Oceanic crust-seismic structure, lithology and the cause of the 2A Event at borehole 504B
Oceanic crust-seismic structure, lithology and the cause of the 2A Event at borehole 504B
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洋壳地震结构、岩性及504B钻孔2A事件的成因
DOI：
10.1093/gji/ggae029
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发表时间：
2024
影响因子：
2.8
通讯作者：
Peirce C
中科院分区：
文献类型：
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作者：
Peirce C
该研究重点研究了504B钻孔周围~ 7 myr -old海洋地壳的三维速度结构和厚度，该钻孔位于距离哥斯达黎加裂谷（巴拿马盆地）中蔓延的~ 235 km处。它研究了三维断层扫描确定的地震结构与实际岩性的对比，从而研究了在多通道地震数据中观察到的振幅异常2A事件的起源和原因。我们的p波模式显示了一个厚约0.3 km的沉积层，其速度介于~ 1.6和1.9 km s - 1之间（梯度为1.0 s - 1），在其底部被一个速度阶梯束缚在海洋地壳第2层顶部的4.8 km s - 1。根据4.5 km深度的垂直速度梯度变化，第2层本身被细分为两个主要单元（2A和2B），其中上面（4.8-5.8 km s−1）的梯度为1.7 s−1，下面（5.8-6.5 km s−1）的梯度为0.7 s−1。而第2层的基底则由5.6 km深度的梯度变化来定义。在此之下，第三层的速度范围为6.5-7.5 km s−1，梯度为~ 0.3 s−1。对应的波火成岩层速度和梯度为：2A层、2.4 ~ 3.1 km s−1和1.0 s−1；2B层，3.1-3.7 km s−1和0.5 s−1；第3层，3.7-4.0 km s−1和0.1 s−1。三维层析模型，加上重力模型，表明整个地区的地壳厚度约为6公里，总体上是平坦的莫霍。虽然纵波和横波模型是光滑的，但它们的速度和梯度与504B内记录的主要岩性分层划分非常一致。因此，使用速度梯度的变化作为代理，第2层被解释为~ 1.8 km厚，第3层被解释为~ 3.8 km厚，在整个3- d体积中垂直变化很小。然而，横向梯度变化的走向并非与哥斯达黎加裂谷平行，而是遵循当今相邻的厄瓜多尔裂谷的方向，表明哥斯达黎加裂谷扩展脊轴的重新定向。在确定其与岩性实相一致后，将得到的纵波模型作为波传播有限差分计算的基础，寻找2A事件的震源。我们的建模显示，生成此事件不需要明显的接口或转换。相反，它是由地震波在第2层传播和散射时对非均匀物理性质进行平均引起的。因此，我们得出结论，2A事件的起源和传播完全在2A层的下部，高于平均深度到2B层堤顶。从我们的合成数据中，我们得出结论，使用地震反射剖面上的2A事件作为代理来确定2A/2B层边界的深度将导致高达数百米的高估，其程度取决于在叠加之前为正常移动校正选择的特定速度。
This study focuses on the 3-D velocity structure and thickness of ∼7-Myr-old oceanic crust surrounding borehole 504B, located ∼235 km from the intermediate-spreading Costa Rica Rift (Panama Basin). It investigates how well seismic structure determined by 3-D tomography compares with actual lithology and, consequently, what the origin and cause might be of an amplitude anomaly, the 2A Event, that is observed in multichannel seismic data. OurP-wave model shows an ∼0.3-km-thick sediment layer of velocity between ∼1.6 and 1.9 km s−1(gradient 1.0 s−1), bound at its base by a velocity step to 4.8 km s−1at the top of oceanic crustal Layer 2. Layer 2 itself is subdivided into two main units (2A and 2B) by a vertical velocity gradient change at 4.5 km depth, with a gradient of 1.7 s−1above (4.8–5.8 km s−1) and 0.7 s−1below (5.8–6.5 km s−1). The base of Layer 2, in turn, is defined by a change in gradient at 5.6 km depth. Below this, Layer 3 has a velocity range of 6.5–7.5 km s−1and a gradient of ∼0.3 s−1. CorrespondingS-wave igneous layer velocities and gradients are: Layer 2A, 2.4–3.1 km s−1and 1.0 s−1; Layer 2B, 3.1–3.7 km s−1and 0.5 s−1; Layer 3, 3.7–4.0 km s−1and 0.1 s−1. The 3-D tomographic models, coupled with gravity modelling, indicate that the crust is ∼6 km thick throughout the region, with a generally flat-lying Moho. Although theP- andS-wave models are smooth, their velocities and gradients are remarkably consistent with the main lithological layering subdivisions logged within 504B. Thus, using the change in velocity gradient as a proxy, Layer 2 is interpreted as ∼1.8 km thick and Layer 3 as ∼3.8 km thick, with little vertical variation throughout the 3-D volume. However, the strike of lateral gradient variation is not Costa Rica Rift-parallel, but instead follows the orientation of the present-day adjacent Ecuador Rift, suggesting a reorientation of the Costa Rica Rift spreading ridge axis. Having determined its consistency with lithological ground-truth, the resultingP-wave model is used as the basis of finite difference calculation of wave propagation to find the origin of the 2A Event. Our modelling shows that no distinct interface, or transition, is required to generate this event. Instead, it is caused by averaging of heterogeneous physical properties by the seismic wave as it propagates through Layer 2 and is scattered. Thus, we conclude that the 2A Event originates and propagates exclusively in the lower part of Layer 2A, above the mean depth to the top of the dykes of Layer 2B. From our synthetic data we conclude that using the 2A Event on seismic reflection profiles as a proxy to determine the Layer 2A/2B boundary's depth will result in an overestimate of up to several hundred metres, the degree of which being dependent on the specific velocity chosen for normal moveout correction prior to stacking.