Inversion of rupture process of the Japan M_W7.6 earthquake on January 1，2024

To obtain a detailed understanding of the rupture process of the M_W7.6 earthquake that occurred in Japan on January 1, 2024, this study integrates seismic data from multiple agencies and researchers. By analyzing the focal mechanism solutions provided by these institutions, we derived two nodal planes of the centroid moment tensor （CMT） solution for this event. Through an in-depth analysis of the depth distribution of aftershocks within 24 hours following the mainshock, we concluded that the southeast-dipping nodal plane in the CMT solution is more appropriate to be identified as the causative fault plane for this earthquake. This determination is based on the spatial alignment of aftershock depths and their correlation with the inferred fault geometry.

To further refine the rupture characteristics, we employed broadband P-wave waveform data from 35 seismic stations within the Global Seismographic Network （GSN）, located at distances between 30° and 90° from the epicenter. Using the waveform inversion method for earthquake rupture processes, we iteratively tested and optimized the parameters to reconstruct the rupture behavior of this earthquake. This methodology allowed us to achieve a reliable solution that delineates the spatiotemporal evolution of the rupture.

The results of the inversion reveal several key characteristics of the earthquake’s rupture process:

1） Rupture duration and sub-events: The total rupture duration of this earthquake was approximately 40 seconds. The source time function shows three distinct sub-events, each corresponding to different stages of the rupture. The largest sub-event occurred around 30 seconds after the initial rupture, releasing a significant portion of the earthquake’s total seismic moment. This pattern suggests a complex rupture process involving multiple fault segments or asperities.

2） Rupture zone and slip distribution: The primary area of concentrated slip is located southwest of the epicenter. This region experienced the most significant fault slip during the rupture process. The maximum slip on the fault plane reached 4.28 meters, while the maximum slip velocity was measured at 1.01 meters per second. The spatial distribution of slip indicates that the rupture propagated predominantly in a unilateral direction, with a notable concentration of energy release in the southwest quadrant of the fault plane.

3） Seismic moment and faulting mechanism: The moment magnitude of this earthquake was determined to be approximately M_W7.54, consistent with previous estimates from global seismic networks. The rupture mechanism is characterized as a thrust faulting event, typical of subduction zone earthquakes. The fault geometry and slip distribution suggest that this earthquake was associated with a reverse fault, dipping to the southeast, in alignment with regional tectonic stress orientations.

This earthquake is noteworthy for its complex rupture dynamics and its ability to generate large-scale fault slip over a relatively short time span. The rupture process involved multiple stages, with significant energy release concentrated in a few key sub-events. The identification of the southeast-dipping fault plane as the causative structure for this earthquake provides valuable insight into the tectonic behavior of the region. Additionally, the analysis of aftershock depth distributions supports the conclusion that this plane is the primary fault responsible for the mainshock.

The implications of this study extend to the broader understanding of seismic hazard in Japan, particularly in regions prone to thrust faulting events. The detailed characterization of the rupture process and fault slip behavior contributes to the growing body of knowledge on earthquake mechanics in subduction zones. Furthermore, the findings underscore the importance of integrating diverse data sources, such as focal mechanism solutions, aftershock distributions, and seismic waveform inversions, to achieve a comprehensive understanding of earthquake rupture processes.

The results of this study may also have broader applications in seismic hazard assessments and earthquake forecasting efforts, as the detailed reconstruction of rupture processes can help inform models of future seismic activity in the region. By identifying key areas of concentrated slip and energy release, future studies can focus on the potential for aftershock activity or further seismic events along the same fault structure.

In summary, the 2024 M_W7.6 earthquake in Japan exhibits a complex rupture process with multiple sub-events and a concentrated area of slip southwest of the epicenter. The southeast-dipping fault plane is identified as the most likely causative fault, based on aftershock distribution and focal mechanism analysis. This earthquake represents a significant seismic event in the region, and the detailed study of its rupture process provides important insights into the nature of earthquake mechanics in subduction zones.