南北地震带地壳结构多参数成像及强震触发机制研究

Crustal structure imaging of multi-geophysical parameters and generating mechanisms of large earthquakes in North-South Seismic Zone

通过反演由大量的纵、横波地震数据组成的综合数据集,获得了南北地震带地壳的多参数三维精细结构,探讨和分析了南北地震带的高地震活动性和强震频发的原因.成像结果表明,尽管1976年松潘一平武地震(M7.2)与2008年汶川地震(M8.0)以及2013年芦山地震(M7.0)均发生在高速、低泊松比异常区域,并且在其震源的下方均有一低速、高泊松比异常区域.我们认为,上述三个地震的触发与流体侵入导致的地壳形变之间有密切的联系.1955年炉霍地震(M7.4)和1973年康定地震(M7.1)均发生在鲜水河断裂带上,其震源中心区域表现为低速、高泊松比异常,可以解释为下地壳中的流体沿断层面上涌.在震源区的周边区域兼有高速、高泊松比异常,低速、高泊松比异常以及高速、低泊松比异常,可能分别与含流体的岩石、沿断裂带发育的变质岩以及坚硬的克拉通块体对应.流体的侵入不仅能够改变断层面上的应力情况,还能降低岩石骨架的岩石力学强度,进而触发地震.1970年云南通海大地震(M7.1)发生在哀牢山一红河断裂带附近的曲江断裂上,其震源处于高速度、低泊松比异常与低速度、高泊松比异常之间的边界区域,被认为是流体挤压后的应变能积累,最终导致脆性破裂,以至于发生地震.根据本次研究获得的多参数结构图像,结合前人的研究成果,我们认为南北地震带地壳强烈形变与流体侵入是造成该区域地震活动性较高及强震频发的两个主要因素.

Crustal structures of multi-geophysical parameters are imaged with high resolution in the North-South Seismic Zone （NSSZ） by inverting a combined dataset of a large number of arrival-time data of P- and S-waves. Based on the high-resolution seismic images we focus on discussing generation mechanism of the large crustal earthquake （M〉7.0） along the NSSZ. Our study revealed that the 1976 Songpan-Pingwu earthquake （M7.2）, 2008 Wenchuan earthquake （M8.0） and 2013 Lushan earthquake （M7.0） occurred in the high velocity areas with low Poisson＇s ratio anomalies. We consider that there could be a close relation between the generation of these three large earthquakes and the crustal deformation caused by intrusive fluids. On the contrary, the 1955 Luhuo earthquake （M7.4） and the 1973 Kangding earthquake （M7.1） occurred in Xianshuihe fault zone and their hypocenters were located in the low-velocity areas with high Poisson＇s ratio anomalies. This might be interpreted as upwelling fluids from the lower crustal channel flow. The surrounding areas of these earthquake hypocenters show strong variations of anomalous high-velocity and high Poisson＇s ratio, high-velocity and low Poisson＇s ratio anomalies, and low-velocity and low Poisson＇s ratio perturbations. These anomalies could be possibly and respectively corresponding to fluid-filled cracked rock, old and steady craton and growing metamorphous rocks along their corresponding faults. The intrusive fluids could initiate large crustal earthquakes by both changing the stress condition on the fault plane and weakening the mechanical strength of the rock matrix. The 1970 Tonghai earthquake （M7.1） in Yunnan occurred on the Qujiang fault near Ailaoshan-Honghe fault zone. The hypocenter of this earthquake is located in a distinctive boundary where the seismic velocity changes dramatically from low to high whilst the Poisson＇s ratio varies from high to low across the suture zone. We considered that the crustal strain energy accumulation after fluid extrusion from the lower crust led to the brittle fracture of the source rock hence the Tonghai earthquake. Therefore, we conclude that the strong crustal deformation and intrusive fluids from lower crust could have played major roles in active seismicity and generation of large crustal earthquakes in the NSSZ.