sábado, 25 de mayo de 2013


SEE YOU IN MÉXICO.  MAY 14-17 2013


An unusual seismic swarm has been striking the Torreperogil-Sabiote Hill (TSH) in southern Spain since October 2012. Its origin is unknown as it is located in an inactive intraplate setting. Last year was hydrologically anomalous in the region with a long-lasting drought period, interrupted by three months of intense rainfalls marking a historical record for the last 25 years. A series of arguments suggest a cause-effect relationship between intense rainfall and seismicity (hydroseismicity): 

1) Earthquakes began one month after heavy precipitations and they basically peak together if one observes the regional record since 2010 (other isolated seismic events occurring before also coincided with rainfall). 

2) The seismic swarm of the TSH is unusual: more than 2000 minor events concentrate in a 10km-diameter subcircular area with low magnitudes and depths (1.0 to 3.7 Mw, between 0 and 5 km) often accompanied by “natural explosive sounds” and migrating spatially with time.

3) The geological structure of the TSH is particularly favorable to the downwards percolation of water towards a series of short fault segments in the basement, thus contributing to lubricate/activate them. It is constituted by a massive edifice up to 750 m thick of upper Miocene detrital sediments defining a bowl-shaped depocenter, underlain by a “gruyere-like” basement with cavernous-like Triassic evaporitic Keuper facies capped by karstified Jurassic carbonates, transected by minor faults beneath the depocenter. 

4) The main aquifer located within the Jurassic carbonates (between 500 and 800 m) has been severely overexploited for the past 20 years by numerous irrigation wells.

5) The maximum concentration of water wells, as well as the main recharge area and convergent subterranean water-fluxes of this aquifer seem to coincide spatially with the subcicular seismic area.

We suggest a mixed (natural and induced) hydroseismic hypothesis for the TSH seismic swarms:

1) According to Assumpçao et al. (2010, Water Resources Research, W07527), irrigation wells connecting aquifer horizons that are under different pore-pressure regimes can trigger earthquakes: in the TSH there is a 400 m fall between the superficial Miocene and the confined Jurassic aquifer, and it is conceivable that the water might have plunged down generating avalanches, collapses, and other instabilities. 

2) The huge Giribaile dam built on the cavernous Triassic Keuper facies in the northern boundary of the TSH was overfilled by the massive rains and it leaked its extra water load along the S-dipping strata towards the buried/faulted center of the Miocene depocenter.

3) The water-soaked thick Miocene sedimentary series added an extra weight that contributed to the general instability of the TSH.

4) The “gruyere-like” basement of the Miocene edifice with cavernous-like Triassic evaporitic Keuper facies capped by the overexploited karstified Jurassic carbonates was violently refilled by the percolating waters.

MÉXICO, MAY 14-17 2013

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