Around-Tear
When there is an earthquake, our home planet shows the power it contains. The earth's crust cracks, and within minutes a fissure runs thousands of kilometers through the subsurface. He can even do huge hooks. On June 23, 2001, a severe earthquake shook southern Peru. The massive tremor was felt as far away as Bolivia and caused major damage. The old town of the megacity Arequipa – declared a World Heritage Site by Unesco – lost its landmark when a tower of the cathedral collapsed. A severe aftershock terrified people again the next day.
Earthquakes are still among the most threatening natural events. Despite global seismic measuring stations, they are hardly predictable, which is why geoscientists try to reconstruct them as precisely as possible. The aftershocks in particular play a key role: As a kind of rule of thumb, weak aftershocks are to be expected after earthquakes with a strong displacement of the crust parts. The basic idea behind this is that the tension in the earth's crust has already been discharged with the first rum, so to speak, and only little energy is left for aftershocks.
David Robinson of the University of Oxford found, however, that the aftershocks from the Arequipa tremor were much stronger than expected and that they were also distributed in a completely random manner. When he and his colleagues then traced the exact course of the earthquake, they discovered that the propagating rift had circled two obstacles. "After initially propagating 70 kilometers to the southeast, the rupture skirted a 6,000-square-kilometer barrier, then resumed its original direction and then skirted a second, smaller barrier," explains Robinson.
But even the hard lumps couldn't withstand the tension for long: after 90 seconds they burst - at a speed of 10,000 kilometers per hour the innermost forces had disintegrated both. This was probably responsible for most of the destructive power of the Arequipa earthquake, conclude Robinson and his colleagues.
But what did the Rift stumble upon? To find out, the researchers used ships to survey the seabed in the Pacific Ocean off the coast of Peru. Here, the Nazca Plate pushes 50 millimeters further under the South American continent every year. However, the oceanic crust is not a smooth conveyor belt that smoothly descends beneath South America. The undersea landscape has cracks, folds or mountains that are swallowed up.
Robinson and his team identified a fracture zone and numerous submarine hills that are moving toward the precise areas of the earthquake region where the researchers had located the barriers. So in the case of the Arequipa quake, the remnants of a swallowed oceanscape may have gotten in the way of the earthquake's main direction-forcing the rift to take a detour before it ruptured them, too.
