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181614121086disallowablefrom frictionallock-updip > 75°n = 61DEXTRALallowable dextralGREENDALE FAULTfavorablyoriented45° 45°σ 1SINISTRALallowable sinistralfavorablyorienteddisallowablefrom frictionallock-up420030° 040° 050° 060° 070° 080° 090° 100° 110° 120° 130° 140° 150° 160° 170° 180° 190° 200°STRIKE AZIMUTH▲ ▲ Figure 6. Azimuthal distribution of nodal plane strikes for close-to-pure strike-slip CMT focal mechanisms (both planes dipping >75°)from the Canterbury earthquake sequence (GeoNet catalog http://www.geonet.org.nz), shown in relation to the inferred σ 1 direction.western termination the fault appears to transform into localareas of normal faulting to the north and reverse faulting to thesouth (Figure 4).While the dominant rupture in the 22 February aftershocksclearly involves dextral-reverse oblique slip, the subordinatesubvertical plane (080°/87° S) lying subparallel to theGreendale fault (Beavan et al. 2011, page 789 of this issue)is at close to the ideal Andersonian orientation for strike-slip.This part of the sequence may therefore represent competitionbetween inherited and newly formed fault segments. The twodiffuse aftershock lineaments trending 140°–155° (Figure 3)are appropriately oriented for left-lateral strike-slip on verticalfaults conjugate to the right-lateral Greendale fault with whichthey form a dihedral angle of ~ 50°–70°. Combining the CMTfocal mechanism (161°/67° WSW) with the fault model for the13 June M w 6.0 aftershock (153°/55° SW) suggests predominantlyleft-lateral strike-slip on a moderately-to-steeply dippingplane with the slip vector raking only 6°, not too dissimilar tothe ideal Andersonian relationship. However, the suggestionof a nonvertical rupture with a degree of oblique slip makes itlikely that rupturing involved the reactivation of an inheritedbasement structure. These arguments are explored further byexamining the distribution of strike azimuths, with respect tothe inferred σ 1 direction, of aftershock nodal planes for closeto-purestrike-slip CMT focal mechanisms (GeoNet catalog,http://www.geonet.org.nz) where both nodal planes dip >75°(Figure 6). Because of the ambiguity as to which nodal planerepresents the rupture plane, the distribution repeats at 90°intervals, separating potential dextral from potential sinistralstrike-slip faults. Theoretical and field studies suggest thatfaults containing the σ 2 direction undergo frictional lock-up at55°–60° to σ 1 (Collettini and Sibson 2001), reducing the allowablerange of strike-slip fault orientations. Potential strike-sliporientations are thus reduced to three categories: dark-shadedcolumns are inadmissible because of frictional lock-up; lightshadedcolumns are positively discriminated as either dextralor sinistral strike-slip ruptures; and moderate-shaded columnscould represent either dextral or sinistral strike-slip. Severalfeatures of the distribution are notable. First, despite its lengthand continuity, the Greendale fault trend is not dominant instrike-slip aftershock orientations. Moreover, a significant proportionof the positively discriminated mechanisms involvesinistral strike-slip on faults that commonly strike 135°–145°,conjugate to the dextral Greendale fault. However, by far thedominant azimuthal trend is 070° and/or 160°. Note first thatthese trends lie at ±45° to inferred σ 1 defining the orientationsof vertical planes with maximum shear stress, the expectedorientation for ductile shear zones developing in the basementbelow the brittle seismogenic crust (Figure 3). However, the070° trend also lies subparallel to the Hope fault and the presentinterplate slip vector, suggesting the possibility of somekinematic control.830 Seismological Research Letters Volume 82, Number 6 November/December 2011