川滇活动块体中-北部主要活动断裂带现今应力状态的分区特征

恢复2009年1月1日-2015年12月31日间川滇活动块体中-北部1012次2.0≤M_L≤5.0地震的震源谱,计算标量地震矩M_o、震源尺度r和应力降△_σ等震源参数并拟合各参数间的定标关系,基于区域地震构造背景、活动断裂展布以及地震活动的成丛分布将研究区划分成四个统计单元,分别讨论各构造单元的应力分布特征、地震应力降随地点位置的变化以及应力-应变加载作用与区域变形的动力学过程的关联.结果显示:中小地震释放的应力降△_σ在0.1~10 MPa;标量地震矩M_o与近震震级M_L呈现较好线性关系(lgM_0=0.92M_L+10.46);应力降与地震大小的关系与Nuttli的增加应力降(ISD)模型比较吻合(1g△_σ=0.31 1gM_0-3.92).震源应力降结果显示:①金沙江断裂端部为低应力区,断裂单元整体滑动速率较高、强震活动极少,不具备强震发生的应力高度集中条件;在3条次级断裂构成的条带断裂结构中,理塘断裂上的应力-应变加载作用自北西向南东逐渐减弱,相对闭锁的北西段较其他部位更易积累应变.②鲜水河断裂带的地震应力降以康定为界南低北高,南段(康定-石棉)短期内难以积累较高应变,北段(甘孜-康定)应力水平较高,已发生的中强地震尚未能填充地震矩释放的亏空区,段落局部仍有较高的应力积累.③安宁河-则木河断裂上高应力降地震事件集中,该单元的应变积累强、应力水平最高,地震危险性大.④丽江—小金河断裂上不同震级地震的应力降特征并不相同,推测与当地复杂的构造背景有关,具体原因尚需深入探讨;木里地区应力背景较低,可能受当地构造环境的影响.研究表明,地震应力降随地点位置而系统变化,高应力降地震事件多发生在断裂与断裂的交汇部位,而断裂无闭锁条件、断裂以蠕滑为主且断面松弛、断裂端部为高温或破碎塑性变形带时,多以低应力降地震事件为主;与通常所认为的"走滑断裂不易积累应力"相反的是,鲜水河断裂带、安宁河—则木河断裂带均表现出较高的应力水平,其原因一方面可能是因为已发生的中强地震无论数量还是强度都尚不足以释放已经积累的能量,另一方面也许是在区域变形的复杂动力学过程中,当构造单元间阻碍断层运动和协助积累应力的作用占主导时,相同震级的地震会释放更多的应力.

Regional characteristics of stress state of main seismic active faults in mid-northern part of Sichuan-Yunnan block

We restore the seismic source spectrums of 1012 earthquakes (2.0≤M_L≤5.0) in the mid-northern part of Sichuan-Yunnan seismic block between January 1, 2009 and December 31, 2015, then calculate the source parameters (e.g., seismic moments M₀, focal scale r and stress drop Δσ) and fit the calibration relationship between these parameters. Based on the regional seismic tectonic background, the distribution of active faults and seismicity, the study area is divided into four statistical units. For each unit the stress distribution characteristics, change of stress drop with location, correlation between the stress-strain loading and the dynamic process of regional deformation, are discussed respectively. The results show that seismic moments M₀ are consistent with the magnitude-moment relation lgM₀=0.92M_L+10.46. The relationship between stress drop and magnitude is consistent with the result gained by Nuttli that intraplate earthquake follows the ISD model, with a statistical relationship lgΔσ=0.31 lgM₀-3.92.
Seismic source stress drop results show the following, (1) The stress at the end of the Jinshajiang fault is low, the overall sliding rate of the fault unit is high, and strong earthquake activity is very rare. In the fault belt consisting of three secondary faults, stress-strain loading deceases gradually from northwest to southeast along Litang fault, the northwest section which is relatively locked is more likely to accumulate strain than southeast section. (2) Stress drop of Xianshuihe fault zone is divided by Kangding, the southern section is low and northern section is high. Southern section (Kangding-Shimian) is difficult to accumulate higher strain in the short term, but in northern section (Garzê-Kangding), moderate and strong earthquakes have not filled the gaps of seismic moment release, there is still a high stress accumulation in partial section. (3) High stress-drop events were concentrated on Anninghe-Zemuhe fault zone, strain accumulation of this unit is strong, and stress level is the highest, earthquake risk is high. (4) On Lijiang-Xiaojinhe fault zone, stress drop characteristics of different magnitude earthquakes are not the same, which is related to complex tectonic setting, the specific reasons still need to be discussed deeply. Stress background in the Muli area is low and may be affected by the local tectonic environment. The study also shows that, (1) Stress drops display a systematic change with different faults and locations, high stress-drop events occur mostly on the fault intersection area. Faults without locking condition and mainly creeping, are mainly characterized by low stress drop. (2) Contrasting to what is commonly thought that "strike-slip faults are not easy to accumulate stress", Xianshuihe fault zone and Anninghe-Zemuhe fault zone all exhibit high stress levels, which may be due to that the magnitude and intensity of medium-strong earthquakes are not enough to release the accumulated energy. On the other hand, in the complex dynamics of regional deformation, when the tectonic unit blocking fault movement and its contribution to accumulation of stress play a key role, the earthquake of same magnitude will release higher stress drop.