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Title:
The 2013 September-October seismic sequence offshore Spain: a case of seismicity triggered by gas injection?
Authors:
Cesca, Simone; Grigoli, Francesco; Heimann, Sebastian; González, Álvaro; Buforn, Elisa; Maghsoudi, Samira; Blanch, Estefania; Dahm, Torsten
Affiliation:
AA(GFZ German Research Centre for Geosciences, Potsdam, Germany. ; Institute of Earth and Environmental Sciences, University of Potsdam, Germany), AB(Institute of Earth and Environmental Sciences, University of Potsdam, Germany), AC(GFZ German Research Centre for Geosciences, Potsdam, Germany), AD(GFZ German Research Centre for Geosciences, Potsdam, Germany. ; Departamento de Ciencias de la Tierra, Universidad de Zaragoza, Spain), AE(Departamento de Geofísica y Meteorología, Universidad Complutense de Madrid, Spain; IGEO, UCM-CSIC, Madrid, Spain), AF(Institute of Earth and Environmental Sciences, University of Potsdam, Germany), AG(Observatori de l'Ebre, Roquetes, Tarragona, Spain), AH(GFZ German Research Centre for Geosciences, Potsdam, Germany; Institute of Earth and Environmental Sciences, University of Potsdam, Germany)
Publication:
Geophysical Journal, Volume 198, Issue 2, p.941-953 (OUP Homepage)
Publication Date:
08/2014
Origin:
OUP
Keywords:
Earthquake dynamics, Earthquake source observations
Abstract Copyright:
The Authors 2014. Published by Oxford University Press on behalf of The Royal Astronomical Society.
DOI:
10.1093/gji/ggu172
Bibliographic Code:
2014GeoJI.198..941C

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 lasting 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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