Fast 1-D Velocity Optimization Inversion to 3D Velocity Imaging: A Case Study of Sichuan Maerkang Earthquake Swarm in 2022
Abstract
To obtain an accurate one-dimensional velocity model, we developed the EA_VELEST method based on the evolutionary algorithm and the VELEST program. This method can quickly generate a suitable 1D velocity model and finally input it into the 3D velocity inversion process using the TomoDD method. We adopt TomoDD methods to inverse the high-resolution three-dimension velocity structure and relative earthquake hypocenters for this sequence. This system processing flow was applied to the Sichuan Maerkang earthquake swarm in 2022. By collecting the seismic phase data of the Maerkang area between 1 January 2009 and 15 June 2022, we relocated the historical earthquakes in the area and obtained accurate 3D velocity imaging results. The relocated hypocenters reveal a SE-trending secondary fault, which is located ~5 km NW of the Songgang fault. In the first ten-hour of the sequence, events clearly down-dip migrated toward the SE direction. The inverted velocity structure indicates that the majority of earthquakes during the sequence occurred along the boundaries of the high and low-velocity zones or high and low-VP/VS anomalies. Especially both the two largest earthquakes, MS 5.8 and MS 6.0, occurred at the discontinuities of high and low-velocity zones. The EA_VELEST method proposed in this paper is a novel method that has played a very good enlightenment role in the optimization of the one-dimensional velocity model in geophysics and has certain reference significance. The 3D velocity results obtained in this paper and the analysis of tectonic significance provide a reference for the seismogenic environment of this Maerkang earthquake and the deep 3D velocity of the Ganzi block.
为了获得准确的一维速度模型，我们开发了基于进化算法和VELEST程序的EA_VELEST方法。该方法可以快速生成合适的一维速度模型，最后用TomoDD方法将其输入三维速度反演过程。我们采用TomoDD方法反演了该序列的高分辨率三维速度结构和相对地震带中心。该系统处理流程应用于2022年的四川马尔康地震群。通过采集2009年1月1日至2022年6月15日期间马尔康地区的地震相位数据，我们对该地区的历史地震进行了重新定位，得到了准确的三维速度成像结果。重新定位的次中心揭示了一个东南走向的次级断层，它位于松岗断层西北方向~5公里处。在该序列的第一个10小时内，事件明显地朝东南方向下倾迁移。倒置的速度结构表明，该序列中的大部分地震都发生在高、低速度带或高、低VP/VS异常区的边界上。特别是两个最大的地震，MS5.8和MS6.0，都发生在高低速带的不连续处。本文提出的EA_VELEST方法是一种新颖的方法，对地球物理学中一维速度模型的优化起到了很好的启蒙作用，具有一定的参考意义。本文得到的三维速度结果和构造意义的分析，为本次马尔康地震的成震环境和甘孜区块的深部三维速度提供了参考。