Lessons for a destabilising planet: insights from the 2009 South Pacific earthquake-tsunami disaster



The South Pacific was rocked by two nearly simultaneous earthquakes and a devastating tsunami.
AAP Image/Tamara McLean

Dale Dominey-Howes, University of Sydney

Just before 7am on September 29, 2009, a magnitude 8 earthquake struck the sea floor in the central South Pacific, about 190kms south of Samoa. It was exactly the sort of earthquake – in fact, it was two almost simultaneous quakes – that create devastating tsunami.

The Earth’s crust tore apart, triggering a region-wide tsunami. Within minutes it inundated Samoa’s coastline, before rolling on to American Samoa and Tonga.




Read more:
Explainer: after an earthquake, how does a tsunami happen?


While a tsunami warning was issued by the Pacific Tsunami Warning Center and relayed by Samoan officials, it was not rebroadcast everywhere. Regardless, the tsunami arrived too quickly for many to escape. In Samoa, 189 people died when the tsunami reached up to 14 metres above normal sea level and many more were injured across the region. Hundreds of millions of dollars of damage was done.

Ten years on from this tragedy, it’s time to look back at the lessons we learned – and how they can help us adapt to a rapidly changing climate, which is making similar natural disasters more and more likely.

Location of earthquake shown by yellow star in South Pacific south of Samoa (Source: USGS)

A new approach to post-disaster research

As is common after a major disaster, Samoa’s government and emergency services quickly began assessing the damage and the needs of affected communities, to direct relief and recovery efforts. The government did an excellent job given the logistical challenges that face small island developing states after such events.

It is also common for researchers from a wide range of disciplines, from earth sciences to engineering to health studies, to visit impacted areas to study the causes and effects of such disasters, and make suggestions to improve future disaster management planning and practice.

Such researcher-led field reconnaissance surveys are usually small, comprising just a few individual researchers and usually from the same discipline. These expeditions are quickly organised, and the researchers get in and out fast – too often focused on their own interests, and not working with the government or scientists of the affected country. This means serious ethical issues can arise.




Read more:
Making waves: the tsunami risk in Australia


After this earthquake and tsunami, I thought we could do better.

I proposed bringing incoming researchers from multiple disciplines, to collaborate with local communities and Samoan researchers and officials. This proposal was accepted, and I was quickly appointed to head up what became the largest ever international post-tsunami survey team.

Our team ended up comprising nearly 200 local and international participants working in multidisciplinary teams for up to a month between October and November 2009.

Critically, for the first time, the team negotiated between incoming scientists with specific questions, and local scientists and the government to ensure their research really did benefit Samoan communities.

As team leader, I reported daily to the prime minister and King of Samoa updating them on research findings, gave local TV and radio interviews on how things were going and worked with researchers to nudge field activity in directions that benefited everyone.

At the end of the survey the team provided a report to the prime minister and government on our findings. This set a new global benchmark for how post-disaster surveys could be done.

A boy looks through the wreckage of a Samoan village after the 2009 tsunami.
Paul Miller/AAP

The lessons we learned

As a consequence of this approach, I was appointed by the United Nations to co-lead a global working group that rewrote the handbook on how post-disaster surveys should be organised and operated.

We learned so much from the Samoan survey team, and from continuing research efforts over the last ten years following other major disasters such as the 2011 Japan earthquake-tsunami and 2013 Philippines Typhoon Haiyan disasters. Key lessons include that:

  • geological studies show us these large hazard events occur much more frequently than we realised before

  • natural ecosystems on which humans depend exhibit both great vulnerability and resilience to the forces of nature, but human management of those ecosystems really affects resilience

  • different types of buildings experience damage and destruction in different ways. This knowledge can be used in land use zoning and improving building codes and design standards

  • despite continuing public education campaigns about natural hazards and disasters, individuals, families and communities still don’t always do what emergency management agencies want them to do (for example, evacuate to high ground if you feel a strong earthquake at the coast)

  • human beings are the most remarkable of species – capable of incredible resilience and generosity in the aftermath of disasters

  • there is still so much we do not understand about natural hazards and disasters

  • as a global community, we must work hard to reduce inequality which makes too many people vulnerable to disasters, and rise to the challenge presented by human-induced climate change.

