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Is it an earthquake, a nuclear test or a hurricane? How seismometers help us understand the world we live in

Is it an earthquake, a nuclear test or a hurricane? How seismometers help us understand the world we live in

Although traditionally used to study earthquakes, like today’s M 8.1 in Mexico,  seismometers have now become so sophisticated they are able to detect the slightest ground movements; whether they come from deep within the bowels of the planet or are triggered by events at the surface. But how, exactly, do earthquake scientists decipher the signals picked up by seismometers across the world? And more importantly, how do they know whether they are caused by an earthquake, nuclear test or a hurricane?  

To find out we asked Neil Wilkins (a PhD student at the University of Bristol) and Stephen Hicks (a seismologist at the University of Southampton) to share some insights with our readers.


Seismometers are highly sensitive and they are able to detect a magnitude 5 earthquake occurring on the other side of the planet. Also, most seismic monitoring stations have sensors located within a couple of meters of the ground surface, so they can be fairly susceptible to vibrations at the surface. Seismologists can “spy” on any noise source, from cows moving in a nearby field to passing trucks and trains.

A nuclear test

On Sunday the 3rd of September, North Korea issued a statement announcing it had successfully tested an underground hydrogen bomb. The blast was confirmed by seismometers across the globe. The U.S.  Geological Survey registered a 6.3 magnitude tremor, located at the Punggye-ri underground test site, in the northwest of the country. South Korea’s Meteorological Administration’s earthquake and volcano center also detected what is thought to be North Korea’s strongest test to date.

However they occur, explosions produce ground vibrations capable of being detected by seismic sensors. Mining and quarry blasts appear frequently at nearby seismic monitoring stations. In the case of nuclear explosions, the vibrations can be so large that the seismic waves they produce can be picked up all over the world, as in the case of this latest test.

It was realised quite early in the development of nuclear weapons that seismology could be used to detect such tests. In fact, the need to have reliable seismic data for monitoring underground nuclear explosions led in part to the development of the Worldwide Standardized Seismograph Network in the 1960s, the first of its kind.

Today, more than 150 seismic stations are operating as part of the International Monitoring System (IMS) to detect nuclear tests in breach of the Comprehensive Test-Ban Treaty (CTBT), which opened for signatures in 1996. The IMS also incorporates other technologies, including infrasound, hydroacoustics and radionuclide monitoring.

The key to determining whether a seismic signal is from an explosion or an earthquake lies in the nature of the waves that are present. There are three kinds of seismic wave seismologists can detect. The fastest, called Primary (P) waves, cause ground vibrations in the same direction that they travel, similar to sound waves in the air. Secondary (S) waves cause shaking in a perpendicular direction. Both P and S waves travel deep through the Earth and are known collectively as body waves. In contrast, the third type of seismic waves are known as surface waves, because they are trapped close to the surface of the Earth. In an earthquake, it is normally surface waves that cause the most ground shaking.

In an explosion, most of the seismic energy is released outwards as the explosive material rapidly expands. This means that the largest signal in the seismogram comes as P waves. Explosions therefore have a distinctive shape in the seismic data when compared with an earthquake, where we expect S and surface waves to have higher amplitude.

Forensic seismologists can therefore make measurements of the seismic data to determine whether there was an explosion. An extra indication that a nuclear test occurred can also be revealed by measuring the depth of the source of the waves, as it would not be possible to place a nuclear device deeper than around 10 km below the surface.

Yet while seismic data can tell us that there has been an explosion, there is nothing that can directly identify that explosion as being nuclear. Instead, the IMS relies on the detection of radioactive gases that can leak from the test site for final confirmation of what kind of bomb was used.

The figure shows (at the bottom) the seismic recording of the latest test in North Korea made at NORSAR’s station in Hedmark, Norway. The five upper traces show recordings at the same station for the five preceding tests, conducted by North Korea in 2006, 2009, 2013 and 2016 (two explosions in 2016). The 2017 test, is as can be seen from this figure, clearly the strongest so far. Credit: NORSAR.

When North Korea conducted a nuclear test in 2013, radioactive xenon was detected 55 days later, but this is not always possible. Any detection of such gases depends on whether or not a leak occurs in the first place, and how the gases are transported in the atmosphere.

