Abstract: The main rupture of Ludian M_S6.5 earthquake is directed to the northwest, which occurred in the east of Xianshuihe-Xiaojiang fault zone. The epicenter is in the transitional zone of the Sichuan-Yunnan block and the South China block, where there are many slip and nappe structures. Some controversy still remains on the earthquake tectonic environment. So, Bouguer gravity anomalies calculated by EGM2008 were broken down into 1-5 ranks using the way of Discrete Wavelet Transform(DWT), then we get the lateral heterogeneity in different depths of the crust. The distribution characteristics of Bouguer gravity anomaly are analyzed using measured gravity profile data. We also get its normalized full gradient(NFG)picture, and study the differences between different depths in crust. The results show that: (1)the characteristic of Buoguer gravity anomaly in southwest to northeast is high-low-high between the Lianfeng Fault(LFF)and Zhaotong-Ludian Fault(ZLF). The mainshock and aftershocks are distributed in the middle of the low-value zone, which means that the east moving materials of Qinghai-Tibet plateau broke through the southern section of Lianfeng Fault(LFF), moving along the Baogunao-Xiaohe zone(low-value belt)to the southeast, stopped by the Zhaotong-Ludian Fault(ZLF), and then earthquake occurred.(2)The third-order discrete wavelet transform(DWT)details show that: there is a good consistency between the negative gravity anomaly in upper crust and the distribution of major faults, which reflects that the rupture caused by the movements of the faults in crust has reduced gravity anomaly. There is a NW-trending negative anomaly belt near the epicenter, which may has some relationship to the southward development of the Daliangshan Fault(DLSF). So we speculate that the southward movement of Daliangshan Fault is the main direct force source of Ludian earthquake.(3)In the picture of the fourth-order DWT details, there is an obvious positive gravity anomaly under the epicenter of Ludian earthquake, which confirms the presence of a high-density body in the middle crust. While the fifth-order DWT details show that: A positive anomaly belt is below the epicenter too, which may be caused by mantle material intruding to the lower crust. Tensile force in crust caused by mantle uplift and extrusion-torsion force caused by Indian plate push are the main force source in the tensile and strike slip movement of the Ludian earthquake.(4)The normalized total gradient of Bouguer gravity anomalies of Huili-Ludian-Zhaotong profile shows that: there is obvious ‘deformation’ in the Xiaojiang fault zone which dips to the east and controls the local crust movement. There is a local ‘constant body’ at the bottom of the epicenter. The stable constant body in density has limiting effects to the earthquake rupture, which is the reason that the earthquake rupture' scale in strike and in depth are limited.(5)The ability of earthquake preparation in Zhaotong-Ludian Fault is lower than the Xianshuihe-Xiaojiang fault zone, and the maximum earthquake capacity in this area should be around magnitude 7.

Key words: Ludian earthquake, Bouguer gravity anomaly, discrete wavelet transform, normalized full gradient, crustal structure

摘要： 鲁甸地震发生在鲜水河-小江断裂带东侧，主破裂为NW走向，其震中区处于川滇块体与华南块体过渡地带，走滑和推覆构造发育，目前对该地震的孕震构造环境认识上还存在一些争议。基于此，利用EGM2008计算的布格重力异常进行1-5阶离散小波变换，分离得到反映不同深度场源特征的地壳介质横向不均匀性；利用实测重力剖面数据，分析其布格重力异常分布特征，计算并绘制其归一化总梯度（NFG）图像，研究了地壳密度变化的差异性。分析结果表明： 1）莲峰断裂与昭通-鲁甸断裂之间布格重力异常呈现SW-NE向的"高-低-高"型异常特征，鲁甸主震及余震均分布于低值带内，反映出青藏高原物质东流沿包谷垴-小河一带（低异常条带）向SE推进，至昭通鲁甸断裂受阻，能量积累到一定程度后发震；2）第3阶小波细节显示：上地壳负重力异常与主要断裂分布一致性非常好，反映了由断裂运动造成地壳浅部物质破碎而呈现出重力异常减小，震中附近NW向负异常条带可能与大凉山断裂向S发展有关，推测NW走向大凉山断裂向S扩展运动是鲁甸地震发震的直接动力来源；3）第4阶小波细节显示鲁甸震中存在明显的正异常，印证了中地壳内部高密度体的存在；第5阶小波细节显示鲁甸震中处于正异常区，预示着地幔物质的侵入，地幔隆升产生地壳引张力和印度板块推挤产生的压扭力是鲁甸地震震源拉张和走滑的动力来源；4）剖面布格重力异常归一化总梯度图像显示："变形"显著且E倾的小江断裂带为区域控制性断裂，鲁甸震源底部存在稳定的局部"不变体"，稳定的密度不变体对此次地震破裂具有限制作用，是导致震源新生破裂规模有限且切割较浅的原因；5）通过与鲜水河-小江断裂系比较，认为昭通-鲁甸断裂孕震能力明显偏弱，其最大发震能力应在7级左右。

关键词: 鲁甸地震, 布格重力异常, 离散小波变换, 归一化总梯度, 地壳结构