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Environmental Seismology – Monitoring More than Earthquakes

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Over the last 100 years, the sensitivity of seismic instruments has improved by many orders of magnitude.

A modern seismometer, like the ones installed in the permanent monitoring network in Finland, is capable of measuring movements in the earth as small as nanometers per second (0.000000001 meters/s). Such incredible sensitivity allows seismologists at the Institute of Seismology to detect signals from earthquakes all over the world or see small explosions from mining operations in Sweden. In between earthquakes, though, there exists an entire world of signals recorded on seismometers, hidden in what we used to call “background noise.”

When Noise is not Noise

As more and more sensitive instruments were installed around the world, seismologists noticed that, instead of being quiet, the earth was constantly humming with energy. This energy was usually quieter than signals generated by earthquakes, but easily detected with high-quality instruments.

Figure 1: Energy recorded at different seismic periods over 5 years of data from station KEV in Finland and station WRAB in Australia. Peak values at periods of 3-5 seconds (red arrows) are related to energy generated by ocean waves. Plots were generated using EarthScope MUSTANG web services [1].

Figure 1 [1] shows the energy recorded at two different seismometers over 5 years. Station KEV is located near the village of Kevo in Lapland, and station WRAB is in central Australia. Even though they are in two very different places geologically, the energy pattern looks quite similar, with a peak of energy around 3-5 seconds period (0.2-0.33 Hz in frequency). This energy pattern is seen every day, on nearly every seismic station on earth, and while scientists long suspected that this noise is coming from waves interacting in the oceans [2], it was only recently that this was able to be accurately verified [3].

Air, Ice, and Rock

Other earth processes also generate signals that either older seismic equipment could not measure, or we lacked the necessary number of stations nearby, or we did not have the necessary computational power to decode the signals until recently. For example, scientists have been able to track thunderstorms moving across the southern United States [4], and calculate the strength of cyclones in the Pacific Ocean [5], using only seismic data.

As glaciers have been rapidly retreating due to climate change, seismology can also be used to detect glacial surges [6] or small caving events, even being able to determine the direction that the ice fell [7].

Seismic data is even being used to monitor for landslides in near real-time, watching for a landslide that could generate a potentially dangerous tsunami (figure 2) [8]. Monitoring for signals like these are similar in theory to how regular earthquakes are detected and located but using completely new methods to interpret the seismic data and doing it rapidly enough to potentially alert people in danger before the threat arrives.

Figure 2: From Karasözen and West (2024), nine automatically detected landslides at fjords in Alaska using seismic data.

We are in an exciting era of seismology, where many of the technological limitations of previous generations have been lifted. We can use seismology to move past only monitoring for earthquakes and use the tools and our creativity to learn more about the earth system as a whole – the atmosphere, the oceans, and the rock beneath our feet.

Matt Gardine, seismologist

References

[1] Casey, Robert, Mary E. Templeton, Gillian Sharer, Laura Keyson, Bruce R. Weertman, and Tim Ahern. “Assuring the Quality of EarthScope Data with MUSTANG”. Seismological Research Letters 89 (2A) (2018): 630-639. https://doi.org/10.1785/0220170191

[2] Longuet-Higgins, Michael. S. “A theory of the origin of microseisms”. Proc. R. Soc. London Ser. A 243 (1950): 1–35.

[3] Kedar, Sharon, Michael Longuet-Higgins, Frank Webb, Nicholas Graham, Robert Clayton, and Cathleen Jones. “The origin of deep ocean microseisms in the North Atlantic Ocean”. Proc. R. Soc. London Ser. A 464 (2008): 1–35. https://doi.org/10.1098/rspa.2007.0277

[4] Tytell, Jonathan, Frank Vernon, Michael Hedlin, Catherine de Groot Hedlin, Juan Reyes, Bob Busby, Katrin Hafner, and Jennifer Eakins. “The USArray Transportable Array as a Platform for Weather Observation and Research”. Bulletin of the American Meteorological Society 97.4 (2016): 603-619. https://doi.org/10.1175/BAMS-D-14-00204.1

