The science of landslides, and why they’re so devastating in PNG



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A magnitude 7.5 earthquake took place on February 25, 81km southwest of Porgera, Papua New Guinea.
US Geological Survey

Benjy Marks, University of Sydney

A magnitude 7.5 earthquake struck the Southern Highlands region of Papua New Guinea on February 25, 2018. This was followed by a series of aftershocks, producing widespread landslides that have killed dozens and injured hundreds. The same landslides have cut off roads, telecommunications and power to the area.

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The PNG government has declared a state of emergency in the region. There is growing concern over several valleys that have been dammed by landslides and are beginning to fill with water – now ready to collapse and surge downstream, directly towards more villages.

Why is Papua New Guinea so susceptible to landslides? It’s a combination of factors – steep terrain, earthquakes and aftershocks plus recent seasonal rains have created an environment that is prone to collapse.




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How land becomes unstable

The Earth around us is generally pretty stable, but when the ground shakes during an earthquake it can start to move in ways we don’t expect.

Pressure changes during an earthquake create an effect in the soil called liquefaction, where the soil itself acts as a fluid.

When wet soil is exposed to physical pressure, other physical changes take place.

When lots of water is present in the soil, as is the case now during the monsoon season in Papua New Guinea, liquefaction can happen even more easily.

When liquefaction occurs, the earthquake creates changes due to friction. Imagine a visit to the greengrocer, where an accidental bumping of a carefully stacked pile of apples can cause cause them all to suddenly collapse. What was holding the pile together was friction between the individual apples – and when this disappears, so does the pile.

In an earthquake, two tectonic plates slip past one another deep underground, rubbing together and cracking the nearby rocks. The effects of this movement up at the surface can vary depending on the nature of the earthquake, but one feature is fairly common: small objects bounce around. The sand grains just below the surface do the same thing, but a bit less excitedly. A few metres down, grains could be bouncing around just enough to lose contact with each other, removing the friction, and becoming unstable.

A 2012 landslide in the southern highlands of Papua New Guinea.
dfataustralianaid/flickr, CC BY

Things are normally stable because they’re sitting on top of something else. When that support suddenly disappears, things tend to fall down – this is the classic dodgy folding chair problem experienced by many.

In engineering, we call this “failure” – and in the building industry it usually occurs immediately before the responsible engineer receives a call from a lawyer. Mechanically, this failure happens when the available friction isn’t enough to support the weight of the material above it.




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When soil acts like fluid

Once a slope fails, it starts to fall downhill. If it really slides, then we’re back to the same situation of grains bouncing around. Now, none of the grains are resting against each other, and the whole thing is acting like a fluid.

A couple of interesting things happen at this point. First, as the grains are bouncing around, small particles start to fall through all the newly formed holes that have opened up. This occurs for the same reason that you find all the crumbs at the bottom of your cereal box, and all of the unpopped kernels at the bottom of your bowl of popcorn. Once these smaller fragments accumulate at the bottom of the flowing landslide, they can help it slide more easily, accelerating everything and increasing its destructive power.

Second, landslides typically flow faster at the surface than below, so as large particles accumulate at the top they are also the ones moving the fastest, and they start to collect at the front of the landslide. These large particles, often boulders and trees, can be incredibly damaging for any people or structures in their path.

Simulation of a landslide impacting a structure.
Benjy Marks/USyd

The image above shows a laboratory simulation of a landslide flowing down a slope and hitting a fixed wall. The spherical particles are coloured by size (small is yellow; large is blue). Data from these sorts of studies can help predict the forces that an object will feel if it gets hit by a landslide.

Watching and waiting

These complex dynamics mean that we really need to know a lot about the geography and geology of a particular slope before any kind of reliable prediction could be made about the behaviour of a particular landslide.

In the remote areas of Papua New Guinea, accumulating this data at every point on every slope is a tough challenge. Luckily, huge advancements have recently been made in remote sensing, so that planes and satellites can be used to extract this vital information.

Using sophisticated sensors, they can see past foliage and map the ground surface in high resolution. As satellites orbit quite regularly, small changes in the surface topography can be monitored. Scientists hope that by using this information, unstable regions that haven’t yet failed can be identified and monitored.




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Papua New Guinea is located on an active fault line and has had nine major earthquakes in the past five years. Combined with the often remote and steep terrain, together with a monsoon season that delivers repeated heavy rainfall events, it is a particularly active area for landslides to develop.

The ConversationThe dry season in Papua New Guinea will not arrive until June. During the current wet season we may see even more slopes fail due to destabilisation by the recent earthquakes.

Benjy Marks, Lecturer in Geomechanics, University of Sydney

This article was originally published on The Conversation. Read the original article.

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I’ve always wondered: do nuclear tests affect tectonic plates and cause earthquakes or volcanic eruptions?


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A detection station for seismic activity at Bilibion, a remote corner of Russia.
The Official CTBTO Photostream (Copyright CTBTO Preparatory Commission) , CC BY

Jane Cunneen, Curtin University

This is an article from I’ve Always Wondered, a new series where readers send in questions they’d like an expert to answer. Send your question to alwayswondered@theconversation.edu.au


Do underground nuclear tests affect Earth’s tectonic plates, and cause earthquakes or volcanic eruptions? – Anne Carroll, Victoria

Apart from escalating global fears about conflict, North Korea’s recent nuclear tests have raised questions about geological events caused by underground explosions.

