Temporal and spatial evolution of hypocentres and anisotropy from the Darfield aftershock sequence: implications for fault geometry and age

Abstract

The first four months of aftershocks of the Darfield earthquake have been studied using data from temporary and permanent seismic stations to investigate the fault geometry, stress field and evolution of seismicity and seismic properties. Earthquake relocations illuminate fault segments and show that the majority of aftershocks occurred beyond the areas of highest slip during the Darfield earthquake. Seismic anisotropy shows a mixture of fast directions parallel to the maximum horizontal stress and fault-parallel fast directions. This, combined with the lack of observable growth of seismicity along fault segments, suggests that the Greendale Fault broke a pre-existing fault plane.

Evolution of the 2010–2012 Canterbury earthquake sequence

“…Both geodetic Beavan et al 2012) and seismological models indicate that most of the displacement was very shallow, and was mainly confined to the upper c. 5 km. Fault models developed from the inversion of InSAR measurements indicate slip on some of the fault segments reached 8 m at 4 km depth, indicating large stress drops of c. 10 MPa (Elliott et al 2012), while aftershock relocations by Syracuse et al (2012) along the central Greendale Fault segment indicate event depths from 5.9 to 14 km, mainly several kilometres below the regions of highest slip inferred by Beavan et al (2010).…”

High‐resolution relocation of aftershocks of the Mw 7.1 Darfield, New Zealand, earthquake and implications for fault activity

“…[] modeled the dip of this segment to be 78° to the south. As observed in earlier relocations of a portion of this data set [ Syracuse et al ., ], there was little aftershock seismicity in the areas of geodetically determined largest slip; throughout the region, most aftershocks occurred below the areas of greatest slip, between 5 and 11 km depth (Figure ).…”

“…The W‐E orientation of the central section of the subvertical Greendale Fault striking 085° is consistent with that expected for either a newly formed fault or an optimally oriented reactivated fault (orientations of 085° or 145°) in the regional stress field of σ 1 oriented at 115° and σ 2 vertical [e.g., Sibson et al ., ; Syracuse et al ., ]. However, recent shear wave splitting measurements and inversions of focal mechanisms show that the local stress field rotates close to the central part of the Greendale Fault, so that both fast directions and the maximum horizontal stress are fault parallel, indicating a portion of the fault that is poorly oriented for formation or reactivation and possibly indicating a weak portion of the fault (Holt et al, submitted manuscript, 2013).…”

“…The differing focal mechanisms are likely caused by the regional and teleseismic data being sensitive to different parts of the rupture, particularly due to the attenuation of high‐frequency signals in the teleseismic data, resulting in the latter solution being unable to recover smaller‐scale motion early on in the rupture. Geodetic and seismic inversions indicate additional buried fault segments to the west, north, and east of the surface rupture with a variety of types of motion along them [ Holden et al ., ; Beavan et al ., ; Elliott et al ., ], which also corresponds to relocated aftershock distributions and focal mechanisms of aftershocks [ Gledhill et al ., ; Syracuse et al ., ].…”

“…Based on GNS Science catalog locations and relocations in a one‐dimensional (1D) velocity model [ Syracuse et al ., ], aftershock patterns show an evolution of fault activity following the Darfield earthquake. During the first day following the main shock, aftershocks were distributed close to the Greendale Fault surface trace, but one week later, the majority of the aftershocks were occurring mainly either north of the rupture along a N‐S trend or east of the surface rupture, closer to Christchurch and the location of the M w 6.3 Christchurch earthquake that occurred five and a half months later, in February 2011.…”