鲁甸M_S6.5地震震源区地壳结构及孕震环境研究

本文利用由中国地震局在鲁甸地震震区附近架设的35个流动观测台站记录的远震事件记录,采用接收函数H-k扫描方法和CCP叠加成像方法获取了鲁甸地震震源区的地壳精细结构,结果显示鲁甸地震发生在地壳厚度和泊松比变化较剧烈的地区.昭通断裂西南段和东北段地壳物质组分差异明显,西南段断裂两侧地壳组分均显示为中泊松比分布,东北段断裂两侧泊松比从低泊松比快速变化为高泊松比,表明东北段西南侧壳内含有更多铁镁质组分,造成昭通断裂西南段和东北段对青藏高原下地壳物质向东南运移的阻挡有所差异,导致壳内应变积累,从而引起鲁甸地震的发生.在震源区地壳内部存在的低速层,可能为此次地震提供了可能的孕震环境.鲁甸地震与芦山地震虽然均没有产生明显的地表破裂带,但两者的震源机制以及孕震环境存在着明显的差异.本文也认为未来应关注青藏高原东缘断裂的历史地震空段发生大地震的可能性.本文研究结果对于理解青藏高原东缘区域的孕震背景具有一定的意义.

The crustal structure and seismogenic environment in the Ludian MS6.5 earthquake region

Teleseismic events used in this study were recorded by 35 seismic stations that operated by China Earthquake Administration in the Ludian epicenter and surrounding region. We used H-k stacking method and Common Conversion Point stacking method to reveal the detailed crustal structure beneath the Ludian earquake. The results show the Ludian earthquake occurred in the place where the crustal thickness and the Poisson's ratio varies strongly. The crustal compositions are very different between the southwest part and the northeast part of Zhaotong fault,the Poisson's ratio is at the middle level in the southwest part, however, the Poisson's ratio changes rapidly from the lower level to higher level across the fault in the northeast part, indicates that there is more mafic minerals in the crust beneath the northeast part of the Zhaotong fault. The significant difference of the crustal composition between the southwest part and the northeast part of Zhaotong fault played a different blocking role to the southeast escaping of Tibetan Plateau materials and caused the strain accumulated in the crust, that induced the Ludian earthquake. The low velocity layer beneath the epicenter may provide the seismogenic environment. Although no surface rupture zone is observed in the epicenter of the Ludian earthquake and Lushan earthquake, there are significant different focal mechanism and seismogenic environment between the Ludian earthquake and Lushan earthquake. We also suggest that the historical seismic gaps in the eastern margin of the Tibetan plateau should get more attention in the furture. Our results are very useful to understand the seismogenic process in the eastern margin of the Tibetan plateau.