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The 2013 September-October seismic sequence offshore Spain: a case of seismicity triggered by gas injection?

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Cesca, Simone and Grigoli, Francesco and Heimann, Sebastian and Alvaro, González and Buforn Peiro, Vicenta María Luisa and Maghsoudi, Samira and Blanch, Estefania and Dahm, Torsten (2014) The 2013 September-October seismic sequence offshore Spain: a case of seismicity triggered by gas injection? Geophysical journal international, 198 (2). pp. 941-953. ISSN 0956-540X

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Official URL: http://dx.doi.org/ 10.1093/gji/ggu172


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http://gji.oxfordjournals.org/UNSPECIFIED


Abstract

A spatially localized seismic sequence originated few tens of kilometres offshore the Mediterranean coast of Spain, close to the Ebro river delta, starting on 2013 September 5, and asting at least until 2013 October. The sequence culminated in a maximal moment magnitude Mw 4.3 earthquake, on 2013 October 1. The most relevant seismogenic feature in the area is the Fosa de Amposta fault system, which includes different strands mapped at different distances to the coast, with a general NE–SW orientation, roughly parallel to the coastline. However, no significant known historical seismicity has involved this fault system in the past. The epicentral region is also located near the offshore platform of the Castor project, where gas is conducted through a pipeline from mainland and where it was recently injected in a depleted oil reservoir, at about 2 km depth. We analyse the temporal evolution of the seismic sequence and use full waveform techniques to derive absolute and relative locations, estimate depths and focal mechanisms for the largest events in the sequence (with magnitude mbLg larger than 3), and compare them to a previous event (2012 April 8, mbLg 3.3) taking place in the same region prior to the gas injection. Moment tensor inversion results show that the overall seismicity in this sequence is characterized by oblique mechanisms with a normal fault component, with a 30◦ low-dip angle plane oriented NNE–SSW and a subvertical plane oriented NW–SE. The combined analysis of hypocentral location and focal mechanisms could indicate that the seismic sequence corresponds to rupture processes along shallow low-dip surfaces, which could have been triggered by the gas injection in the reservoir, and excludes the activation of the Amposta fault, as its known orientation is inconsistent with focal mechanism results. An alternative scenario includes the iterated triggering of a system of steep faults oriented NW–SE, which were identified by prior marine seismics investigations.


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© The Authors 2014. This article has been accepted for publication in Geophysical journal international Published by Oxford University Press on behalf of the Royal Astronomical Society..
We are thankful to Dr J. Wassermann and two anonymous reviewers for their comments. This work has been funded by the German BMBF ‘Geotechnologien’ project MINE (Grant of project BMBF03G0737) and Ministerio de Ciencia e Innovación project ALERT-ES (CGL2010-19803C03-01). AG was supported by the Caja Madrid Foundation (Spain) and the Spanish Government grant FIS2010-19773. We are thankful to the Seismic Network of the National Geographical Institute of Spain (IGN), to the Catalan Seismic Network of the Geological Institute of Catalunya (IGC) and to the Ebro Observatory for providing seismic waveform and catalogue data.

Uncontrolled Keywords:Earthquake dynamics; Earthquake source observations
Subjects:Sciences > Physics > Geophysics
Sciences > Physics > Meteorology
ID Code:29325
Deposited On:10 Apr 2015 07:53
Last Modified:10 Apr 2015 07:53

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