Seismic data recorded from a temporary network deployed at the western edge of the Pyrenees is used to study the aftershocks series following a magnitude 4.1 earthquake that took place on 2002 February 21, to the NW of Pamplona city. Aftershock determinations showed events distributed between 1 and 4 km depth in a small active area of about 4 km2, E-W oriented delineating the southern sector of the Aralar thrust unit. This seismogenic feature is supported by focal solutions showing a consistent E-W nodal plane with normal faulting following the main strike-slip rupture. The Aralar structure with its shallow activity may be interpreted as a conjugate system of the NE-SW deep-seated Pamplona active fault nearby. Cross-correlation techniques and relative location of event clusters further constrained the epicentral domain to 2 km long and 1 km wide. Statistical relations and parameters established indicate a rather low b-value of 0.8 for the Gutenberg-Richter distribution, denoting a region of concentrated seismicity, and a P-parameter of 0.9 for the Omori's law corresponding to a low decay of the aftershock activity in this area. More than 100 aftershocks were accurately located in this high-resolution experiment, whereas only 13 of them could be catalogued by the permanent agencies in the same period, due to a much sparser distribution. The results enhance the importance of using dense temporary networks to infer relevant seismotectonic and hazard constraints. © 2006 The Authors Journal compilation © 2006 RAS.

This work was carried out with the support of the MARCONI (REN2001-1734) project from the Spanish MCYT. MR and CL benefited from a PhD grant from the Spanish MCYT. We would like to thank to the responsible of the IGN, RENASS, OMP and CEA databases for sharing the seismological data used in this study.

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