computer models have been developed to calculate wave transformation in specific coastal environments. However, most of this progress has been limited to incoming waves from large and distant sources. Waves of intermediate size generated locally, such as the Japan Sea central region tsunami, have received only limited attention. The occurrence of the May 26, l983, tsunami provided new information for scientists and engineers working to understand the tsunami phenomenon and created new challenges in finding ways to reduce tsunami damage. Based on this reconnaissance study, there are a number of areas in which further research is needed to help solve engineering and social problems. TSUNAMI THEORIES There are four main aspects of the May 26, l983, tsunami that appear to warrant further investigation. First, current theories of tsunami generation predict that a tsunami for this earthquake should have consisted of a single wave with an amplitude of about l m and a period of l0 minutes. This prediction is consistent with the observed height and length of the first wave of the tsunami. However, there were three to four main waves in the tsunami with periods of about l0 minutes. 26
27 Since the ground motion is related to the response time of the overlying waters, and since the distance from the source of the tsunami to the shore was less than one wavelength, it is not clear how several waves with 10-minute periods could have been generated. Second, the evolution of short-period waves on the long main wave is both surprising and fortunate. It is surprising because the appearance of these waves would not have been predicted based on previous experience. It is fortunate because without the appearance of these waves the energy contained in them would not have been dissipated by early breaking, and damage would have been more severe. Third, the conditions under which bores begin to form and migrate across the entire wave crest deserve further study. Fourth, the propagation of "edge" bores also deserves more study. COASTAL PROTECTION Coastal protection structures designed for storm waves can provide some protection against tsunamis when short-period waves evolve. But aside from extensive seawalls or "set-back" lines for coastal development, there appears to be little that can be done to avoid coastal flooding by waves with periods of l0 minutes or more. It should be emphasized that most of the damage from this tsunami was caused by floating structures colliding with other structures. In areas with ample tsunami warning, a management scheme for the orderly removal of ships and floating structures to offshore areas would be very useful. There were many sites in Japan and Korea that would have benefited from such efforts during this tsunami. WARNING AND PUBLIC EDUCATION The tsunami warning service of the Japan Meteorological Agency functioned properly by issuing a warning from Tokyo within l4 minutes of the earthquake. However, the nearness of the source of the tsunami to
28 the Japan coast negated the utility of this warning in many localities. In other areas local agencies failed to broadcast the warning promptly. In still other areas people responded to the warning by rushing to the shore to see the wave. Figure l4 shows docks in Korea lined with people during the tsunami inflow. Apparently, to people who do not respect the danger of tsunamis, a warning can be an enticement to travel to the endangered area. A similar lack of public education is revealed by the fact that most of the l00 people killed by the tsunami could have been saved had they left the coastal area immediately after the earthquake occurred. Effective warnings must be coupled with more public education about the dangers of tsunamis.