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

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 M w 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.

Earthquake static stress transfer in the 2013 Gulf of Valencia (Spain) seismic sequence

“…The reservoir itself is located roughly 1.7 to 2.5 km beneath the seabed. The fault system derived from Geostock (2010), as in Cesca et al (2014), is considered here (Fig. 1c).…”

“…(b) Sketch of the UGS structure after the Shell seismic profile in Seeman et al (1990); vertical scale is in two-way travel time. A rough approximation of the reservoir body is depicted in yellow, and stratigraphic markers are shown in blue and green (modified from Cesca et al, 2014). (c) Detailed view of known faults in the area modified from Cesca et al (2014) according to the red area highlighted in panel (a).…”

“…A rough approximation of the reservoir body is depicted in yellow, and stratigraphic markers are shown in blue and green (modified from Cesca et al, 2014). (c) Detailed view of known faults in the area modified from Cesca et al (2014) according to the red area highlighted in panel (a). The map view is plotted at the approximate depth indicated by the discontinuous line in panel (b).…”

“…Nonconfidential literature in relation to the Castor case has already addressed problems in earthquake locations and focal mechanism computation, as well as the frequencymagnitude relationship (Institut Cartogràfic i Geològic de Catalunya, ICGC, 2013;IGME, 2013;Instituto Geografico Nacional, IGN, 2013;Cesca et al, 2014;González, 2014;Gaite et al, 2016). Cesca et al (2014) included some discussion in relation to the mapped faults that would have been more likely to slip based on background stress.…”

Two-phase flow effects on the CO2injection pressure evolution and implications for the caprock geomechanical stability

“…Even though fault reactivation will not lead to CO 2 leakage [14,15], it may induce earthquakes that may be felt by the local population. Felt seismicity induced by fluid injection in deep geological formations has led to the closure of several geo-energy related projects (e.g., a geothermal project at Basel, Switzerland [16], the seasonal gas storage project of Castor, Spain [17] and a wastewater disposal project at Guy-Greenbrier, Arkansas [18]). Thus, felt induced seismicity should be avoided to achieve a successful deployment of geological carbon storage.…”