Seismic rupture parameters based on the source directivity analysis of the broadband teleseismic P-waves for deep earthquakes in the NW Japan subduction zone

Seismic moment-dependent rupture characteristics are investigated for deep earthquakes within the northwestern Japan subduction zone, focusing specifically on rupture velocity, source duration, and rupture length. Based on the source directivity analysis and stacking of the teleseismic P-wave data, reliable estimates of source parameters are obtained from earthquakes with magnitudes Mw 5.7–7.3 occurring at depths between 300 and 600 km. Results reveal that smaller earthquakes exhibit generally higher rupture velocities, while larger earthquakes composed of multiple subevents demonstrate comparatively slower rupture velocities, possibly due to delayed ruptures between subevents and/or to the dual rupture propagation mechanism in the metastable olivine wedge and the surrounding spinel zone with increased stress-drops for larger earthquakes. Using estimated rupture velocities, a directivity deduction process is presented and applied to the source durations that are primarily obtained from the stacking process. Despite these directivity corrections, the slope of the log-log linear relationship between duration and seismic moment remains around 0.26, significantly smaller than the theoretical value 1/3 expected from a constant (moment-independent) stress-drop model. Estimated rupture lengths, ranging from approximately 8.6 to 30.9 km, scale positively and linearly with seismic moment in log-log space. Compared with shallow crustal earthquake rupture scaling, the deep-focus earthquakes analyzed here show rupture lengths approximately 40–100% of those from the crustal earthquakes, with a smaller percentage for large earthquakes. The observed moment-dependent rupture characteristics provide important constraints on the mechanisms of deep earthquakes and highlight the need for further comparative studies across different subduction zones.