Strong Upper‐Plate Heterogeneity at the Hikurangi Subduction Margin (North Island, New Zealand) Imaged by Adjoint Tomography
Strong Upper‐Plate Heterogeneity at the Hikurangi Subduction Margin (North Island, New Zealand) Imaged by Adjoint Tomography
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DOI：
10.1029/2021jb022865
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发表时间：
2022-01
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通讯作者：
B. Chow;Y. Kaneko;C. Tape;R. Modrak;N. Mortimer;Stephen Bannister;John Townend
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作者：
B. Chow;Y. Kaneko;C. Tape;R. Modrak;N. Mortimer;Stephen Bannister;John Townend
我们使用基于地震的伴随层析成像来反演新西兰北岛和邻近的Hikurangi俯冲带的三维结构。该研究区域对北岛下方的板块界面具有较浅的深度，为利用陆基测量对活动俯冲带的物质性质进行成像提供了难得的机会。我们从射线层析成像得到的初始模型出发，利用谱元素和伴随模拟进行迭代模型修正，以拟合周期从4-30 S的波形。我们使用L-BFGS优化算法进行了28次模型修正，改善了数据拟合，引入了高达±30%的P波和S波速度变化。利用点扩展函数进行的分辨率分析表明，我们的测量对30公里以上的非均质性最为敏感。最显著的速度变化与活动的Hikurangi俯冲带有关的区域重合。5公里以上的横向速度结构与新西兰地质有很好的相关性。反转揭示了边缘沿走向非均质性的增强。在库克海峡，我们观察到一个解释为深部沉积盆地的低速带。在北岛中部，低速异常与地表地质有关，我们将深部的速度结构与陶波̄火山带下方的地壳岩浆活动联系起来。我们的速度模型相对于初始模型提供了更准确的合成地震记录，更好地约束了小(<；$<；$50公里)、浅(<；$<；$15公里)和近海速度结构，并提高了我们对与活动Hikurangi俯冲带相关的火山和构造结构的理解。
We use earthquake‐based adjoint tomography to invert for three‐dimensional structure of the North Island, New Zealand, and the adjacent Hikurangi subduction zone. The study area, having a shallow depth to the plate interface below the North Island, offers a rare opportunity for imaging material properties at an active subduction zone using land‐based measurements. Starting from an initial model derived using ray tomography, we perform iterative model updates using spectral element and adjoint simulations to fit waveforms with periods ranging from 4–30 s. We perform 28 model updates using an L‐BFGS optimization algorithm, improving data fit and introducing P‐ and S‐wave velocity changes of up to ±30%. Resolution analysis using point spread functions show that our measurements are most sensitive to heterogeneities in the upper 30 km. The most striking velocity changes coincide with areas related to the active Hikurangi subduction zone. Lateral velocity structures in the upper 5 km correlate well with New Zealand geology. The inversion reveals increased along‐strike heterogeneity on the margin. In Cook Strait we observe a low‐velocity zone interpreted as deep sedimentary basins. In the central North Island, low‐velocity anomalies are linked to surface geology, and we relate velocity structures at depth to crustal magmatic activity below the Taupō Volcanic Zone. Our velocity model provides more accurate synthetic seismograms with respect to the initial model, better constrains small ( < $< $ 50 km), shallow ( < $< $ 15 km) and near‐offshore velocity structures, and improves our understanding of volcanic and tectonic structures related to the active Hikurangi subduction zone.