2007年6月3日宁洱6．4级地震地表变形的构造分析和解释

Structural analysis and interpretation of the surface deformation associated with the Ning'er,Yunnan Province,China M_S6.4 earthquake of June 3,2007

以2007年6月3日云南省宁洱6.4级地震地表裂缝、喷砂冒水、地震滑坡、地震崩塌等资料为基础,结合震区的地质构造、震源机制解、余震分布等资料,研究了本次地震的发震断层及其强震频繁发生的动力学机制.地震地表裂缝资料表明,北西向的宁洱断裂班海段具有右旋走滑的特征,北北东向的断层具有左旋走滑的性质.地震地质灾害集中分布在330°方向上的长13.5km、宽4km的范围内.等震线的长轴方向亦为330°,Ⅷ度区长轴长度为17km.震源机制解资料显示,宁洱6.4级地震的北西向节面为右旋错动,北东向节面为左旋错动.大于等于2.0级的余震分布优势方向为330°,深度为3—12km,优势深度为3—10km,余震分布与地震地质灾害集中分布带一致.以上资料说明,宁洱断裂班海段为这次地震的发震断层.最后以活动地块理论为基础,讨论了宁洱地震的动力来源为印度板块的向北推挤使青藏高原向东滑移,在滇西南地区转化为向南南东方向的挤出,使宁洱附近网络状的北西向断裂发生右旋活动,北东向断裂发生左旋活动.这种构造格局可能是该区频繁发生6.0—6.9级地震的原因.

The seismogenic fault and the dynamic mechanism of the Ning＇er, Yunnan Province Ms6.4 earthquake of June 3, 2007 are studied on the basis of the observation data of the surface fissures, sand blow and water eruption, landslide and collapse associated with the earthquake, incorporating with the data of geologic structures, focal mechanism solutions and aftershock distribution for the earthquake area. The observation of the surface fissures reveals that the Banhai segment of the NW-trending Ning＇er fault is dominated by right-lateral strike-slip, while the NNE-trending fault is dominated by left-lateral strikeslip. The seismo-geologic hazards are concentrated mainly within a 330°-extending zone of 13.5 km in length and 4 km in width. The major axis of the isoseismal is also oriented in 330° direction, and the major axis of the seismic intensity area is 13.5 km long. The focal mechanism solutions indicate that the NWtrending nodal plane of the Ning＇er Ms6.4 earthquake is dominated by right-lateral slip, while the NE-trending nodal plane is dominated by left-lateral slip. The preferred distribution orientation of the aftershocks of Ms≥2 is 330°, and the focal depths are within the range of 3-12 km, predominantly within 3-10 km. The distribution of the aftershocks is consistent with the distribution zone of the seismo-geologic hazards. All the above-mentioned data indicate that the Banhai segment of the Ning＇er fault is the seismogenic fault of this earthquake. Moreover, the driving force of the Ning＇er earthquake is discussed in the light of the active block theory. It is believed that the northward pushing of the Indian plate has caused the eastward slipping of the Qinghai-Tibetan Plateau, which has been transformed into the southeastern-southernward squeezing of the southwest Yunnan region. As a result, the NW-trending faults in the vicinity of the Ning＇er area are dominated by right-lateral strike-slip, while the NE-tren- ding faults are dominated by left-lateral strike-slip. This tectonic framework might be the main cause of the frequent occurrence of Ms6.0-6.9 earthquakes in the area.