Future disasters on a rapidly changing planet

Disasters are on the rise and climate change will only make things much worse. Earth is destabilising rapidly.

Climate change (coupled with environmental degradation and an increasing population which increases exposure to hazard events) are driving more frequent and intense disasters.




Read more:
Explainer: are natural disasters on the rise?


In fact, according to the United Nations Office for Disaster Risk Reduction, weather and climate related disasters have more than doubled over the last 40 years. They have said,

90% of recorded major disasters caused by natural hazards from 1995 to 2015 were linked to climate and weather including floods, storms, heatwaves and drought.

Number of people affected by disasters of different types between 1998 and 2017.
UNISDR

A heart wrenching factor is the poorest people around the world always bear the greatest burden of loss to natural disasters due to inequality and poverty. Layered on top of the particular vulnerability of poorer people to disasters, global statistics show the Asian region experiences the most disasters of all types.

Occurrence of different types of disasters by regions. Cylinders show the percentage of each particular disaster in a given region in relation to the whole world.
Alcantara-Ayala, 2002

Earth is unique, dynamic, fragile and dangerous. Human activity is driving changes that, if not addressed soon, will result in disasters in the near future that are outside our experience and capacity to cope with.The Conversation

Dale Dominey-Howes, Professor of Hazards and Disaster Risk Sciences, University of Sydney

This article is republished from The Conversation under a Creative Commons license. Read the original article.

A ‘seiche’ wave can outpace a tsunami, and both can be triggered by meteorites and earthquakes



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Waves can be generated in lakes and other bodies of water when seismic energy travels through land.
Leo Roomets / Unsplash, CC BY

Craig O’Neill, Macquarie University

A catastrophic event occurred on Earth 66 million years ago. A huge meteorite struck our planet in what is now Mexico, triggering mass extinctions of the dinosaurs and most other living creatures.

A new paper shows the first recorded victims of this impact were fish and other marine animals, stranded by a wave that left them high and dry in an ancient river in North Dakota, at a site called Tanis.

For scientists unpacking the evidence around the event, a full picture of the cataclysm has involved looking into the details of planetary surface physics during giant impacts.

But beyond the first layer of fascinating results – little glass impact beads stuck in the gills of fish, for example – one really interesting aspect of this work is around how water behaves when it’s exposed to extreme forces.

If you’ve never heard of a form of wave called a seiche, this is your chance to catch up.

This is a seiche – a standing wave – in a swimming pool, during a large earthquake in Nepal.

Waves of damage

The Chicxulub meteorite crater in coastal Mexico is strongly associated with the mass extinction of the dinosaurs (and 75% of all species), 66 million years ago.

The first victims were right at the site. Any marine creatures close to the point of impact would have been instantly vaporised (sadly leaving no fossil record), along with much of the surrounding rock.

Around the periphery, the energy of the impact melted and ejected tonnes of molten rock, which together with condensing rock vapour, formed little glass beads (“impact spherules”) that can be found in a layer around the world at this time.

The shock wave itself pulverised the adjacent rock enough to metamorphise it, forming features like “shocked quartz” – fractured quartz indicative of enormous pressures. It carried the energy equivalent of a magnitude 11 earthquake – 1,000 times more energy than the 2004 Boxing Day quake which killed almost 230,000 people.

Vast inland sea now gone

North Dakota is more than 3,000km away from the Chicxulub crater, and was a similar distance at the time of the meteorite impact event.

Separating them back then, however, was a vast inland sea that covered much of midwest USA, from Texas up to the Dakotas. Feeding into that inland sea was a river system upon which the Tanis site in North Dakota was formed. This site has preserved the earliest recorded deaths of the Chicxulub impact.

Different views of the Tanis site. A: Tanis (starred) within a regional context (large map) and on a national map (inset). B: Photo and interpretive overlay of an oblique cross-section through Tanis. C: Simplified schematic depicting the general deposits at the site (not to scale). Most fish carcasses were found at point 3.
Robert A DePalma and colleagues

The site itself is unusual. The deposition of sediments can tell us about the flow of water in the river.

