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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.

The hydrocarbon-rich southeastern margin of the South China Sea is divided by NW-SE lineaments into a series of sharply contrasting segments distinguished by, among other things, abrupt changes in gravity patterns. The Sabah segment is bounded to the SW by the Tinjar or West Baram line and to the NE by the Balabac line at the southwest margin of the Sulu Sea. The most prominent gravitational feature of this segment is the strong freeair gravity low associated with the Sabah Trough which lies about 150 km offshore. Seismic reflection data suggest that loading by prograding sediments and gravity driven thrust sheets has depressed the extended continental crust of the South China Sea below its level of local isostatic compensation, producing the trough as a foreland basin in which sedimentation has failed to keep pace with subsidence. The load masses themselves, supported in part by the rigidity of the underlying crust and lithosphere, are above their levels of local compensation and deep Neogene sedimentary basins lie on the flanks of Bouguer and free-air gravity highs. Gravity values decrease across the Sabah coast so that the Crocker Ranges, including Mt Kinabalu, rest in rough isostatic equilibrium on presumably weaker lithosphere. The free-air gravity anomaly associated with the Sabah Trough is smaller than would have been predicted from the thickness of the water column, suggesting crustal thinning beneath the trough axis. This is not a characteristic of normal foreland basins and can therefore be assumed to predate basin formation. It can be concluded that the NE-SW trending belts of parallel gravity anomaly and geomorphology, of which the Sabah Trough is the most obvious, have been controlled by the pre-existing fabric of the crust and lithosphere of the South China Sea since they are discordant to the Palaeogene geological trends in Sabah. Reconstructions of the Tertiary history of the Sabah segment can be based on this assumption, which also suggests that sediments deposited in rift basins formed during the Palaeogene break-up of the South China margin were the source for much, if not all, of the hydrocarbon reserves of the area.

We investigate seismic signatures of fracturing in a newly ruptured strike‐slip fault by determining the wavefield polarization in the New Zealand Canterbury Plains area and across the Greendale Fault, which was responsible for the 3 September 2010 Darfield Mw 7.1 earthquake. Previous studies suggested that fractured rocks in fault damage zones cause directional amplification and ground motion polarization in the fracture‐perpendicular direction as an effect of stiffness anisotropy, and cause velocity anisotropy with shear wave velocity larger in the fracture‐parallel component. An array of 14 stations was installed following the Darfield earthquake. We assess polarization both in the frequency and time domains through the individual‐station horizontal‐to‐vertical spectral ratio and covariance matrix analysis, respectively, and compare the results to previously reported anisotropy measurements from shear wave splitting. Stations installed in the Canterbury Plains have an amplification peak between 0.1 and 0.3 Hz for both earthquakes and ambient noise. We relate the amplification to the resonance of a considerable thickness (c. 1 km) of soft sediments lying over the metamorphic bedrock. Analysis of seismic events revealed the existence of another peak in amplification between 2 and 5 Hz at two on‐fault stations, which was not visible in the noise analysis. In contrast to the lower frequency peak, the ones between 2 and 5 Hz are more strongly anisotropic, attaining amplitudes up to a factor of 4 in the N52° direction. To interpret this effect we model the fracture pattern in the fault damage zone produced by the fault kinematics. We conclude that the horizontal polarization is orthogonal to extensional fractures, which predominate in the shallow layers (<2 km) with an expected strike of N139°. Fracture orientation is consistent with coseismic surface rupture observations, confirming the reliability of the model. S wave splitting is produced by velocity anisotropy in the entire rock volume crossed along the seismic path; thus, it is affected by deeper material than the amplification study. We explain the rotation of S wave fast component observed by Holt et al. (2013) near the fault in terms of the dominant synthetic cleavages at greater depths (>2 km), expected in N101° direction on the basis of the model. Thus, different fracture distribution at different depths may explain different results for amplification compared to anisotropy. We propose polarization amplification analysis as a complementary method to S wave splitting analysis. Polarization analysis is rapidly computed and robust, and it can be applied to either earthquakes or ambient noise recordings, giving useful information about the predominant fracture patterns at various depths.

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