Recurrence interval of surface-rupturing earthquakes on the Pyu segment of the Sagaing fault, Myanmar, revealed by a paleoseismic investigation

The 28 March 2025 Mandalay earthquake (M w 7.7) on the right-lateral Sagaing fault caused severe human and structural damage to the densely populated central part of Myanmar. The earthquake filled a previously identified seismic gap along the Sagaing fault, illustrating the importance of data on the past behavior of active faults. Despite its high slip rate and several destructive earthquakes over the past ~ 100 years, the paleoseismic history of the Sagaing fault is poorly constrained compared with that of other large active faults worldwide. We conducted three trenching surveys at Taungthonlon Village, Kyauktaga Township, Bago Region, across the Pyu segment that ruptured during the December 1930 Pyu earthquake (M w 7.3) but not during the 2025 earthquake. The faults exposed on the trench walls dip steeply and rotated ~ 20° clockwise relative to the strike of the excavated fault scarp, suggesting that they are Riedel shears related to dextral slip of the fault. We identified geologic evidence for three seismic events, including the December 1930 earthquake. An OxCal model of the earthquake chronology and historical records of destructive earthquakes in central and southern Myanmar suggests that the penultimate earthquake may correlate with the 1830 earthquake, and the antepenultimate earthquake with either the 1757 or 1768 earthquake. These data indicate that an average recurrence interval of surface-rupturing earthquakes on the Pyu segment is 80–90 years. This, in turn, suggests that the next earthquake may be due in the southernmost part of the Sagaing fault, since the fault segments last ruptured in 1930.