Late Quaternary Strong Earthquake History of the Leibo Fault on the southeastern margin of the Tibetan Plateau

The Yingjing-Mabian-Yanjin tectonic blet (YMYTB) serves as a critical boundary structure between the southeastern margin of the Tibet Plateau and the Sichuan Basin. Although seismicities has been frequent since the late Quaternary, the activity of individual faults within the tectonic belt remains unclear, introducing significant uncertainty in understanding and assessing the current regional crustal deformation patterns and seismic hazards. Particularly, the southern segment of the tectonic belt near the Leibo fault has experienced the 1216 Mahu M7 earthquake and several strong earthquake swarms of magnitude 6 or above. However, research on this fault zone is limited, and there is still a lack of reliable evidence to determine its most recent activity period and its relationship with nearby major earthquakes.

To address this issue, this study conducted paleoseismic trenches on the northern, central, and southern branches of the Leibo fault, based on the interpretation of high-resolution remote sensing imagery and field geological-geomorphological investigations. The following conclusions were drawn:

(1) Based on paleoseismic event identification markers, three, three, and five paleoseismic events were revealed on the three branch faults, respectively. Dating results of radiocarbon samples constrained the occurrence times of the three paleoseismic events on the northern branch fault to 21,190–20,590 BC (EP1), 20,550–12,120 BC (EP2), and after 12,090 BC (EP3). The timings of the three strong seismic activities on the central branch fault were 7,400–6,320 BC (EY1), 5,690–2,620 BC (EY2), and 2,220 BC–170 AD (EY3). The occurrence times of the five surface-rupturing seismic events on the southern branch fault were 14,660–9,300 BC (ES1), 9,270–7,560 BC (ES2), 600–640 AD (ES3), 740–1,440 AD (ES4), and 1,650–1,900 AD (ES5). The paleoseismic results indicate that all branch faults of the Leibo fault zone are Holocene active faults.

By comparing the occurrence times of paleoseismic events on each branch fault, it is determined that the Leibo fault zone has experienced at least 10 surface-rupturing paleoseismic events since the Late Pleistocene. The corresponding age ranges are 21,190–20,590 BC (E1), 14,600–9,300 BC (E2), 12,090–11,820 BC (E3), 9,270–7,560 BC (E4), 7,400–6,320 BC (E5), 5,690–2,620 BC (E6), 2,220 BC–170 AD (E7), 600–640 AD (E8), 740–1,440 AD (E9), and 1,650–1,900 AD (E10). The paleoseismic history of the Leibo fault zone reveals that the strong seismicities of the three branch faults exhibit significant spatial independence and temporal clustering, indicating that the branch faults of the Leibo fault zone are independent seismogenic structures.

(3) Based on historical earthquake records and paleoseismic research results, this study proposes that the seismogenic structure of the 1216 Mahu M7 earthquake is the southern branch of the Leibo fault. Additionally, the Leibo fault likely participated in the rupture of the 1935–1936 Mabian M6¾ earthquake swarm.

(4) By collecting and analyzing the magnitudes of strike-slip earthquake events that generated surface ruptures in western China since 1920, it is inferred that the lower limit of the magnitudes of paleoseismic events revealed on the Leibo fault zone is 6.5. Furthermore, based on the fault length and empirical relationship, it is estimated that the Leibo fault has the capability to generate earthquakes with magnitudes of 7.0 or higher.