Late quaternary slip rate and paleoseismic sequence of the Cuopuhu section of the Litang-Yidun fault, western Sichuan, China

Abstract

Objective The Litang-Yidun fault is a left-lateral strike-slip active fault zone extending approximately 130 km in the Sichuan-Yunnan rhombic block in the Holocene. As a significant seismogenic structure controlling seismic activity in the Litang area of western Sichuan, research on both paleoseismicity and surface ruptures primarily focuses on the Litang and Damaoyaba sections, with relatively limited study on the Cuopuhu section in the northern part. Detailed investigation of the Cuopuhu section can provide fundamental information on its activity characteristics, paleoseismic events, and slip rates. Methods The Cuopuhu section of the Litang-Yidun fault was investigated using field surveys, high-precision mapping, trenching, and 14C dating methods to explore its slip rate and paleoseismic events. Two trench sites were excavated at the foothills of Dongou Mountain to identify the relationships between faulting and strata, sedimentary characteristics, and fault motion. Conclusion Four paleoseismic events were identified: Event Ⅰ occurred before BC 3382±60 a; Event Ⅱ occurred between BC 3382±60 a and BC 1094±51 a; Events Ⅲ and Ⅳ occurred after AD 1330±44 a. The recurrence intervals of the four earthquakes are approximately 0.4±0.3 ka, 2.42±0.1 ka, and 2.40±0.1 ka, respectively. Based on the calculated intervals, Events Ⅰ and Ⅱ, and Events Ⅱ and Ⅲ, Ⅳ, have recurrence intervals of about 2.4 ka. Events Ⅲ and Ⅳ occurred after AD 1330±44 a, making it difficult to determine their sequence and exact timing. It can be inferred that the Cuopuhu section of the Litang-Yidun fault likely has a recurrence interval of about 2.4 ka for paleoseismic events, with a possibility of seismic events with recurrence intervals of 0.4±0.3 ka. By comparing the research data between the Cuopuhu section and the Litang and Damaoyaba segments, differences in paleoseismic events between the Cuopuhu section and the other sections are evident. However, the seismic activity of different fault sections has shown a sustained strengthening trend since the Holocene. Based on mapped fault scarps and moraine ridges from the last glacial period, the average slip rate of the Cuopuhu section since the Late Pleistocene is estimated to be 4.15±0.5 mm/a, similar to the slip rates of different branches of the Litang-Yidun fault in the late Quaternary period. Significance This study provides information on the tectonic features, paleoseismicity, and slip rates of the Litang-Yidun fault, aiding in a better understanding of the seismic history and structural deformation patterns in the area and giving more data for medium- and long-term earthquake prediction in the future. It also contributes to the seismic risk assessment of relevant projects along the Sichuan-Tibet Railway.

Keywords

• active fault
• litang-yidun fault
• paleoearthquake
• slip rate
• cuopuhu
• eastern tibetan plateau