Examining the influence of 2.5-D ultra-low velocity zone morphology on ScP waveforms and estimated elastic parameters

IF 2.8 3区 地球科学 Q2 GEOCHEMISTRY & GEOPHYSICS Geophysical Journal International Pub Date : 2024-08-14 DOI:10.1093/gji/ggae285
Surya Pachhai, Michael S Thorne, Sebastian Rost
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Abstract

Summary Ultra-low velocity zones (ULVZs) have been identified as regions of extremely low velocity anomalies in the Earth's lowermost mantle using seismic observations from reflected, refracted, and diffracted arrivals along the mantle side of the core-mantle boundary (CMB). Estimation of ULVZ geometrical (i.e., shape and size) and elastic (i.e., velocity and density) parameters with uncertainties is crucial in understanding the role of ULVZs in the ongoing dynamic processes within the Earth's mantle; however, these parameters are still poorly known due to uncertainties and tradeoffs of the seismically resolved ULVZ geometries and elastic parameters. Computation of synthetic waveforms for 2-D and 3-D ULVZs shapes is currently computationally feasible, but past studies utilize higher dimensional waveform modeling of mostly only low-frequency diffracted waves. Most studies focusing on high-frequency core-reflected waveforms (e.g., ScP) still use 1-D modeling approaches to determine ULVZ properties. This approach might lead to wrong results if the imaged structures have inherently 3-D geometries. This study investigates high-frequency synthetic ScP waveforms for various 2.5-D ULVZ geometries showing that additional seismic arrivals are generated even when the ScP geometrical ray path does not directly strike the location of the ULVZ. The largest amplitude additional phases in the 2.5-D models are post-cursor arrivals that are generated at the edges of the finite-length ULVZs. These newly identified ScP post-cursors can arrive within the ScsP post-cursor time window traditionally analyzed in 1-D ULVZ studies. These post-cursors might then be misidentified or constructively/destructively interfere with the ScsP postcursor, leading to incorrect estimation of ULVZ parameters. In this study we investigate the bias introduced by the 2.5-D morphologies on the 1D estimated ULVZ elastic properties in a Bayesian waveform inversion scheme. We further expand the Bayesian method by including the data noise covariance matrix in the inversion and compare it to an autoregressive noise model that was utilized in previous studies. From the application to the observed ScP data, we find that the new approach converges faster, particularly for the inversion of data from multiple events, and the new algorithm retrieves ULVZ parameters with more realistic uncertainties. The inversion of 2.5-D synthetic ScP waveforms suggests that the retrieved ULVZ parameters can be misleading with unrealistically high confidence if we do not consider the data noise covariance matrix in the inversion. Our new approach can also retrieve the shape of a multi-dimensional Gaussian ULVZ if its length is 12o or longer in the great circle arc direction. However, 2.5-D synthetic waveforms show additional waveform complexity which can constructively interfere with the ScP wavefield. Hence, in many cases the estimation of ULVZ properties using 1-D forward modeling can provide incorrect ULVZ parameters. Hence previous ULVZ modeling efforts using 1-D parameter estimation methods may be incorrect.
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研究 2.5-D 超低速度区形态对 ScP 波形和估计弹性参数的影响
摘要 利用沿地核-地幔边界(CMB)地幔一侧的反射、折射和衍射到达的地震观测数据,确定了地球最下层地幔中的超低速度异常区(ULVZs)。估算具有不确定性的超低变异区几何参数(即形状和大小)和弹性参数(即速度和密度)对于了解超低变异区在地幔内持续动态过程中的作用至关重要;然而,由于地震解析的超低变异区几何参数和弹性参数存在不确定性和折衷,这些参数仍然知之甚少。目前,计算 2-D 和 3-D ULVZs 形状的合成波形在计算上是可行的,但过去的研究大多只利用低频衍射波的高维波形建模。大多数侧重于高频岩心反射波形(如 ScP)的研究仍然使用一维建模方法来确定超低真空区的特性。如果成像结构本身具有三维几何结构,这种方法可能会导致错误的结果。本研究对各种 2.5-D 超低电压区几何形状的高频合成 ScP 波形进行了研究,结果表明,即使 ScP 几何射线路径没有直接击中超低电压区的位置,也会产生额外的地震到达。在 2.5-D 模型中,最大振幅的附加相位是在有限长度 ULVZ 边缘产生的后源到达。这些新发现的 ScP 后源相可能会在 1-D ULVZ 研究中传统分析的 ScsP 后源相时间窗内到达。这些后ursor可能会被错误识别,或对 ScsP 后ursor 产生建设性/破坏性干扰,从而导致对 ULVZ 参数的错误估计。在本研究中,我们研究了贝叶斯波形反演方案中 2.5-D 形态对一维估算的 ULVZ 弹性特性所带来的偏差。通过在反演中加入数据噪声协方差矩阵,我们进一步扩展了贝叶斯方法,并将其与之前研究中使用的自回归噪声模型进行了比较。通过对观测到的 ScP 数据的应用,我们发现新方法的收敛速度更快,尤其是对来自多个事件的数据进行反演时,新算法检索出的 ULVZ 参数具有更真实的不确定性。对 2.5-D 合成 ScP 波形的反演表明,如果我们在反演中不考虑数据噪声协方差矩阵,检索到的 ULVZ 参数可能会产生误导,使置信度过高。如果多维高斯超视距波形在大圆弧方向的长度为 12o 或更长,我们的新方法也能检索出其形状。然而,2.5 维合成波形会显示出额外的波形复杂性,从而对 ScP 波场产生建设性干扰。因此,在许多情况下,使用 1-D 正演建模估算 ULVZ 特性可能会提供不正确的 ULVZ 参数。因此,以前使用一维参数估计方法进行超低电压区建模的工作可能是不正确的。
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来源期刊
Geophysical Journal International
Geophysical Journal International 地学-地球化学与地球物理
CiteScore
5.40
自引率
10.70%
发文量
436
审稿时长
3.3 months
期刊介绍: Geophysical Journal International publishes top quality research papers, express letters, invited review papers and book reviews on all aspects of theoretical, computational, applied and observational geophysics.
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