Traceable calibration of a broadband seismometer down to 5 mHz

Broadband seismometers have been widely employed in geophysical observation and disaster prevention. To ensure the reliability of the measurement, traceable calibration of their sensitivities following ISO 16063-11 down to 8 mHz is required. However, the calibration accuracy generally degrades below 0.1 Hz due to tilt-coupling error. In this paper, two tilt-coupling correction methods are investigated: one is a dynamic tilt measurement method using an autocollimator, and the other is a comparison method using a reference accelerometer. These methods were introduced to the primary calibration system to calibrate a seismometer between 5 mHz and 200 Hz. As a result, the tilt-coupling error was corrected with an estimated uncertainty of less than 2% at 5 mHz, and the bandpass response of the seismometer sensitivity was confirmed with the traceable method.