Aftershock characteristics of the 2024 Noto Peninsula earthquake (Mw7.5) through centroid moment tensor analysis using a 3-D seismic velocity structure model
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A crustal earthquake of the 2024 Noto Peninsula earthquake with a moment magnitude of 7.5 occurred on January 1, 2024, and was followed by many aftershocks distributed in both onshore and offshore regions. The mainshock was characterized as a reverse fault with NW-SE pressure- (P-) axes. We conducted centroid moment tensor (CMT) inversions for the aftershocks using a three-dimensional seismic velocity structure model to capture the detailed stress state and fault geometries around the source region. CMT solutions were obtained for 221 aftershocks with moment magnitudes of 3.2–6.1 at depths shallower than 15 km. Our approach showed substantial improvement in depth determination of CMT solutions, compared to that of the hypocenter determination using P- and S-wave arrival times, even for the early aftershock period when no seismic station was available close to the earthquake source region. Our CMT solutions were characterized as follows: (1) reverse faults with an NW-SE P-axis, which is consistent with that of the mainshock mechanism; (2) strike-slip faults in predominantly shallower regions compared to type 1; (3) reverse faults with ENE-WSW P-axes, possibly activated following the mainshock in the shallow southwestern aftershock region; and (4) earthquakes predominantly featuring normal and strike-slip faults localizing at a depth of approximately 5 km around the dip transition zone in geologically constructed fault models. Additionally, we conducted the same CMT inversion for earthquakes that occurred between 2007 and 2023 to further understand the effect of the mainshock on the aftershock dynamics. We confirmed that CMT solutions of types 3 and 4 appeared as new earthquake categories after the mainshock, suggesting that the mainshock could have triggered them. Our results provide a deeper understanding of complex stress fields and fault geometries in the source region.
