"Stone-breakers" clarify earthquake roots

Connection between brittle behavior in the rock and tectonic disturbances

Destroyed houses after earthquake © NGDC / NOAA
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Earthquakes still hold many secrets. So it is unclear what happens in the rock, when the voltages discharge jerkily. A Berlin geologist has now for the first time confirmed the relationship between brittle and ductile behavior in rocks and the development of a major tectonic fault using unusual methods.

Can you make stones speak? For the geologist Mark Handy of the Free University of Berlin there is only one answer, and it drives him. "Although the magnitude and distribution of earthquakes can be measured very accurately with seismographs, we must understand the conditions in the Earth's interior that lead to earthquakes." For it is still unclear what happens in the rock, when suddenly the gigantic tension that has accumulated between interconnected tectonic plates over centuries to millennia drops.

According to Handy's conviction, new insights are only available if one can better interpret the traces of the entire earthquake cycle - that is, the time from one big earthquake to the next - in the rock. An increase in knowledge is important here - because it may even be possible to better protect yourself from earthquakes. After all, megacities like Istanbul or San Francisco are on the threshold between tectonic plates and are threatened.

Costa Brava as research object

As part of a project of the German Research Foundation (DFG), the geologist initially searched for regions in which the crust of the earth concurrently shows traces of two different types of rock: brittle fracture and ductile flow. It had been deduced from laboratory experiments that the largest earthquakes occur more frequently in this so-called brittle-ductile transition in the earth's crust. The choice of scientists fell to the north of the Costa Brava. The area at the easternmost point of the Iberian peninsula provided ideal conditions for understanding the processes involved in the deformation of the Earth's interior. There, so-called tectonic uplift and erosion, former shear areas on the edge of tectonic plates originally 10 to 15 kilometers deep have entered the earth's surface.

Glass as a earthquake

What interested the coastal scientists most was minerals that were transformed or even melted by deformation at depth. Because glass as a remnant of molten rock in such shingles is witness to an earthquake. "At high stress and above all high deformation speed along wedged earth plates, it can heat up more than 1, 200 degrees at these depths for a short time", explains the tectoniker. display

The rapid release of pressure releases energy, which leads to the formation of rock glass and seismic waves within fractions of a second. In the top 30 kilometers of the earth's crust lies the hypocenter of most earthquakes, the source of seismic waves, explains Handy. Even the strongest ever measured quake in 1960 in Chile with 9.5 on the Richter scale had its hypocenter at a depth of about 30 kilometers.

Dragon with camera as a surveying tool

In their investigations in Spain, the researchers had to resort to unusual methods. Their trick: Two team members got a huge kite flying to which a remote-controlled digital camera was attached. The area was mapped so piece by piece by high-resolution photos from the dragon's perspective. These images and measurements as well as a number of rock samples were examined by the scientists in the laboratory. It cooperated with various specialists: with material scientists, geophysicists, and geochemists; the cooperation reached as far as Oslo and Liverpool.

The results were similar to finding a needle in a haystack: "For the first time, we have demonstrated the connection between brittle and ductile behavior in the rock and the development of a major tectonic disturbance, " explains Handy. Thus, not only the spatial but also the temporal course of the intersection of shear planes at depth during a phase of the earthquake cycle could be read off.

tsel of the "failing" strong earthquakes

The fossil quakes discovered in Spain occurred about 270 million years ago, according to specialists' calculations, and reached strengths of one to two on the Richter scale. "We are actually interested in strengths from six to eight with an area of ​​hundreds to thousands of square kilometers, " emphasizes the geoscientist. Why the traces of large earthquakes in the rock are not better preserved, remains a mystery for the time being: "Perhaps it is the selective memory of rocks, or the property of large earthquakes, their own Blurs tracks in depth, "says Handy.

Nevertheless, the analysis in Spain allows conclusions about active earthquake regions: The discovery of how shorers network may explain why earthquakes in space and time are not randomly distributed, but rather occur in linear patterns: " People talk about communicating earthquakes, "says Handy. Based on the findings from Spain, the team now wants to use mathematical models to analyze under what conditions and to what extent stresses along growing shingles will break up and down again.

Between split seconds and millions of years

The challenges for research are enormous: "We have to combine methods to capture on the one hand the movement history of tectonic faults on a continental scale and on the other hand to characterize nanoscale processes", f Turn off the phone. "And we need to understand processes that occur in fractions of a second as well as changes that have evolved over millions of years." A fundamental problem is that people lived shorter than the cycle between two earthquakes, Handy emphasizes: "So we are prisoners of our time."

However, this is no reason to give up, says the tectonician, because the rapid technical progress offers the research manifold possibilities. Thus, the one-year satellite network "Galileo" of the European Union will offer better opportunities to detect even tiny movements of the earth's surface. But one thing is certain for Handy: that one dares to predict exactly the time for the destruction of a metropolis, so exactly one will never understand the stones.

(Free University Berlin, 22.02.2007 - NPO)