Postseismic Deformation Associated with Recent New Zealand Earthquakes

This paper summarizes measurements of postseismic deformation associated with the July 2009 Dusky Sound earthquake (MW7.8) and the 2010-2012 Christchurch earthquake sequence determined using a combination of InSAR analysis and time series analysis of continuous GPS (cGPS) and campaign GPS data. Using the cGPS time series and repeated campaign measurements we can measure post-seismic relaxation over distances in excess of 365 km. We have modeled the fit of the time series using exponential, power-law and logarithmic functions. For stations nearest to the epicenter, logarithmic or power-law functions provide marginally better fits than the exponential function; for stations located farther away, the three functions cannot be distinguished. The amplitude of the postseismic deformation is less than a third of the coseismic deformation for the near stations but for the more distant stations the two signals are comparable. For the vertical component, however, the postseismic signal is nearly always larger. Preliminary results from campaign GPS and cGPS stations indicate that postseismic deformation is continuing more than two years after the 4 September 2010 Darfield (Canterbury) earthquake (MW6.2) and over a year after the last major earthquake in the sequence. GPS solutions indicate dextral shear along an extension of the rupture plane of the Darfield earthquake (i.e. the Greendale fault). Using a modified version of the Small Baseline Subset (SBAS) method of InSAR time-series analysis, we are able remove the effects of coseismic strain associated with large aftershocks, including the damaging M6.2 earthquake of 22 February 2011, from the longer-term postseismic signals.