Editors’ Vox is a blog from AGU’s Publications Department.

For the past 30 years, Earth scientists have been monitoring the entire planet from surface processes down to the innermost core using near real-time digital seismic data. These data are collected by global seismographic networks and are free and openly available to anyone. These networks operate seismic stations that are often in very remote locations such as Pitcairn Island in the middle of the south Pacific Ocean or the South Pole in Antarctica.

A recent article published in Reviews of Geophysics explores the history and resulting scientific achievements of Global Seismographic Networks. We asked the lead author to give an overview of how Global Seismographic Networks evolved, what they’ve uncovered, and what challenges remain.

What are “Global Seismographic Networks” and how are they used? 

Global seismographic networks are collections of seismic stations that measure near real-time ground motion (movement of the earth from earthquake shaking or other sources) and send those data to scientists. The instruments at these stations are so sensitive that they can record earthquakes from all over the world. This information is then used to locate the earthquake, determine its size, and resolve how the fault that generated the earthquake moved.

a) Global Seismographic Network as of 2021 with stations colored by primary sensor type. (b) Nanometrics T-360 GSN sensor (red); (c) Nanometrics T-120 borehole sensor (purple); (d) Streckeisen STS-6 sensor (orange); (e) Streckeisen STS-1sensor (blue); (f) GeoTech KS-54000 sensor (green).  These instruments range from a height of approximately 16 centimeters (e) to 2 meters (f) and are not shown to scale. Credit: Ringler et al. [2022], Figure 7

These data are also used to study the interior of the Earth. By using earthquakes as sources of seismic energy, scientists estimate the internal properties of the Earth via tomography, which is similar to how a computed tomography (CT) scan works using seismic waves instead of X-rays.

In the absence of ground motions generated by earthquakes, some of the next largest signals that these instruments detect are smaller seismic waves generated through the interaction of ocean waves with Earth’s crust. Therefore, the long-running history of global seismographic network stations can also be used to track changes in global ocean wave activity, which can be important for climate science.

What did the first seismic network look like and how has it evolved? 

The first global seismographic networks were relatively small (20 to 30 stations) and recorded data on paper records. In the 1960s, these networks evolved to be much larger (100 to 150 stations), such as the Worldwide Standardized Seismographic Network (WWSSN) to monitor for nuclear testing.

Although the WWSSN was state of the art at the time, it was later realized that the network was unable to record the slowest oscillations of great earthquakes, also known as normal modes. Large earthquakes cause normal modes to oscillate through the Earth much like ringing a bell. The Earth rings for many days, and each oscillation takes several minutes to complete. These normal modes provide unique information about the properties of Earth’s core and lower mantle that seismic waves from smaller earthquakes can’t tell us.

Advances in technology and a continued interest to study normal modes by scientists led to the development of the GEOSCOPE network and the Global Seismographic Network (GSN) starting in the 1980s, with continual improvements to today. These state-of-the-art networks digitally record an exceptional range of ground motion amplitudes, from movements as small as the size of an atom to accelerations capable of collapsing buildings.

Current instrumentation used in the Global Seismographic Network (GSN). Credit: Ringler et al. [2022], Figure 8

How have GSNs advanced our understanding of the Earth? 

Global seismographic networks have provided a wealth of information about earthquakes, properties of the Earth’s interior, and surface processes such as ocean storms, large volcanic eruptions, and glacial calving events.

Through locating and determining slip mechanisms of earthquakes, the long-running history of these networks has also helped quantify plate tectonics through the characterization of earthquakes along tectonic plate boundaries.

Global seismographic networks have contributed to several foundational observations of Earth’s interior, including providing the first unambiguous evidence that the inner core is solid.

Although not an original goal of these networks, they have recently been used to provide insight into environmental changes in the oceans and polar regions, as well as unique observations about how large volcanic eruptions oscillate the Earth’s atmosphere.

In addition, these networks have helped scientists and engineers understand regions of potential hazard and develop building codes that mitigate loss of life and property after large earthquakes.

How can scientists continue to advance the quality of seismic data and networks? 

Continued long-term and freely accessible monitoring data provided by global seismographic networks and support from the international scientific community to ensure high-quality data would be beneficial to advancement. Many scientific discoveries made using global seismic data were only possible after decades of data collection. For example, monitoring the rotation of the inner core, which is responsible for Earth’s magnetic field, required long running high-quality data records from globally distributed stations.

Are there any additional interdisciplinary uses for GSNs? 

The very broadband nature and multidisciplinary development of global seismic networks makes them very well-suited to be used for interdisciplinary studies. Earth scientists have been able to use global seismic network data to study changes in climate and oceans. Many global seismic stations not only record seismic data, but also atmospheric data (such as pressure) and the Earth’s magnetic field. These additional data streams can be used to study things like how large volcanoes, such as the volcanic eruption near Tonga on 15 January 2022, erupted and how the energy seismically coupled into the Earth.

The very broadband nature and multidisciplinary development of global seismic networks makes them very well-suited to be used for interdisciplinary studies.

What are some remaining challenges where additional research, data or modeling efforts are needed? 

Global seismographic networks have been widely successful at imaging the interior of the Earth, quantifying where tectonic plate boundaries are, and reducing geological hazards. Similar to how new and improved telescopes produce higher resolution images of distant galaxies, continued improvements to the infrastructure and instrumentation of global seismic networks will lead to new discoveries and an improved understanding of the Earth’s structure, atmospheric interactions, and geologic hazards.

Many questions about the Earth still remain for which seismic data may help provide answers. For example, seismic data could provide one of the key tools for better understanding the evolution of the interior of the Earth and how it interacts with Earth surface processes. The long-running data streams from global seismographic networks could also help us understand increasing extreme climate activity that interacts with the Earth’s surface. Additionally, global station coverage is sparse in some regions, including in ocean basins and in central Africa, which limits our ability to detect earthquakes and thus obtain clear images on Earth structure in these regions.

—Adam T. Ringler (aringler@usgs.gov; ORCID logo0000-0002-9839-4188), United States Geological Survey

Editor’s Note: It is the policy of AGU Publications to invite the authors of articles published in Reviews of Geophysics to write a summary for Eos Editors’ Vox.

Citation: Ringler, A. T. (2022), Global Seismic Networks: recording the heartbeat of the Earth, Eos, 103, https://doi.org/10.1029/2022EO225025. Published on 9 September 2022.
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