eprintid: 1397 rev_number: 20 eprint_status: archive userid: 2 dir: disk0/00/00/13/97 datestamp: 2015-02-10 13:57:01 lastmod: 2017-02-08 12:21:41 status_changed: 2015-04-27 12:10:49 type: article metadata_visibility: show creators_name: Ruiz, Fernández, Mario creators_name: Gaspà, O. creators_name: Gallart, Muset, Josep creators_name: Díaz, Cusí, Jordi creators_name: Pulgar, J.A. creators_name: García-Sansegundo, J. creators_name: López-Fernández, C. creators_name: González-Cortina, J.M. corp_creators: Institute of Earth Sciences Jaume Almera - ICTJA-CSIC title: Aftershocks series monitoring of the September 18, 2004 M=4.6 earthquake at the western Pyrenees: A case of reservoir-triggered seismicity? ispublished: pub subjects: RSP1 subjects: SR subjects: SS divisions: EPOS-P full_text_status: none abstract: On September 18, 2004, a 4.6 mbLg earthquake was widely felt in the region around Pamplona, at the western Pyrenees. Preliminary locations reported an epicenter less than 20 km ESE of Pamplona and close to the Itoiz reservoir, which started impounding in January 2004. The area apparently lacks of significant seismic activity in recent times. After the main shock, which was preceded by series of foreshocks reaching magnitudes of 3.3 mbLg, a dense temporal network of 13 seismic stations was deployed there to monitor the aftershocks series and to constrain the hypocentral pattern. Aftershock determinations obtained with a double-difference algorithm define a narrow epicentral zone of less than 10 km2, ESE-WNW oriented. The events are mainly concentrated between 3 and 9 km depth. Focal solutions were computed for the main event and 12 aftershocks including the highest secondary one of 3.8 mbLg. They show mainly normal faulting with some strike-slip component and one of the nodal planes oriented NW-SE and dipping to the NE. Cross-correlation techniques applied to detect and associate events with similar waveforms, provided up to 33 families relating the 67% of the 326 relocated aftershocks. Families show event clusters grouped by periods and migrating from NW to SE. Interestingly, the narrow epicentral zone inferred here is located less than 4 km away from the 111-m high Itoiz dam. These hypocentral results, and the correlation observed between fluctuations of the reservoir water level and the seismic activity, favour the explanation of this foreshock-aftershock series as a rapid response case of reservoir-triggered seismicity, burst by the first impoundment of the Itoiz reservoir. The region is folded and affected by shallow dipping thrusts, and the Itoiz reservoir is located on the hangingwall of a low angle southward verging thrust, which might be a case sensible to water level fluctuations. However, continued seismic monitoring in the coming years is mandatory in this area to infer more reliable seismotectonic and hazard assessments. date: 2006 date_type: published publication: Tectonophysics volume: 424 number: 3-4 publisher: Elsevier pagerange: 223-243 id_number: doi:10.1016/j.tecto.2006.03.037 refereed: TRUE issn: 0040-1951 official_url: http://dx.doi.org/10.1016/j.tecto.2006.03.037 access_IS-EPOS: limited owner: Publisher citation: Ruiz, Fernández, Mario and Gaspà, O. and Gallart, Muset, Josep and Díaz, Cusí, Jordi and Pulgar, J.A. and García-Sansegundo, J. and López-Fernández, C. and González-Cortina, J.M. (2006) Aftershocks series monitoring of the September 18, 2004 M=4.6 earthquake at the western Pyrenees: A case of reservoir-triggered seismicity? Tectonophysics, 424 (3-4). pp. 223-243. DOI: https://doi.org/10.1016/j.tecto.2006.03.037