Abstract
The Northern Anqiu–Juxian Fault (NAJF) is one of the most active faults in the Tan-Lu Fault Zone (TLFZ), which produced the Anqiu M 7 earthquake in 70 BC. However, there is no clear understanding of the surface rupture caused by this historical earthquake. In this study, we determined the earthquake rupture characteristics of the NAJF based on high-precision surveying, geophysical exploration and drilling profiles. Based on an analysis of 87 horizontal offsets of gullies, we estimated a characteristic offset of ∼ 5 m along the NAJF for a rupture length about 130 km. Geophysical exploration results revealed a shallow geometric distribution of stepovers in the NAJF. We concluded that the ∼ 5 m offset and the rupture length of about 130 km are both in agreement with an empirical relationship among the magnitude, offset, and rupture length and imply that the ∼ 1 km wide stepover could not have terminated ruptures in the Anqiu M 7 earthquake. The relationship among the coseismic offset, magnitude, and surface rupture length of a strike-slip fault show that the 70 BC Anqiu earthquake was more likely to have had a magnitude of M ∼ 7.5.
Highlights
The Tan-Lu Fault Zone (TLFZ) is about 2,400 km long and the largest active strike-slip fault zone cutting through the lithosphere in Eastern China (Zhang and Tang, 1988; Zhang et al, 2010) (Figure 1A) The Anqiu–Juxian Fault (AJF) is a significant branch of the TLFZ from Anqiu to Tancheng in Shandong Province (Figure 1B)
Field geological survey results show that the Northern Anqiu–Juxian Fault (NAJF) trends 20–30° N and begins at Juxian in the south, extending approximately 135 km to Anqiu in the north
Our survey showed that the NAJF consists of three discontinuously exposed fault segments that form right-stepping faults (Figure 1B)
Summary
The Tan-Lu Fault Zone (TLFZ) is about 2,400 km long and the largest active strike-slip fault zone cutting through the lithosphere in Eastern China (Zhang and Tang, 1988; Zhang et al, 2010) (Figure 1A) The Anqiu–Juxian Fault (AJF) is a significant branch of the TLFZ from Anqiu to Tancheng in Shandong Province (Figure 1B). The AJF is part of the TLFZ but is ~ 340 km long and divided into two segments by the Juxian Basin: the Northern Anqiu–Juxian Fault (NAJF) from Anqiu to Juxian and the Southern Anqiu–Juxian fault (SAJF) from Juxian to Tancheng (Zheng et al, 1988). Multiphase activities caused the TLFZ to form five parallel faults, which trended NNE in the Shandong and Jiangsu provinces in East China in the Early Cretaceous (Wang et al, 2015) (Figure 1B) and controlled geomorphologic development (Lin et al, 1998; Zhu et al, 2011) (Figure 1B). The AJF is divided into two segments by the Juxian Basin: the NAJF from Anqiu to Juxian and the SAJF from Juxian to Tancheng (Figure 1B). The AJF is divided into two segments by the Juxian Basin: the NAJF from Anqiu to Juxian and the SAJF from Juxian to Tancheng (Figure 1B). (Zheng et al, 1988)
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