Seismotectonic constraints at the western edge of the Pyrenees: Aftershock series monitoring of the 2002 February 21, 4.1 Lg earthquake

Abstract

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.