Late Quaternary paleoseismology and faulting behavior of the Yilan-Yitong fault zone and implications for seismic hazards of the Tanlu fault zone, eastern China

Abstract Along the north branch of the Tanlu fault zone (TLFZ), the Yilan-Yitong fault zone (YYFZ) displaces Quaternary geomorphic features, forming a ∼70-km-long fresh surface rupture. In this study, we investigated offset landforms and paleoearthquake history by excavating four trenches along the YYFZ within the Fangzheng graben combined with Quaternary dating of landforms and trench horizons. Four paleoearthquakes are identified with their timings constrained at 1730 ± 40 a BP to 770 ± 30 a BP, 28.5 ± 1.2 ka BP to 21.7 ± 0.9 ka BP, 59.4 ± 2.4 ka BP to 33.1 ± 1.1 ka BP, and before 59.4 ± 2.4 ka BP, respectively. Seismic recurrence behavior of the YYFZ follows a quasi-periodic pattern with a very long interval of 10-20 ka. Geological and geomorphic features show that the YYFZ is dominated by right-lateral strike-slip movement with a low dextral slip rate of ∼0.2-0.3 mm/a in late Quaternary, in accord with present-day geodetic observations. As with the YYFZ, paleoearthquakes along the TLFZ commonly share a recurrence interval of ∼10-20 ka. We suggest that active segments of the TLFZ that have been dormant for thousands of years may pose significant earthquake hazards in the future. Moreover, strain transfer from the bounding faults to those more central in the graben may be significant on the TLFZ and should be given consideration when assessing regional seismic hazards. Insight into past earthquakes and faulting behavior in a low-strain environment is crucial for earthquake-hazard analysis and often requires multidisciplinary approaches.