{"id":7212,"date":"2013-11-06T08:30:52","date_gmt":"2013-11-06T08:30:52","guid":{"rendered":"http:\/\/blogs.agu.org\/landslideblog\/?p=7212"},"modified":"2013-11-06T08:30:52","modified_gmt":"2013-11-06T08:30:52","slug":"landslide-trigger","status":"publish","type":"post","link":"https:\/\/blogs.agu.org\/landslideblog\/2013\/11\/06\/landslide-trigger\/","title":{"rendered":"Our strange desire to find a landslide trigger"},"content":{"rendered":"<p>When I was studying Physics at school, we were shown the famous regelation experiment in which a wire is placed over a block of ice and a weight is attached to each end.\u00a0 Though time the pressure on the wire causes melting of the ice, and the wire slowly cuts its way through the block, and eventually the weights and wire fall to the ground.\u00a0 As the wire passes through, the water refreezes, such that the wire appears to pass magically through the ice.<\/p>\n<p>There are various versions of this experiment on Youtube, of which this is about the best:<\/p>\n<p><iframe loading=\"lazy\" title=\"Ice Cutting Experiment\" width=\"500\" height=\"281\" src=\"https:\/\/www.youtube.com\/embed\/qQCVnjGUv24?feature=oembed\" frameborder=\"0\" allow=\"accelerometer; autoplay; clipboard-write; encrypted-media; gyroscope; picture-in-picture; web-share\" referrerpolicy=\"strict-origin-when-cross-origin\" allowfullscreen><\/iframe><\/p>\n<p>..<\/p>\n<p>The interesting part of this experiment starts at about 1:22 and ends at 1:40 (in the speeded up sequence).\u00a0 Of course when the wire finally cuts through the ice the weights collapse to the floor with a great crash &#8211; the very last moments before this are shown below:<\/p>\n<p><a href=\"https:\/\/blogs.agu.org\/landslideblog\/files\/2013\/11\/13_11-ice-wire-1.png\"><img loading=\"lazy\" decoding=\"async\" class=\"aligncenter size-full wp-image-7213\" alt=\"landslide trigger\" src=\"https:\/\/blogs.agu.org\/landslideblog\/files\/2013\/11\/13_11-ice-wire-1.png\" width=\"396\" height=\"349\" srcset=\"https:\/\/blogs.agu.org\/landslideblog\/files\/2013\/11\/13_11-ice-wire-1.png 396w, https:\/\/blogs.agu.org\/landslideblog\/files\/2013\/11\/13_11-ice-wire-1-300x264.png 300w\" sizes=\"auto, (max-width: 396px) 100vw, 396px\" \/><\/a><\/p>\n<p>..<\/p>\n<p>This experiment, though fun, illustrates an important aspect of first time failures in rock slopes.\u00a0 If you weren&#8217;t watching the experiment closely then the collapse looks spontaneous &#8211; I remember when we were shown the experiment one of my class rates immediately asked &#8220;What caused that?&#8221;.\u00a0 However, the collapse had no trigger &#8211; it was a spontaneous event caused by a progressive process.\u00a0 Now, if towards the end of the experiment that same classmate had leant on one of the weights then he would have triggered the collapse.\u00a0 However, he didn&#8217;t cause the failure, which was generated by the wire cutting its way through the block.<\/p>\n<p><a href=\"https:\/\/blogs.agu.org\/landslideblog\/2013\/11\/04\/tumbi-quarry-landslide-paper\/\">The Tumbi Quarry landslide<\/a> brings this into sharp focus.\u00a0 Very often in the aftermath of large slope failures we spend a great deal of time looking for a landslide trigger.\u00a0 We analyse rainfall records, look at seismicity, examine temperature data and investigate the actions of people.\u00a0 Sometimes we even look at a combination of all these things.\u00a0 And, lo and behold, eventually we find the event, or combination of events, that caused the collapse to occur at that specific moment in time.