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[SCG65-04] Approaches for utilizing OBS’s amplitude data for Earthquake Early Warning
Keywords:Earthquake Early Warning, Ocean Bottom Seismometers, Amplification, Tilting, Swinging
The JMA magnitude in EEW is basically determined from the maximum amplitude of three-component vector summation of displacement waveform. We found that the difference between the JMA EEW magnitude estimated by using OBS data and the JMA catalogue magnitude (Mjma) increases in proportion to input acceleration (PGA). We suppose that the difference is explained by (1) Tilting of OBS housing, (2) Swinging of OBS housing, (3) Amplification of horizontal component at stations near the epicenter. Since tilting and swinging of OBS causes contaminations of gravitational acceleration on waveform record, the DC offset change in acceleration is recorded at each component during earthquake. The integration of such records causes significantly large amplitudes and contributes to the overestimation of the JMA EEW magnitude. The acceleration offset caused by tilting is larger on the horizontal component perpendicular to the cable line than that on the vertical component at in-line cable type OBS stations.
We here propose an alternative magnitude estimation for OBS data by using the maximum amplitude of the vertical component of displacement waveform in order to suppress the effects of tilting and amplification of the horizontal component. The proposed approach also contributes to reducing the difference of amplification factors among sites. For amplifications associated with swinging on the vertical component, we are developing several logics to exclude the anomalous amplified data when estimating the magnitude.