2025 Volume 78 Pages 23-34
To promptly issue seismic intensity information and earthquake warnings immediately after an earthquake occurs, it is necessary to construct and maintain an earthquake observation network and a telecommunication network, and to install seismographs that can retrieve high-quality data in an appropriate environment. In the railway field, to improve the reliability of early earthquake warning (EEW) systems, EMC standards are applied to railway seismographs that measure and record seismic motions, mainly installed in substations to reduce the influence of electromagnetic waves. However, as for seismographs that measure and record seismic motions, an investigation that the seismic motions recorded satisfy certain standards, such as the seismic intensity inspection by the Japan Meteorological Agency, has not been performed. Thus, the reliability of seismographs installed in railways has not been verified. Therefore, in this study, to comprehend the reliability of earthquake warnings in the railway field, we investigated seismic data quality affecting earthquake warnings applied to railways through temporary earthquake ground motion observation with multiple types of seismographs at the Kasama station (KSM) in the Metropolitan area maintained by the Railway Technical Research Institute.
First, we investigated the self-noise of seismographs by the calibration test for microtremors. We showed that the seismographs need to measure the power spectral density of microtremors of about －120 dB in the frequency range of more than 0.12 Hz, to properly record microtremors at KSM on the rock site. In addition, the average power of microtremors recorded by all seismographs was less than 10-4 cm2/s4, indicating no particular problems with data accuracy when measuring vibrations with an amplitude of 0.01 cm/s2 or more. Second, we investigated seismic data quality affecting two types of earthquake warnings applied to railways (the threshold method and the C－Δ method). As for the threshold method, the variability can be seen in the case of values of 10.1 to 10.7 cm/s2 exceeding 10 cm/s2 for all seismographs (average: 10.4 cm/s2, standard deviation: 0.2 cm/s2). It is revealed that the timing of exceeding the threshold varies by a few tenths of a second among the seismographs in this observation data. And, the residual average ± the standard deviation with the unified earthquake catalog of Japan Meteorological Agency (JMA) of the epicentral distance estimation by the C－Δ method were within the range of 0.05 to 0.6 km (Common logarithm), and the impact of this observation data affecting the C－Δ method to estimate the epicentral distance was not very large. On the other hand, the residual average ± the standard deviation with the unified earthquake catalog of JMA of the epicentral direction estimation by the C－Δ method were within the range of －90° to 90°, and the impact affecting the C－Δ method to estimate the epicentral direction was relatively large compared to the epicentral distance estimation.
