3:45 PM - 4:00 PM
[SSS12-08] Non-Double-Couple Compoonents of the 2024 Noto Earthquake Aftershocks: Influence on Focal Mechanism Estimation
Keywords:The 2024 Noto Earthquake, Moment tensor inversion, Non-double-couple component, Short-period ocean-bottom seismometer
Moment tensor solutions are an important tool for estimating the mechanisms of earthquakes and the geometry of fault planes. For many small to moderate earthquakes, fault slip can often be approximated as pure shear slip. Consequently, the non-double-couple (non-DC) components of moment tensor solutions have frequently been regarded as mere artifacts of analysis or assumed to be nonexistent. However, recent studies have utilized non-DC components to infer the complexity of source processes and structural heterogeneities.
In the case of the 2024 Noto Peninsula Earthquake, aftershock activity was observed using short-period ocean-bottom seismometers (4.5 Hz velocity sensors) starting approximately two weeks after the mainshock. Shinohara et al. (2025, submitted to EPS) analyzed the initial P-wave polarities recorded by these observations and estimated focal mechanisms for the aftershocks. Their results revealed strike-slip mechanisms for many of the aftershocks. In contrast, moment tensor solutions obtained through F-net’s routine analysis (hereafter referred to as F-net solutions) predominantly showed reverse faulting mechanisms, resulting in a discrepancy between the two sets of results.
In this study, we estimated moment tensor solutions using P-wave polarities and amplitudes recorded by short-period ocean-bottom seismometers, demonstrating that the differences between initial focal mechanisms and F-net solutions are attributed to non-DC components. First, P-wave amplitudes were measured for 798 aftershock events, and the effects of local site condition and attenuation along propagation paths were statistically corrected. Then, the corrected amplitudes and P-wave polarities were used as inputs for Bayesian inversion based on the Markov Chain Monte Carlo (MCMC) method. The results showed that many events exhibited reverse faulting mechanisms with positive isotropic components (volume expansion) and negative CLVD components (where the pressure axis serves as the symmetry axis). Additionally, it was found that when focal mechanisms were estimated without considering non-DC components and using only P-wave polarities, strike-slip solutions were often obtained for such events.
Moment tensor solutions with positive isotropic and negative CLVD components cannot be easily explained by simple crack opening or closure, and their cause remains unclear. Potential explanations include the complexity of fault plane geometry and structural anisotropy. Furthermore, it remains to be determined whether the prominence of non-DC components is unique to this aftershock sequence or represents a universal characteristic independent of region or time. Further analysis is required to address these questions.
In the case of the 2024 Noto Peninsula Earthquake, aftershock activity was observed using short-period ocean-bottom seismometers (4.5 Hz velocity sensors) starting approximately two weeks after the mainshock. Shinohara et al. (2025, submitted to EPS) analyzed the initial P-wave polarities recorded by these observations and estimated focal mechanisms for the aftershocks. Their results revealed strike-slip mechanisms for many of the aftershocks. In contrast, moment tensor solutions obtained through F-net’s routine analysis (hereafter referred to as F-net solutions) predominantly showed reverse faulting mechanisms, resulting in a discrepancy between the two sets of results.
In this study, we estimated moment tensor solutions using P-wave polarities and amplitudes recorded by short-period ocean-bottom seismometers, demonstrating that the differences between initial focal mechanisms and F-net solutions are attributed to non-DC components. First, P-wave amplitudes were measured for 798 aftershock events, and the effects of local site condition and attenuation along propagation paths were statistically corrected. Then, the corrected amplitudes and P-wave polarities were used as inputs for Bayesian inversion based on the Markov Chain Monte Carlo (MCMC) method. The results showed that many events exhibited reverse faulting mechanisms with positive isotropic components (volume expansion) and negative CLVD components (where the pressure axis serves as the symmetry axis). Additionally, it was found that when focal mechanisms were estimated without considering non-DC components and using only P-wave polarities, strike-slip solutions were often obtained for such events.
Moment tensor solutions with positive isotropic and negative CLVD components cannot be easily explained by simple crack opening or closure, and their cause remains unclear. Potential explanations include the complexity of fault plane geometry and structural anisotropy. Furthermore, it remains to be determined whether the prominence of non-DC components is unique to this aftershock sequence or represents a universal characteristic independent of region or time. Further analysis is required to address these questions.