We have developed a velocity-type strong-motion seismometer that can record strong ground motion over a wide range of periods with high resolution in the near-field of large earthquakes. The pendulum of this seismometer is a coupled pendulum with a natural period of several seconds. This pendulum is able to consistently cancel out the influence of transverse acceleration, that is, the acceleration perpendicular to the measuring direction. We immersed the coupled pendulum in a highly viscous silicon oil. The silicon oil overdamps the pendulum so that it becomes a wide-band, velocity-type seismometer. This seismometer can record ground velocity from 2 × 10–7 m/sec to 1 m/sec in a period range of 0.01–1000 sec. Furthermore, the silicon oil has effects on maintaining the strength of the pendulum system and suppressing its parasitic oscillations. This article discusses the design principles and presents test results and observed seismograms from large earthquakes.

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