Some media reports suggest the tests triggered earthquakes in South Korea. Others report the explosions may trigger a volcanic eruption at Paektu Mountain, about 100km from the test site.

So can an underground test cause an earthquake? The short answer is yes: a nuclear explosion can cause small earthquakes. But it is unlikely to affect the earth’s tectonic plates or cause a volcanic eruption.

Although a nuclear explosion releases a lot of energy in the immediate region, the amount of energy is small compared to other stresses on tectonic plates.


Read more: What earthquake science can tell us about North Korea’s nuclear test


What are tectonic plates?

Tectonic plates are slabs of the earth’s crust which move very slowly over the surface of the earth. Mountain ranges form at the edges of the plates when they collide, and ocean basins form when they move apart.

Tectonic plates are slabs of the earth’s crust.
Designua/shutterstock

Volcanoes occur mostly where plates are colliding. One plate overrides another, pushing it down to where it may partly melt. The partially melted rock – also known as lava – then rises to the surface, causing a volcano.

The movement of tectonic plates also causes earthquakes, which is why 90% of them occur at the plate boundaries. All but the deepest earthquakes occur along faults, which are breaks in the crust where rocks can move past each other in response to stress. This stress can be from both natural events and human activities.

Human induced earthquakes

Induced seismicity” is the term used to describe earthquakes caused by human activities.

Human induced earthquakes can be caused by anything that changes the stresses on rocks beneath the surface. These include processes that add or remove great loads from the surface, such as mining, building dams or tall buildings.

Other processes that change the amount of pressure on rocks can include fluid injection from drilling, or extraction of water from aquifers.

Human-induced earthquakes have been reported from every continent except Antarctica. Induced earthquakes only occur where there is already some stress on the rocks. The human activity adds enough stress to the rocks to reach the “tipping point” and trigger the earthquake.

Nuclear explosions can induce small earthquakes along existing faults near a test site. Some underground nuclear tests have fractured the ground surface above the explosions, causing movement on faults adjacent to explosion sites.

Earthquakes from nuclear testing

The 3 September 2017 North Korean nuclear test generated shock waves equivalent to a magnitude 6.3 earthquake. Eight minutes later, a magnitude 4.1 event was detected at the same site. This may have been linked to a collapse of a tunnel related to the blast.

Several small earthquakes measured since the event may have been induced by the nuclear test, but the largest is only a magnitude 3.6. An earthquake of this size would not be felt outside of the immediate area.


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The largest induced earthquake ever measured from nuclear testing was a magnitude 4.9 in the Soviet Union. An earthquake of this size can cause damage locally but does not affect the full thickness of the earth’s crust. This means it would not have any effect on the movement of tectonic plates.

Historical data from nuclear testing (mostly in the USA) shows that earthquakes associated with nuclear testing typically occur when the explosion itself measures greater than magnitude 5, 10–70 days after the tests, at depths of less than 5km, and closer than around 15km to the explosion site. More recent studies have concluded that nuclear tests are unlikely to induce earthquakes more than about 50km from the test site.

Volcanic eruptions

Concerns have also been raised about the risk of volcanic eruptions induced by the nuclear tests in North Korea. Paektu Mountain is about 100km from the test site and last erupted in 1903.

Mount Paektu is an active volcano on the border between North Korea and China.
Google Maps

In the 1970s, the USA conducted a number of nuclear tests in the Aleutian Islands, a volcanic island arc chain containing 62 active volcanoes.

One of the blasts, named Cannikin, was the largest underground nuclear test ever conducted by the USA. There were fears that the blast would cause a huge earthquake and tsunami. The blast did result in some induced earthquakes, but the largest was a magnitude 4.0 and there was no increase in volcanic activity.

Based on this evidence, it seems unlikely a nuclear test by North Korea will trigger an eruption of Paektu Mountain. If the volcano was on the verge of erupting, then an induced earthquake from a nuclear blast could influence the timing of the eruption. However, given the distance from the test site then even this is not likely.

Monitoring nuclear tests

The Comprehensive Nuclear Test Ban Treaty Organisation (CTBTO) has a global monitoring system to detect nuclear tests, including seismometers to measure the shock waves from the blast and other technologies.

Global network of seismic monitoring stations.
CTBTO / The Conversation, CC BY-NC-ND

Seismologists can analyse the seismic data to determine if the shock waves were from a naturally occurring earthquake or a nuclear blast. Shock waves from nuclear blasts have different properties to those from naturally occurring earthquakes.

Testing was much more common before the CTBTO was formed: between 1945 and 1996 more than 2,000 nuclear tests were conducted worldwide, including 1,032 by the USA and 715 by the Soviet Union.

The ConversationSince 1996 only three countries have tested nuclear devices: India, Pakistan and North Korea. North Korea has conducted six underground nuclear tests at the same site between 2006 and 2017.

Jane Cunneen, Research Fellow, Curtin University

This article was originally published on The Conversation. Read the original article.

Man-Made Earthquakes Becoming Common


Random Thoughts

I guess it was only a matter if time before our meddling with the earth via fracking became a major problem, or perhaps better put, a bigger problem. Man-made earthquakes are now a reality, but this article suggests they have been around a lot longer than fracking.

For more visit:
http://www.geek.com/science/man-made-earthquakes-are-becoming-a-real-problem-1576464/

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