Prepare for a magnitude 7 earthquake

The Japan Meteorological Agency's seismic intensity scale is set to 10 levels, from 0 to 7 (with seismic intensities 5 and 6 divided into weak and strong). Before the Great Hanshin-Awaji Earthquake, the seismic intensity was judged by meteorological observatory staff based on bodily sensations and damage to objects and buildings. The maximum seismic motion of intensity 7 was deemed to cause 30% or more of houses to collapse, and landslides, cracks in the ground, and faults, so it was only judged after the damage situation was surveyed after the earthquake. In fact, during the Great Hanshin-Awaji Earthquake, the areas with the most damage were certified as having seismic intensity 7 several days later. Since 1996, by incorporating computers into seismometers, the system has been improved to determine seismic intensity directly from the measured values of the shaking. As a result, if the shaking exceeds the limit of intensity 6+, it is immediately judged to be intensity 7. In this revision, seismic intensities 5 and 6 were divided into weak and strong, but intensity 7 remains the maximum, and in theory it includes shaking up to infinity.

The level of earthquake resistance safety stipulated in the Building Standards Act is to prevent damage from "rare earthquakes" and to prevent collapse or destruction from "extremely rare earthquakes." Rare earthquakes are interpreted as having a seismic intensity of about lower 5, and extremely rare earthquakes as having a seismic intensity of about upper 6.

Of course, a building that is safe for a seismic intensity of 6+ will not collapse if it exceeds that level. The elastoplastic design method currently used in earthquake-resistant design for large earthquakes is considered to have a margin of safety even for earthquake motions that exceed the assumed seismic motion. This is because the structure absorbs seismic energy by being damaged in parts, and the absorption capacity is effective, and the more deformation progresses, the greater the energy is absorbed. Therefore, if the details of the parts that exert this effect are carefully designed, the building can withstand earthquakes of considerable strength. Also, although it is not specified in the law, buildings with basements and buildings with good shapes and balance have a great advantage in terms of safety. For ease of understanding, methods of increasing safety by multiplying the seismic force equivalent to a large earthquake by 1.2 or 1.5 times, or methods of limiting deformation to a small amount to increase the margin, are also used to increase safety. In the case of the method of inputting seismic motion into the foundation of the building and performing vibration analysis, which is used in super-high-rise buildings and seismic isolation structures, seismic motion equivalent to the seismic intensity of 7 is assumed from the beginning.

Given these facts, it may not be easy to prevent damage to buildings when a magnitude 7 earthquake causes ground deformation such as slope collapse or landslides, but it is safe to say that carefully and meticulously designed buildings can withstand even if the ground shakes slightly more than a magnitude 6+ earthquake.

However, this refers to fatal damage such as collapse, and if seismic motion of intensity 7 is predicted in the near future, we must think about ways to minimize damage. The issue is that buildings must be able to be lived in and repaired after an earthquake, and the safety of people inside and outside the building must be ensured. To achieve this, it is necessary to improve performance against rare earthquakes so that buildings will not suffer damage even from intensity 5+ or 6-. Generally, even designs for rare earthquakes include a margin for various reasons, but we must ensure that this margin is in place. It is also necessary to inspect not only the building itself, but also the finishes, equipment, etc., and ensure that furniture is earthquake-resistant.