Additionally, the seismic data cannot indicate the size of the nuclear device or whether it could be attached to a ballistic missile, as the North Korean government claims.

What seismology can give us is an idea of the size of the explosion by measuring the seismic magnitude. This is not straightforward, and depends on knowledge of exactly how deep the bomb was buried and the nature of the rock lying over the test site. However, by comparing the magnitude of this latest test with those from the previous five tests conducted in North Korea, we can see that this is a much larger explosion.

The Norwegian seismic observatory NORSAR has estimated a blast equivalent to 120 kilotons of TNT, six times larger than the atomic bomb dropped on Nagasaki in 1945, and consistent with the expected yield range of a hydrogen bomb.

Hurriquakes?

Nuclear tests are not the only hazard keeping our minds busy in the past few weeks. In the Atlantic, Hurricanes Harvey, Irma and Katia have wreaked havoc in the southern U.S.A, Mexico and the Caribbean.

Hurricanes in the Atlantic can occur at any time between June and November. According to hurricane experts, we are at the peak of the season. It is not uncommon for storms to form in rapid succession between August, September and October.

The National Hurricane Centre (NHC) is the de facto regional authority for producing hurricane forecasts and issuing alerts in the Atlantic and eastern Pacific. For their forecasts, meteorologists use a combination of on the ground weather sensors (e.g. wind, pressure, Doppler radar) and satellite data.

As hurricane Irma tore its way across the Atlantic, gaining strength and approaching the Caribbean island of Guadeloupe, local seismometers detected its signature, sending the global press into a frenzy. It may come as a slight surprise to some people that storms and hurricanes also show on seismometers.

However, a seismometer detecting an approaching hurricane is not actually that astonishing. There is no evidence to suggest that hurricanes directly cause earthquakes, so what signals can we detect from a hurricane? Rather than “signals”, seismologists tend to refer to this kind of seismic energy as “noise” as it thwarts our ability to see what we’re normally looking out for – earthquakes.

The seismic noise from a storm doesn’t look like distinct “pings” that we would see with an earthquake. What we see are fairly low-pitched “hums” that gradually get louder in the days and hours preceding the arrival of a storm. As the storm gets closer to the sensor, these hums turn into slightly higher-pitched “rustling”. This seismic energy then wanes as the hurricane drifts away. We saw this effect clearly for Hurricane Irma with recordings from a seismometer on the island of Guadeloupe.

What causes these hums and rustles? If you look at the frequency content of seismic data from any monitoring station around the globe, noise levels light up at frequencies of ~0.2 Hz (5 s period). We call these hums “microseism”. Microseism is caused by persistent seismic waves unrelated to earthquakes, and it occurs over huge areas of the planet.  One of the strongest sources of microseism is caused by ocean waves and swell. During a hurricane, swell increases and ocean waves become more energetic, eventually crashing into coastlines, transferring seismic energy into the ground. This effect is more obvious on islands as they are surrounded by water.

As the hurricane gets closer to the island, wind speeds dramatically increase and may dwarf the noise level of the longer period microseism. Wind rattles trees, telegraph poles, and the surface itself, transferring seismic energy into the ground and moving the sensitive mass inside the seismometer. This effect causes higher-pitched “rustles” as the centre of the storm approaches. Gusts of wind can also generate pressure changes inside the seismometer installation and within the seismometer itself, generating longer period fluctuations.

During Hurricane Irma, a seismic monitoring station located in the Dutch territory of St. Maarten clearly recorded the approach of the storm, leading to an intense crescendo as the eyewall crossed the area. As the centre of the eye passed over, the seismometer seems to have recorded a slightly lower noise level. This observation could be due to the calmer conditions and lower pressure within the eye. The station went down shortly after, probably from a power outage or loss in telemetry which provides the data in real-time.

Seismometers measuring storms is not a new observation. Recently, Hurricane Harvey shook up seismometers located in southern Texas. Even in the UK, the approach of winter storms across the Atlantic causes much higher levels of microseism.

It would be difficult to use seismometer recordings to help forecast a hurricane – the recordings really depend on how close the sensor is to the coast and how exposed the site is to wind. In the event of outside surface wind and pressure sensors being damaged by the storm, protected seismometers below the ground could possibly prove useful in delineating the rough location of the hurricane eye, assuming they maintain power and keep sending real-time data.

