Source parameters, seismogenic structures of the 1950 Medog-Zayu M_S8.6 earthquake and seismicity in the surrounding areas

The collision between the Indian and Eurasian plates formed the 2500 km long Himalayan orogenic belt . At 22:09 on August 15, 1950, Beijing time, an M_S8.6 earthquake occurred in Medog-Zayu area in the eastern Himalayan syntaxis, which is the largest continental earthquake in the world ever recorded since the historical earthquake records have become available. It has been felt by people across the entire Tibetan Plateau and the adjacent plains of India and caused extensive economic and property damages. Based on previous studies about the seismic hazards, earthquake locations and focal mechanisms, we systematically review the source parameters, deep structures of the earth beneath the source area and the features of the seismogenic faults.

In addition, we collected earthquake catalogues provided by different agencies and summarized a relatively complete earthquake catalogue with magnitudes greater than or equal to 5.0 within 20 years before and 10 years after the great earthquake. We then analysed the seismicity in four different special and temporal stages. Using the modern earthquake catalogues and waveform data recorded by seismic stations we deployed in the nearby areas, we discussed the mechanisms of small and medium earthquakes occurring since 1964, the 2017 M_S6.9 Mainling earthquake, and the 2019 M_S6.3 Medog earthquake.

The tectonic background of the eastern Himalayan syntaxis is complex. There are three secondary syntaxis which migrate from north to south in different directions, i.e., the Namche Barwa syntaxis, the Sang syntaxis and the Assam syntaxis. In the youngest Assam syntaxis, the Indian plate subducts not only toward NE along the Mishmi Thrust but also toward NW along the Main Frontal Thrust, resulted strong compressive crustal deformation in the Zayu island arc and in Sang syntaxis. Under the continuous high strength of tectonically compressional regime, the Main Frontal Thrust and the Mishmi Thrust ruptured simultaneously, led to the formation of the M_S8.6 Medog-Zayu great earthquake. Large aftershocks and clear surface deformation are widely observed in the areas of the Mishmi hills and the Abor hills. The uncertainties of the mechanism of early great earthquakes need to be further analyzed. The integration of seismological, geological, remote sensing and field investigations may provide observations in more comprehensive ways.