Re-Evaluating the Surface Rupture and Slip Distribution of the AD 1609 M7 1/4 Hongyapu Earthquake Along the Northern Margin of the Qilian Shan, NW China: Implications for Thrust Fault Rupture Segmentation

Abstract

The Hexi Corridor is located beyond the northeastern edge of the Tibetan Plateau, and it is bounded by a series of active thrusts along the northern margin of the Qilian Shan and the southern piedmont of the Longshou Shan. Historically, five destructive earthquakes have occurred along the Hexi Corridor, which indicates that this region poses high potential seismic risks. The 1609 Hongyapu earthquake occurred along the Fodongmiao-Hongyazi fault in the northern Qilian Shan, China, and it killed more than 840 people and destroyed a large number of buildings. Presently, there are different opinions as to the distribution and length of the surface rupture of this event along the Fudongmiao–Hongyazi fault. Thus, we searched all of the fault scarps on the Holocene surfaces and suspected surface rupture locations related to the 1609 earthquake based on previous studies and developed detailed remote-sensing interpretations along the fault. An abundance of north-facing scarps on the younger fans and terrace faces, slightly higher than the active modern stream bed, were found along the Fodongmiao-Hongyazi fault in the area ranging from the Hongshuiba River (39.52°N, 98.41°E) in the west to the Shuiguan River (39.07°N, 99.37°E) in the east. Based on our research, we estimate a surface rupture length as ∼98 km based on the distribution of the fault scarps on Late Holocene surfaces and constraints provided by age dating. Most of the surface ruptures are preserved as fault scarps and indicate an average vertical surface offset of ∼1.0 m, a value found consistently in three segments of the fault. The surface rupture features indicate that segments of the fault ruptured together coseismically during the 1609 earthquake, i.e., a multisegment rupture. Using the surface fault traces, length of 98 or 90 km (without the Shuiguan River section), dip of 30° inferred from previous reflection profiles, a rigidity of 3.3 × 1010 N/m2, and dip slip average as 1.9 m converted from our observations of the offsets, we computed the magnitude of this event as ca. Mw 7.2–Mw 7.4.

Post‐20 ka Earthquake Scarps Along NE‐Tibet's Qilian Shan Frontal Thrust: Multi‐Millennial Return, ∼Characteristic Co‐Seismic Slip, and Geological Rupture Control

“…Former studies of the 1609 Hongyazi earthquake were mostly focused on local fault trace mapping and vertical offset measurements near Hongyapu village (e.g., Liu et al., 2014; Xu et al., 2010; Yang et al., 2017), and on the assessment of the long‐term slip rate history, based on sparsely distributed offset markers (e.g., Yang, Yang, Huang, et al., 2018; Yang et al., 2017; Hetzel et al., 2019; Liu et al., 2019). However, although recent studies integrated more evidence from several local UAV data swaths (Huang, Yang, Yang, Hu, et al., 2021), the full co‐seismic rupture length and slip distribution of the 1609 event remained poorly constrained. Based on the interpretation of our high‐resolution (0.5 m), broad‐scale DEM, our additional geomorphic mapping and offset measurements, and on previously published dating results (Huang, Yang, Yang, Yang, et al., 2021; Xu et al., 2010), we confirm that the surface rupture length and average throw of the 1609 earthquake were ∼118 km and ∼1.8 ± 1 m (average), respectively (Figure 17a).…”

“…However, at this stage, there has been insufficient work aimed at reconstructing slip distributions along the thrust and earthquake rupture lengths, including that of the 1609 earthquake, which thus remain controversial (e.g., Huang et al., 2018; Huang, Yang, Yang, Hu, et al., 2021; Liu et al., 2014; Xu et al., 2010). From mole tracks, ground fissures, and fault scarps observations, the 1609 rupture was first inferred to extend over a total length of ∼58 km, with maximum surface throws on the order of ∼1–2 m (Institute of Geology and Lanzhou Institute of Seismology, 1993).…”

“…Its throw rate (vertical displacement rate) has long been estimated to be on order of ∼1 mm/a (Hetzel et al., 2019; Liu et al., 2019; Meyer et al., 1998; Yang et al., 2017; Yang, Yang, Huang, et al., 2018) (Figure 3b). Historical documents indicate that it is the most likely source of the 1609 Hongyazi earthquake (Huang et al., 2018; Huang, Yang, Yang, Hu, et al., 2021; Liu et al., 2014; Xu et al., 2010; Yang, Yang, Huang, et al., 2018) (Figure 2). Although well‐preserved terraces and fans are conspicuously offset by the FFT (e.g., Hetzel et al., 2019; Liu et al., 2014, 2019; Tapponnier et al., 1990; Xu et al., 2010; Yang, Yang, Huang, et al., 2018; Yang et al., 2017, 2020), only a few of previous studies focused on systematic measurement and reconstruction of the cumulative vertical slip distribution along that thrust (e.g., Avouac, Tapponnier, et al., 1993; Tapponnier et al., 1990; Xu et al., 2010).…”

“…Due to the episodic deposition of coarse alluvial sedimentary sequences, evidence for multiple events in relatively shallow (∼2 m) excavations (trenches) have remained controversial (e.g., Huang et al., 2018; Huang, Yang, Yang, Yang, et al., 2021; Liu et al., 2014), hampering paleoseismic history reconstructions and a credible assessment of seismic hazards. The 1609 earthquake surface‐rupture length and offsets are also disputed (e.g., Huang et al., 2018; Huang, Yang, Yang, Hu, et al., 2021; Institute of Geology and Lanzhou Institute of Seismology, 1993; Liu et al., 2014; Xu et al., 2010). Hence, important questions concerning earthquake ruptures segmentation and termination, and events return‐times remain unanswered.…”

Variable Thrust Rates of the Eastern Qilianshan Mountain Front, Northeastern Margin of the Tibet Plateau and Its Implication to the Topography of the Yongchangnan Shan

“…Though no seismic events have been recorded on the FF in recent years, earthquakes are abundant in the region (Figure 1B): The 180 AD Gaotai M7.5 earthquake on the Yumu Shan Fault (Xu et al, 2010), the 1,609 Hongyazi M7.25 earthquake on the Fodongmiao-Hongyazi fault (Xu et al, 2010;Huang, 2021), and the 1954 Shandan Ms7.25 earthquake (Zheng W. J. et al, 2013) on the North Longshou Shan fault. The most recent seismic activities in this region includes the 2003 Minle-Shandan Ms6.1 and Ms5.8 earthquakes near Minle County (Zhang et al, 2004), and the 2016 Menyuan Ms6.4 earthquake (Wang et al, 2017;Liu et al, 2018) on the Haiyuan fault (Figure 1B).…”