At least several seismic monitoring stations in the northern Antilles region were put out of action by the effects of the Hurricane. Given the total devastation on some islands, it is likely that it will take at least several months to bring these stations back online. The Lesser Antilles are a very tectonically active and complex part of Earth; bringing these sensors back into operation will be crucial to earthquake and volcano hazard monitoring in the region.

By Neil Wilkins (PhD student at the University of Bristol) and Steven Hicks (a seismologist at the University of Southampton)

References and further reading

GeoSciences Column: Can seismic signals help understand landslides and rockfalls?

NORSAR Press Release: Large nuclear test in North Korea on 3 September 2017

The Comprehensive Nuclear-Test-Ban Organization Press Release: CTBTO Executive Secretary Lassina Zerbo on the unusual seismic event detected in the Democratic People’s Republic of Korea

First Harvey, Then Irma and Jose. Why? It’s the Season (The New York Times)

NOAA  National Hurricane Center

IRIS education and outreach series: How does a seismometer work?

August GeoRoundUp: the best of the Earth sciences from around the web

August GeoRoundUp: the best of the Earth sciences from around the web

Drawing inspiration from popular stories on our social media channels, as well as unique and quirky research news, this monthly column aims to bring you the best of the Earth and planetary sciences from around the web.

Major Stories

On August 25th Hurricane Harvey made landfall along the southern coast of the U.S.A, bringing record breaking rainfall, widespread flooding and a natural disaster on a scale not seen in the country for a long time. In fact, it’s the first time since 2005 a major hurricane has threatened mainland U.S.A. – a record long period.

But Harvey’s story began long before it brought destruction to Texas and Louisiana.

On August 17th,the National Space Agency (NASA) satellite’s first spotted a tropical depression forming off the coast of the Lesser Antilles. From there the storm moved into the eastern Caribbean and was upgraded to Tropical Storm Harvey where it already started dropping very heavy rainfall. By August 21st, it had fragmented into disorganised thunderstorms and was spotted near Honduras, where heavy local rainfall and gusty winds were predicted.

Over the next few days the remnants of the storm travelled westwards towards Nicaragua, Honduras, Belize and the Yucatan Peninsula. Forecasters predicted that, owing to warm waters of the Gulf of Mexico and favorable vertical wind shear, there was a high chance the system could reform once it moved into the Bay of Campeche (in the southern area of the Gulf of Mexico) on August 23rd. By August 24th data acquired with NASA satellites showed Harvey had began to intensify and reorganise. Heavy rainfall was found in the system.

Harvey continued to strengthen as it traveled across the Gulf of Mexico and weather warnings were issued for the central coast of Texas. Citizens were told to expect life-threatening storm surges and freshwater flooding. On August 25th, Harvey was upgraded to a devastating Category 4 hurricane, when sustained wind speeds topped 215 kph.

Since making landfall on Friday and stalling over Texas (Louisiana is also affected) – despite being downgraded to a tropical storm as it weakened – it has broken records of it’s own. “No hurricane, typhoon, or tropical storm, in all of recorded history, has dropped as much water on a single major city as Hurricane Harvey is in the process of doing right now in Houston (Texas)”, reports Forbes. In fact, the National Weather Service had to update the colour charts on their graphics in order to effectively map it. This visualisation maps Harvey’s destructive path through Texas.

A snaptshot from the tweet by the official Twitter account for NOAA’s National Weather Service.

So far the death toll is reported to be between 15 to 23 people, with the Houston Police Chief saying 30,000 people are expected to need temporary shelter and 2,000 people in the city had to be rescued by emergency services (figures correct at time of writing).

Many factors contributed toward making Hurricane Harvey so destructive. “The steering currents that would normally lift it out of that region aren’t there,” J. Marshall Shepherd, director of the atmospheric sciences program at the University of Georgia, told the New York Times. The storm surge has blocked much of the drainage which would take rainfall away from inland areas. And while it isn’t possible to say climate change caused the hurricane, “it has contributed to making it worse”, says Michael E Mann. The director of the Earth System Science Center at Pennsylvania State University argues that rising sea levels and ocean water temperatures in the region (brought about by climate change) contributed to greater rainfall and flooding.

A man carries his cattle on his shoulder as he moves to safer ground at Topa village in Saptari. Credit: The Guardian.

