Non-double-couple components of the 2024 Noto earthquake aftershocks: influence on focal mechanism estimation

Abstract Aftershocks of the 2024 Noto earthquake exhibited a notable discrepancy between focal mechanisms estimated from P-wave polarities and those derived from routine moment tensor (MT) analyses based on waveform inversion of regional data. While first-motion solutions suggest prevalent strike-slip events, the MT solutions indicate thrust-type mechanisms. This study attributes the discrepancy to the influence of non-double-couple (non-DC) components in the MTs and verifies this hypothesis through Bayesian inversion of polarity and amplitude data. To ensure the reliability of amplitude information, we applied regression-based corrections for attenuation and site amplification, isolating the radiation patterns required for our MT inversion. Using data from ocean-bottom and onshore seismic stations, we obtained 79 well-constrained MT solutions for aftershocks. Many of these exhibit significant non-DC components, with positive isotropic and negative compensated linear vector dipole (CLVD) components being particularly common. We show that these non-DC components can bias first-motion solutions and lead to the misidentification of thrust-type events as strike-slip events. Our findings highlight the necessity of incorporating both amplitude data and non-DC components to accurately resolve focal mechanisms. The observed non-DC characteristics, particularly volumetric expansion implied by positive ISO components, may suggest involvement of deep fluid migration. However, the dominance of negative CLVD components cannot be explained by simple tensile crack opening alone, implying that the non-DC components originate from more complex faulting processes or the presence of seismic anisotropy. Graphical Abstract