Slip Kinematics of the 2021 Yangbi Earthquake: Fore-Main-Aftershock Sequence Rupture along an Unknown Secondary Fault of the Weixi–Qiaohou Fault

Abstract

On 21 May 2021, an earthquake sequence occurred at the Weixi–Qiaohou fault that is considered as a boundary of the Chuandian block in Southwest China, providing a significant opportunity to understand the seismogenic tectonics of the fault and its secondary faults. We used Interferometric Synthetic Aperture Radar and Global Navigation Satellite System observations to obtain surface displacements and slip kinematics of the Mw 6.1 mainshock. The Mw 6.1 event ruptured along a previously unknown secondary fault of the Weixi–Qiaohou fault; this newly identified southwest-dipping fault is dominated by right-lateral strike slip with a small normal component. Fault slip was concentrated between depths of 3 and 8 km, with a maximum slip of ∼0.8 m. Given that there is minor slip and lack of foreshocks and aftershocks rupture in 0–3 km depth, a systemic shallow slip deficit is observed in the depth. In addition, stress change simulations suggest that the Mw 5.4 foreshock promoted the Mw 6.1 event. The increased coulomb stress change region extends approximately 16 km (from 25.65° N to 25.52° N) along the Weixi–Qiaohou fault triggered by the Mw 6.1 mainshock. Combined with modeling results of the Yangbi earthquake sequence and magnetotelluric model across the Weixi–Qiaohou fault, the fore-main-aftershock sequence initiate in depths of 8–16 km (corresponding to relatively low resistivity) and further promote larger slip upward along depths of 3–8 km (relatively high resistivity). Finally, aftershocks rupture and release stress below slip depths of the mainshock.

Seismological Characterization of the 2021 Yangbi Foreshock‐Mainshock Sequence, Yunnan, China: More than a Triggered Cascade

“…However, it is probable that both the stress shadow and the fault step over (Figure 6b) slow down the rupture cause the termination near F 3. This rupture extension is consistent with the coseismic slip model (Li et al., 2022; Wang, He, et al., 2022), the MPS results (Figures 6b and Section 3.3), the distribution of repeating aftershocks (Figures 7b and 10c), and an ∼20‐min delay in the occurrence of aftershocks on SE (Figure 10b). We attribute the post‐seismically triggered aftershocks (Figures 10b and 10f) to the effect of static stress transfer and post‐seismic deformations.…”

Spatial distribution and tectonic significance of the landslides triggered by the 2021 Ms6.4 Yangbi earthquake, Yunnan, China

“…Through seismic sequence pinpointing, many studies showed that the Yangbi seismic sequence distributes along the NW-SE direction, which is approximately parallel to the WQWF with a distance of 3-10 km (Li et al, 2021;Long et al, 2021). Several studies show that the seismogenic fault dips to the southwest based on earthquake distribution (Long et al, 2021;Li et al, 2022;. According to the landslides are prone to occur on the hanging wall, the seismogenic fault should be moved eastward from the location inferred by Li et al (2022) (Figure 11B).…”

“…Several studies show that the seismogenic fault dips to the southwest based on earthquake distribution (Long et al, 2021;Li et al, 2022;. According to the landslides are prone to occur on the hanging wall, the seismogenic fault should be moved eastward from the location inferred by Li et al (2022) (Figure 11B). Therefore, we can infer one model that the Yangbi earthquake's seismogenic fault is a parallel associated fault of the WQWF, with the NW-SE striking, dipping toward the southwest with a large dip angle (Figure 11C).…”

“…The east limit of the seismogenic fault should be in the strip of 20-22 km according to the peaks of LND and LN (Figure 9B), shown with a red dotted line in Figure 11B. Li et al (2022) used InSAR and GNSS observations to obtain coseismic deformation and the inferred seismogenic fault, a secondary fault of the WQWF (the black dotted line in Figure 11B). The suspected seismogenic fault is about 10 km away from the WQWF.…”

“…Recently, many scholars have studied the deep tectonic background and seismogenic mechanism of the Yangbi earthquake through field investigation, geomagnetic, and seismic sequence, etc. (Li et al, 2021(Li et al, , 2022Yang et al, 2021;Liu et al, 2021;Long et al, 2021;Ye et al, 2021;Liu X et al, 2022;. Since no apparent surface rupture phenomenon was found during the earthquake fieldwork (Li et al, 2021), there are some difficulties in understanding the seismogenic structure.…”