While all eyes are on Houston, India, Bangladesh and Nepal are also suffering the consequences of devastating flooding brought about a strong monsoon. The United Nations estimates that 41 million people are affected by the disaster across the three countires. Over 1200 people are reported dead. Authorities are stuggling with the scale of the humanitarian crisis: “Their most urgent concern is to accessing safe water and sanitation facilities,” the UN Office for the Coordination of Humanitarian Affairs (OCHA) said earlier this week, citing national authorities. And its not only people at risk. Indian authorities reported large swathes of a famous wildlife reserve park have been destroyed. In Mumbai, the downpour caused a building to collapse killing 12 people and up to 25 more are feared trapped.Photo galleries give a sense of the scale of the disaster.

Districts affected by flooding. Credit: Guardian graphic | Source: ReliefWeb. Data as of 29 August 2017

What you might have missed

In fact, it’s highly unlikely you missed the coverage of this month’s total solar eclipse over much of Northern America. But on account of it being the second biggest story this month, we felt it couldn’t be left out of the round-up. We particularly like this photo gallery which boasts some spectacular images of the astronomical event.

This composite image, made from seven frames, shows the International Space Station, with a crew of six onboard, as it transits the Sun at roughly five miles per second during a partial solar eclipse, Monday, Aug. 21, 2017 near Banner, Wyoming. Credit: (NASA/Joel Kowsky)

Since the end of July, wildfires have been raging in southwest Greenland. While small scale fires are not unheard of on the island otherwise known for its thick ice cap and deep fjords, the fires this month are estimated to extend over 1,200 hectares. What started the fires remains unknown, as do the fuel sources and the long-term impacts of the burn.

The U.S.A’s National Oceanic and Atmospheric Administration highlighted that the fires are a source of sooty “black carbon”. As the ash falls on the pristine white ice sheet, it turns the surface black, which can make it melt faster. Greenland police recently reported that unexpected rain haf all but extinguished the massive fires; though the situation continues to be monitored, as smouldering patches run the risk of reigniting the flames.

 

 

 

Links we liked

The EGU story

Do you enjoy the EGU’s annual General Assembly but wish you could play a more active role in shaping the scientific programme? Now is your chance! Help shape the scientific programme of EGU 2018.

From today, until 8 Sep 2017, you can suggest:

  • Sessions (with conveners and description), or;
  • Modifications to the existing skeleton programme sessions
  • NEW! Suggestions for Short courses (SC) will also take place during this period
  • From now until 18 January 2018, propose Townhall and splinter meetings

And don’t forget! To stay abreast of all the EGU’s events and activities, from highlighting papers published in our open access journals to providing news relating to EGU’s scientific divisions and meetings, including the General Assembly, subscribe to receive our monthly newsletter.

 

GeoTalk: Meet the EGU’s President, Jonathan Bamber

GeoTalk: Meet the EGU’s President, Jonathan Bamber

GeoTalk interviews usually feature the work of early career researchers, but this month we deviate from the standard format to speak to Jonathan Bamber, the EGU’s President. Jonathan has a long-standing involvement with the Union, stretching back almost 20 years. Following a year as vice-president, Jonathan was appointed President at this year’s General Assembly in Vienna. Here we talk to him about his plans for the Union, how scientists can stand up for science at a time when it is coming under attack and how the Union plans to foster the involvement of early career scientists (ECS) in its activities.

In the unlikely event that some of our readers don’t know who you are, could you introduce yourself and tell us a little about your career path so far and also about your involvement with the EGU over the years?

I started out with a degree in Physics. I’ve spent the last 20 years in the geography department at the University of Bristol focusing on Earth Observation. In that time, I’ve covered a lot of topics: from oceanography to land surface processes, but glaciology is my core discipline and research area. Most of my work has broadly been in the area of climate change and climate research but also solid Earth geophysics.

I’ve been involved with EGU (actually, it was EGS then) since the late 90s. I used to attend the meetings and I realised there was a gap in the market for cryospheric sciences. I approached Arne Richter [the former General Secretary of EGS] to form the Division of Cryospheric Sciences. I put together a proposal and became secretary of the division at the time and later became president of the division when EUG & EGS merged to form EGU. I spent five years in that role, towards the end of which I proposed (and launched) the open access journal The Cryosphere, which just celebrated its 10th anniversary and publishes about 220 papers per year.  I’m very proud of those contributions to the community and feel that they have helped develop the discipline and strengthen it.