Most ripples (or flame structures) indicate a southerly flow of the river before and after the Tanis deposit. However, these flow indicators point the wrong way during the time the Tanis unit formed. Water was flowing upstream, fast.

At the site are also found the fossilised remains of species, like sharks and rays, that occupied brackish water, rather than the freshwater of the stream. These had to be brought inland from the sea by something, and left to die, smothered in sediment, on a riverbank.

Stranded in Dakota

The obvious candidate is an impact tsunami. Perhaps the impact of the meteorite hitting the ocean generated a huge wave that carried fish from the inland sea, and against the flow of fresh water, to leave the creatures stranded in Dakota?

But there are problems with this hypothesis. The tiny impact spherules that formed in Chicxulub can be found throughout the deposit (many clogging the gills of fish), and pockmarks in the sedimentary layers means rocks were still raining down. This means the surge of water occurred within around 15 minutes to two hours of the impact itself.

For a tsunami to travel the 3,000km from the point of impact, to the Tanis site across the inland sea, would have taken almost 18 hours. Something else killed these creatures.

The seismic waves from the impact would have travelled through the Earth much faster than a tsunami travelled across water – and arrived near Tanis between 6-13 minutes later. The authors of the Tanis study suggest these seismic waves may have triggered an unusual type of wave in the inland sea, called a seiche.

Standing waves

Seiches are standing waves in bodies of water, and are often found in large lake systems during strong winds. The winds themselves cause waves and water displacement, which can have a harmonic effect, causing the water to slosh side to side like an overfull bathtub.

However, earthquakes are also known to cause seiches. Particularly dramatic seiches are often seen in swimming pools during large quakes. The interaction of the seismic wave’s period (the time between two waves) with the timescale of waves sloshing in a pool can amplify their effect.

But seiches can affect larger bodies of water too.

During the 2011 Tohuku earthquake in Japan, seiches over 1m high were observed in Norwegian fjords more than 8,000km away. With an energy more than 1,000 times greater, the Chicxulub event could quite conceivably have generated bigger than 10 metre swells in the North American inland sea – the scale implied by the deposition of the Tanis site.

These waves in Norwegian fjords were created by seismic waves from the 2011 Tohoku earthquake in Japan.

Given a seiche can be driven by seismic waves, it’s conceivable that one drove the surge that stranded marine creatures at Tanis, resulting in the short time between the impact debris and the surge deposit.

Still lots of questions

But a lot remains unclear regarding exactly what did happen 66 million years ago.

Could the fish stranding have been driven by the first seismic activity to appear at Tanis (the P and S waves in science parlance, which travel through the interior of the Earth, arriving at Tanis 6 and 10 minutes after impact, respectively), or the more destructive but slower surface waves at the top of the Earth’s crust, which arrived 13 minutes after impact?

How might seiche waves have interacted with global hurricane-strength wind storms caused by the impact?

Would the period of sloshing of a seiche be consistent with the scale of the inland sea? (The inland sea was much larger than most lakes seiches are traditionally observed in – and may or may not have been open to the ocean). Given so little is really known about the dimensions of the inland sea, this is hard to constrain.

The Tanis site has given us an incredible window into the first few hours of a mass-extinction. But it has also highlighted how little we have probed into the fatal surface physics of these extreme events.The Conversation

Craig O’Neill, Director of the Macquarie Planetary Research Centre/Associate Professor in Geodynamics, Macquarie University

This article is republished from The Conversation under a Creative Commons license. Read the original article.

Satellite measurements of slow ground movements may provide a better tool for earthquake forecasting



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The 2016 Kaikoura earthquake shattered the surface and twisted railway lines.
Simon Lamb, CC BY-ND

Simon Lamb, Victoria University of Wellington

It was a few minutes past midnight on 14 November 2016, and I was drifting into sleep in Wellington, New Zealand, when a sudden jolt began rocking the bed violently back and forth. I knew immediately this was a big one. In fact, I had just experienced the magnitude 7.8 Kaikoura earthquake.

