渤海海峡及周边区域地壳结构的层析成像特征

渤海海峡是连接中国东部山东半岛和辽东半岛的重要途径,其跨海通道的地壳稳定性研究受到高度关注.本文利用地震层析成像方法重建三维P波速度模型,揭示了渤海海峡及周边区域地壳和上地幔的构造特征.结果表明,渤海海峡的速度结构存在明显的非均匀性,海峡北部地壳速度较高,结构较为完整,断层活动不明显,与现今较弱的地震活动相吻合,但是地壳底部存在低速薄层,它有可能成为地壳和上地幔之间的滑脱带,需要开展进一步的研究加以确认.相比之下,海峡南部地壳速度偏低,附近区域地震活动频繁,与张家口—蓬莱断裂带通过于此有着密切的联系,该断裂持续不断的地震活动对海峡南部的地壳结构产生了较大的影响.在渤海南部,郯庐断裂带东、西两侧的地壳结构明显不同,西侧速度偏高,东侧至渤海海峡速度偏低,这一特征可能与此地区广泛发育的断层和地震活动有关.另外,受华北克拉通破坏及地幔上涌的影响,渤海地区地壳深部和上地幔速度偏低,郯庐断裂带及渤海海峡附近显示出深部热流的活动迹象,反映了岩石圈减薄和软流圈的局部抬升.

Tomographic images of crustal structure beneath the Bohai Strait and surrounding areas,eastern China

The Bohai Strait is an important channel to link the Liaodong peninsula and the Shandong peninsula of eastern China. To estimate the stability of the crustal structure of the strait and adjacent areas, we used seismic tomography to construct a three-dimensional P-wave velocity model for this region. Distinct velocity variations are observed beneath the Bohai Strait. Its northern part is featured by a high-velocity curst with stable structure, less affected by tectonic alternations, with weak seismicity. However, a thin low-velocity layer is found in the lowermost crust, which extends from the northern strait toward the Liaodong peninsula, and may be a detachment layer between the crust and the upper mantle. We need to confirm this layer through future studies. On the contrary, the southern part of the strait is featured by a low-velocity crust with active seismicity likely associated with the Zhangjiakou-Penglai fault zone. Tectonic activity along this fault zone greatly affects the stability of the crustal structure of the southern strait. In the southern Bohai Sea, prominent velocity variations are observed in the east and west of the Tan-Lu fault zone. Compared to the high-velocity crust in the west, the east of the fault zone adjacent to the Bohai Strait is featured by the low-velocity crust. This is probably related to a broadly distributed fault system and frequent earthquakes of the area. Additionally, affected by the North China Craton destruction and mantle upwelling, low velocity characterizes the deep crust and upper mantle of the Bohai Sea. Evidence of heat flows can be found in the uppermost mantle beneath the Tan-Lu fault zone and the Bohai Strait, which reflects the thinning of the lithosphere and the local rise of the asthenosphere.