It was 2007 when I stepped down from the EGU Council all together although I still attended the General Assembly, of course, and convened various sessions. It was 2015 when the then EGU vice-president, Hans Thybo, suggested I stand in the next presidential elections. I wasn’t at all certain I wanted to take on the role, but decided to go for it because I think it is important to serve the scientific community and colleagues and EGU is an organisation that is close to my heart.

At this year’s General Assembly, you were appointed Union President (after serving as Vice-President for a year). What are the main things you hope to achieve during your two-year term?

There are two main areas that I am very keen to promote and foster:

First, I want to make the organisation [the EGU] more attractive to early career scientists (ECS) and offer them more opportunities, be that more and better short courses, career support and other benefits of attending. For some years now there has been a strong ECS network within the Union and there have been great advances in that direction already.

Second, I’d like to increase the EGU’s opportunities, and those of members, to be involved in policy activities.

Why those two in particular?

There are many things one could do; but having attended the General Assembly for 15 years, there is no doubt that ECS are the future of the discipline, so if we don’t make the meeting attractive and useful for them, what are we here for?

In terms of policy, there are a number of events which have happened in the past few years which make it come into focus.

Certainly, in the UK, it is important that the science we do has impact, and just as important is that we [researchers] understand what the impact of the research we do has. Ultimately, tax payers pay for the research we do, so it is important not to get detached from the role we have in benefiting society in broad terms but also through specific opportunities and activities.

From many years attending the AGU Fall Meeting, I am aware the American Geophysical Union (AGU) has a very well developed and successful policy related programme. It is, of course, simpler for them, as the policy landscape is restricted to one nation and AGU’s headquarters are in Washington. Nonetheless, despite those differences, EGU is not, currently, providing opportunities for engagement in the policy realm in the way we could, for example, with the European Commission and its funding instruments.

Science for policy is not suited to all scientists, and all disciplines that we represent. However, it is important for a large cohort of our membership.

EGU President, Jonathan Bamber (centre left) and EGU Vice-President, Hans Thybo (centre right), stand along side the 2016 EGU Outstanding Student Poster and PICO (OSPP) awardees. Credit: EGU/Pflugel

ECS make up a significant proportion of the Union’s membership. EGU is a bottom up organisation and there is no doubt that ECS have a say in many matters of the Union already, but how do you plan on including ECS further in decision-making processes in the future?

I wouldn’t necessarily classify ECS separately. They are simply geoscientists, just like the majority of our members. It is important, however, for us to show them and highlight the opportunities available for them to be involved in the General Assembly and the Union as a whole.

We have a Union-wide ECS Representative on Council – this gives ECS a good understanding of how the organisation works and gives the individual experience of the machinery involved in running all the activities of EGU. Roles like this give the next generation skills to take on leadership roles in the future too. How do they know how organisations operate if they don’t have opportunities like this?

There are also no barriers to them being involved in convening sessions, organising short courses and proposing activities for the Union to prepare.

It can be intimidating as a junior scientist to be involved in these activities, so it’s important that we make it accessible to them. I think we are making great progress in this direction.

As an established scientist, what advice would you give ECS starting out in their career?

Accountancy pays very well!

More seriously: get involved!

Also, look at your most successful and respected senior colleagues and identify what about them makes them successful and what do you admire in them. Positive role models are very important.

Recently, the scientific process has come under attack. Initiatives such as the March For Science have given scientists opportunities to make their voices heard. What role can the Union play in supporting members wanting to stand up for science?

We can put together advice for how scientists can get their voice heard. The Union’s Outreach Committee is quite active in this regard already.

Trying to make sure that the voice of the geoscience community is heard within Europe is another area where we can contribute. We’ve been involved in an EU Parliamentary meeting, representing EGU, where discussions focused on improving the integration of science and collaboration across Europe.

We also offer policy makers and institutions the opportunities to contact scientists, through our database of experts.  We need to make European policy-makers more aware that we can provide that service.

In terms of funding for scientific research, we’ve established links with the President of European Research Council. Jean-Pierre Bourguignon gave a talk at this year’s General Assembly and participated in one of our Great Debates. We also hosted a meeting where senior members of the EGU’s council met with Bourguignon to discuss how the EGU could support the ERC in the future.

