During earthquakes, oblong structures (such as those found on railways) are exposed to earthquake waveforms with different characteristics of amplitude, phase and frequency. These structures will also be significantly affected by permanent fault displacement where the epicenter fault is shallow. As measures to prevent the earthquake-related disasters and ensure the safety of train running, it is therefore essential to quantify the earthquake motion of the ground in the area containing the epicenter fault.

In this research, the ground is modeled as a horizontal stratification, and the fracture of fault as an external force acting to the ground (Fig. 1). A rigidity matrix is introduced for the whole fault-containing ground system. And a method is developed to theoretically compose the waveforms of earthquake motion of the ground, taking account of permanent fault displacement. Unlike the conventional method, it simultaneously satisfies the two contradictory requirements of (1) two-dimensional calculation of earthquake motion over a wide area, and (2) the efficiency (in terms of speed and stability) of data processing. This method is particularly effective for railway infrastructure, as the earthquake motion is evaluated linearly and two-dimensionally.

Figure 2 shows the results of a simulation of spatial and time changes of a transcurrent fault in an earthquake. On the ground surface, clockwise and counterclockwise eddies appear and transmit in the direction of the fault fracture. As a result, large amplitudes emerge in opposite directions within an area of several hundred meters, forming a complicated pattern of spatial amplitude distribution. Clarification of the eddy transmission phenomenon, which was not taken into consideration previously, has enabled evaluation of the relationship among the transmission characteristics of earthquake motion, the response of the ground and structures and rolling stock running performance.
This research was executed with a subsidy from the Ministry of Land, Infrastructure and Transport.