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The Mj6.5 (Mw6.2) event that occurred on 5 May 2023 near the northern shoreline of the northeastern tip of the Noto Peninsula, central Japan, is the largest event to date in a long‐lasting, intense earthquake swarm. Here we have created a more precise aftershock catalog associated with the 2023 Mj6.5 and the second‐largest 2022 Mj5.4 sequence to understand the rupture process of this largest earthquake. Most of the aftershocks are aligned along a ∼45° SE‐dipping plane. The mainshock initially ruptured the same deep section of the fault zone that had been ruptured by the 2022 Mj5.4 event, before propagating rapidly to shallow depths and to offshore along the ruptured fault plane. The aftershock front migrated at a speed of ∼20 km/hr. This rapid upward migration of the immediate aftershocks might be driven by upwelling of crustal fluids along the intensely fractured and permeable fault zone via mainshock dynamic rupture. Plain Language Summary Near the northeastern tip of the Noto Peninsula, central Japan, a long‐lasting, intense earthquake swarm has continued since November 2020. On 5 May 2023, the largest Mj6.5 (Mw6.2) event to date occurred. We precisely located the aftershock distribution following the 2023 Mj6.5 and the second‐largest 2022 Mj5.4 sequence and enhanced the catalog by searching events based on waveform similarity to understand the rupture process of this largest earthquake. The 2023 Mj6.5 event initially ruptured the same deep section of the fault that had been ruptured by the 2022 Mj5.4 event, before propagating rapidly to shallow depths and to offshore along the ruptured fault. We can see that the aftershock front moved at a speed of ∼20 km/hr, which is a rare case that constrains the rapid movement of aftershocks. The rapid upward movement of the aftershocks may have been caused by the upwelling of crustal fluids along the permeable fault zone created by the dynamic rupture of the mainshock. Key Points We constructed a more precise aftershock catalog associated with the two major ruptures during a long‐lasting intense seismic swarm We identify a rapid migration of early aftershocks following the largest 2023 Mj6.5 earthquake to date during the seismic swarm Upwelling of crustal fluids along the fractured permeable fault zone could drive the rapid migration of early aftershocks

In this study, we compiled the Central Weather Bureau (CWB) data in order to study the Gutenberg-Richter magnitude-frequency slopes (i.e., b-values) and seismicity rates of significant earthquake sequences in the area of Hualien. A total of ten events between 1973 and 2018 were selected for analysis. Using time windows 72 h before and after the main shock, we first examined the existence of detectable foreshocks and then applied the Gutenberg-Richter law and Omori's law to determine the b-value and seismicity rate, respectively. The compiled results were used to assess the abnormalities and other characteristics of the 2018 Hualien earthquake for their forecast potential. We concluded that seismicity rates alone are not sufficient to forecast whether a greater main shock is forthcoming. The foreshock sequence of the 2018 Hualien earthquake was characterized by a low b-value and a high seismicity rate. Another earthquake with a prominent foreshock sequence occurred in 1990, but it showed a different relationship between the magnitude and the seismicity rate. For both the 1990 and the 2018 Hualien earthquakes, we found that the b-values of the foreshocks were lower than those of the respective aftershocks. The b-values for earthquake sequences are depressed relative to the background seismicity in the area. The mechanisms proposed for temporal variation in b-values are briefly reviewed to explain the observed b-value patterns. Finally, we established an empirical relationship with moment magnitude (M_w) in order to estimate the spatial range of aftershock distributions in the area of Hualien for shallow earthquakes (hypocenter depth ≤ 20 km) with M_w ≥ 5.3.