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 [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.

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

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).