Rupture and tsunami generation process of the 2015 Mw 5.9 Bonin earthquake revealed by in-situ pressure gauge array observations and an integrated simulation of seismic and tsunami waves
We removed the ocean tides and applied the lowpass filter (cutoff of 0.033 Hz). Large impulsive signals (~0.02 Hz) were observed just after the focal time and then the low frequency waves (< ~0.005 Hz) and pressure offset changes were recorded. We simulated the pressure changes including seismic waves, tsunamis, and crustal deformation (Saito 2019), to find that the impulsive signals were generated by the dynamic pressure changes due to seismic motion, and the subsequent low frequency signals and offset changes were by tsunamis and permanent crustal deformation, respectively.
We then constructed the finite fault model. We found the pressure offsets and tsunamis (characterized by slow propagation velocity) contributed to constrain the fault size, horizontal location and seismic moment. The center of the main rupture was located at ~15 km east and ~10 km northwest from the USGS and GCMT centroids, respectively. The estimated fault model had the seismic moment of 9.5 × 10 18 Nm (Mw 5.9), consistent with the GCMT and USGS solutions. We also obtained the stress drop of 0.51 MPa. The dynamic pressure changes contributed to constrain the rupture duration Tr. Although the lowpass filter (0.033 Hz) made it difficult to resolve the duration Tr in detail, Tr > ~30 s could not explain the observed dynamic pressure changes at all. We also investigated the ocean acoustic wave signals (> ~0.33 Hz) to find Tr ~5–6 s explained the observed signals well. These results suggest that this earthquake was an ordinary earthquake rather than a tsunami earthquake as pointed by Fukao et al. (2020).