M 7.3 - Southern Qinghai, China

Tectonic Summary

The May 21, 2021 M7.3 earthquake in central China occurred as the result of strike-slip faulting with a component of normal faulting. The earthquake occurred on either an east-west left-lateral fault, or on a north-south striking right-lateral fault. The regional geologic context is consistent with left -lateral strike slip motion. The extension component of the earthquake is oriented northwest-southeast. This M7.3 earthquake was preceded ~4.5 hours by an M6.1 earthquake also on May 21, 2021. However, the earlier M6.1 occurred ~1000 km southeast of the M7.3 event, so the two earthquakes are too distant to be considered a foreshock-mainshock pair. The distance between these two events indicates that the two earthquakes are likely not directly related.

The earthquake occurred in the eastern portion of the Tibetan Plateau where the Tibetan Plateau extends eastward. The Tibetan Plateau forms as a result of the northward collision of the India plate with the Eurasia plate and rests north of the Himalaya mountain range. The Tibetan Plateau is the largest and highest plateau in the world, and as a result, it currently extends to the east and west as a response to gravitational forces. This extension is accommodated by a complex combination of fault motions that include left-lateral strike-slip motion along the Kunlun fault (located north of the M7.3 earthquake), normal faulting within the Tibetan Plateau, and reverse faulting as the Tibetan Plateau impinges onto more stable continental crust in central China. In the past century, two earthquakes larger than M7 have occurred within 250 km of the M7.3 earthquake, including an M7.8 earthquake ~100 km northwest of the May 21 earthquake that occurred on or near the Kunlun fault in January 1937. The May 2021 earthquake additionally occurred 223 km northeast of the April 14, 2010 M6.9 Yushu, China earthquake that caused >2000 fatalities.

While commonly plotted as points on maps, earthquakes of this size are more appropriately described as slip over a larger fault area. Strike-slip faulting events of the size of the May 21 earthquake are typically about 90 x 15 km in size (length x width).