The 2021 Mw 7.3 Madoi, China Earthquake: Transient Supershear Ruptures on a Presumed Immature Strike-Slip Fault

We used seismic data recorded at local and teleseismic distances to analyze the May 22, 2021 Mw 7.3 Madoi, China earthquake sequence, which occurred along a low-slip-rate (0.3-1.0 mm/yr) and small-cumulative-displacement (4-5 km) fault & mdash;the Kunlun Mountain Pass-Jiangcuo Fault (KMPJF). We first restored the clipped waveforms recorded at local distances, and obtained more accurate phase arrivals for relocating the earthquake sequence. The relocated earthquakes illuminate a similar to 170 km-long complex northwest-southeast (NW-SE) striking ruptured fault with apparent bifurcated ends at both tips and variations in dip angles along strike. We further performed backprojection analyses to image the rupture process of the mainshock using seismic data recorded at four teleseismic arrays (Europe, Australia and Southeast Asia, Japan, and Alaska). The results show that the earthquake propagated bilaterally in the NW and SE directions with maximum rupture speeds of similar to 4.0 km/s. This observation was further refined by multiple array backprojections onto the fault that was determined by the fine relocations of the earthquake sequence. Synthetic and realistic tests validated the resolutions of the backprojection analyses (within 5-20 km). The fast supershear rupture speeds were only transient but exhibited spatial correlation with the geometric complexities of the KMPJF. This suggests that the transient supershear ruptures are due to the varying geometry of the KMPJF, which is likely an immature fault. Our results suggest that the potential for supershear ruptures along immature strike-slip faults should be emphasized in earthquake hazard assessment.