Tipos de Desastres Naturales Tras Un Terremoto: Procesos Superficiales Terrestres

Tras el terrible terremoto que ha afectado a China, conviene reflexionar y analizar los efectos sobre las perturbaciones de los procesos superficiales terrestres que acaecen, tanto antes como después de un seísmo. En este post nos centraremos en los eventos de esta naturaleza que ocurren en plena placa continental, soslayando los que se generan en los fondos marinos, y que dan lugar a maremotos o tsunamis, como el que ha afectado trágicamente a Birmania. No hablaremos tampoco de las consecuencias humanitarias. Un sismo de la magnitud que ha sacudido a China conlleva una profunda alteración de la superficie terrestre. Tanto la fisiografía, como la hidrología y los suelos-regolito suelen ser afectados sobremanera. Como os exponemos en la noticia que ofrece al respecto Terradaily, al final del post, el Gobierno de aquél país se dispone a prepararse para la segunda factura, es decir la post-sísmica. Nuevas víctimas y tragedias acechan, cuando el ciudadano comienza a olvidarse de la noticia. Como veréis, la perturbación de la cobertura de suelos es una de las principales generadoras de estas lamentables secuelas de la furia de la naturaleza.     

 

 

Megaripples generados por una gigantesca inundación

 

Obviamente, los procesos superficiales que serán alterados dependen tanto de la intensidad del sismo, como de la zona en donde se encuentra ubicada la región afectada. En consecuencia, ni podemos generalizar en exceso, ni ser exhaustivos. Sin embargo, lo que digamos servirá como un botón de muestra que ayudará a que os percatéis de la gravedad del asunto.

 

Los Terremotos pueden llegar a acarrear unas transformaciones severas de la cortaza terrestre, a menudo agravadas por el hecho de que otros sismos de menor intensidad suelen ocurrir antes y después del acontecimiento principal. Si adicionalmente existen  infraestructuras de gran tamaño y/o riesgo en la zona (embalses, centrales nucleares, etc.), puede legar a darse el caso de que los efectos sean más demoledores de los que generan las sacudidas de mayor envergadura.

 

Como ya hemos comentado, el temblor de la tierra genera que suelos y regolitos se deslicen por las pendientes como en una pila de arena cuando se menea su soporte. De este modo, se generan graves problemas de erosión del suelo por deslizamientos y avalanchas. El problema puede empeorar si, adicionalmente, estamos ubicados en zonas montañosas con coberturas nivales y/o glaciares conspicuas, que como los suelos correrán vertientes abajo arrastrando y sepultando todo lo que encuentren a su paso. Pueblos enterrados y laderas erosionadas serán algunas de las consecuencias. Estas suelen destrozar infraestructuras viarias, entorpeciendo la ayuda a las víctimas.

 

Como en el caso de China, no es infrecuente que tal avalancha de sedimentos hacia los cauces fluviales interrumpa el fluir natural de los ríos generando represas de agua que pueden llegar a almacenar volúmenes muy importantes de este líquido elemento.  Una vez generados, estos pueden quebrase por la un temblor menor, o por la misma erosión del sedimento allí acumulado. En consecuencia, tanto si se han producido grietas en una presa artificial, como si se trata de estas represas pos-sísmicas naturales, su desmoronamiento amenazará con generar inundaciones y avenidas cauca abajo, cuyos efectos estarán condicionados por la magnitud del agua represada y la carga de sedimentos que arrastran.

 

Se han documentado casos en los que los propios cursos fluviales pueden cambiar de rumbo y desviarse notablemente de cómo transcurrían antes del sismo. Tal hecho comporta una transformación profunda de los paisajes y los ecosistemas afectados. Sin embargo, a lo largo de la historia se han producido fenómenos asombrosamente mucho más dramáticos. Expongamos un ejemplo.

 

 

 

Paisaje de Scablands

 

Tras las glaciaciones, se han producido de forma natural represas de hielos de tamaños gigantescos (casi inimaginables). Ya fuera por el deshielo, ya por un pequeño temblor, al resquebrajarse enormes cantidades de agua son derramadas súbitamente, desparramándose sobre territorios extensos y modificando la fisiografía regional a lo largo de miles y miles de kilómetros cuadrados. Este fue el caso de la cabecera del Río Columbia en EE.UU., o de algún curso de las Montañas de Altai en Siberia (como también en el Tibet). En el primer caso mentado, estudios realizados por Victor Baker  (uno de los expertos mundiales en el tema) y mi colega y amigo Gerardo Benito (actualmente en el CCMA (CSIC, Madrid) constataron que se vertieron al mar en pocos días el mismo volumen de agua que acarrean todos los ríos del mundo en un año. Este trabajo fue publicado en Science en la década de 1990. ¿Os imagináis los efectos sobre el paisaje? ¡Os aseguro que no! Incluso pueden alterar las circulaciones marinas (como las corrientes termohalinas) generando cambios climáticos regionales de enorme magnitud. Junto con el agua, son arrastradas ingentes cantidades de sedimentos que al decantarse cubrirían el paisaje precedente con las típicas formas que produce el oleaje marino en una playa, pero de dimensiones desorbitadas (como os ripples). También dio lugar a extrañas formas denominadas “scablands” Aunque parezca curioso, tales modelados pueden llegar a confundirse con los mares de dunas de los desiertos. Se ha dado el caso, de que los científicos tengan que hacer uso de estudios e instrumentales muy concienzudos y sofisticados para poder discernir si se trataba de un relieve eólico o hídrico. No es un disparate pensar que algunos de los hoy considerados antiguos desiertos relictos, de climas precedentes, sean en realidad inundaciones como las descritas, especialmente si se encuentran localizados a la salida de de vertientes montañosas que albergaron glaciares.

