Publication: Datación mediante liquenometría de los desprendimientos rocosos asociados a la sismicidad histórica en Lorca
(Murcia, SE de España)
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2012-10
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Instituto Geológico y Minero de España
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Abstract
El terremoto de Lorca (Murcia, SE de España) del año 2011 (Mw 5,2, profundidad 4 km) provocó una serie de caídas de rocas con un volumen total inferior a los 2000 m3. Estos desprendimientos de origen sísmico se localizan principalmente en la Sierra de las Estancias y en la Sierra de La Tercia, ambos relieves formados por calizas tortonienses con escarpes superiores a 30 metros de altura, así como con diferentes orientaciones. En estos mismos sitios se han encontrado evidencias de antiguas caídas de bloques. En consecuencia, se ha aplicado la técnica de la liquenometría para estimar la edad de bloques antiguos en dos desprendimientos de la Sierra de las Estancias, con el fin de comprobar su posible origen sísmico. Con este fin, se ha utilizado una tasa de crecimiento de 0,24 mm/año para líquenes calcícolas en climas templados. Cabe recordar que Lorca ha experimentado sismicidad histórica de campo cercano, concretamente con dos terremotos en 1674 (EMS VIII) y en 1818 (EMS VI), así como los efectos de campo lejano del terremoto de Lisboa del año 1755 (EMS VI). Los resultados indican que estos desprendimientos rocosos se originaron durante el terremoto de 1674, movilizando un volumen rocoso veinte veces mayor al estimado para el terremoto del año 2011. Según esto, es posible especular que el sismo de 1674 estuviese asociado al mismo segmento que generó el sismo del 2011, el segmento Lorca Totana de la Falla de Alhama de Murcia, y que tuviese una magnitud entre 6,1 y 6,8 en relación a una posible profundidad entre 5 y 15 km, respectivamente.
During the earthquake at Lorca (Murcia, SE Spain) in 2011 (5.2 Mw, 4km depth) several rock-falls occurred, mobilizing an estimated volume of close to 2,000 m3. All these rock-falls took place within the Estancias and La Tercia mountain ranges, the topography of which is composed of Tortonian calcarenitic sandstones with steep scarps more than 30 m in height. We have conducted a lichenometric study to obtain the age of the ancient rock-falls within the Las Estancias Range. We have assumed an annual growth rate of 0.24 mm for lichen species classified as calcicolous and related to warm climatic conditions. Our aim was to corroborate the hypothesis that seismic events triggered these massive rock-falls. The city of Lorca had experienced two nearfield historic earthquakes (1674 EMS VIII and 1818 EMS VI) and one far-field tremor during the great Lisbon earthquake in 1755 (EMS VI). Results obtained here indicate that the earthquakes of 1674 and 2011 were quite similar, except that the 1674 one mobilised a greater quantity and twenty times the volume of blocks mobilised during the 2011 earthquake. Therefore, we conclude that the size of the earthquake of 1674 was possibly between 6.0 < M < 6.8, assuming similar focal and seismotectonic conditions to those of the instrumentally measured earthquake of 2011.
During the earthquake at Lorca (Murcia, SE Spain) in 2011 (5.2 Mw, 4km depth) several rock-falls occurred, mobilizing an estimated volume of close to 2,000 m3. All these rock-falls took place within the Estancias and La Tercia mountain ranges, the topography of which is composed of Tortonian calcarenitic sandstones with steep scarps more than 30 m in height. We have conducted a lichenometric study to obtain the age of the ancient rock-falls within the Las Estancias Range. We have assumed an annual growth rate of 0.24 mm for lichen species classified as calcicolous and related to warm climatic conditions. Our aim was to corroborate the hypothesis that seismic events triggered these massive rock-falls. The city of Lorca had experienced two nearfield historic earthquakes (1674 EMS VIII and 1818 EMS VI) and one far-field tremor during the great Lisbon earthquake in 1755 (EMS VI). Results obtained here indicate that the earthquakes of 1674 and 2011 were quite similar, except that the 1674 one mobilised a greater quantity and twenty times the volume of blocks mobilised during the 2011 earthquake. Therefore, we conclude that the size of the earthquake of 1674 was possibly between 6.0 < M < 6.8, assuming similar focal and seismotectonic conditions to those of the instrumentally measured earthquake of 2011.