Calculating regional stresses for northern Canterbury: the effect of the 2010 Darfield earthquake

<p>We model regional stresses before and after the <i>M</i>_w 7.1 Darfield earthquake of September 2010 in Canterbury, New Zealand including crustal structure derived from seismic tomography. Models show that the Banks Peninsula volcanic assemblage acts as a strong, rigid block that pinches out ductile layers in the mid-crust but has little effect on shallower principal stress orientations. Static stress changes from the Darfield earthquake are everywhere <25 MPa except within 5 km of the fault. When added to regional stresses, these create only small rotations of <5° in the orientation of maximum horizontal stress <i>S</i>_Hmax, even near the fault. Predicted stress rotations do not correlate strongly with those inferred from aftershock focal mechanisms. The perturbations caused by earthquake stresses are not significant enough to explain either the magnitude or the sense of <i>S</i>_Hmax rotations near the fault at seismogenic depths.</p

Microseismicity and P–wave tomography of the central Alpine Fault, New Zealand

<p>We utilise seismic data from the central section of the Alpine Fault to locate earthquakes and image crustal structure in three dimensions. Tomography results from c. 6500 sources reveal the fault as either a southeast-dipping low-velocity zone or a marked velocity contrast in different parts of the study region. Where our model is best resolved, we interpret the Alpine Fault to be listric in nature, dipping steeply in the upper crust (50–60°) and flattening to 25–30° in the lower crust. The base of the seismogenic zone shallows from c. 15 km beneath the footwall and Alpine Fault to c. 6 km beneath the Southern Alps Main Divide, and then deepens to c. 15 km by c. 10 km further southeast. The shallow brittle–ductile transition overlies a broad low-velocity zone, which together likely result from the presence of fluids and elevated temperatures brought about by enhanced exhumation rate in this section of the Alpine Fault.</p

Crustal Fault Connectivity of the Mw 7.8 2016 Kaikōura Earthquake Constrained by Aftershock Relocations

©2019. American Geophysical Union. All Rights Reserved. The 14 November 2016 Mw7.8 Kaikōura earthquake in the northern South Island, New Zealand, involved highly complex, multifault rupture. We combine data from a temporary network and the permanent national seismograph network to repick and relocate ~2,700 aftershocks of M≥3 that occurred between 14 November 2016 and 13 May 2017. Automatic phase-picking is carried out using REST, a newly developed hybrid method whose pick quality is assessed by comparing automatic picks for a subset of 138 events with analysts' picks. Aftershock hypocenters computed from high-quality REST picks and a 3-D velocity model cluster almost exclusively in the shallow crust of the upper plate and reveal linkages at depth between surface-rupturing fault segments. Only eight aftershocks are relocated on a deeper structure positioned between patches of geodetically detected afterslip. This indicates that afterslip has not triggered significant earthquake activity on the subduction interface during the period of aftershock activity analyzed