Since NASA's InSight mission landed on Mars in November 2018 and installed a seismometer, the Marsquake Service, led by ETH Zurich and involving the Swiss Seismological Service (SED), has been regularly analysing the recorded seismic data. In addition to identifying numerous marsquakes, researchers used these data to make statements about the structure of the planet's interior. They built a profile of the planet's crust, mantle and core but could not yet reveal much about the near-surface structures. However, shallow subsurface is vital to understanding Mars's geological history.

Rather than using marsquake signals to look into the subsurface, the new study utilises the ambient noise recorded at times without marsquakes. On Earth, such noise is generated by ocean waves, wind and human activity. Over the past few decades, the SED has developed methods to analyse ambient noise. These methods are used to define the structure of the local geology and to determine whether the local subsurface tends to attenuate or amplify seismic waves. This information is crucial for determining a site's earthquake hazard and analysing unstable landslide zones on mountains or in lakes.

On Mars, ambient noise is triggered by the wind, which generates seismic waves when interacting with the planet's surface. Based on analyses of this ambient noise, researchers can now image for the first time the shallow subsurface of Mars and study its geological history at depths ranging from a few dozens to two hundred metres. In contrast to Earth, Mars has never been home to any active plate tectonics. The planet has been shaped by phases of active volcanism that covered large areas with basaltic lava plateaus. The new analyses provide now a detailed image of the subsurface at the InSight landing site and show a top layer of three meters sand (regolith) and loose rock about 20 metres thick that has been fissured by thousands of meteorite impacts. Below are layers of lava flows that covered the planet between 1.7 and 3.6 billion years ago. These lava layers are divided by sediments lying at around 30 to 75 metres deep. The seismic image of the layer-cake geological stratification allows researchers to trace, for the very first time, the most important geological events that have occurred at the InSight landing site on Mars over the last three billion years.

When humans land on Mars one day, they need to know what lies under their feet. The question of whether these near-surface layers contain water is, for example, particularly interesting. The results of this latest study demonstrate that established techniques to investigate Earth are helping to answer such questions on Mars.

Hobiger, M., Hallo, M., Schmelzbach, C. et al. The shallow structure of Mars at the InSight landing site from inversion of ambient vibrations. Nat Commun 12, 6756 (2021).

https://doi.org/10.1038/s41467-021-26957-7

The SED, as well as other research groups within the ETH Zurich Department of Earth Sciences, is a long-standing partner of the Mont Terri rock laboratory, which has grown from a basic research facility in a side tunnel of the motorway tunnel between Saint-Ursanne and Courgenay into an internationally recognised institution. While research into the storage of radioactive waste was initially the laboratory's primary focus, in recent years work at the facility has increasingly been centred around the underground storage of CO₂. The SED has also been active in this field for several years as part of ELEGANCY, a research project funded by the Swiss Federal Office of Energy (SFOE) and the EU intended to investigate whether CO₂ extracted from industrial production (from waste incineration plants or the atmosphere, for instance) can be captured and safely stored deep underground permanently, whether in Switzerland or elsewhere. Several such carbon capture and storage (CCS) projects have already been rolled out around the world. One of the challenges here lies in ensuring that CO₂ does not slowly migrate to the earth's surface through fault zones (fracture zones deep underground) in the caprock and thus re-enter the atmosphere. To prevent this from occurring, we need a better understanding of the physical and chemical processes that influence whether and how CO₂ could escape through fault zones. We also need to look into whether the injected CO₂ could trigger microquakes. 

To this end, researchers from the SED and partner institutions injected several litres of CO₂-enriched saltwater into a fault zone in the Opalinus Clay over a period of several months at varying pressures, using geophysical and geochemical measurement sensors to monitor exactly what was happening in the rock. In principle, Opalinus Clay is an ideal caprock for a CO₂ storage facility because it is extremely impermeable to water. Until now, however, it was unclear whether CO₂ could migrate through fault zones in the clay. Initial results of the research conducted at Mont Terri show that the CO₂ injected near the natural fault zone rises as expected. However, rather than just taking the path of least resistance, i.e. along the fault, it also spreads in the surrounding area in a complex pattern, mixing with the CO₂ already present in the fault zone. As such, though CO₂ does move towards the earth's surface, it does so very slowly. In addition, the clay swells as soon as it comes into contact with the CO₂-saltwater mixture. This causes any cracks to close again, so there are no ways for CO₂ to rise. We can therefore assume that Opalinus Clay is a very efficient caprock and that no CO₂ would escape from the reservoir for thousands of years. In the medium term, CO₂ becomes part of the rock or is mineralised and is then permanently bound. The research findings are currently being prepared for scientific publication. Mont Terri's research thus contributes to achieving the UN climate goals, in which negative emissions using CCS play an important role. 