[5] Gualtieri, Lucia, Suzana J. Camargo, Salvatore Pascale, Flavio M.E. Pons, and Göran Ekström. “The persistent signature of tropical cyclones in ambient seismic noise”. Earth and Planetary Science Letters 484 (2018): 287-294. https://doi.org/10.1016/j.epsl.2017.12.026

[6] Nettles, Meredith, and Göran Ekström. “Glacial earthquakes in Greenland and Antarctica”. Annual Review of Earth and Planetary Sciences 38:1 (2010): 467-491. https://doi.org/10.1146/annurev-earth-040809-152414

[7] Olsen, Kira G., and Meredith Nettles. “Constraints on terminus dynamics at Greenland glaciers from small glacial earthquakes”. Journal of Geophysical Research: Earth Surface 124 (2019): 1899–1918 https://doi.org/10.1029/2019JF005054

[8] Karasözen, Ezgi, and Michael E. West. “Toward the Rapid Seismic Assessment of Landslides in Coastal Alaska”. The Seismic Record 4 (1) (2024): 43–51 https://doi.org/10.1785/0320230044

About timescales in seismology

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Finland’s first seismograph began operating in Helsinki a hundred years ago. It is a milestone worth noticing and a long time for humans.

However, one hundred years is a brief time to collect evidence of earthquakes. In many regions, no strong earthquake has occurred in over a century, and even in regions with recent large earthquakes, a century is too brief to properly assess the long-term seismic hazard of the region. Although seismologists generally do not work on geological timescales spanning up to millions to billions of years, their work does include extending the series of earthquake observations as far back in the past as possible and forward into the future.

Earthquakes in the distant past can be investigated by using non-instrumental seismological methods. The likelihood of a strong earthquake occurring goes up the longer the time of observation is. The price of looking very far back, however, is an increase in uncertainty about the earthquake size – but there are ways of using available evidence to model sizes of earthquakes in the past.

Before measuring devices became common, the impacts of earthquakes could be observed directly by the naked eye. Especially after earthquakes caused damage, people have written documents about the event for authorities and other contemporaries, and sometimes also for scientific purposes. The most reliable of these reports are from eyewitness accounts. In the best cases, detailed written records have survived to modern times and been found by researchers. Through modern research methods, the seismic history of a target location showing how often strong earthquakes have occurred and how those earthquakes have impacted society over the centuries, or a couple of millennia in some parts of the world, can be extracted.

The geological traces left by strong earthquakes in the natural environment can go back to at least ten thousand years ago. The suddenness and violence of the earthquake remains as displacement movements, subsidence or rise of the ground compared to the environment, old landslides, and ancient water paths in the soil. The location and timing of these traces adds information about the frequency of strong earthquakes. Searching the landscape for evidence of large earthquakes is called paleoseismology.

The Fennoscandian Shield in Northern Europe has ancient bedrock and is seismically relatively quiet. There are written records of earthquake effects as far north as Lapland for almost three centuries and in the south for a slightly longer time. At the end of the last Ice Age, the retreat of the ice mass changed the stress conditions of the earth’s crust, causing strong earthquakes associated with several southwest-northeast trending fault escarpments in northern Fennoscandia. They have been dated to about 9–11 thousand years ago.

In the 2000s, high-resolution LiDAR (Light Detection and Ranging)-images, trenching, and seismic reflection campaigns have advanced paleoseismology in Fennoscandia. Traces of paleoearthquakes have been identified in lake sediments in the south. It has been proposed that paleoseismicity had three maxima in the Finnish territory around 10-12 thousand years ago, 5-7 thousand years ago, and 1.5-3 thousand years ago. The highest magnitude earthquakes, likely around Mw7 and above, were associated with the oldest earthquakes, and lower magnitudes (around Mw6) with the younger events (https://doi.org/10.1016/j.tecto.2019.228227). Traces of strong earthquakes 700–4000 years ago have been reported from the Stuoragurra postglacial fault in northern Norway(https://doi.org/10.1017/9781108779906.015). What is special is their size range – up to magnitude 7 – which challenges the old notion that the strongest earthquakes occurred relatively shortly after the Ice Age.

