The 2014 Mw6.2 Eketahuna earthquake, Hikurangi subduction zone – normal faulting in the subducted Pacific Plate crust

Abstract:

The 2014 January 20^th M6.2 Eketahuna earthquake occurred in the subducted crust of the Pacific plate at the Hikurangi subduction zone, beneath North Island, New Zealand. Moment tensor analysis together with aftershock relocations show that this event was an oblique-normal faulting intraplate event, with hypocentre depth ca.30 km, and with rupture on a northwest-dipping fault extending through the subducted crust up to the subduction megathrust at ca.18-20 km depth. More than 3500 aftershocks were subsequently recorded by the New Zealand GeoNet network, with only minor migration of the aftershocks away from the inferred mainshock rupture, and with very few aftershocks within +/- 1 km of the subduction megathrust. The megathrust in this particular region is inferred to be interseismically locked with no seismic or aseismic slip, although slow slip is occurring ca.15-30 km down-dip (Wallace et al, 2013).

Similar oblique-normal faulting events have previously occurred along the Hikurangi subduction margin, including in 1985 (M_L5.7) and 1990 (M_w6.2). Earlier earthquakes in 1942 (M_w6.8) and 1921 (M_w6.8) are also inferred to have occurred at a similar depth within the subducted crust. The 1990 earthquake sequence occurred ~40 km along-strike from the 2014 Eketahuna event, and involved a M_w6.2 oblique-normal faulting event in the subducted crust, which was quickly followed by a M_w6.4 event in the overlying crust, with both thrust and dextral strike-slip components, possibly responding to deeper aseismic slip.

Deeper earthquakes of similar type at other subduction margins are thought to be high stress drop. We calculate the stress drops of the mainshock and larger aftershocks, using a direct wave, empirical Green’s function (EGF) approach that includes measurement uncertainties and objective criteria for assessing the quality of each spectral ratio (Abercrombie, 2013). We compare the results to those for earthquakes in other tectonic regions of New Zealand, calculated using the same method. We investigate whether there are any well-resolved differences in stress drop between these relatively deep earthquakes in the subducted plate and those in other tectonic environments.

Wallace, L.M., et al., AGU 2013 Fall meeting abstracts, 2013.

Abercrombie, 2013, JGR v118, 2013.