Discussion on source parameters of the M6¾ Lingshan, Guangxi, earthquake occurred in 1936

The M6¾ Lingshan earthquake occurred on the north section of Fangcheng-Lingshan fault on April 1, 1936, and it was also the largest earthquake occurred on the continental part of South China seismic belt ever since the historical earthquake records began. The source parameters of this earthquake still remain controversial for the lack of modern seismographic record and detailed field investigation on the surface rupture zone. Furthermore, the long-time interval and human activities between earthquake and the later field investigation also cause uncertainty on the inferred source parameters. In recent years, there are some reports about determination of source parameters such as epicenter, magnitude, intensity and seismogenic structure through investigation of earthquake surface rupture zone caused by historical earthquake without modern seismographic records. Most of these researches were carried out in arid area in western China where the climate is suitable for the reservation of earthquake traces, although it is difficult to identify earthquake traces such as earthquake surface rupture zone caused by historical earthquake in the South China moderate-strong seismic tectonic zone due to the specific seismic structure, humid weather and human activities. On the other hand, it is uncertain to some extent to estimate source parameters of historical moderate-strong earthquake without modern seismographic records in South China, because such source parameters were often indirectly calculated from intensity zoning map acquired by investigation of damaged buildings or surface ruptures in the quake-hit area. To reduce this uncertainty, in this paper we introduce the newly discovered earthquake surface rupture zone caused by the M6¾ Lingshan earthquake. Our newly investigation shows that earthquake surface rupture zone marked by earthquake fissures, earthquake scarp, earthquake fault, earthquake-induced landslide and gully displacement developed along the north section of Lingshan fault at the northwest foothill of Luoyang mountain with total length about 12.5 km. The surface rupture zone strikes from NE to ENE, its west branch stretches from Gaotang to Liumeng and the east branch stretches from Jiaogenping to Hekou. The co-seismic motion along this earthquake surface rupture zone is normal dextral slip. Furthermore, by utilization of parameters of the newly discovered earthquake surface rupture zone, we discuss the source parameters of this earthquake. Our research result shows that: ① The magnitude of the Lingshan earthquake occurred on 1 April 1936 is about M6.8; ② The intensity in the meizoseismal area reaches Ⅸ degree plus; ③ The macro-epicenter is located near the intersection of northern segment of Lingshan fault with Youliao-Jiaogenping fault; ④ The Lingshan fault located at the northwest foothill of Luoyang mountain is the seismogenic fault of the Lingshan earthquake, meanwhile Youliao-Jiaogenping fault, Sizhou fault and other NW-NNW-trending small faults located at the south foothill of Luoyang mountain are the earthquake-controlling faults of the Lingshan earthquake. The main shock caused triggered motion along these small NW-NNW-trending faults, resulting in local intensity augmentation along these small faults accordingly.