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<p>Understanding how stress and strength conditions evolve before and after large earthquakes remains a fundamental challenge in seismology. Here, we introduce an observationally grounded metric—the ratio of the summed moment tensor to scalar seismic moment (<b>Mstk/M₀</b>)—to track temporal changes in deformation behavior surrounding earthquake faults. Using high-resolution focal mechanism datasets, we analyze two well-instrumented sequences with M6-class foreshocks followed by M7-class mainshocks: the 2016 Kumamoto and 2019 Ridgecrest events. We find that <b>Mstk/M₀</b> remains elevated after M6-class foreshocks but decreases sharply after M7-class mainshocks, indicating a transition toward a more heterogeneous deformation state. To evaluate broader applicability, we examined nine inland M6–M7 earthquake sequences in Japan (2000–2020). Among ten events that exhibited high Mstk/M₀ during foreshock activity, five maintained high values after the M6-class event, and three of these were subsequently followed by even larger earthquakes. In contrast, all other sequences not followed by larger events showed clear decreases in Mstk/M₀ after the initial large earthquake. These observations suggest that sustained high Mstk/M₀ after an M6-class earthquake may indicate conditions favorable for continued rupture growth, whereas decreases may reflect reduced likelihood of further large rupture. Relationships between Mstk/M₀ and inelastic strain rate further support a physical interpretation linking deformation consistency with the ambient stress field to the potential for large earthquake occurrence. Monitoring Mstk/M₀ in the immediate aftermath of large earthquakes may therefore provide useful information for assessing whether an even larger event is likely to follow.</p>

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Seismic moment efficiency reveals the potential for larger earthquakes after M6-class events

  • 期刊论文

  • 出版模式：

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  • 发表日期：

    2026年4月28日

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  • Satoshi Matsumoto,
  • Koki Aizawa,
  • Takeshi Matsushima

摘要

Understanding how stress and strength conditions evolve before and after large earthquakes remains a fundamental challenge in seismology. Here, we introduce an observationally grounded metric—the ratio of the summed moment tensor to scalar seismic moment (Mstk/M₀)—to track temporal changes in deformation behavior surrounding earthquake faults. Using high-resolution focal mechanism datasets, we analyze two well-instrumented sequences with M6-class foreshocks followed by M7-class mainshocks: the 2016 Kumamoto and 2019 Ridgecrest events. We find that Mstk/M₀ remains elevated after M6-class foreshocks but decreases sharply after M7-class mainshocks, indicating a transition toward a more heterogeneous deformation state. To evaluate broader applicability, we examined nine inland M6–M7 earthquake sequences in Japan (2000–2020). Among ten events that exhibited high Mstk/M₀ during foreshock activity, five maintained high values after the M6-class event, and three of these were subsequently followed by even larger earthquakes. In contrast, all other sequences not followed by larger events showed clear decreases in Mstk/M₀ after the initial large earthquake. These observations suggest that sustained high Mstk/M₀ after an M6-class earthquake may indicate conditions favorable for continued rupture growth, whereas decreases may reflect reduced likelihood of further large rupture. Relationships between Mstk/M₀ and inelastic strain rate further support a physical interpretation linking deformation consistency with the ambient stress field to the potential for large earthquake occurrence. Monitoring Mstk/M₀ in the immediate aftermath of large earthquakes may therefore provide useful information for assessing whether an even larger event is likely to follow.