As an organisation, it should be our goal to provide our members with a mechanism by which they can communicate with the European Commission and policy-makers.

Last month, the EGU issued a statement condemning President Trump’s decision to pull the USA out of the Paris Climate Agreement. Why is this decision so troubling and, in your opinion, what can Union members do to raise awareness of the challenges facing the globe?

We should communicate the importance of our science: what we know, what we understand, the evidence based facts.

In the absence of evidence based science, how do policy makers reach decisions? They rely on gut instinct, on beliefs, on prejudices… But they should be making them on evidence based science. So, it is crucial that we communicate what we know to the public and policy-makers.

In Europe, a large majority don’t question human influence on climate. They understand it is real and that it’s an issue of upmost importance.

Trump’s decision was about politics not science; it is important to remember that. He didn’t deny that climate change was real, but he was making the decision on an economic basis and that is something else again. Whether it was a wise economic decision or an entirely myopic one is another question altogether.  I speak about this in more detail in an open editorial I wrote shortly after the decision was announced.

Geoscientists are, perhaps, more important in terms of policy and the health of the planet than they ever have been before. All the work we are doing in the geosciences has huge implications for policy and for safeguarding our future on the planet.

Jonathan, thank you for talking to me today about a whole range of topics. I’d like to finish this interview by bringing the conversation back around to EGU. We’ve discussed, at some length, what the Union hopes to do for its members and highlighted that there are plenty of opportunities to get involved. So, how exactly do they go about taking a more active role in the Union’s activities?

One of the easiest ways to have your voice heard is by getting involved through your scientific division. Attend your division(s)’s business meeting. Each division has quite a few officers: a secretary, vice-president, secretaries for sub disciplines and so on. There are lots of opportunities there. In general, anyone who wants to put the time in will be welcomed by division presidents because it’s always good to have enthusiastic, dedicated volunteers.

When it comes to the General Assembly in Vienna, anybody can propose a session. If you want to organise a session or a short course, just fire it out there! The call-for-sessions is currently open [until 8th September]. You’ll find all the details online.

If you are interested in policy-related activities do complete the register of experts questionnaire.  It doesn’t take long and you’ll find details on our webpages. Make sure you provide as much detail about your expertise as possible. That way we’ll be able to match you up with those who make inquires and opportunities in the most effective way.

Interview by Laura Roberts Artal (EGU Communications Officer)

 

 

 

 

Imaggeo on Mondays: Nor’Wester in the Southern Alps of New Zealand

Imaggeo on Mondays: Nor’Wester in the Southern Alps of New Zealand

Stephan Winkler’s 2017 Imaggeo Photo Contest finalist photo showcases an unusual weather phenomenon…

The image shows a typical weather situation in the Southern Alps of New Zealand with a moist, westerly airflow pushing over the Main Divide [which separates the water catchments of the more heavily populated eastern side of the island from those on the west coast] to create a typical foehn wind [dry and warm winds which form on the downside of a mountain range] pattern (locally called Nor’Wester) in the region. Immediately west of this Main Divide, annual precipitation of up to 15,000 mm has been estimated.

The upper part of Tasman Glacier, as other glaciers around and immediately east of the Main Divide, receive impressive amounts of snow due to an overspill effect and can still be regarded as maritime.

In the image, however, the situation is displayed when right at the Main Divide the clouds disappear due to increasing temperatures when flowing over the Divide. The foehn wind developing with such weather pattern can be very strong. However, the image nicely shows how the glaciation of the central Southern Alps is influence by the availability of moisture and the dynamic character of the regional climate.

Description by Stephan Winkler (Senior Lecturer in Quaternary Geology and Palaeoclimatology at the University of Canterbury), as published previously on imaggeo.egu.eu

Imaggeo is the EGU’s online open access geosciences image repository. All geoscientists (and others) can submit their photographs and videos to this repository and, since it is open access, these images can be used for free by scientists for their presentations or publications, by educators and the general public, and some images can even be used freely for commercial purposes. Photographers also retain full rights of use, as Imaggeo images are licensed and distributed by the EGU under a Creative Commons licence. Submit your photos at http://imaggeo.egu.eu/upload/.

 

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