ACADEMIA Letters
Monitoring the Earth: the Near-Future Developments in
Seismology
Mourad Bezzeghoud
Marco Manso
University of Évora, Portugal
Seismology deals with the study of the activity of physical forces responsible for the origin
of earthquakes and the seismic waves generated within the Earth. All structures located from
the center of the Earth to its surface are the subject of study in this discipline. Seismology
therefore pursues the understanding of the Earth’s internal structure and the physical processes
that cause earthquakes, resorting to advanced instruments for observation and measurements.
This paper presents an overview of important milestones in the seismological field, followed
by revolutions in the instrumentation and observation of seismological events.
The Earth
Earth is our natural habitat. Human beings and economic and social development depend
on the planet’s resources, which are not inexhaustible. In fact, the way resources will be managed during the 21st century will be decisive: only their moderate and rational exploitation
will allow the Earth to host and sustain the 10 billion human population estimated for the end
of this century. Developing a thorough knowledge and understanding of the functioning of
our planet is therefore essential to develop our society in a harmonious and sustainable way.
It is also our legacy to teach future generations our understanding of the Earth.
The Earth is a fascinating “entity” and discovering it from a physical point of view is an
even greater adventure. Understanding its structure, its dynamics and its shape imposes answering questions across different domains, because several physical phenomena of different
Academia Letters, March 2021
©2021 by the authors — Open Access — Distributed under CC BY 4.0
Corresponding Author: Mourad Bezzeghoud, mourad@uevora.pt
Citation: Bezzeghoud, M., Manso, M. (2021). Monitoring the Earth: the Near-Future Developments in
Seismology. Academia Letters, Article 581. https://doi.org/10.20935/AL581.
1
scales are involved, from “astronomical” scale (e.g., interaction with other celestial bodies)
to “atomic” scale (e.g., radioactive emission).
In the 20th century, the acquisition and analysis of massive amounts of observations and
information was made possible by progresses in instrumentation, electronics and information
technology. However, as opposed to Jules Verne’s novel “Journey to the Center of the Earth”,
so far, knowing the interior of our planet is only possible through observations and records
made on the surface, i.e., indirect observations.
The Physics of the Earth
Our knowledge of the Earth’s interior was still rudimentary at the beginning of the 20th
century [1], especially when compared with the scientific advances obtained about the “infinitely small” (discovery of radioactivity by Bequerel in 1896; identification of the electron by
Thomson in 1897; formulation of the quantum theory by Planck in 1900) and “infinitely large”
(theory of gravitation by Newton in 1687; foundations of celestial mechanics by Laplace in
1799; formulation of the theory of general relativity by Einstein in 1915). Knowledge of the
interior of the Earth mainly results from work conducted in the 20th century: in 1887 John
Milne (1850-1913) identified the crust, Lord Rayleigh, Lord Rutherford and Emil Wiechert
the mantle; the limit between the crust and the mantle was defined by Andrya Mohorovicic in
1909 (discontinuity of Mohorovicic/Moho); in 1906, Oldham’s remarkable work determined
the size of the Earth’s outer core; Beno Gutenberg (1889-1960), in 1912 in his doctoral thesis,
defined the boundary between the outer nucleus and the mantle. This interface between the
asthenosphere and the endosphere is called Gutenberg’s Discontinuity; in 1926, Sir Harold
Jeffreys (1891-1989) discovered that the outer core is liquid; in 1936 Inge Lehman (18881993) provided the key for the identification of the Earth’s inner core; inner core which, in
1946, is identified as solid by Keith Edward Bullen (1906–1976). In 1935, H. Jeffreys and
K.E. Bullen published the famous travel time tables of the seismic waves that bear their names
(Jeffreys-Bullen tables) and which served as a reference for seismologists and geophysicists
for half a century. The previously mention discoveries about the structure of the Earth, from
John Milde to K.E. Bullen, were based on the study of earthquakes and the propagation of
seismic waves. It is also important to underline one of the great steps taken to understand
internal geodynamics, due to the Irish Robert Mallet (1810-1881) and to the French, Alexis
Perrey (1807-1882) and Count Fernand Jean Batiste Marie de Montessus de Ballore (18511923) who dedicated a significant part of their work to the collection of information regarding
earthquakes that occurred throughout the planet. The revolutionary discovery of the ocean
floor expansion in 1963 by Drummond Hoyle Matthews and his student Fred J. Vine, also inAcademia Letters, March 2021
©2021 by the authors — Open Access — Distributed under CC BY 4.0
Corresponding Author: Mourad Bezzeghoud, mourad@uevora.pt
Citation: Bezzeghoud, M., Manso, M. (2021). Monitoring the Earth: the Near-Future Developments in
Seismology. Academia Letters, Article 581. https://doi.org/10.20935/AL581.
