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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—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 ∼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 ∼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. Plain Language Summary: On May 22, 2021 (local time), an Mw 7.3 earthquake occurred in Madoi County, Qinghai province and on the Kunlun Mountain Pass‐Jiangcuo Fault (KMPJF) in the Bayan Har block of the Tibetan Plateau. We examined the earthquake sequence with high‐resolution relocation of aftershocks and backprojection analyses using seismic data recorded at local distances, and four regional arrays in Europe, Australia and Southeast Asia, Japan, and Alaska. The relocated earthquakes indicate a ∼170 km‐long complex NW‐SE striking rupture, with bifurcation at both ends and variations of dips along the fault. Backprojection analyses show several episodes of transient supershear ruptures that corresponded to the segmentation of the KMPJF. This segmentation, along with the low slip rate of 0.3–1.0 mm/yr and small cumulative displacement of 4–5 km, implies that the KMPJF is an immature fault. If so, this might be the first example of transient supershear ruptures along an immature strike‐slip fault. Supershear ruptures could generate strong ground motion and cause extensive damage surrounding the ruptured faults. Previously, supershear ruptures were considered to occur mainly along straight and mature strike‐slip faults. Yet, our results suggest that the potential for supershear ruptures along immature strike‐slip faults should also be emphasized in earthquake hazard assessment. Key Points: Relocated aftershocks for the Madoi earthquake delineate a ∼170 km‐long NW‐SE striking fault with variations of dips along strikeBackprojection analyses show that the mainshock propagated bilaterally in the NW and SE directions with a maximum speed of ∼4.0 km/sThe Madoi earthquake is the first known example of transient supershear rupture along a presumed immature strike‐slip fault [ABSTRACT FROM AUTHOR]

We apply a novel method to estimate the magnitude of the 23 January 2018 M7.9 Alaska earth-quake using seismic stations recorded at local to regional distances in Alaska, US. We determine the source duration from back-projection results derived from the Alaska stations in a relatively compact azimuth range. Then we calculate the maximum P-wave displacements recorded on a wide azimuth range at distances of 8 to 15 degrees. Combining the source duration and the maximum P-wave displacements, we obtain magnitudes of 7.86–8.03 for the 23 January 2018 earthquake in 3–5 min, very close to the Mw 7.9 determined by the USGS and GCMT. This example validates the new approach for determining magnitude of large earthquakes using local to regional stations, and its time efficiency that magnitudes of large earthquakes can be accurately estimated within in 3–5 min after origin time. Therefore, further application of this new method would help accurate estimation of size of earthquakes that occur off shore and might cause tsunami hazards. [ABSTRACT FROM AUTHOR]