Velocity structure in the Izu-Bonin seismic zone and the depth of the olivine-spinel phase transition in the slab
Velocity structure in the Izu-Bonin seismic zone and the depth of the olivine-spinel phase transition in the slab
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伊豆-小笠宁地震带速度结构及板片中橄榄石-尖晶石相变深度
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发表时间：
1985
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通讯作者：
S. Roecker
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作者：
S. Roecker
在这项研究中，提出了一种在地震相对重新定位的同时确定板片原位弹性波速度的方法。该方法是一种结合了现实模型的扩展到达时间差（ATD）方案。为了利用模型的真实性，同时又不完全牺牲 ATD 技术固有的简单性，我们对板内和周围的射线路径的特性做出了一些假设。通过将这些近似光线与使用三维光线追踪例程计算的更精确光线进行比较来估计由这些近似光线引入的误差。一般来说，比较表明这些误差通常远小于 0.1 秒。反演方法在同一组假设数据上进行测试。这些测试的结果表明，对到达时间差影响最大的参数是板片内慢度异常的幅度以及离开主事件的射线的方位角和起飞角。到达时间差对描述板块的其他参数（走向、倾角、半宽度和主事件的位置）相对不敏感，这意味着该方案对描述板块内慢度变化的解析函数的特定选择相当不敏感。测试中逆例程收敛速度快；通常需要不到五次迭代才能获得稳定的解决方案。看来板参数和射线方向的解之间的耦合是微不足道的。与标准 ATD 方法不同，震源和速度的解决方案在此方案中并不直接耦合，尽管需要位置良好的事件来确保震源不会因结构变化而过度偏差。一旦确定了板块的速度结构，该方案就可以常规地使用该信息来重新定位记录较少的事件。该方法适用于伊豆-小笠南俯冲带 200 至 600 公里深度的地震到达时间数据。 200-300公里深度范围内的结构无法解析，并且500-600公里深度范围内的结果被证明对噪声过于敏感。来自 27°N 至 31°N 之间事件的数据表明，400 至 500 km 深度的板片内的 P 波速度比周围地幔的 P 波速度高 4-6%。在 300 至 400 公里深度之间，板状结构因地区而异。在北纬 32.5°以北，板片内 180 至 375 公里深度的 P 波速度比周围环境高 3-4%。相比之下，北纬32.5°以南的速度从375-410公里深度范围内比环境高出6-7%增加到325-375公里深度范围内比环境高出8-10%，并且在270-325公里深度范围内似乎比环境高出9%。事实证明，该区域 325 至 375 km 深度之间高速的确定与数据的选择和起始模型的选择无关。然而，在 325 公里深度以上，速度的确定在一定程度上取决于起始模型的选择。由于这种依赖性，无法解决高速深度的上限。对于 32.5°N 以南板片内 P 波速度异常高的一个合理解释是，板片的这一部分中橄榄石-尖晶石相变边界升高，而 32.5°N 以北则没有显着升高。目前尚不清楚相变是否在北纬 31°以南升高，但该区域 400 至 500 公里深度之间的速度表明相变并未降低。北纬 32.5°以南的高速带位于标志着地震间隙开始的区域，该区域的宽度向南增加，介于深部和浅部地震活动之间。在该区域的南部，板片的倾角也逐渐变得陡峭。相比之下，北纬 32.5°以北的较低速度发生在地震活动相当连续且沿浅倾平面排列的区域。如果北纬 32.5° 以南的地震间隙代表高速区域，那么南部板片的垂直倾角增加可能是由于延伸到周围橄榄石-尖晶石相边界上方的多余质量提供的巨大重力所致。向北裂缝宽度逐渐减小表明伊豆-小笠原地震带的演化受到来自南部的不稳定传播的控制。
In this study a method is presented for determining the in situ elastic wave velocities of slabs simultaneously with the relative relocations of earthquakes. The method is an extended arrival time difference (ATD) scheme that incorporates a realistic model. Several assumptions are made about the characteristics of ray paths in and around the slab in order to take advantage of the realism of the model without completely sacrificing the simplicity inherent in the ATD technique. The errors introduced by these approximate rays are estimated by comparing them with more exact rays calculated with a three-dimensional ray-tracing routine. In general, the comparisons suggest that these errors are usually much less than 0.1 s. The inverse method is tested on the same set of hypothetical data. The results of these tests show that the parameters that influence the arrival time differences the most are the amplitude of the slowness anomaly within the slab and the azimuths and takeoff angles for rays leaving the master event. The arrival time differences are relatively insensitive to the other parameters describing the slab (strike, dip, half width, and the position of the master event), which implies that the scheme is fairly insensitive to the particular choice of analytic function to describe the slowness variation within the slab. Convergence of the inverse routine is rapid in the tests; usually less than five iterations are required before a stable solution is reached. It appears that coupling between solutions for slab parameters and ray directions is insignificant. Unlike the standard ATD approach, solutions for hypocenters and velocities are not directly coupled in this scheme, although well-located events are required to insure that the hypocenters are not unduly biased by variations in structure. Once the velocity structure of the slab is determined, that information can be used routinely by this scheme to relocate less well-recorded events. This method was applied to arrival time data from earthquakes located between 200 and 600 km depth in the Izu-Bonin subduction zone. Structure in the 200–300 km depth range could not be resolved, and results in the 500–600 km depth range proved to be too sensitive to noise. Data from events between 27°N and 31°N show that P wave velocities within the slab from 400 to 500 km depth are 4–6% higher than those of the ambient mantle. Between 300 and 400 km depth the slab structure varies regionally. North of about 32.5°N the P wave velocity within the slab is 3–4% higher than ambient from 180 to 375 km depth. In contrast, the velocities south of 32.5°N increase from 6–7% higher than ambient in the 375–410 km depth range to 8–10% higher in the 325–375 km depth range and appear to be 9% higher than ambient in the 270–325 km depth range. The determination of the high velocities in this region between 325 and 375 km depth proved to be independent of the selection of data and of the choice of starting model. Above 325 km depth, however, the velocity determinations are somewhat dependent on the choice of starting model. As a result of this dependence, no upper bound on the depth of the high velocities could be resolved. A plausible explanation for the exceptionally high P wave velocities within the slab south of 32.5°N is that the olivine-spinel phase change boundary is elevated in this part of the slab, while north of 32.5°N it is not significantly elevated. It is not known if the phase change is elevated south of 31°N, but the velocities between 400 and 500 km depth in this region imply that it is not depressed. The high-velocity zone south of 32.5°N lies in an area that marks the beginning of a seismic gap, increasing in breadth to the south, between deep and shallow seismicity. The dip of the slab also becomes gradually steeper to the south of this region. In contrast, the lower velocities north of 32.5°N occur in a region where the seismicity is fairly continuous and is aligned along a shallow dipping plane. If the seismic gap south of 32.5°N represents a region of high velocity, then the increasingly vertical dip of the slab in the south could be due to a substantial gravitational force provided by the excess mass extending above the ambient olivine-spinel phase boundary. The decreasing width of the gap to the north suggests that the evolution of the Izu-Bonin seismic zone has been governed by a propagating instability from the south.