2
dependently brought by Lawrence Morley, were essential elements for the acceptance of the
Plate Tectonics theory .
Recording earth motion and earthquakes - the first (r)evolution in
high-density deployments
Seismic events can be extreme, and severe threats to humanity. Helping to understand
these phenomena, seismic networks have been deployed in increasing number, filling in gaps
in the global coverage and improving our understanding of the physical processes that cause
earthquakes. Several countries have made significant efforts to deploy Broadband seismic
networks incorporating seismological stations supporting real-time monitoring of the earthquake activity. However, these stations are installed several kilometers from each other, thus
limiting the overall spatial resolution of the observations. It is important to highlight that
the revolutionary contribution of broadband seismic instrumentation, in which principles of
feedback accelerometers and zero-length leaf spring were implemented. Construction, functionality and measurement results of the first vertical STS-1 broad-band (BB) seismometer
was published by Wielandt and Streckeisen [2] and the advancing the STS-1 to a novel digital
Very-Broadband-Seismograph (VBB) was published by Wielandt and Steim [3].
A paradigm change occurred in the United States with the deployment of high density seismic networks with the capability to record the propagation of seismic activity in high resolution: The California Institute of Technology established the Community Seismic Network an
earthquake monitoring system based on a dense array of low-cost acceleration sensors (more
than 1000) aiming to produce block-by-block strong shaking measurements during an earthquake [4]. The University of Southern California’s Quake-Catcher Network began rolling out
6000 tiny sensors in the San Francisco Bay Area, being part of the densest network of seismic
sensors ever devoted to study earthquakes in real time [5].
A high dense network-enabled seismic network operating in the principle of “live” data
brings the opportunity to explore new applications in seismology, including real-time earthquake detection, as well as the generation of Shakemaps (i.e., spatial representation of ground
motion amplitudes).
Low-cost sensors: the second (r)evolution and the near-future developments
In the last years, sensors and sensing network technology evolved at a fast pace, resulting
Academia Letters, March 2021
©2021 by the authors — Open Access — Distributed under CC BY 4.0
Corresponding Author: Mourad Bezzeghoud, mourad@uevora.pt
Citation: Bezzeghoud, M., Manso, M. (2021). Monitoring the Earth: the Near-Future Developments in
Seismology. Academia Letters, Article 581. https://doi.org/10.20935/AL581.
3
in improved performance, operation and connectivity at significant cost reduction. Low-cost
Micro-Electro Mechanical Systems (MEMS) accelerometers, in particular, demonstrated the
capability to generate relevant data for seismic analysis in dense deployment contexts [6].
MEMS technology has enabled the mass production of small size accelerometers. Capacitive accelerometers, in particular, are highly popular due to reduced cost, their simple
structure, and the ability to integrate the sensor close to the readout electronics. When subjected to an acceleration, the inertial mass shifts cause a proportional change in capacitance.
By measuring the capacitance change, the acceleration can be calculated.
In order to properly exploit its data, it is important to take into account MEMS benefits
and limitations [7-10]. MEMS accelerometers have adequate range (several times the standard
gravity g), sensitivity and frequency response (typically around 1k Hz) but exhibit high-levels
of instrumental self-noise. As such, they are especially fit to measure strong seismic activity (M>3), high frequencies (>40 Hz) and can measure the gravity acceleration component.
Importantly, MEMS accelerometers complement broadband seismometers in what regards
strong motion and high frequency measurements.
In Portugal, as part of the SSN-Alentejo project [11], the University of Évora is planning
a deployment of up to 300 network-enabled stations in the Évora region, complementing the
existing network that is comprised by 15 broadband stations. SSN-Alentejo will be used to
monitor ground motion activity - caused by natural and/or human activity - in high detail,
including in Évora city given its high patrimonial value and cultural heritage.