 

Visto desde la distancia espacio-temporal desde la que escribo, resultan eventos fascinantes. Sin embargo, los fragmentos de la biosfera que arrastraron no dejan de ser cataclismos naturales difíciles de imaginar. Como veis, muchos de los efectos de los terremotos proceden del movimiento de tierras y de la erosión del suelo.  Que yo sepa, no se han realizado estudios que analizaran la magnitud de los procesos de erosión-deposición producidos. La génesis de los suelos y regolitos profundos raramente aparecen en las zonas más activas de las placas tectónicas, sino en sus porciones más estables, como lo puede ser  el craton por el que transcurre la Cuenca del Amazonas. No obstante, estos padecerán a la larga, pérdida de fertilidad, mientras que las coberturas de suelos de las zonas que sufren sismos son más jóvenes y delgadas, pero por ello también más ricas en nutrientes (fértiles, a menudo).

 

 

En búsqueda del Arca de Noé

 

Otro caso espectacular, que aun está por demostrar, es el del Mar Negro. Estudios realizados, hace no muchos años, parecen indicar que este mar era en realidad un lago de agua dulce cuyas dimensiones eran 1/3 del actual. Sin embargo, hace unos 5.500 años, algunos investigadores defienden que, probablemente debido a un terremoto, el actual Estrecho de Dardanelos vio modificada su configuración de tal forma que el lago fue invadido por una cataclísmica entrada de agua del mar Mediterráneo. Según algunos estudiosos, la devastación fue total, generándose el mito del “diluvio universal” que hoy aparece tanto en los escritos mesopotámicos como en los de la Biblia. En este último enlace nos informan de que la mitología mapuche depara una leyenda similar. Ya hablaremos del tema, por cuanto quizás no sea una mera coincidencia………. 

 

 

Bósforo y Dardanelos (fuente Wikipedia)

 

Veamos seguidamente lo que las autoridades chinas temen tras al reciente terremoto que a devastado parte de su territorio.            

 

Juan José Ibáñez

 

More disaster awaits China’s quake zone: official says


Aftershocks, avalanches and flooding could bring more geological disasters to China’s mountainous southwest following last week’s devastating earthquake, a senior government official said Thursday. The 8.0-magnitude earthquake has increased risks along China‘s Dragon Gate seismic fault, a well-known geological landmark, Yun Xiaosu, vice head of the Ministry of Land and Resources, told reporters.

 

«As rain increases during the rainy season and coupled with the continuous aftershocks, it is very probable that geologic hazards in the disaster area will increase and again bring a large amount of losses,» he said. Heavy rains were likely to bring greater instability to the mountains that had already been greatly shaken by the earthquake and its nearly 170 aftershocks, Yun said.

 

«This has made the work of monitoring, prevention and mitigation of potential geologic disasters great and the tasks before us arduous,» he said. The quake, the biggest to hit China in more than 30 years, has left over 80,000 people dead or missing, and more than five million homeless. Most of the flattened towns and villages have been along the 200-kilometre (120-mile) long Dragon Gate seismic fault in the Himalayan foothill region of Sichuan province.

 

Yun said one of the biggest concerns was the creation of 34 bodies of water known as «quake» or «barrier» lakes that formed after landslides blocked rivers. He said they could burst and threaten populated areas. «We must be aware that the barrier lakes that we have identified are very serious,» said Liu Yuan, vice director of the ministry’s geologic environment department. «I want to be clear about this in order to ensure safety — if these barrier lakes burst, the after effects will be severe over a wide area.»

 

Officials had ordered people in all threatened areas near the quake lakes to evacuate, he said. Experts were studying ways to engineer channels around or through the barriers to alleviate rising waters and reduce the potential dangers, especially on the Minjiang River, a major tributary of the Yangtze, he said. The officials further warned that more quake lakes could be formed if the frequency of aftershocks, landslides and avalanches in the mountainous region increased during the rainy season, which begins next month. On Thursday, the government announced that 167 aftershocks over 4.0 magnitude had struck along the Dragon Gate seismic fault since the May 12 quake, with four registering over magnitude 6.0. The fault, also known as the Longmenshan fault, runs along the rugged and steep Dragon Gate mountains, which form a geological border between the plain of Sichuan province and the towering Himalayan mountains, the world’s highest.

 

Yun said his ministry had identified more than 4,900 geological hazards along the Dragon Gate fault before the earthquake struck, of which 158 of them were considered major hazards.

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