Knowing the seismicity of the past is critical in assessing the seismic risk in the near future. Because we cannot predict exactly when and where an earthquake will occur, probabilites must be used. The usual way of talking about seismic risk is that an earthquake of a given size or a given ground motion occurs in a target area with a given probability in 30 or 50 years. This 30-to-50-year period is related to construction and engineering building codes, and the hazard maps are aimed at ensuring that appropriate seismic building codes are used so that strong ground movements do not damage buildings and society’s infrastructure.

Seismologists therefore have at their disposal a catalog of instrumental measurements from the last 100 years or so, and usually a longer period of non-instrumental observations. Combined, these give seismologists the best data available for working with engineers to plan for the strong earthquakes that will inevitably occur in the future.

Päivi Mäntyniemi, University Researcher

The Finnish mobile seismic instrument pool

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Seismologists are scientists who study the earth, from the surface to the core, using physics theories combined with observations from seismic waves travelling through the planet. These observations are required to better understand earthquakes, faults, volcanoes, seismic hazards, landslides, and earth structures, as well as for economic applications such as identifying subsurface mineral or petroleum deposits.

Seismometers are the primary instrument needed to make these observations and so seismologists rely on the availability of these extremely sensitive instruments. In recent years, the production of more cost-effective sensors and access to large-scale data storage and computational facilities has led to an evolution in the number of seismometers used in seismic studies. However, individual sensors are generally too expensive for individual researchers to purchase and operate and thus have been restricted to large private companies or through well-funded but restricted government research consortiums.

To help the access to such equipment to the scientific community, the Finnish mobile seismic instrument pool was created in 2021. It is owned and operated by seven Finnish academic and research institutions: the University of Helsinki, the University of Oulu, the Geological Survey of Finland, the National Land Survey, Aalto University, the Technical Research Center of Finland and the University of Turku. The pool is funded through the Research Council of Finland call for research infrastructures (FIRI),. through the FLEX-EPOS project, under the FIN-EPOS* umbrella.

The initial funding for the project was started in 2021, with the instrument pool build-up phase ending in 2024. After that, the pool will continue to operate indefinitely. By the end of 2024, the pool is expected to include 46 Güralp broadband seismometers and 5 Güralp strong-motion accelerometers, as well as 1229 Geospace and 71 SmartSolo self-contained geophone units. When complete, this pool represents one of the largest mobile seismic instrument pools in Europe available in the public sector.

The pool is made available to researchers and supports domestic and international collaborative projects that enhance data-driven subsurface and environmental applications. Projects can last a few days up to a few years.

Between October 2021 and December 2023, 30 projects have been completed using the pool , generating around 28TB of data – equivalent to 280 Helsinki central libraries Oodi. These projects have helped seismologists further study diverse topics such as ground water, faults, frost quakes, ore bodies, crustal structure and local sedimentary overburden, with many more to come.

If you want to know more, you can check the FLEX-EPOS and seismic instruments wikipage: https://wiki.helsinki.fi/xwiki/bin/view/FLEX/Flex-epos%20Home/#. The pool will also be presented through a poster at the EGU24 meeting (14-19.4.2024), hall X1 at board number X1.120.

Roméo Courbis, University Researcher

 

FLEX-EPOS is funded through the Research Council of FInland FIRI2019 call (funding decisions no. 328984, 328776, 328778-328782, 328784 and 328786).

*FIN-EPOS is a Finnish national node of EPOS (European Plate Observing System).