Using regional moment tensors to constrain the kinematics and stress evolution of the 2010-2013 Canterbury earthquake sequence, South Island, New Zealand

On September 3, 2010, a M_W 7.0 (U.S. Geological Survey moment magnitude) earthquake ruptured across the Canterbury Plains in South Island, New Zealand. Since then, New Zealand GNS Science has recorded over 10,000 aftershocks M_L 2.0 and larger, including three destructive ~M_W 6.0 earthquakes near Christchurch. We treat the Canterbury earthquake sequence as an intraplate earthquake sequence, and compare its kinematics to an Andersonianmodel for fault slip in a uniform stress field. We determined moment magnitudes and double couple solutions for 150 earthquakes having M_W 3.7 and larger through the use of a waveform inversion technique using data from broadband seismic stations on South Island, New Zealand. The majority (126) of these double couple solutions have strike-slip focal mechanisms, with right-lateral slip on ENE fault planes or equivalently left-lateral slip on SSE fault planes. The remaining focal mechanisms indicate reverse faulting, except for two normal faulting events. The strike-slip segments have compatible orientations for slip in a stress field with a horizontal σ₁ oriented ~N115°E, and horizontal σ₃. The preference for right lateral strike-slip earthquakes suggests that these structures are inherited from previous stages of deformation. Reverse slip is interpreted to have occurred on previously existing structures in regionswith an absence of existing structures optimally oriented for strike-slip deformation. Despite the variations in slip direction and faulting style, most aftershocks had nearly the same P-axis orientation, consistent with the regional σ₁. There is no evidence for significant changes in these stress orientations throughout the Canterbury earthquake sequence.