A tectonic zip

The complex fracture pattern created by the earthquake in Concepción (Chile) on 27 February 2010 was to a certain extent predictable. GPS observations from the years before the earthquake showed the pattern of stresses that had accumulated through the plate movements during the past 175 years in this area. The stress distribution derived from the observations correlates highly with the subsequent fracture distribution. In all likelihood the tremor removed all the stress that had accumulated since the last earthquake in this region, which was observed by Charles Darwin in 1835. An earthquake of similar magnitude in this area is therefore unlikely in the near future. This result was presented by scientists of the GFZ German Centre for Geosciences (Helmholtz Association) in the latest edition of the scientific journal Nature (09 September 2010).

"The Maule earthquake near Concepción, Chile, on the 27 Februar registered with a momentum magnitude of 8.8, makes it one of the largest earthquakes to have been recorded in its entirety via a modern network of space-geodetic and geophysical instruments on the ground," says Professor Onno Oncken, head of the Department "Geodynamics" at GFZ. "It thus offers a unique opportunity to compare detailed observations prior to the earthquake with those taken during and after it, and to re-evaluate hypotheses regarding the predictability of such events."

Measurements using the satellite navigation system GPS showed that the seafloor of the Nazca plate in the Pacific Ocean does not slide evenly under the western boundary of the South American continent. Rather, it appears from the GPS measurements that some parts of the ocean floor got locked with the subsurface of the continent. In the gaps, however, the Nazca plate continued to push under South America. The resulting uneven stress pattern was released by the earthquake of the 27 February in such a way that, just like a zipper, the locked patches were ruptured one after the other. As a result, this seismic gap off the Chilean westcoast is now closed, one last gap remains in northern Chile. Here, the GFZ scientists set up a plate boundary observatory, in order to make use of the entire range of geoscientific instruments to record the conditions before, during and after an earthquake- an important step in understanding the processes of plate tectonics.

Modern Earth science may still not be able to predict the location, time and magnitude of an earthquake. But the present study offers an optimistic perspective concerning the predictability of possible fracture patterns and magnitudes of expected earthquakes.

Note: This story has been adapted from a news release issued by the Helmholtz Association of German Research Centres

Glaciers help high-latitude mountains grow taller

Glaciers can help actively growing mountains become higher by protecting them from erosion, according to a University of Arizona-led research team.

The finding is contrary to the conventional view of glaciers as powerful agents of erosion that carve deep fjords and move massive amounts of sediment down mountains. Mountains grow when movements of the Earth's crust push the rocks up.

The research is the first to show that the erosion effect of glaciers - what has been dubbed the "glacial buzzsaw" - reverses on mountains in colder climates.

The researchers were surprised, said first author Stuart N. Thomson, a research scientist in the UA department of geosciences. "We were expecting to see the buzzsaw."

The team discovered the protective effects of glaciers by studying the Andes Mountains in the southernmost region of South America, known as Patagonia.

UA co-author Peter W. Reiners said, "It's been thought that glaciers limit the height of mountain ranges worldwide."

The key is climate. Glaciers atop mountains in temperate latitudes flow downhill, scouring away the surface of the mountain. Over millennia, such erosion can reduce the height and width of a mountain range by miles.

However in very cold climates such as the Patagonian Andes, rather than scraping away the surface of the mountain, the team found that glaciers protect the mountain top and sides from erosion.

The team dubs the action of the cold-climate glaciers "glacial armoring."

"Climate, especially through glaciers, has a really big impact on how big mountains get," said Reiners, a UA professor of geosciences.

"What we're seeing is that below certain latitudes, glacial buzzsaws clearly and efficiently operate, but south of about 45 degrees, it not only doesn't work - it has the opposite effect," he said. "The glaciers actually protect the surface and allow the mountains to grow higher."

He and his colleagues anticipate that glacial armoring also occurs on cold-climate mountains very far north, such as those in Alaska.