Our research, published today, shows how the slow build-up to this earthquake, recorded by satellite GPS measurements, predicted what it would be like. This could potentially provide a better tool for earthquake forecasting.




Read more:
New Zealand’s Alpine Fault reveals extreme underground heat and fluid pressure


Shattering the landscape

The day after the quake, I heard there had been huge surface breaks in a region extending for more than 170 km along the eastern part of the northern South Island. In some places, the ground had shifted by 10 metres, resulting in a complex pattern of fault ruptures.

In effect, the region had been shattered, much like a fractured sheet of glass. The last time anything like this had happened was more than 150 years ago, in 1855.

Quite independently, I had been analysing another extraordinary feature of New Zealand. Over the past century or so, land surveyors had revealed that the landscape is moving all the time, slowly changing shape.

These movements are no more than a few centimetres each year – but they build with time, relentlessly driven by the same forces that move the Earth’s tectonic plates. Like any stiff material subjected to excessive stress, the landscape will eventually break, triggering an earthquake.

I was studying measurements made with state-of-the-art global positioning system (GPS) techniques – and they recorded in great detail the build-up to the 2016 Kaikoura earthquake over the previous two decades.

A mobile crust

GPS measurements for regions at the edges of the tectonic plates, such as New Zealand, have become widely available in the last 15 years or so. Here, the outer part of the Earth (the crust) is broken up by faults into numerous small blocks that are moving over geological time. But it is widely thought that even over periods as short as a few decades, the GPS measurements still record the motion of these blocks.

New Zealand straddles the boundary between the Australian and Pacific tectonic plates, with numerous active faults. Note the locked portion of the underlying megathrust.
Simon Lamb, CC BY

The idea is that at the surface, where the rocks are cold and strong, a fault only moves in sudden shifts during earthquakes, with long intervening periods of inactivity when it is effectively “locked”. During the locked phase, the rocks behave like a piece of elastic, slowly changing shape over a wide region without breaking.

But deeper down, where the rocks are much hotter, there is the possibility that the fault is slowly slipping all the time, gradually adding to the forces in the overlying rocks until the elastic part suddenly breaks. In this case, the GPS measurements could tell us something about how deep one has to go to reach this slipping region, and how fast it is moving.

From this, one could potentially estimate how frequently each fault is likely to rupture during an earthquake, and how big that rupture will be – in other words, the “when and what” of an earthquake. But to achieve this understanding, we would need to consider every major fault when analysing the GPS data.

Invisible faults

Current earthquake forecasting “reverse engineers” past distortions of the Earth’s surface by finding all the faults that could trigger an earthquake, working out their earthquake histories and projecting this pattern into the future in a computer model. But there are some big challenges.

The most obvious is that it is probably impossible to characterise every fault. They are too numerous and many are not visible at the surface. In fact, most historical earthquakes have occurred on faults that were not known before they ruptured.

Our analysis of the GPS measurements has revealed a more fundamental problem that at the same time opens new avenues for earthquake forecasting. Working with statistician Richard Arnold and geophysicist and modeller James Moore, we found the GPS measurements could be better explained if the numerous faults that might rupture in earthquakes were simply ignored. In other words, surface faults seemed to be invisible when looking at the slow movements recorded by GPS.

There was only one fault that mattered – the megathrust that runs under much of New Zealand. It separates the Australian and Pacific tectonic plates and only reaches the surface underwater, about 50 to 100km offshore. Prior to the Kaikoura earthquake, the megathrust was locked at depths shallower than about 30km. Here, the overlying Australian plate had been slowly changing shape like a single piece of elastic.

Slip at depth on the megathrust drives earthquakes in New Zealand, including the M7.8 Kaikoura Earthquake.
Simon Lamb, CC BY

The pacemaker for future quakes

In the conventional view, every big fault has its own inbuilt earthquake driver or pacemaker – the continuously slipping part of the fault deep in the crust. But our analysis suggests that these faults play no role in the driving mechanism of an earthquake, and the pacemaker is the underlying megathrust.

