Geological origin of Jinggu earthquake swarm in 2014 in southwest Yunnan: A response to propagation process of the Chafang-Puwen fault zone

Yongping Town in Jinggu of Yunnan Province experienced three times of strong earthquakes from October to December in 2014, which were Ms6.6, Ms5.8, Ms5.9 respectively. In order to determine the cause of this earthquake swarm in the geological structure area, the authors, based on the investigation at the surface in combination with the geological structure, intensity and the aftershock distribution and focal mechanism solution, have reached the conclusion that the macro epicenter of the earthquake swarm activity was located in the mountain area on the southeast side of Yongping basin. The seismogenic fault was the NW-trending dextral strike slip fault which had no obvious geological and geomorphological features. The southeastward migration phenomenon of the aftershocks indicates, due to the temporary block of the oblique connecting position of the rock bridge fault, there was a brief stagnation when Ms6.6 earthquake rupture propagation was spread southeastward. Nevertheless, on December 6th, after breaking through barriers, the earthquake rupture further triggered the Ms5.8 and Ms5.9 earthquakes. This shows that the earthquake swarm activity accorded with the behavior of focal rupture along the NW seismogenic fault segment rupture. The remote sensing interpretation of regional active faults indicates that the seismogenic fault lay just between Chafang NW-trending right strike-slip faults and Puwen NW-trending right strike-slip faults. The area belongs to the incoherent area of Chafang-Puwen fault zone. The authors point out that the strong earthquake swarm activities should be tectonic activities during the coalescence of Chafang-Puwen fault in the upper crust. Combined with the historical seismic data of the Simao block, it is held that the grades of all the seismic activities were less than or equal to 6.8, and the seismogenic structures were mostly NW-trending faults. These phenomena indicate that, in the present tectonic stress field, the NW-trending fault activity is more obvious relative to the NE-trending fault and belongs to the main earth-quake controlling structure, which deserves much attention in furture seismic geological work.