Micro-seismic measurements of cliff motion under wave impact and implications for the development of near-horizontal shore platforms
Micro-seismic measurements of cliff motion under wave impact and implications for the development of near-horizontal shore platforms
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波浪冲击下悬崖运动的微震测量及其对近水平海岸平台开发的影响
DOI：
10.1016/j.geomorph.2012.01.006
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发表时间：
2012
期刊：
影响因子：
3.9
通讯作者：
Rachael M. Pentney
中科院分区：
文献类型：
--
作者：
M. Dickson;Rachael M. Pentney
在岩石海岸上，很少有高分辨率的过程-形式相互作用测量，但最近在加州的研究表明，便携式地震仪可以有效地替代测量波能传递到悬崖。在这里，我们描述了20多天的高频率地面运动的测量悬崖形成的沉积（复理石）岩石在Okakari点，北部的奥克兰，新西兰。三个传感器位于悬崖顶部内陆的海岸正常阵列中，第四个传感器用螺栓固定在悬崖底部上方2米的窗台上。利用5个测波站组成的岸法阵对悬崖和海岸平台前的近岸海岸波浪场进行了监测。这些仪器提供了波能传递和随之而来的地面运动的测量结果，包括对悬崖顶部和底部运动的首次观测。结果表明：崖顶水平地震动占主导地位，崖底垂直地震动占主导地位。功率谱显示，几个高频峰值出现在悬崖底部的数据中，而一个单一的，更广泛的峰值频率出现在悬崖顶部，这是由于地震波通过几十米的悬崖岩石时信号发生了变化。悬崖底部100米宽的海岸平台从根本上控制着观测到的能量输送模式。海岸平台几乎是水平的，在高水位附近升高，在其向海边缘突然陷入水深> 10 m的水中。正如预期的那样，在较大的波浪期间，所有传感器的地面运动幅度最大。测量进一步表明，悬崖底部和顶部的地面运动在低潮时最强，在高潮时最弱。这一观测结果与在圣克鲁斯观测到的相反，在圣克鲁斯，地面运动在高潮时最大。在Okakari点，最显著的高频地面运动发生在低潮时，当波浪被迫破碎（有时是剧烈的）在海岸平台的向海边缘。在1 ~ 50 Hz之间的4个频率峰值随潮汐水位下降而增大，表明波浪在海岸平台外缘破碎是一个重要的振动源。本研究未提供对能量源（例如短时间冲击压力）和岩石共振的详细了解。然而，量化能量输送的空间和时间模式非常强调海岸平台几何形状在过滤传递到悬崖的波能中的重要作用。在为期20天的实验中，大部分波能被传递到海岸平台的外边缘，而不是悬崖趾。波浪冲击产生的高频震动的地貌作用仍有待明确证明，但如果波浪冲击能够侵蚀岩石，则本研究的数据意味着，在目前条件下，海岸平台的外缘可能比悬崖坡脚遭受更高的侵蚀率。海岸平台目前可能正在被摧毁，而不是被建立，但需要一个更长的测量方案来检验这一概念。
Few high-resolution measurements of process-form interactions have been taken on rock coasts, but recent studies in California have shown that portable seismometers enable useful proxy measurements of wave-energy delivery to cliffs. Here we describe measurements over 20days of high frequency ground motion of cliffs formed in sedimentary (flysch) rocks at Okakari Point, north of Auckland, New Zealand. Three sensors were located in a shore-normal array inland from the cliff top and a fourth sensor was bolted to a ledge 2m above the cliff toe. The nearshore wave field in front of the cliff and shore platform was monitored using a shore-normal array of 5 wave gauges. The instrumentation provided measurements of wave-energy delivery and consequent ground motion, including the first observations of motion at the top and bottom of cliffs. Results showed that horizontal ground motion is dominant at the cliff top, whereas vertical motion is dominant at the cliff toe. Power spectra show that several high frequency peaks occur in data from the cliff toe, whereas a single, broader peak frequency occurs at the cliff top resulting from signal modification as seismic waves pass through tens of metres of cliff rock. A 100m wide shore platform at the cliff toe fundamentally controls the patterns of observed energy delivery. The shore platform is nearly horizontal, elevated close to high water level, and abruptly plunges into water >10m deep at its seaward edge. As expected, the magnitude of ground motion at all sensors is greatest during larger waves. Measurements further show that ground motion, both at the bottom and top of the cliff, is strongest at low tide and weakest at high tide. This observation is opposite to that noted at Santa Cruz, where ground motion was greatest at high tide. At Okakari Point the most significant high frequency ground motions occur at low tide when waves are forced to break (sometimes violently) against the seaward edge of the shore platform. Four distinctive frequency peaks between 1 and 50Hz increase in magnitude as tidal stage drops, implying that wave breaking against the outside edge of the shore platform represents an important source of vibration. A detailed understanding of the energy source (e.g. short duration shock pressures) and rock resonance is not provided by this study. However, quantifying the spatial and temporal patterns of energy delivery places strong emphasis on the important role of shore platform geometry in filtering wave-energy delivery to the cliff. During the 20-day experiment most wave energy was delivered to the outside edge of the shore platform, not the cliff toe. The geomorphic role of high-frequency shaking from wave impacts remains to be clearly demonstrated, but if wave impacts are capable of eroding rock then the data from this study imply that under present conditions the outside edge of the shore platform may be subject to higher erosion rates than the cliff toe. It is possible that the shore platform is currently being destroyed rather than created, but a longer programme of measurements is required to test this notion.