Seismology: More than Earthquakes

Modern seismology is expanding from a solid Earth into an indispensable planetary and environmental discipline, says professor Gregor Hillers.

As a seismologist I am occupied with diverse Earth Science phenomena but at its core it is all about elastic wave propagation (in contrast to electromagnetic or gravity waves) in the Earth. These waves propagate in the solid Earth, and as acoustic waves in the oceans and in the atmosphere. They are excited by natural internal and external processes including earthquakes, volcanic activity, landslides, glacier and ice sheet dynamics, water runoff, bolides, and storms, by anthropogenic activity, but also by large mammals.

I find it interesting to image and monitor subsurface deformation processes that evolve on human time scales that are short compared to geological time scales. In our team we apply seismic data processing and numerical modeling to understand the properties and the functioning of systems such as fault zones, geoenergy systems, and ice sheets, and the interrelations with their environments. 

As an example of our multidisciplinary collaboration with other domain specialists, I have been working on a seismic imaging technique that uses the ambient noise wavefield to compile images of the subsurface structure and properties. The approach has been developed in ultrasound medical imaging. The analogy between modern dense seismic arrays that contain a large number of seismic stations and multi-element ultrasonic medical sensors supports this transfer. 

I share the above iconic image with you that was obtained from data collected in the Helsinki area (courtesy R. Courbis). It shows the propagation of Rayleigh surface waves in the 0.1 to 7.5 Hz frequency range. The fact that the image shows only one single line of arrival means that waves at all included frequencies travel with the same velocity of about 3 km/s. This is fast for surface waves at these frequencies, and the arrivals are typically much more fanned out. This peculiar linear feature is governed by the properties of the Earth’s crust in Finland. Doing seismology in Finland also means we tinker with the clean seismograms of the small earthquakes that occur in the Fennoscandian Shield to explore some fundamental wave propagation effects. 

Where and how does the topic of your research have an impact?

You never know! I like to think that our method development and associated high-resolution results are interesting to be further developed and applied to various targets to help improve our understanding of what lies below. A better understanding of how the different spheres interact—solid Earth, hydrosphere, cryosphere, biosphere, atmosphere— seems similarly timely to support sound decision making.

What is particularly inspiring in your field right now?

These are interesting times—machine learning tools help us to analyze the ever increasing data volumes that are collected by a growing number of different sensors. Whale behavior is monitored using whale song sensed by ocean bottom telecommunication cables. Patterns of precursory signals observed in laboratory rock experiments and along fault zones can lead to insights into the preparatory phase of earthquakes. Observations of gravity waves excited by large earthquakes imply critical improvements of early warning systems. Seismic metamaterials tame otherwise damaging earthquake waves. Sensors deployed on Alpine glaciers, in Antarctica, and on floating sea ice reveal stunning deformation patterns. After placing seismometers on Earth and on the moon, planetary seismology has taken the next step by deploying an instrument on Mars. Taken together, its focus on elastic wave propagation is transforming seismology into an indispensable planetary and environmental discipline.

Gregor Hillers is Professor of Seismology at the Faculty of Science.

Newly appointed professors of the University of Helsinki will give their inaugural lectures on Wednesday, 29 May 2024, starting at 14.15.