We think the 2016 Kaikoura earthquake provides the vital clue that we are right. The key observation is that numerous ruptures were involved, busting up the boundary between the two plates in a zone that ran more-or-less parallel to the line of locking on the underlying megathrust. This is exactly what we would anticipate if the slow build-up in stress was only driven by slip on the megathrust and not the deeper parts of individual crustal faults.

I remember once watching a documentary about the making of the Boeing 777 aircraft. The engineers were very confident about its design limits under flying conditions, but the Civil Aviation Authority wanted it tested to destruction. In one test, the vast wings were twisted so that their tips arced up to the sky at a weird angle. Suddenly, there was a bang and the wings snapped, greeted by loud cheering because this had occurred almost exactly when predicted. But the details of how this happened, such as where the cracks of metal fatigue twisted the metal, were something that only the experiment could show.

I think this is a good analogy for realistic goals with earthquake prediction. The Herculean task of identifying every fault and its past earthquake history may be of only limited use. In fact, it is becoming clear that earthquake ruptures on individual faults are far from regular. Big faults may never rupture in one go, but bit by bit together with many other faults.

But it might well be possible to forecast when there will be severe shaking in a region near you – surely something that is equally as valuable.The Conversation

Simon Lamb, Associate Professor in Geophysics, Victoria University of Wellington

This article is republished from The Conversation under a Creative Commons license. Read the original article.

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.




Read more:
Five active volcanoes on my Asia Pacific ‘Ring of Fire’ watch-list right now


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.




Read more:
Explainer: after an earthquake, how does a tsunami happen?


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.




Read more:
Controversial artist Elizabeth Durack gave us a sensitive insight into the lives of Papuan women


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.

California’s other drought: A major earthquake is overdue


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Fires break out across San Francisco after the April 18, 1906 earthquake.
USGS

Richard Aster, Colorado State University

California earthquakes are a geologic inevitability. The state straddles the North American and Pacific tectonic plates and is crisscrossed by the San Andreas and other active fault systems. The magnitude 7.9 earthquake that struck off Alaska’s Kodiak Island on Jan. 23, 2018 was just the latest reminder of major seismic activity along the Pacific Rim.

Tragic quakes that occurred in 2017 near the Iran-Iraq border and in central Mexico, with magnitudes of 7.3 and 7.1, respectively, are well within the range of earthquake sizes that have a high likelihood of occurring in highly populated parts of California during the next few decades.

The earthquake situation in California is actually more dire than people who aren’t seismologists like myself may realize. Although many Californians can recount experiencing an earthquake, most have never personally experienced a strong one. For major events, with magnitudes of 7 or greater, California is actually in an earthquake drought. Multiple segments of the expansive San Andreas Fault system are now sufficiently stressed to produce large and damaging events.

The good news is that earthquake readiness is part of the state’s culture, and earthquake science is advancing – including much improved simulations of large quake effects and development of an early warning system for the Pacific coast.

The last big one

California occupies a central place in the history of seismology. The April 18, 1906 San Francisco earthquake (magnitude 7.8) was pivotal to both earthquake hazard awareness and the development of earthquake science – including the fundamental insight that earthquakes arise from faults that abruptly rupture and slip. The San Andreas Fault slipped by as much as 20 feet (six meters) in this earthquake.

Although ground-shaking damage was severe in many places along the nearly 310-mile (500-kilometer) fault rupture, much of San Francisco was actually destroyed by the subsequent fire, due to the large number of ignition points and a breakdown in emergency services. That scenario continues to haunt earthquake response planners. Consider what might happen if a major earthquake were to strike Los Angeles during fire season.

Collapsed Santa Monica Freeway bridge across La Cienega Boulevard, Los Angeles after the
Northridge earthquake, Jan. 17, 1994.

Robert A. Eplett/FEMA

Seismic science

When a major earthquake occurs anywhere on the planet, modern global seismographic networks and rapid response protocols now enable scientists, emergency responders and the public to assess it quickly – typically, within tens of minutes or less – including location, magnitude, ground motion and estimated casualties and property losses. And by studying the buildup of stresses along mapped faults, past earthquake history, and other data and modeling, we can forecast likelihoods and magnitudes of earthquakes over long time periods in California and elsewhere.

