Documentation and monitoring of active structures at El Hierro: potential of flank collapse from the present seismo-volcanic crisis and eruption

经过四个月的低震级地震活动和持续变形，加那利群岛的耶罗岛于2011年10月10日爆发。自那时以来，地震活动的震级一直在缓慢增加，自11月1日以来发生了超过34次Ml>3的地震（其中4次Ml> 4）。最大的一次Ml 4.6，强度（EMS）IV-V地震发生在2011年11月11日，整个耶罗岛以及邻近的特内里费岛和拉帕尔马群岛都有震感。目前的喷发范围也有所扩大，现在有几个喷口发生了活动，在离岸不远处大致呈南北走向。最不祥的是，这条直线平行于一个突出的年轻的锥状带，可能是向北延伸穿过该岛的裂谷，这一地区现在已经处于红色警戒状态。此外，这种排列与目前将该岛一分为二的地震活动模式以及明显受结构控制的悬崖海岸线部分平行。耶罗岛虽然是加那利群岛中最小的岛屿，但它只是一座非常大的火山的最高1500米，该火山延伸到海平面以下3公里多。其独特的扇形海岸线是至少四次大规模侧翼塌陷的结果，最近发生在1.5万年前，目前的变形和地震活动可能引发新的侧翼塌陷或重新激活其中一个旧的塌陷。负责监测地震活动的西班牙官方机构（IGN）也在2011年7月中旬安装了4个GPS站，以记录这些点之间的距离变化，而负责火山研究的主要当地组织（INVOLCAN）主要侧重于地球化学监测（二氧化碳和氡的排放），但有另外6个全球定位系统站，一些早在危机，测量绝对移动站，数据由日本名古屋大学人员处理。这些数据集虽然有价值，但仅提供来自分散点的信息，并不代表目前提出的最近和潜在未来整体变形的综合结构文件和分析。迫切需要进行这一结构分析，以便抓住科学机会，获取基线数据，并提供火山构造框架，将上述监测数据置于其中。通过这种方式，可以记录和模拟潜在的侧翼不稳定性的进展，这项工作对于理解这种广泛的和潜在的灾难性现象至关重要，但目前还不存在。2011年10月27日，因当前危机而成立的科学委员会成员举行了一次会议。PI参加的这次会议证实，没有任何小组正在调查该领域的活动裂缝和其他结构，强调了以下紧迫性：1）建立这一基线结构框架，2）制定监测网络和程序，记录该框架变形的模式和速率。因此，我们在本项目中提出了一项实地活动，以绘制和分析活火山-地震-该项目的目的是在岛上建立一个重力结构，并建立一个控制系统，监测这些结构的演变。这项工作将主要有助于建立模型和了解火山喷发期间火山体的总体变形以及不稳定地区的产生和演变。此外，这项工作有可能在当前的危机中做出有用的预测和警告。一旦初步实地活动结束，将由西班牙有关机构、INVOLCAN和热核实验堆的人员进行重复测量。最终目的是在后续多学科项目提案中与这些同事进一步合作。

After four months of low magnitude seismic activity and continuous deformation, the Island of El Hierro in the Canarian Achipelago erupted on 10 October 2011. Since that date, seismic activity has been slowly increasing in magnitude, with more than 34 Ml>3 earthquakes (four of which have Ml> 4) occurring since 1 November. The largest, a Ml 4.6, intensity (EMS) IV-V earthquake occurred on the 11/11/11 and was felt across El Hierro as well as in the neighbouring Tenerife and La Palma Islands. The current eruption has also extended, with activity now taking place from several vents describing an approximate north-south alignment a short distance offshore. Most ominously this alignment parallels a prominent band of young cinder cones and possible rifting that extends northwards across the island and this area has now been placed on red alert status. Moreover, this alignment parallels the pattern of present seismicity that bisects the island, as well as apparently structurally controlled segments of the cliffed coastline. El Hierro, although the smallest island in the Canary group, represents just the uppermost 1500 metres of a very large volcano that extends more than three kilometres below sea level. Its distinctive scalloped coastline is the result of at least four large-scale flank collapses, occurring as recently as 15 thousand years ago, and the current deformation and seismic activity could trigger a new flank collapse or reactivate one of the older collapses. The official Spanish agency (IGN) responsible for monitoring the seismicity also installed 4 GPS stations in mid July 2011 to record variations in distance between these points, while thekey local group for volcanological research (INVOLCAN) focuses mainly on geochemical monitoring (CO2 and radon emissions) but has a further 6 GPS stations, some pre-dating the crisis, which measure absolute movement of the stations, with data processed by personnel from Nagoya University inJapan. These datasets, although valuable, only provide information from scattered points, and do not represent the integrated structural documentation and analysis of recent and potential future overall deformation presently proposed. This structural analysis needs to be carried out urgently, to seize the scientific opportunity to capture baseline data and to provide a volcano-tectonic framework in which to place the monitoring data outlined above. In this way the progression of potential flank instability can be documented and modelled, work that is crucial to understanding this widespread and potentially catastrophic phenomenon, yet presently does not exist. On 27 October 2011, a meeting took place of the members of the scientific committee set up as a result of the present crisis. This meeting, attended by the PI, confirmed that no group is investigating active fractures and other structures in the field, emphasising the urgency of, 1) establishing this baseline structural framework and, 2) enacting monitoring networks and procedures to record the pattern and rate at which this framework deforms.We therefore propose in this project a field campaign to map and analyse active volcano-seismo-gravitational structures on the island and to set up a control system that will monitor the evolution of these structures. The work will contribute principally to modelling and understanding the overall deformation of volcanic edifices during eruptions and the generation and evolution of areas of instability. Additionally, the work has the potential to make predictions and warnings useful in the current crisis. Once the initial field campaign is finished, repeat measurements will be performed by personnel from the relevant Spanish agencies, INVOLCAN and ITER. The ultimate aim is to further collaborate with these colleagues in a follow-up multi-disciplinary project proposal.