An analysis of the intraplate earthquake (2016M5.8_GY) that occurred in the Gyeongsang Basin in the SE of the Korean Peninsula, based on 3-D modelling of the gravity and magnetic field.

An earthquake (2016M5.8_GY) occurred on 2016 September 12 due to the SW−NE movement of the Yangsan fault in the Gyeongsang Basin in the southeastern part of the Korean Peninsula and caused more than 600 aftershocks within a 1-yr period. To find out the causes and process of the 2016M5.8_GY and its aftershocks, we conducted 3-D underground structural modelling of the Gyeongju area using a combined interpretation of geological, petrophysical and potential field data and hence calculated the density- and magnetic-field distribution along the Yangsan fault. The results of the 3-D modelling indicate that the 2016M5.8_GY occurred in the ductile region of the fault, which is tectonically very unstable, and is therefore highly likely to be significantly affected by any future seismological stress. The crustal structure most affected by the 2016M5.8_GY is the Cretaceous Granite, which has a vertical relief of approximately 12 km with a relatively lower density material than the surrounding country rock and rests on highly ductile material. Therefore, the lower part of the granite is enough unstable that it was probably crushed by the main shock. That is also the reason, why so many aftershocks occurred beneath the granites. [ABSTRACT FROM AUTHOR]