Dynamic characteristics of an active coastal spreading area using ambient noise measurements—Anchor Bay, Malta
Dynamic characteristics of an active coastal spreading area using ambient noise measurements—Anchor Bay, Malta
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使用环境噪声测量的活跃沿海扩散区域的动态特征——马耳他锚湾
DOI：
10.1093/gji/ggu318
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发表时间：
2014
影响因子：
2.8
通讯作者：
D. Farrugia
中科院分区：
文献类型：
--
作者：
P. Galea;S. D’Amico;D. Farrugia
S U M M A R Y锚湾及周边地区位于地中海中部马耳他岛的西北海岸。该地区的特点是沿海悬崖环境，硬珊瑚石灰岩（UCL）的露头层位于厚（高达50米）的粘土和泥灰岩（Blue Clay，BC）层上。这种结构引起海岸不稳定效应，特别是横向扩展现象和罗克瀑布。之前和正在进行的研究已经确定了每年几毫米的横向扩展率和垂直运动。该地区是一个有趣的天然实验室，因为可以确定并容易进入许多不同阶段的海岸剥离过程。我们调查现场的动态特性，本研究区通过记录环境噪声的时间序列在30多个点，超过0.07平方公里的面积，使用便携式三分量地震仪。时间序列进行处理，以给出水平到垂直的频谱比图（H/V）以及频率相关的偏振分析。H/V图说明并量化了由于底层地质以及部分或全部分离块体的机械共振而引起的场地共振效应。极化图显示了振动效应的线性程度和主要方向。靠近悬崖边缘的H/V曲线在较高的频率下显示出复杂的响应，这是单个分离块体的动态行为的特征。粒子运动与较高的频率显示出强烈的方向性极化和高度的线性在定义良好的频率，正常模式振动的指示。另一方面，稳定的平台区显示出简单的单峰H/V曲线，代表了底层分层，没有主要的极化方向。这些结果将与该地区其他实验的结果进行比较，对了解地质活跃和不稳定的沿海环境中正在进行的过程具有重要意义。
S U M M A R Y Anchor Bay and surrounding regions are located on the northwest coast of the island of Malta, Central Mediterranean. The area is characterized by a coastal cliff environment having an outcropping layer of hard coralline limestone (UCL) resting on a thick (up to 50 m) layer of clays and marls (Blue Clay, BC). This configuration gives rise to coastal instability effects, in particular lateral spreading phenomena and rock falls. Previous and ongoing studies have identified both lateral spreading rates and vertical motions of several millimetres per year. The area is an interesting natural laboratory as coastal detachment processes in a number of different stages can be identified and are easily accessible. We investigate the site dynamic characteristics of this study area by recording ambient noise time-series at more than 30 points, over an area of 0.07 km2, using a portable three-component seismograph. The timeseries are processed to give both horizontal-to-vertical spectral ratio graphs (H/V) as well as frequency-dependent polarisation analysis. The H/V graphs illustrate and quantify aspects of site resonance effects due both to underlying geology as well as to mechanical resonance of partly or wholly detached blocks. The polarization diagrams indicate the degree of linearity and predominant directions of vibrational effects. H/V curves closer to the cliff edge show complex responses at higher frequencies, characteristic of the dynamic behaviour of individual detached blocks. Particle motion associated with the higher frequencies shows strongly directional polarization and a high degree of linearity at well-defined frequencies, indicative of normalmode vibration. The stable plateau areas, on the other hand, show simple, single-peak H/V curves representative of the underlying stratification and no predominant polarization direction. These results, which will be compared with those from other experiments in the area, have important implications for the understanding of ongoing processes in geologically active and unstable coastal environments.