2. A method for efficiently and precisely calculating site-specific design earthquake motions

In the seismic design of railway structures, site-specific design earthquake motions must be calculated using the strong ground motion prediction method. In this method, calculations are performed by individually setting (1) the characteristics of source, (2) the propagation characteristics from the hypocenter to the seismic bedrock directly beneath the construction site, and (3) the site characteristics representing the ground's shaking susceptibility (Figure 1).
Calculating the design earthquake motion while considering each characteristic requires an enormous number of computations of approximately 1000 cases, which takes a significant amount of time and money (approximately several months). Therefore, in practice, standard design earthquake motions assumed to be applicable throughout Japan were often used, leading to the use of earthquake motions that could be significantly different from those that could actually occur at construction sites.

In response to this, we developed a method that can calculate site-specific design earthquake motions within about a day. The method involves selecting one seismic bedrock waveform corresponding to the assumed source from a database of ground motion waveforms at the seismic bedrock location, which were comprehensively calculated based on various assumed source characteristics and propagation characteristics, and using that waveform and the amplitude parameter from the site characteristics. We have now introduced a new calculation method that takes into account not only the amplitude parameter but also the duration parameter, making it possible to reproduce the characteristics of earthquake observation records more accurately than before (Figure 2).

Figure 3 shows the structural response (response spectrum) due to earthquake motion calculated at two locations based on the assumption of a magnitude 7.0 earthquake occurring directly beneath. Using these results, the seismic design of railway structures appropriate for each construction site can be performed based on an appropriate understanding of the earthquake motions that may occur at the construction site.