The network-enabled high-density seismic network generates data in real-time enabling
the following applications [9]:
• Seismic detection (strong motion) for near and “far” earthquakes (far being in the order
of hundreds of kms).
• Study of local events and characterize the structure of the seismogenic zone by performing waveform analysis of nearby small events and ambient noise.
• Analyze the impact produced by human activity and cultural noise on buildings and
monuments: Urban seismic noise is usually dominated by traffic and industrial activity
with peak frequencies below 25 Hz. A continuous exposure to urban tremors can cause
a cumulative and progressive degradation on fragile buildings and monuments, which
could cause irreparable damage in human heritage.
• Generation of Shakemaps that can be used by civil protection authorities for postearthquake response, including assessing structural integrity risks in buildings and slopes.
Academia Letters, March 2021
©2021 by the authors — Open Access — Distributed under CC BY 4.0
Corresponding Author: Mourad Bezzeghoud, mourad@uevora.pt
Citation: Bezzeghoud, M., Manso, M. (2021). Monitoring the Earth: the Near-Future Developments in
Seismology. Academia Letters, Article 581. https://doi.org/10.20935/AL581.
4
• Provide to the scientific community with new open-access high-resolution seismic data.
• Facilitate access to education in seismology, resulting from open access to low-cost
technology that can be installed in high schools and integrated in projects and activities.
Conclusion
Seismology is a relatively young scientific discipline, that has significantly evolved since
the beginning of the 20th century, benefiting from significant advances in theories and technology. Broadband seismic networks brought the capability to perform real-time monitoring
of the earthquake activity and subsequent high-density deployments allowed further increasing spatial resolution of the observations for a more accurate characterization (high resolution) of earthquake motion. Recent developments in low-cost MEMS accelerometers have
found numerous real-world applications, including in seismology and risk hazard assessment
of buildings and human heritage. Being low-cost, it facilitates their widespread adoption
enabling the deployment of high-density networking providing high resolution observation
and massive amount of data that may feed intensive processing techniques like big data and
artificial intelligence, applying machine learning techniques and pattern matching-based processing that are much more sensitive than the power detectors used in current seismic systems
[12] making them especially relevant in the presence of noise and weak signals.
The deployment of high-density network-enabled seismic networks represents an important step in our road towards understanding the functioning of the Earth, including its internal
structure and physical processes that cause earthquakes, while at the same time contributing
towards a safer and more sustainable society.
Academia Letters, March 2021
©2021 by the authors — Open Access — Distributed under CC BY 4.0
Corresponding Author: Mourad Bezzeghoud, mourad@uevora.pt
Citation: Bezzeghoud, M., Manso, M. (2021). Monitoring the Earth: the Near-Future Developments in
Seismology. Academia Letters, Article 581. https://doi.org/10.20935/AL581.
5
References
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[3] Wielandt, E., Steim, J.M. (1986) A digital very broad band seismographe, Annales Geophysicae, 4, B, 3, 227-232.
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[9] Manso M., Bezzeghoud M., Caldeira B. (2017) Design and Evaluation of a High Throughput Seismic Sensor Network - Tools for Planning, Deployment and Assessment. In Proceedings of the 6th International Conference on Sensor Networks - Volume 1: SENSORNETS, ISBN 978-989-758-211-0, pages 129-134. DOI: 10.5220/0006127701290134
[10] Manso, M., Bezzeghoud, M., Borges, J., Caldeira, B., Abdelhakim (2020) High-density
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[11] See https://www.uevora.pt/en/Research/projects?id=3812, accessed 2021/01/15. The
SSN-Alentejo project is funded by the Science Foundation of Portugal (FCT) under grant
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Academia Letters, March 2021
©2021 by the authors — Open Access — Distributed under CC BY 4.0
Corresponding Author: Mourad Bezzeghoud, mourad@uevora.pt
Citation: Bezzeghoud, M., Manso, M. (2021). Monitoring the Earth: the Near-Future Developments in
Seismology. Academia Letters, Article 581. https://doi.org/10.20935/AL581.
6