What Controls Slip Directions of Diffuse Microseismicity in a Zone of Continental Transpression, South Island, New Zealand?

Abstract:

In South Island, New Zealand, oblique convergence of the Australian and Pacific plates is accommodated in a wide zone, up to 150 km wide, with ~75% of the total motion (~37 mm/yr) taken up on the Alpine Fault. In this study, we examine diffuse shallow crustal (0—20 km) microseismicity in a region up to 100 km to the SE of the Alpine Fault, where most of the remaining relative plate motion is accommodated. In the Central Otago region of southern South Island, focal mechanisms have been determined for 152 events, recorded in a broadband seismic network (COSA) over a 15 month period spanning June 2012—October 2013. One approach is to interpret these mechanisms in the context of a uniform stress field, assuming slip occurs in the direction of maximum shear stress on randomly orientated fractures, with an equal probability of slip on either nodal plane. This way, a stress field inversion of these mechanisms produces an estimate of S_Hmax oriented at 114°, an orientation inconsistent with estimates of the principal contraction direction derived from GPS (105°) and gravitational stresses due to topography and crustal thickness variations. However, a rose diagram for the dip directions of all nodal planes indicates that slip may be simply occurring subparallel to the plate convergence direction (~070°) on a limited set of linked fractures, organized into strike-slip and thrust segments. In other words, the earthquake slip directions may be kinematically controlled, accommodating the plate convergence, rather than the response of randomly orientated fractures to a uniform stress field. Near-vertical fractures aligned with the plate convergence direction are preferentially slipping and dominate the microseismicity in the Central South Island. We show that both large >M5.5 and small (2 > M < 4) events are controlled by the same kinematic process, using aftershock data from the 04/05/15 M_L = 5.8 Wanaka earthquake, with the mainshock occurring on a steeply dipping dextral fault aligned at 070° and subparallel to the plate convergence vector. The segmentation observed in the microseismicity also occurs for neotectonic faulting along the main trace of the Alpine Fault, which is broken up into strike-slip and thrust segments at a 1-10km scale.