\u00a0 Occasionally, we can&#8217;t even find a landslide trigger, as was the case in the <a href=\"http:\/\/www.teara.govt.nz\/en\/photograph\/8785\/aorakimt-cook-landslide\">Mount Cook landslide in 1991<\/a>.<\/p>\n<p>The point is not that triggers are unimportant &#8211; of course they are, especially in slopes in weak materials, and it is sensible to try to understand the relationship between the number of landslides and the magnitude of large triggering events in any landslide-prone environment.\u00a0 However, for individual first time failures in rock slopes the search for a landslide trigger is often a red herring.\u00a0 The reality is that the slope has weakened through time until it is on a hair-trigger; when a large enough event comes along the collapse occurs.\u00a0 If no event occurs then the slope collapses spontaneously; this is a true progressive failure.<\/p>\n<p>So, spending time looking for a trigger for the Tumbi Quarry landslide is interesting and worthwhile, and I have no problem with this sort of work.\u00a0 However, we must first recognise that the basic question should be whether there is a landslide trigger at all, and if so what?\u00a0 It may well be the case that no landslide trigger can be identified.\u00a0 Second, we should not allow the search for a landslide trigger to distract us from the real issue &#8211; i.e. what were the processes that led to the rock mass becoming unstable, such that the trigger became effective?<\/p>\n<p>To me the key issues at Tumbi Quarry remain unanswered, and the multiple victims of the landslide mean that they deserve more attention.<\/p>\n<!-- AddThis Advanced Settings generic via filter on the_content --><!-- AddThis Share Buttons generic via filter on the_content -->","protected":false},"excerpt":{"rendered":"<p>The Tumbi Quarry landslide in Papua New Guinea raises the issue of the importance, or otherwise, of landslide triggers in rock slopes<!-- AddThis Advanced Settings generic via filter on wp_trim_excerpt --><!-- AddThis Share Buttons generic via filter on wp_trim_excerpt --><\/p>\n","protected":false},"author":22,"featured_media":7213,"comment_status":"open","ping_status":"closed","sticky":false,"template":"","format":"standard","meta":{"footnotes":""},"categories":[1],"tags":[],"class_list":["post-7212","post","type-post","status-publish","format-standard","has-post-thumbnail","hentry","category-uncategorized"],"_links":{"self":[{"href":"https:\/\/blogs.agu.org\/landslideblog\/wp-json\/wp\/v2\/posts\/7212","targetHints":{"allow":["GET"]}}],"collection":[{"href":"https:\/\/blogs.agu.org\/landslideblog\/wp-json\/wp\/v2\/posts"}],"about":[{"href":"https:\/\/blogs.agu.org\/landslideblog\/wp-json\/wp\/v2\/types\/post"}],"author":[{"embeddable":true,"href":"https:\/\/blogs.agu.org\/landslideblog\/wp-json\/wp\/v2\/users\/22"}],"replies":[{"embeddable":true,"href":"https:\/\/blogs.agu.org\/landslideblog\/wp-json\/wp\/v2\/comments?post=7212"}],"version-history":[{"count":0,"href":"https:\/\/blogs.agu.org\/landslideblog\/wp-json\/wp\/v2\/posts\/7212\/revisions"}],"wp:featuredmedia":[{"embeddable":true,"href":"https:\/\/blogs.agu.org\/landslideblog\/wp-json\/wp\/v2\/media\/7213"}],"wp:attachment":[{"href":"https:\/\/blogs.agu.org\/landslideblog\/wp-json\/wp\/v2\/media?parent=7212"}],"wp:term":[{"taxonomy":"category","embeddable":true,"href":"https:\/\/blogs.agu.org\/landslideblog\/wp-json\/wp\/v2\/categories?post=7212"},{"taxonomy":"post_tag","embeddable":true,"href":"https:\/\/blogs.agu.org\/landslideblog\/wp-json\/wp\/v2\/tags?post=7212"}],"curies":[{"name":"wp","href":"https:\/\/api.w.org\/{rel}","templated":true}]}}