Offshore near-fault pulse-like ground motions: An insight into the DONET1 seafloor ground motions from the 2016 Mw6.0 Off-Mie earthquake in Japan

Abstract

Offshore near-fault ground motions were obtained by the DONET1 seafloor seismic network during the 2016 Off-Mie Mw6.0 earthquake that occurred in the Nankai Trough of Japan. These data provide an opportunity for exploring the characteristics of offshore near-fault ground motions, especially pulse-like ground motions. A comparison of the offshore and onshore near-fault ground motions indicates that the recorded offshore spectral acceleration (SA) in the period of 0.5–5.0 s and peak ground velocity (PGV) for this earthquake are higher than those of the onshore values, and the observed maximum spectral value (βmax) of the offshore near-fault ground motions in this event is underestimated by the design spectra in the seismic codes of China, the United States, and Japan. Based on the Hilbert-Huang transform method, eight offshore pulse-like ground motion records are identified and extracted in the Off-Mie earthquake. The primary analysis indicates that the pulse-like ground motion may be caused by the directivity effect and the radiation pattern. Comparing the pulse parameters, time-frequency parameters, and response spectra of offshore and onshore pulse-like ground motions, the findings demonstrate that within the context of this earthquake event, approximately half of the offshore pulse amplitude (Vp) exceeds the predicted values of onshore models; the low-frequency components of offshore pulse-like ground motions of this event are more pronounced, constituting a larger portion of the total ground motion energy; offshore pulse-like ground motions exhibit a tendency to possess higher maximum instantaneous energy (Einst) compared to onshore pulse-like ground motions with similar rupture distances. Moreover, the βmax of offshore pulse-like ground motions is found to be greater than that of onshore pulse-like ground motions in medium to long periods (>0.5 s). The identified and characteristic recognition of offshore near fault pulse-like ground motion revealed by this earthquake event raises new concerns about seismic design for offshore engineering.