虚拟震源方法研究青藏高原中部莫霍面起伏

The previous studies show that there are significant variations for the crustal thickness beneath the smooth surface of the Tibetan plateau. It has been proposed that the upper crust is decoupled with the lower crust and the dynamic processes of the upper crust and the lower crust may be different based on this contrast. For example, the north-south trending rifts and the conjugate strike-slip faults in the southern and central Tibet are considered as the result of the gravitational collapse of upper crust. However, the material which comes from the upper mantle implies that the west-east extension may extend to the lower crust even the whole lithosphere. The relationship between the crustal thickness and the surface tectonics is the key clue to figure out this problem. At present, the difficulties to detect the crustal thickness of Tibetan plateau are from three aspects: firstly, the seismic wave is absorbed and damped by the thick Tibetan crust; secondly, the amplitude of receiver function’s Ps phases is weaken by the high wave velocity at the lower crust and the complexity of the crust-mantle transition zone; finally, the relevant data is insufficient to image the lateral variations of crustal thickness. Virtual Deep Seismic Sounding is a method, which uses SsPmp phases totally reflected at the Moho with a strong amplitude, to detect the crustal thickness, so the influences of the absorption and attenuation during propagation and the complexity of the structure is negligible. We will introduce both the instantaneous phase analysis and 2D spatial variation of teleseismic travel time residuals into VDSS and use the waveform data recorded by a 2D broadband seismic array, which is deployed by our research group at both sides of the Bangong-Nujiang suture, to detect the crustal thickness beneath the conjugate strike-slip extension area. The new information will provide more constraints on the dynamics process of the eastern extrusion in central Tibet.

青藏高原近似平坦的地表下方，地壳厚度有着剧烈的横向变化，据此人们推测高原上地壳与下地壳解耦，深浅动力学过程存在明显差异，例如高原中南部的南北向裂谷和共轭走滑被认为是高原上地壳垮塌的表现。然而，南北向裂谷中幔源物质的发现暗示高原东西向拉张可能涉及到下地壳甚至整个岩石圈。而地壳厚度与地表构造的对应关系是解决该问题的关键线索。目前，青藏高原地壳厚度研究受限于巨厚地壳对地震波的吸收衰减、下地壳高速和壳幔过渡带的复杂性、以及资料匮乏。虚拟震源地震测深方法利用远震SsPmp震相在莫霍面全反射能量大的特点，可避免地震波衰减和结构复杂性的影响，适合于高原的地壳厚度探测。本申请拟将瞬时相位分析和二维远震到时空间变化引入该方法，使用学科组在班公湖-怒江缝合带两侧布设的二维宽频带台网远震波形数据，探测高原中部共轭走滑拉张区下方地壳厚度的横向变化，为高原中部东向挤出的动力学研究提供深部约束。

新生代以来，印度与欧亚大陆之间的陆-陆碰撞和后期的持续挤压形成了现今世界上规模最大、海拔最高的青藏高原。青藏高原中部是受印度岩石圈作用的直接区域，南北向持续挤压作用下，青藏高原中部发生东西向拉张。近似平坦的地表下方，青藏高原地壳厚度有着剧烈的横向变化，据此人们推测高原上地壳与下地壳解耦，深浅动力学过程存在明显差异。然而，南北向裂谷中幔源物质的发现暗示高原东西向拉张可能涉及到下地壳甚至整个岩石圈。深部结构与地表构造的对应关系是解决该问题的关键线索。.由于巨厚地壳的吸收衰减、壳幔过渡带的复杂性以及资料匮乏等因素，青藏高原及其周缘的地壳厚度仍然存在争议。虚拟震源地震测深方法利用远震SsPmp震相在莫霍面全反射能量大的特点，可避免地震波衰减和结构复杂性的影响，适合于高原地区的地壳厚度探测。本研究将瞬时相位分析引入该方法，并首先应用在位于青藏高原东南缘的峨眉山大火成岩省地区，获得了峨眉山大火成岩省内带的巨厚地壳厚度。通过与接收函数、深地震测深结果的对比，以及射线参数的约束，可以估算峨眉山大火成岩省内带高速下地壳的规模与全球典型大火成岩省的规模相符。.为了探测高原中部拉张区下方地壳厚度的横向变化，为其东向挤出的动力学研究提供深部约束，我们对学科组在班公湖-怒江缝合带两侧布设的二维宽频带台阵（SANDWICH）远震波形数据进行了处理并获得了青藏高原中部地壳厚度横向变化。结果显示，地壳厚度的变化与裂谷及走滑断层的分布有一定的相关性，但由于高原深部地震波速度结构横向变化较大，结果仍需进一步处理。.前人研究认为：青藏高原中部发育的南北向裂谷可能与印度俯冲板片的撕裂有关，但仍缺乏印度板片撕裂的直接证据。本研究利用SANDWICH台阵记录的远震波形数据，提取S波接收函数对青藏高原下方的岩石圈底界面的形态进行共转换点叠加成像，结果显示：印度岩石圈俯冲板片底界面在亚东-谷露裂谷下方发生了明显的错断，为印度板片撕裂提供直接证据。