Constraints on the Seismogenic Stress Tensor in Northeastern South Island, New Zealand

The seismogenic tensorial stresses in the crust beneath northeastern South Island, New Zealand that are responsible for the 2010 $M_{w}7.2$ Darfield and 22 February 2011 $M_{w}6.2$ Christchurch earthquakes of the Canterbury Plains, South Island, New Zealand are compatible with an Andersonian strike-slip regime with a NW–SE trending most compressive stress. The stresses we infer from coseismic slip models of these two earthquakes are consistent with previous estimates of stress from earthquake focal mechanisms near the Alpine Fault. The strike-slip stress regime of the Canterbury Plains differs from the thrust stress regime that we previously estimated from coseismic models of the 2016 $M_{w}7.8$ Kaikōura earthquake. We demonstrate that loading due to thick sediment deposits overlying the crust of the Canterbury Plains is a sufficient mechanism to rotate the principal stresses from a background thrust regime to a strike-slip regime, constraining the absolute tectonic stress in the northeast of South Island to most likely be $\sim$ 26 MPa, and no larger than $\sim$ 80 MPa.