Playing stress ping-pong in the earth crust

Indonesia, early May 2006. The Merapi Volcano is spewing hot, grey ashes. More than 22,000 slope dwellers are evacuated, some all the way to Yogyakarta in the safe south. But exactly how safe is "the south"? The earth is also shaking here. On 27 May, an earthquake destroyed large parts of the province. Several thousand people died, probably including many of those who had earlier been evacuated. But that was not the end of it. The earthquake was directly followed by Merapi becoming more active again. Volcanologist Dr. Thomas Walter from the GFZ GeoForschungsZentrum Potsdam is trying to find out whether the two events are inter-related. In autumn 2005, he took charge of a Young Investigators Group on volcano tectonics, one of the few research teams worldwide to preoccupy themselves with the ping-pong game between magma mass changes and the movement of crustal rock. The scientists want to understand the disaster in two respects. "We're checking whether and how volcanic eruptions and earthquakes influence each other and what mechanisms play a role." The two natural events mostly become simultaneously active in areas where the tectonic plates meet, move away from or overlay each other. Enormous stresses exist there. "To reduce these, the Earth repeatedly shakes itself back into place," says Walter.

At the same time, the stress levels shift in the crust and the nearby magma reserves respond. "Occasionally, the two phenomena give each other a little kick." A subterranean power struggle that fascinates Walter. Actually, he had planned to study mineralogy, "inspired by his grandfather, a passionate collector." Eventually, geology won the day. On an academic year in Hawaii, he discovered his penchant for volcanological research and subsequently did his doctorate at GEOMAR in Kiel with a thesis on landslides in the Canary Islands. He was then drawn back to the United States again. At a satellite ground station run by the University of Miami, he was able to observe volcanoes from space. "Even though not that much happens on the surface, when a volcanic area lifts by just one centimetre the satellite radar can see this."

The Emmy Noether Programme funded by the German Research Foundation (DFG) eventually took Walter to Potsdam, where he does his research today and supervises several doctoral students.  The eyes in space are still among his most important instruments. Earthquake measurements, gravity measurements, structural studies in the field, and computational modelling all serve to complement the researcher's toolbox. Furthermore, Walter and his team simulate volcanic scenarios in lab experiments. "We only find out that something is really happening underneath our feet when several sensors start to register changes." This great methodological breadth also convinced the Geologische Vereinigung. And so, last year, the geological society awarded the young researcher with the Hans Cloos Prize for internationally outstanding contributions to science. In Chile, for example, Walter's team discovered a new giant volcano that is a kind of natural laboratory. Located east of the town of Antofagasta, close to the border with Argentina, the ground lifts over 1000 square kilometres. "Ten years ago, no volcanic activity was known here. After several strong earthquakes, a system developed here in a short period of time that is now more active than the Yellowstone Volcano in the United States." However, Walter cannot see any acute threat or danger: although the magma is flowing quite quickly at three kilometres per year, it is currently only moving horizontally and at a depth of ten kilometres beneath the crust.

In contrast to Merapi. Following last year's earthquake, it began to become really active. The volume and daily number of emissions increased in leaps and bounds. A dangerous search for the geological reasons? "The traffic presented a much greater threat than did working on the volcano." Now the scientists want to find out whether the volcano is triggered by occasional seismic waves, the permanently transformed tensions or even completely different mechanisms. "Once we know this, we will perhaps be able at some time to improve the predictions." And the risk for people and infrastructure could decrease.