ETH Zurich also operates a rock laboratory in a tunnel deep underground, though this lab focuses on a slightly different field. The BedrettoLab is a research facility located about 1.5 kilometres below the earth's surface and in the middle of a 5.2-kilometre-long tunnel connecting Ticino with the Furka railway tunnel. The lab is home to a number of teams of scientists conducting experimental research, more specifically with a view to developing new methods for creating an efficient heat exchanger deep underground without triggering major perceptible or damaging earthquakes. The researchers also want to intentionally trigger very weak earthquakes with magnitudes of 0 to 1, which cannot be felt by humans, so that they can observe the approximately 10- to 30-metre-long fracture process from a few metres away. These experiments produce new scientific findings for geothermal energy and earthquake physics, as well as new techniques and sensors that can be used in this field. The SED is one of the BedrettoLab's key research partners and is responsible for the seismic monitoring of all activities. 

Even before the BedrettoLab opened its doors, the SED was collecting initial, vital data on the connection between geothermal energy and induced earthquakes at another rock laboratory in Switzerland, the Grimsel Test Site. Though on a somewhat smaller scale than in the Bedretto Valley, researchers investigated the physics of induced earthquakes, i.e. quakes stimulated by deep geothermal power projects, for example. The research in rock laboratories is supplemented by small-scale experiments with rock samples performed in the Rock Physics and Mechanics Laboratory at ETH Zurich, where researchers can control the environment better than directly in the rock. In order to advance research, research laboratories – whether in the rock deep below the earth's surface or in ETH's buildings – are key to enabling the scientific community to get to grips with the complex processes under way in the bowels of the earth. 

Schäden sind bei Beben dieser Stärke nicht zu erwarten. Falls Sie eines der Beben gespürt haben, können Sie Ihre Beobachtungen unter  www.seismo.ethz.ch/earthquakes/did-you-feel-an-earthquake melden.

Spürbare Beben sind im Jura vergleichsweise selten. Im Wallis hingegen werden im langjährigen Mittel ungefähr fünf Beben pro Jahr von der Bevölkerung gespürt. Schadensreiche Beben können grundsätzlich in beiden Regionen jederzeit vorkommen. Sie sind jedoch auch im Wallis, der Region mit der höchsten Erdbebengefährdung in der Schweiz, eher selten. Mit einem starken Beben mit einer Magnitude von 6 oder mehr ist etwa alle 50 bis 150 Jahre zu rechnen. Zuletzt ereigneten sich im Jahr 1946 bei Sierre (VS) ein Beben mit einer Magnitude von 5.8. Wo genau das nächste grössere oder kleinere Beben auftritt, lässt sich nicht vorhersagen. 

An algorithm translates the various seismic signals to determine the shape and size of the water arcs. The jets are arranged in four groups of three and can propel the water to heights of over 2.5 metres. One jet in each group produces a spray of water representing either the acceleration, speed or path (displacement) of the recorded ground motion. These three parameters are also fundamental for analysing seismological data. 

A large version of this water feature can be admired at Zurich's Seebad Enge resort, where the Aquaretum fountain also displays real-time signals from the Zürichberg station. The fountain usually shows how the waves of the Atlantic, the Mediterranean or the Baltic Sea are behaving. The seismic station on the Zürichberg continuously records the vibrations of the ocean waves and instantly transmits these signals to the fountain's control system. Roughly once a week, the dynamics of the water feature briefly change whenever a major earthquake occurs somewhere in the world whose vibrations shake Zurich's geological underground. With a bit of luck, you might even see smaller-scale Swiss earthquakes on the fountain at Lake Zurich and its little sibling at the focusTerra exhibition.

focusTerra's special exhibition 'Waves - Dive in!' will run until 5 March 2023.

More information is available here.