Upper mantle seismic structure beneath eastern Mexico determined from P and S waveform inversion and its implications
Upper mantle seismic structure beneath eastern Mexico determined from P and S waveform inversion and its implications
复制标题
DOI：
10.1029/2006jb004304
复制
发表时间：
2006-08
影响因子：
--
通讯作者：
Wei Gao;E. Matzel;S. Grand
中科院分区：
文献类型：
--
作者：
Wei Gao;E. Matzel;S. Grand
[1] 我们提出了墨西哥东南部下方上地幔的压缩（P）和剪切（S）波地震速度模型，这些模型是根据三次地震相位的波形反演得出的。位于墨西哥和危地马拉边境附近的地震产生的地震波形数据由 La Ristra 被动地震台阵记录。 La Ristra 地震台阵由 54 个宽带地震仪组成，从德克萨斯州西部到犹他州东南部呈线性排列。 La Ristra 阵列的方向几乎沿着事件的大圆，并且地震阵列与墨西哥南部的距离（18.5°–26.5°）使得这些数据非常适合研究上地幔的局部地震结构。先前的层析成像和接收器功能研究提供了接收器侧地壳和上地幔结构的先验知识，从中对地震数据进行了静态调整。使用共轭梯度算法对 40 至 1000 km 深度的地幔速度波形进行反演。在反演中，我们评估了一套具有不同深度的 410 公里和 660 公里不连续性以及不同速度梯度的起始模型。最佳拟合模型的纵波和横波速度在 410 公里间断处的速度分别增加了 6.2% 和 7.3%。研究发现，跨越 660 km 间断面的速度跃变，P 波为 3.3%，S 波为 6.3%。我们发现的上地幔不连续性的大小比标准参考模型所暗示的更符合叶熔岩的成分。最佳拟合模型的一个共同特征是 410 km 间断面以上的低速区，该低速区在剪切速度模型中比压缩速度模型中更为突出。这一特征可能是由于过渡区的水释放引起的部分熔化。 410 公里处的整体速度跳跃也比之前发布的具有较低坡度的模型更大。此外，P 波数据需要 490 km 深度处的小不连续性，而 S 数据中无法解决这一问题。最后，S 波数据需要从约 600 公里深度开始一直延伸到 660 公里间断面的异常高梯度。这一特征可能是由于墨西哥东部下方平坦的板块造成的热和/或矿物异常造成的。
[1] We present compressional (P) and shear (S) wave seismic velocity models for the upper mantle beneath southeastern Mexico derived from waveform inversion of triplicated seismic phases. The seismic waveform data produced by an earthquake located near the Mexico-Guatemala border were recorded by the La Ristra passive seismic array. The La Ristra seismic array consists of 54 broadband seismometers arranged linearly from west Texas to southeastern Utah. The orientation of the La Ristra array is nearly along the great circle from the event, and the distance (18.5°–26.5°) of the seismic array from southern Mexico is such that the data are ideal for investigating localized seismic structure of the upper mantle. Previous tomography and receiver function studies provide a priori knowledge of receiver-side crustal and upper mantle structure from which static adjustments were made to the seismic data. The waveforms were inverted for mantle velocity from 40 to 1000 km depth using a conjugate gradient algorithm. In the inversion, we evaluated a suite of starting models with different depths of the 410 km and 660 km discontinuities and varying velocity gradients. The best fitting models have velocity increases across the 410 km discontinuity of 6.2% and 7.3% for P and S wave velocities, respectively. The velocity jump across the 660 km discontinuity was found to be 3.3% for P waves and 6.3% for S waves. The size of the upper mantle discontinuities that we find are more in agreement with a pyrolite composition than standard reference models imply. A common feature of the best fitting models is a low-velocity zone above the 410 km discontinuity that is more prominent in the shear velocity model than the compressional velocity model. This feature may be due to partial melting induced by water release from the transition zone. The overall jump in velocity at 410 km is also larger than in previously published models with a lower gradient below. In addition, the P wave data require a small discontinuity at 490 km depth that is not resolved in the S data. Finally, the S wave data require an unusually high gradient beginning at about 600 km depth extending to the 660 km discontinuity. This feature may be due to a thermal and/or mineralogic anomaly due to a flat lying slab beneath eastern Mexico.