A detailed analysis of the February 1996 aftershock sequence in the eastern Pyrenees, France

Following the 1996 February 18 M-L = 5.2 earthquake in the Agly massif in t he eastern French Pyrenees, we installed a temporary network of seismometer s around the epicentre. In this paper, we analyse 336 well-located aftersho cks recorded from February 19 to February 23 by 18 temporary stations and t wo permanent stations located less than 35 km from the epicentre. Most afte rshocks have been located with an accuracy better than 1.5 km in both horiz ontal and vertical positions. Their spatial distribution suggests the react ivation of a known fault system. We determined 39 fault-plane solutions usi ng P-wave first motions. Despite their diversity, the focal mechanisms yiel d an E-W subhorizontal T-axis. We also determined fault-plane solutions and principal stress axes using the method developed by Rivera & Cisternas (19 90) for the 15 best-recorded events. We obtain a pure-shear-rupture tectoni c regime under N-S subhorizontal compression and E-W subhorizontal extensio n. These principal stress axes, which explain the focal mechanisms for at l east 75 per cent of the 39 aftershocks, are different from the axes deduced from the main shock. The post-earthquake stress field caused by the main-s hock rupture, modelled as sinistral strike slip on three vertical fault seg ments, is computed for various orientations and magnitudes of the regional stress held, assumed to be horizontal. The aftershock distribution is best explained for a compressive stress field oriented N30 degrees E. Most after shocks concentrate where the Coulomb failure stress change increases by mor e than 0.2 MPa. The diversity of aftershock focal mechanisms, poorly explai ned by this model, may reflect the great diversity in the orientations of p re-existing fractures in the Agly massif.