Imaging of an incipient volcanic flank collapse by passive seismic methods: El Hierro, Canary Islands
Authors: Cauchie Léna , Cerfontaine Philippe , Mreyen Anne-Sophie , Havenith Hans-Balder 
Published: March 2021

&lt;p&gt;&lt;span&gt;The proposed &lt;/span&gt;&lt;span&gt;research&lt;/span&gt;&lt;span&gt; aims at the investigation of large mass movements on volcanic islands, &lt;/span&gt;&lt;span&gt;like&lt;/span&gt; &lt;span&gt;the &lt;/span&gt;&lt;span&gt;San Andres landslide &lt;/span&gt;&lt;span&gt;on&lt;/span&gt;&lt;span&gt; El Hierro island &lt;/span&gt;&lt;span&gt;(&lt;/span&gt;&lt;span&gt;Canary Islands, &lt;/span&gt;&lt;span&gt;Spain)&lt;/span&gt;&lt;span&gt;. These coastal and submarine landslides are extremely large (with run-out exceeding tens of k&lt;/span&gt;&lt;span&gt;m&lt;/span&gt;&lt;span&gt;) and voluminous (&lt;/span&gt;&lt;span&gt;up &lt;/span&gt;&lt;span&gt;to hundreds of km&lt;/span&gt;&lt;sup&gt;&lt;span&gt;3&lt;/span&gt;&lt;/sup&gt;&lt;span&gt;). &lt;/span&gt;&lt;span&gt;They &lt;/span&gt;&lt;span&gt;represent therefore a major geological hazard with direct consequences for the population of the islands. Volcanic activity and large earthquakes, as well as factors unrelated to the growth of the island &lt;/span&gt;&lt;span&gt;like&lt;/span&gt;&lt;span&gt; heavy precipitations and sea level change must be &lt;/span&gt;&lt;span&gt;considered&lt;/span&gt;&lt;span&gt; among the important triggering factors. Recent studies also evidenced that &lt;/span&gt;&lt;span&gt;these &lt;/span&gt;&lt;span&gt;large instabilities and failure mechanisms &lt;/span&gt;&lt;span&gt;are&lt;/span&gt;&lt;span&gt; linked to the geomechanical characteristics of the volcanic rocks, especially &lt;/span&gt;&lt;span&gt;the &lt;/span&gt;&lt;span&gt;formation of low strength and high deformability rocks.&lt;br&gt;&lt;/span&gt;San Andres landslide, formed between 176 and 545 ka, has been interpreted as the result of an aborted giant collapse and it represents one of the rarest sites where it is possible to investigate the landslide mass and fault planes of a volcanic collapse structure onshore. &lt;span&gt;While several studies have been performed for the surface characterization, there is still a lack of knowledge about the subsurface properties &lt;/span&gt;&lt;span&gt;of the &lt;/span&gt;&lt;span&gt;San Andres landslide&lt;/span&gt;&lt;span&gt;. &lt;/span&gt;&lt;span&gt;For&lt;/span&gt;&lt;span&gt; this purpose we &lt;/span&gt;&lt;span&gt;conducted a seismological survey &lt;/span&gt;&lt;span&gt;on &lt;/span&gt;&lt;span&gt;El Hierro island &lt;/span&gt;&lt;span&gt;in October 2020 aimed at the &lt;/span&gt;&lt;span&gt;characterization&lt;/span&gt;&lt;span&gt; of the internal properties of the &lt;/span&gt;&lt;span&gt;terrestrial&lt;/span&gt;&lt;span&gt; part of the landslide through seismological measurements. &lt;br&gt;&lt;/span&gt;&lt;span&gt;Three temporary seismic arrays and two seismic profiles were deployed in order to retrieve the elastic properties of the subsurface through the analysis of seismic ambient noise. We applied the f-k analysis and cross-correlation techniques to measure the dispersion of the surface waves, &lt;/span&gt;&lt;span&gt;the &lt;/span&gt;&lt;span&gt;features &lt;/span&gt;&lt;span&gt;of which &lt;/span&gt;&lt;span&gt;were successively inverted to retrieve 1D shear-wave velocity profiles. Furthermore we analysed &lt;/span&gt;&lt;span&gt;3D signals to investigate the site resonance frequencies and &lt;/span&gt;&lt;span&gt;thus&lt;/span&gt; &lt;span&gt;identified&lt;/span&gt;&lt;span&gt; impedance contrast&lt;/span&gt;&lt;span&gt;s&lt;/span&gt;&lt;span&gt; at depth. &lt;/span&gt;&lt;span&gt;We therefore &lt;/span&gt;&lt;span&gt;determin&lt;/span&gt;&lt;span&gt;e&lt;/span&gt;&lt;span&gt;d&lt;/span&gt;&lt;span&gt; the degree of &lt;/span&gt;&lt;span&gt;(&lt;/span&gt;&lt;span&gt;de&lt;/span&gt;&lt;span&gt;)-&lt;/span&gt;&lt;span&gt;consolidation of the sliding mass itself estimated &lt;/span&gt;&lt;span&gt;and&lt;/span&gt; &lt;span&gt;and then &lt;/span&gt;&lt;span&gt;compared &lt;/span&gt;&lt;span&gt;it &lt;/span&gt;&lt;span&gt;to the surrounding rocks of the volcanic island. &lt;/span&gt;&lt;span&gt;During the aforementioned campaign, w&lt;/span&gt;&lt;span&gt;e also performed UAV flights to establish a 3D model of the investigated site. &lt;/span&gt;&lt;span&gt;T&lt;/span&gt;&lt;span&gt;hese &lt;/span&gt;&lt;span&gt;recent investigations&lt;/span&gt;&lt;span&gt; contributed to &lt;/span&gt;&lt;span&gt;the construction of a 3D-geomodel &lt;/span&gt;&lt;span&gt;by including existing geological information&lt;/span&gt;&lt;span&gt;. &lt;br&gt;&lt;/span&gt;&lt;span&gt;In prospect, t&lt;/span&gt;&lt;span&gt;h&lt;/span&gt;&lt;span&gt;e estimation of&lt;/span&gt;&lt;span&gt; the &lt;/span&gt;&lt;span&gt;landslide geometr&lt;/span&gt;&lt;span&gt;y &lt;/span&gt;&lt;span&gt;will contribute&lt;/span&gt;&lt;span&gt; to &lt;/span&gt;&lt;span&gt;the evaluation of &lt;/span&gt;&lt;span&gt;the flank&lt;/span&gt;&lt;span&gt; stability as well as&lt;/span&gt; &lt;span&gt;to the assessment of &lt;/span&gt;&lt;span&gt;the risk&lt;/span&gt;&lt;span&gt;s&lt;/span&gt;&lt;span&gt; associated to &lt;/span&gt;&lt;span&gt;any&lt;/span&gt;&lt;span&gt; possible reactivation&lt;/span&gt;&lt;span&gt;. &lt;/span&gt;&lt;/p&gt;&lt;p&gt;&amp;#160;&lt;/p&gt;