Tomography on a Grand Scale
Geologists investigate mountain forming processes in the Alps with the help of earthquakes at the other end of the world
Aug 13, 2020
Mountains are similar to icebergs. Only a small part of them can be seen. The much larger part is hidden below the surface because mountains have roots! “The ratio of mountains to their roots is about 1:8 – 1:9. But only if the mountains are in isostatic equilibrium,” says Professor Mark Handy, a geologist and specialist in tectonics at Freie Universität Berlin. However, few are in equilibrium, certainly not the Alps. Aged 30 million years, they are comparatively young “upstarts,” are under a lot of pressure, and are still emerging. “With an average height of mountains of 2000 meters, the roots would have to be about 16 to 20 kilometers thick and reach to a depth of about 40 to 45 kilometers – but they are actually 60 kilometers deep!” What’s going on down there? How are forces in the enormous rock masses distributed? That would be useful to know in order to better assess the risk of earthquakes and falling rocks at the surface.
The Fourth Dimension
While research has been conducted on the surface of the Alps for 200 years, very little is known about their depths. Investigating the depths is the focus of a research program funded by the German Research Foundation. The program, Mountain Building in Four Dimensions(4D-MB), is coordinated by Mark Handy. 3D refers to the forms of subsurface structures, whereas the fourth dimension in the title is time. For the work in various interdisciplinary research projects, scientists in the Earth sciences from 23 German universities and federal research institutions use very sensitive seismometers (geophones).
Exploring the Earth’s Interior
The 4D-MB program is the German contribution to the European project AlpArray. The term arrayrefers to a network of because with 610 geophone stations deployed across the entire Alpine region, including the Adria and Liguria. Data from the stations, which are spaced some 40 kilometers apart, are now being used by 64 research institutions in 17 countries. “AlpArray is like a large radio telescope. But instead of being directed into space, it is directed inward, toward the Earth,” explains Handy. “It provides us with important information about Earth’s interior of the Earth, down to a depth of 600 kilometers, possibly beyond.”
The Alps Began Emerging 130 Million Years Ago
The Alps started to form about 130 million years ago, when a small tectonic plate, the Adriatic Plate, started to converge with the European Plate. Today, parts of Italy and the seabed under the Adriatic lie on the Adriatic Plate. For millions of years, it was driven like a wedge into southern Europe. The edges of the plates pushed in and over, folding the Earth’s crust in a 1200-kilometer long and in places up to 250-kilometer wide mountain belt, which today extends from Nice to Vienna. Alpine mountain building is basically a crash in ultra-slow motion that has not ended. Even today, the Adriatic Plate drifts one to two millimeters a year northward, causing the eastern part of the Alps to grow by a few millimeters. In fact, the plate boundary is right at the foot of the mountains in northeastern Italy. The stress in the rock in this “crumple zone” is particularly high, – and with it, the danger of earthquakes. It is no coincidence that earthquakes occur most frequently in the northeastern corner of Italy in the Friuli and Veneto regions, as well as in Slovenia.
A Tomographic Image Is Created
How do the seismic measurements of AlpArray work? “At any given time, the Earth is always shaking somewhere in the world, creating elastic waves. Some of these waves run through the entire Earth and are collected by geophones on the other side.” Since the interior of the Earth is not homogenous, the waves – depending on the nature of the interior – do not reach the surface everywhere at the same time. “When they pass through dense material, such as lithosphere on the edges of plates that have been pushed down during collision, the waves travel faster. If the waves pass through a bit warmer material, like in the asthenosphere, the viscous part of the Earth’s mantle, they are slower,” explains Handy.
The seismometers send their data to centers at the ETH in Zurich, Grenoble, INGV in Rome, and the GFZ German Centre for Geosciences in Potsdam. There, the data from all 610 stations are merged, analyzed, and processed into 3D tomographic images of the interior of the Earth under the Alps.
The image below, which was created in collaboration between Mark Handy and colleagues in geophysics at the University of Bochum, shows deformed parts of old subducted tectonic plates buried under the Alps. In fact, they are no longer plates, but rather blobs or drops of lithosphere that extend to depths of several hundred kilometers. The resolution of these images is only about 10 to 40 kilometers, depending on the spacing of the measuring stations. Nevertheless, they show clearly and impressively that Earth’s deep interior is by no means as homogenous as previously thought.
The Highly Sensitive Geophone
The images are still blurred, but every earthquake, whether in Indonesia, Japan, California, or just around the corner in Italy provides further information and sharpens the images. Yet, what are such images good for? On the one hand, for a more precise identification of earthquake-prone regions. “The sectional images will allow us to create a much more precise model of the Earth’s crust and the Earth’s lithosphere, which we can use to calculatemass distribution at depth and to identify areas that are particularly at risk,” says Handy.
Accurate navigation of aircraft and satellites also depends on the mass distribution within the Earth. An observation made by a colleague in Zurich also showed how highly sensitive the seismometers are: A geophone on a small Adriatic island reported small vibrations every day between 12 noon and 1 p.m. There could not possibly have been such regular tremors there. “The colleague found out that during this period, a biologist went to the island every day by boat to count insects,” laughs Handy. The geophone linked to Zurich recorded every step she took.
The Roots of the Alps
The image shows the Alps as seen from France, with a distant view of the area around the Mediterranean (arrow points north). For the first time, the large structures under the mountains (in blue) are seen in three dimensions. The scientists involved in the project interpret the unusually deformed rock bodies in the depths of the interior of the Earth as the remains of old tectonic plates. Now it’s up to geologists in the 4D-MB project to explain how these structures formed through time, to relate them to the plate tectonic history they have determined from studies of the surface.
White lines mark longitudes and latitudes, and black lines indicate national borders. Red triangles show the locations of geophone stations. The image and its interpretation are based on collaboration of specialists in tectonics and gophysics at Freie Universität Berlin (Mark Handy, Emanuel Kästle) and Ruhr-Universität Bochum (Marcel Paffrath, Wolfgang Friedrich).
This text originally appeared in German on June 21, 2020, in the Tagesspiegel newspaper supplement published by Freie Universität.
Prof. Dr. Mark Handy, Department of Earth Sciences, Freie Universität Berlin, Email: firstname.lastname@example.org