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seismic signals

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?

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

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

From the top of a small gully in the French Alps, a 472 kg block is launched into the chasm. Every detail of it’s trajectory down the slope is scrutinised by two cameras and a network of seismometers. They zealously record every bounce, scrape and tumble – precious data in the quest to better understand landslides.

What makes landslides tick?

In 2016, fatalities caused by landslides tipped 2,250 people. The United States Geological Survey (USGS) estimates that between 25 and 50 people are killed, annually, by landslides in the United States alone. Quantifying the economic losses caused by landslides is no easy task, but the costs are known to be of economic significance.

It is paramount that the mechanisms which govern landslides are better understood in hopes that the knowledge will lead to improved risk management in the future.

But landslides and rockfalls are rarely observed in real-time. Deciphering an event, when all you have left behind is a pile of debris, is no easy task. The next best thing (if not better than!) to witnessing a landslide (from a safe distance) is having a permanent record of its movement as it travels down a slope.

Although traditionally used to study earthquakes, 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. For some year’s now they have been an invaluable tool in detecting mass movements (an all-encompassing term for the movement of bed rock, rock debris, soil, or mud down a slope) across the globe.

More recently, processing recorded seismic signals triggered by large catastrophic events has not only allowed to identify when and where they occurred, but also their force, how quickly they travel, gain speed and their direction of movement.

This approach gives only a limited amount of data for scientists to work with. After all, large, catastrophic, mass movements represent only a fraction of the landslide and rockfall events that occur worldwide. To gain a fuller understanding of landslide processes, information about the smaller events is needed too.

So, what if scientists could use a seismic signal which is generated by all mass movements, independent of their size?

The high-frequency seismic signal

A high-frequency seismic signal is generated as the individual particles, which combined make up a landslide or rockfall, bounce and tumble against the underlying layer of rock. Would it be possible to, retrospectively, find out information about the size and speed at which individual particles traveled from this seismic signal alone?

This very question is what took a team of scientists up into the valleys of the French Alps.

At a place where erosion carves gullies into lime-rich muds, the researchers set-up two video cameras and network of seismometers. They then launched a total of 28 blocks, of weights ranging from 76 to 472 kg, down a 200 m long gully and used the data acquired to reconstruct the precise trajectory of each block.

The impacts of each block on the underlying geology, as seen on camera, were plotted on a 3D representation of the terrain’s surface. From the time of impact, block flight time and trajectory, the team were able to find out the velocity at which the blocks travelled and the energy they carried.

View from (a) the first and (b) the second video cameras deployed at the bottom of the slope. The ground control points are indicated by blue points. (c) Trajectory reconstruction for block 4 on the DEM, built from lidar acquisition, superimposed on an orthophoto
of the Rioux-Bourdoux slopes. Each point indicates the position of an impact and the colour gradient represents the chronology of these impacts (blue for the first impact and red for the last one). K2 is a three-component short-period seismometer and K1, K3 and K3 are vertical-only seismometers. CMG1 is a broad-band seismometer. From Hibert, C. et al., 2017. (Click to enlarge)

As each block impacted the ground, it generated a high-frequency seismic signal, which was recorded by the seismometers. The signals were processed to see if information about the (now known) properties of the blocks could be recovered.

Following a detailed analysis, the team of scientists, who recently published their results in the EGU’s open access journal Earth Surface Dynamics, found a correlation between the amplitude (the height of the wave from it’s resting position), as well as the energy of the seismic signals and the mass and velocities of the blocks before impact. This suggests that indeed, these high-frequency seismic signal can be used to find out details about rockfall and landslide dynamics.

But much work is left to be done.

There is no doubt that the type of substrate on which the particles/blocks bounce upon play a large part in governing the dynamics of mass movements. In the case of the French Alps experiment, the underlying geology of lime-rich muds was very soft and absorbed some of the energy of the impacts. Other experiments (which didn’t use single blocks), performed in hard volcanic and metamorphic rocks, found energy absorption was lessened. To really get to the bottom of how much of a role the substrate plays, single-block, controlled release experiments, like the one described in the paper, should be performed on a variety of rock types.

At the same time, while this experiment certainly highlights a link between seismic signals and individual blocks, rockfalls and landslides are made up of hundreds of thousands of particles, all of which interact with one another as they cascade down a slope. How do these complex interactions influence the seismic signals?

By Laura Roberts Artal, EGU Communications Officer

References and resources:

Hibert, C., Malet, J.-P., Bourrier, F., Provost, F., Berger, F., Bornemann, P., Tardif, P., and Mermin, E.: Single-block rockfall dynamics inferred from seismic signal analysis, Earth Surf. Dynam., 5, 283-292, doi:10.5194/esurf-5-283-2017, 2017.

USGS FAQs: How many deaths result from landslides each year?

The human cost of landslides in 2016 by David Petley, published, 30 January 2017 in The Landslide Blog, AGU Blogosphere.

[Paywalled] Klose M., Highland L., Damm B., Terhorst B.: Industrialized Countries: Challenges, Concepts, and Case Study. In: Sassa K., Canuti P., Yin Y. (eds) Landslide Science for a Safer Geoenvironment. Springer, Cham, (2014)

 

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