Precisamente, una investigación de la Universidad de Cornell (EEUU) -publicada el pasado domingo en la revista Nature Geoscience- puso en duda la teoría de la elasticidad de los terremotos, tras descubrir que éstos pueden agrietar la Tierra de forma permanente.

Según lo detallado por el sitio web LiveScience, los modelos vigentes sugieren que toda la deformación de la placa superior relacionada con los ciclos sísmicos es elástica, es decir, que tras un período de tiempo vuelve a su lugar como lo hace una banda elástica y que, por lo tanto, el daño no es permanente.

No obstante, el nuevo estudio liderado por el geólogo estructural Richard Allmendinger cuestionó estos paradigmas, tras postular que los terremotos de magnitud 7 o superior provocaron que la corteza en el norte de Chile se quebrara de manera permanente.

En este sentido, el experto señaló que durante su indagación en la zona norte de nuestro país se hallaron lugares en los que estas grietas estaban muy expuestas: “Todavía recuerdo haberme sentido impresionado -nunca había visto nada como éstas (grietas) en mis 40 años como geólogo- y también perplejo”.

“¿Qué eran estas características y cómo se formaron? Los científicos odian dejar cosas como éstas sin explicar, así que este tema quedó dando vueltas en mi mente”, agregó Allmendinger.

Gracias al registro de los numerosos sismos en las rocas del norte chileno, los investigadores pudieron examinar su comportamiento durante un largo período de tiempo y, así, descubrir ciertos patrones.

Tras ello, hallaron que entre el 1% y 10% de la deformación de la Tierra causada por entre 2.000 y 9.000 grandes terremotos, en un período de entre 800.000 años y 1 millón de años, era permanente. En este cálculo, se incluyen las grietas en la corteza del Desierto de Atacama.

Con los resultados en mano, Allmendinger y su equipo sugirieron que la corteza tendría menos elasticidad de la que se pensaba hasta ahora.

“Sólo en un lugar como el Desierto de Atacama pueden ser observadas estas grietas. En todos los demás lugares, los procesos de las superficies los borran dentro de días o semanas de su formación, pero en Atamaca se preservaron por millones de años”, añadió el especialista.

Cabe señalar, que los científicos creen que los resultados obtenidos en el norte de Chile pueden ser aplicables en otras áreas.

Al respecto Allmendinger dijo que Iquique, una de las áreas que él y su equipo estudiaron, “es uno de los pocos lugares junto con la parte occidental de Sudamérica que no ha tenido un gran terremoto en los últimos 100 años, por lo que hay una gran probabilidad de que ocurra un gran sismo en las próximas décadas (…) Podríamos poner a prueba nuestras predicciones sobre los terremotos, si el nuevo gran sismo en ese lugar ocurre en las próximas dos décadas”, concluyó.

Fuente: http://www.biobiochile.cl/2013/04/30/geologo-estadounidense-revela-...

ARTICULO:

Deformación permanente causada por los

terremotos de subducción en el norte de Chile

Earthquakes are accompanied by coseismic and post-seismic rebound: blocks of crust on either side of the fault spring back to their initial, undeformed configuration. This rebound is well documented by space geodetic data, such as the Global Positioning System. Thus, all earthquake-induced deformation of the crust is considered non-permanent and is modelled as an elastic or visco-elastic process. Here, however, we show that earthquakes larger than magnitude 7 in northern Chile caused the crust to deform permanently. We identify millimetre- to metre-scale tension cracks in the crust of the Atacama Desert and use cosmogenic nuclides to date the timing of crack formation.

The cracks were formed by between 2,000 and 9,000 individual plate-boundary earthquakes that occurred in the past 0.8–1 million years. We show that up to 10% of the horizontal deformation generated during the earthquakes, recorded by Global Positioning System data and previously assumed to be recoverable, is permanent. Our data set provides a record of permanent strain in the shallow crust of the South American Plate. Although deformation of the deep crust may be predominantly elastic, we conclude that modelling of the earthquake cycle should also include a significant plastic component.

Fuente: http://www.nature.com/ngeo/journal/vaop/ncurrent/full/ngeo1789.html

Earthquakes can permanently crack the Earth, an investigation of quakes that have rocked Chile over the past million years suggests.

Although earthquakes can wreak havoc on the planet's surface, more than a century of research has suggested the Earth actually mostly rebounds after quakes, with blocks of the world's crust elastically springing back, over the course of months to decades, to the way they initially were. Such rebounding was first seen after investigations of thedevastating 1906 San Francisco temblor thathelped lead to the destruction of more than 80 percent of the city. The rebound is well-documented nowadays by satellite-based GPS systems that monitor Earth's movements.

"My graduate students and I originally went to northern Chile to study other features," Allmendinger said. "While we were there, our Chilean colleague, Professor Gabriel González of the Universidad Católica del Norte, took us to a region where these cracks were particularly well-exposed."

"I still remember feeling blown away — never seen anything like them in my 40 years as a geologist — and also perplexed," Allmendinger told OurAmazingPlanet. "What were these features and how did they form? Scientists hate leaving things like this unexplained, so it kept bouncing around in my mind."

Atacama exposed

In northern Chile, "the driest place on Earth, we have a virtually unique record of great earthquakes going back a million years," Allmendinger said. Whereas most analyses of ancient earthquakes only probe cycles of two to four quakes, "our record of upper plate cracking spans thousands of earthquake cycles," he noted.

The record of the vast number of earthquakes captured in northern Chilean rocks allowed the researchers to examine their average behavior over a much longer period of time, which makes it easier to pick out any patterns. They discovered that a small but significant 1 to 10 percent of the deformation of the Earth caused by 2,000 to 9,000 major quakes over the past 800,000 to 1 million years was permanent, involving cracks millimeters to meters large in the crust of the Atacama Desert. The crust may behave less elastically than previously thought.

"It is only in a place like the Atacama Desert that these cracks can be observed — in all other places, surface processes erase them within days or weeks of their formation, but in the Atacama, they are preserved for millions of years," Allmendinger said. "We have every reason to believe that our results would be applicable to other areas, but is simply not preserved for study the way that it is in the Atacama Desert," he added.

Model rethink

This work "calls into question the details of models that geophysicists who study theearthquake cycle use," Allmendinger said. "Their models generally assume that all of the upper-plate deformation related to the earthquake cycle is elastic — recoverable, like an elastic band — and not permanent. If some of the deformation is permanent, then the models will have to be rethought and more complicated material behaviors used.

The area the researchers studied, the Iquique Gap, "is one of the few places along western South America that has not had a great earthquake in the last 100 years and thus has a high probability of a major earthquake in the next couple of decades," Allmendinger added. "We may get to test out predictions about earthquakes if the next great earthquake there happens in the next couple of decades."

The scientists detailed their findings online April 28 in the journal Nature Geoscience.

Fuente: http://www.livescience.com/29091-chile-earthquakes-leave-permanent-...