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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.
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.
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.
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.
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.
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.
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.
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.
Since 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.
This article is based on a paper being presented at the State of Australian Cities Conference in Adelaide, November 28-30.
The green infrastructure of our cities includes both publicly owned, designed and delineated areas and less formal, unplanned areas of vegetation — informal green spaces. These spaces account for a large proportion of urban green areas. However, they are often among the most overlooked and neglected urban spaces, which contributes to negative perceptions, a recent study has found.
Yet informal green spaces represent a largely untapped opportunity to improve liveability and residents’ health and social well-being. Especially in lower socioeconomic areas that lack formal green spaces, improving the condition of informal green spaces can promote their use and enhance neighbourhood liveability.
Further reading: Our cities need more green spaces for rest and play — here’s how
We can’t afford to waste green space
For residents, the benefits include physical, mental and social health and wellbeing. The multiple environmental benefits include ecosystem services, improving microclimate and reducing air pollution, alongside biodiversity conservation.
Owing to such benefits, governments invest a lot in greening projects or improving green spaces. Sometimes these interventions include informal green spaces to increase their accessibility, use and potential benefits to residents.
Upper Stony Creek, an urban waterway restoration in Melbourne’s west, is a good example. Work will soon transform the concrete drainage channel, now separated from the residential area, into an accessible urban wetland and park.
Further reading: How Melbourne’s west was greened
Residents’ perceptions and uses
Clean Air and Urban Landscapes (CAUL) Hub researchers from RMIT University investigated residents’ perceptions and uses of Upper Stony Creek and the adjacent informal green space before the start of the intervention.
Interviews with residents showed overall impressions of the site were negative. An overwhelming majority of them commented on the site’s undesirable features.
Lack of regular maintenance, lack of access, feeling unsafe and litter were among their main concerns. Safety concerns included natural hazards, such as the presence of snakes (encouraged by a lack of regular maintenance), crime and local drug trade. These concerns affected when and how often residents used the site.
The negative perceptions suggested residents were looking forward to the intervention. They believed it would improve the informal green space and their neighbourhood.
In spite of their misgivings, residents found value in using the area for practices typically found in formal green spaces such as dog-walking. They also used it for less typical practices such as motorbike riding. The lack of restrictions in these spaces allows for uses that might not be acceptable in more formalised urban spaces.
In fact, residents appreciated the sense of exploration, informality and feelings of being away from urbanisation that the site provided.
Informal green spaces are filling a niche not met by more formal green spaces. This means interventions to transform informal green spaces should, where possible, take into account residents’ current uses of these areas.
Further reading: More than just drains: recreating living streams through the suburbs
Ensuring work improves these spaces
Our findings highlight the importance of considering and understanding residents’ perceptions and concerns about informal green spaces for informing work on these spaces.
Our case study suggests small interventions, which aim to resolve the main concerns such as lack of maintenance and safe access, can increase the use of informal green spaces without resorting to entirely formalising the space. In fact, understanding residents’ needs and expectations could result in more cost-effective interventions that won’t jeopardise the informal character of such areas.
Each informal green space will be unique in its features and characteristics, as will residents’ perceptions of it. Therefore, understanding these sites and residents’ lived experiences and concerns more completely through in-depth consultation will be important to ensure interventions meet community needs and expectations.
A sound knowledge of how informal green spaces are used, or of why they are not being used, can inform planners and decision-makers when intervening in such spaces to increase the liveability of urban neighbourhoods.
The authors welcome collaboration with local councils and other organisations to help understand residents’ uses and perceptions of informal green spaces when undertaking improvements or waterway restorations.
The year isn’t over yet, but we can already be sure that 2017 will be among the hottest years on record for the globe. While the global average surface temperature won’t match what we saw in 2016, it is now very likely that it will be one of the three warmest years on record, according to a statement issued by the World Meteorological Organization.
What is more remarkable is that this year’s warmth comes without a boost from El Niño. When an El Niño brings warm waters to the tropical east Pacific, we see a transfer of heat from the ocean to the lower atmosphere, which can raise the global average temperatures recorded at the surface by an extra 0.1-0.2℃. But this year’s temperatures have been high even in the absence of this phenomenon.
We can already say with confidence that 2017 will end up being the warmest non-El Niño year on record, and that it will be warmer than any year before 2015. The average global temperature between January to September this year was roughly 1.1℃ warmer than the pre-industrial average.
This trend is associated with increased greenhouse gas concentrations, and this year we have seen record high global atmospheric carbon dioxide concentrations and the biggest recorded surge in CO₂ levels.
A year of extremes
Of course, none of us experiences the global average temperature, so we also care about local extreme weather. This year has already seen plenty of extremes.
At the poles we’ve seen a continuation of the global trend towards reduced sea ice extent. On February 13, global sea ice extent reached its lowest point on record, amid a record low winter for Arctic ice. Since then the Arctic sea ice extent has become less unusual but it still remains well below the satellite-era average. Antarctic sea ice extent also remains low but is no longer at record low levels as it was in February and March of this year.
East Africa saw continued drought with failure of the long rains, coupled with political instability, leading to food insecurity and population displacement, particularly in Somalia.
Storms and fires
This year also saw a very active North Atlantic hurricane season. Parts of the southern United States and the Caribbean were struck by major hurricanes such as Harvey, Irma, and Maria, and are still recovering from the effects.
Other parts of the globe have seen a quieter year for tropical cyclones.
There have also been several notable wildfire outbreaks around the world this year. In Western Europe, record June heat and very dry conditions gave rise to severe fires in Portugal. This was followed by more severe fires across Spain and Portugal in October.
Parts of California also experienced severe fires following a wet winter, which promoted plant growth, and then a hot dry summer.
Australia is now gearing up for what is forecast to be a worse-than-average fire season after record winter daytime temperatures. A potential La Niña forming in the Pacific and recent rains in eastern Australia may reduce some of the bushfire risk.
The overall message
So what conclusions can we draw from this year’s extreme weather? It’s certainly clear that humans are warming the climate and increasing the chances of some of the extreme weather we’ve seem in 2017. In particular, many of this year’s heatwaves and hot spells have already been linked to human-caused climate change.
For other events the human influence is harder to determine. For example, the human fingerprint on East Africa’s drought is uncertain. It is also hard to say exactly how climate change is influencing tropical cyclones, beyond the fact that their impact is likely to be made worse by rising sea levels.
For much of 2017’s extreme weather, however, we can say that it is an indicator of what’s to come.