Focal mechanism analysis of Christchurch Boxing Day aftershocks

Abstract

On 26 December 2010 a series of shallow aftershocks to the moment magnitude (MW) 7.1 Darfield earthquake occurred near the Christchurch city centre. The largest was local magnitude (ML) 4.9 and caused significant damage in the city centre. Focal mechanisms derived from regional moment tensor analysis have been calculated for three events including the ML 4.9. 16 first-motion focal mechanisms have also been calculated including the same three events for which moment tensor solutions have been calculated. The moment tensor solutions are right-lateral strike-slip mechanisms while the corresponding first-motion solutions show more reverse faulting. However, the P-axis orientation is consistent between the two methods. The remaining first-motion mechanisms show mainly reverse faulting with P-axes oriented NW-SE or E-2. All of the mechanisms are tightly clustered within – 2.3 km and do not show any spatial variability.