环渤海地区Pn波速度结构与各向异性

环渤海地区位于华北克拉通的中东部,是岩石圈破坏和减薄的主要地区,同时也是我国大陆东部强震的多发区和油气田产区,一直是国内外学者研究的重点区域.本研究利用环渤海地区1980—2015年期间中国地震台网高质量Pn波到时数据,反演得到环渤海地区Pn波速度结构及各向异性.结果显示,环渤海地区上地幔顶部的Pn波速度结构存在明显的横向不均匀性,且与区域地质构造有一定相关性.在地形隆起区,如太行山隆起、燕山隆起、鲁西隆起、胶辽隆起及苏鲁褶皱带地区,呈现为低波速异常,说明这些隆起区下方的上地幔存在热物质上涌,而凹陷地区,如华北盆地、南黄海北部盆地和南黄海南部盆地,则表现为高波速异常,说明这些凹陷地区上地幔顶部岩石圈强度较大.地壳内强震主要发生于低波速异常区和高低波速异常过渡带上,说明华北地区地壳强震的发生有可能受到上地幔深部构造的影响.太行山造山带地区Pn波各向异性快波方向为近NNE向,苏鲁褶皱带区域的Pn波各向异性快波方向为近NE向,与断裂带的走向基本一致,表明在地壳形变剧烈的地区,可能受上地幔顶部的深部动力学影响较大.华北盆地的北部和南部各向异性方向存在差异,可能与岩石圈的厚度及热状态的不均匀性有关.

Pn-wave velocity and anisotropy around the Bohai Sea areas

The Bohai Sea and surrounding areas are located on the mid-eastern parts of North China Craton, where the lithospheric thinning and destruction occurred and earthquakes frequently took place and oil and gases were generated. Therefore, it is a region attracting much attention of geoscientists to study.#br#The Pn-wave data during 1980 to 2015 used in this study come from the observational bulletins of China Seismological Networks Center, including hand-picked 3068 Pn arrival times. We selected 10056 high-quality Pn data from 569 earthquakes recorded at 265 stations. The ray paths have good coverage in the study region.#br#We inverted Pn-wave velocity structure and anisotropy beneath the Bohai Sea and surrounding areas using the Hearn's method. The high-resolution result shows that Pn-wave velocity is well consistent with the regional geological structure in the Bohai Sea and surrounding areas. The lithospheric thinning and destruction of North China Craton has little effects on the uppermost mantle. In the study area, the uppermost mantle exhibits prominent lateral heterogeneities. The uplifted areas, such as the Taihangshan orogen, Yanshan orogen, Luxi uplift, Jiaoliao uplift and Sulu belt, present low-velocity (low-V) anomalies, indicating the lithosphere is weak beneath these areas. The depressed regions, such as the North China basin, northern South Yellow Sea basin, and southern South Yellow Sea basin, exhibit high-velocity (high-V) anomalies, implying a strong lithosphere there. Crustal earthquakes are mainly distributed along the transition zone of low-V to high-V anomalies, suggesting a possible correlation between the crustal strong earthquakes and the upper mantle structure. The fast directions of Pn anisotropy beneath the Taihangshan orogen are roughly along the NNE direction that is consistent with the trend of the mountains. The fast directions of Pn anisotropy beneath the Sulu belt are generally in the NE direction. It is inferred that the uppermost mantle structure may have affected on the shallow crustal structure. The fast directions of Pn anisotropy are variable in the northern and southern North China basin, which might be related to the lithosphere thickness and thermal conditions beneath the study region.