Wedge geometry, mechanical strength, and interseismic coupling of the Hikurangi subduction thrust, New Zealand Åke Fagereng ⁎ Department of Geological Sciences, University of Cape Town, Private Bag X3, Rondebosch 7701, South Africa abstract article info Article history: Received 11 August 2010 Received in revised form 23 February 2011 Accepted 12 May 2011 Available online 18 May 2011 Keywords: Subduction Interseismic coupling Shear strength Coulomb wedge Accretionary prism Hikurangi margin The Hikurangi subduction thrust interface, east coast North Island, New Zealand, exhibits along-strike variations in seismic style: the southern segment is interseismically locked, while the plate boundary thrust slips aseismically in the north. The geometry of the offshore accretionary prism is also heterogeneous, changing from small to large taper angle (fault dip plus surface slope) from south to north. Along-strike variations in accretionary prism geometry generally reflect changes in Coulomb wedge critical taper angle. Such variations are controlled by the relative strengths of the subduction megathrust and the wedge material. In the southern Hikurangi margin, the small taper angle indicates a relatively weak subduction thrust interface where strong interseismic locking is inferred. In the northern margin, high taper angle reflects a strong shallow décollement relative to the south. Thus, based on critical Coulomb wedge theory, the shallow megathrust appears relatively weak in the locked segment, and stronger in weakly coupled regions. © 2011 Elsevier B.V. All rights reserved. 1. Introduction Why some fault segments slip aseismically and others fail in large earthquakes is a matter of on-going debate. A commonly held view is that displacement by aseismic shearing (weak coupling) occurs on low effective stress fault segments, while locked (coupled) fault patches experience a higher effective normal stress (McCaffrey et al., 2008; Scholz, 1998; Scholz and Campos, 1995; Sibson and Rowland, 2003; Song and Simons, 2003). Other studies, however, have pointed out that large megathrust earthquakes occur on mechanically weak faults under low effective stress (Brown et al., 2003; Fagereng and Ellis 2009; Lamb, 2006; Tassara, 2010; Tobin and Saffer, 2009; Wang, 2010; Wang et al., 1995). There is therefore a need for examples of how relative frictional strength varies along faults exhibiting along- strike changes in seismic style. The Hikurangi subduction zone, along the east coast of the North Island, New Zealand, exhibits significant spatial variations in observed microseismic activity (Reyners and Eberhart-Phillips, 2009), and interseismic coupling (Fig. 1)(Wallace et al., 2004), where inter- seismic coupling is defined by the relative motion of rocks on opposite sides of the fault in the time between major earthquakes. According to best-fit inversions of geodetic and seismic data, the Hikurangi subduction thrust is interseismically locked to 35–50 km depth in the southern North Island, while the depth of the fully locked zone decreases to only 10–15 km offshore from Hawke's Bay and the Raukumara Peninsula (Fig. 1)(Wallace et al., 2004). Thus at depths of 10–15 km, the southern Hikurangi margin is interseismically locked, while north of Hawke's Bay this depth range corresponds to the down-dip limit of the locked zone and the slip zone of episodic slow slip events (Wallace et al., 2009). Two M w 6.9–7.1 earthquakes occurred on the northern segment in 1947, with coseismic slip zones shallower than 20 km (Doser and Webb, 2003) indicating a shallow base of the seismogenic zone. Temperature has been excluded as a main controlling factor of the along-strike variation in coupling depth, based on negligible variation in modeled thermal structure (Fagereng and Ellis, 2009; McCaffrey et al., 2008). The established variations in interseismic coupling, depth of slow slip events, and a variety of well-studied parameters varying in 3-d, make the Hikurangi margin a suitable natural laboratory for study of the relative impact of various factors on seismic style, although the lack of well-documented megathrust ruptures restricts interpretation to the current interseismic period. Fagereng and Ellis (2009) and Reyners and Eberhart-Phillips (2009) suggested that the fluid pressure state on the interface may be a significant control on megathrust mechanical behavior, although it is likely that multiple factors affect the degree of interseismic coupling (Wallace et al., 2009). To assess whether variations in interface strength (caused by a heterogeneous fluid pressure distribution) occur along the margin, and the correlation (if any) between interface strength and inter- seismic coupling, we apply a critical Coulomb wedge model to recently published offshore geometrical data for the accretionary prism (Barker et al., 2009; Barnes et al., 2010; Bell et al., 2010). The analysis is restricted to the offshore prism, thus to depths in the 10– 15 km range, which corresponds to depths at and below the down-dip Tectonophysics 507 (2011) 26–30 ⁎ Tel.: +27 21 650 2926. E-mail address: ake.fagereng@uct.ac.za. 0040-1951/$ – see front matter © 2011 Elsevier B.V. All rights reserved. doi:10.1016/j.tecto.2011.05.004 Contents lists available at ScienceDirect Tectonophysics journal homepage: www.elsevier.com/locate/tecto