Dense and broad-coverage ocean-bottom observation networks enable us to
obtain near-fault displacement records associated with an offshore
earthquake. However, simple integration of ocean-bottom strong-motion
acceleration records leads to physically unrealistic displacement
records. Here we propose a new method using a Kalman filter to estimate
coseismic displacement waveforms using the collocated ocean-bottom
seismometers and pressure gauges. First, we evaluate our method using
synthetic records and then apply it to an offshore Mw 6.0 event that
generated a small tsunami. In both the synthetic and real cases, our
method successfully estimates reasonable displacement waveforms.
Additionally, we show that the computed waveforms improve the results of
the finite fault modeling process. In other words, the proposed method
will be useful for estimating the details of the rupture mechanism of
offshore earthquakes as a complement to onshore observations.