However, the interplay of stresses and faults in the Earth is dauntingly chaotic. And even with continuing advances in basic research and ever-improving data, laboratory and theoretical studies, there are no known reliable and universal precursory phenomena to suggest that the time, location and size of individual large earthquakes can be predicted.

Major earthquakes thus typically occur with no immediate warning whatsoever, and mitigating risks requires sustained readiness and resource commitments. This can pose serious challenges, since cities and nations may thrive for many decades or longer without experiencing major earthquakes.

California’s earthquake drought

The 1906 San Francisco earthquake was the last quake greater than magnitude 7 to occur on the San Andreas Fault system. The inexorable motions of plate tectonics mean that every year, strands of the fault system accumulate stresses that correspond to a seismic slip of millimeters to centimeters. Eventually, these stresses will be released suddenly in earthquakes.

But the central-southern stretch of the San Andreas Fault has not slipped since 1857, and the southernmost segment may not have ruptured since 1680. The highly urbanized Hayward Fault in the East Bay region has not generated a major earthquake since 1868.

Reflecting this deficit, the Uniform California Earthquake Rupture Forecast estimates that there is a 93 percent probability of a 7.0 or larger earthquake occurring in the Golden State region by 2045, with the highest probabilities occurring along the San Andreas Fault system.

Perspective view of California’s major faults, showing forecast probabilities estimated by the third Uniform California Earthquake Rupture Forecast. The color bar shows the estimated percent likelihood of a magnitude 6.7 or larger earthquake during the next 30 years, as of 2014. Note that nearly the entire San Andreas Fault system is red on the likelihood scale due to the deficit of large earthquakes during and prior to the past century.
USGS

Can California do more?

California’s population has grown more than 20-fold since the 1906 earthquake and currently is close to 40 million. Many residents and all state emergency managers are widely engaged in earthquake readiness and planning. These preparations are among the most advanced in the world.

For the general public, preparations include participating in drills like the Great California Shakeout, held annually since 2008, and preparing for earthquakes and other natural hazards with home and car disaster kits and a family disaster plan.

No California earthquake since the 1933 Long Beach event (6.4) has killed more than 100 people. Quakes in 1971 (San Fernando, 6.7); 1989 (Loma Prieta; 6.9); 1994 (Northridge; 6.7); and 2014 (South Napa; 6.0) each caused more than US$1 billion in property damage, but fatalities in each event were, remarkably, dozens or less. Strong and proactive implementation of seismically informed building codes and other preparations and emergency planning in California saved scores of lives in these medium-sized earthquakes. Any of them could have been disastrous in less-prepared nations.

Remington Elementary School in Santa Ana takes part in the 2015 Great California Shakeout.

Nonetheless, California’s infrastructure, response planning and general preparedness will doubtlessly be tested when the inevitable and long-delayed “big ones” occur along the San Andreas Fault system. Ultimate damage and casualty levels are hard to project, and hinge on the severity of associated hazards such as landslides and fires.

Several nations and regions now have or are developing earthquake early warning systems, which use early detected ground motion near a quake’s origin to alert more distant populations before strong seismic shaking arrives. This permits rapid responses that can reduce infrastructure damage. Such systems provide warning times of up to tens of seconds in the most favorable circumstances, but the notice will likely be shorter than this for many California earthquakes.

Early warning systems are operational now in Japan, Taiwan, Mexico and Romania. Systems in California and the Pacific Northwest are presently under development with early versions in operation. Earthquake early warning is by no means a panacea for saving lives and property, but it represents a significant step toward improving earthquake safety and awareness along the West Coast.

The ConversationManaging earthquake risk requires a resilient system of social awareness, education and communications, coupled with effective short- and long-term responses and implemented within an optimally safe built environment. As California prepares for large earthquakes after a hiatus of more than a century, the clock is ticking.

Richard Aster, Professor of Geophysics, Colorado State University

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

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.


Read more: North Korea tests not just a bomb but the global nuclear monitoring system


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.
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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.