Co-seismic Slip Distribution of 2010 Darfield Mw 7.1 Earthquake Derived from InSAR Measurements

The New Zealand islands locating at the boundary between the Pacific and Australian plates, is one of the most seismically active regions in the world. However, the 2010 Darfield earthquake occurred on previously-unknown faults, which absorb only a minor portion of relative plate motion there. We attempt to obtain detailed information about fault geometry and rupture distribution of this event using InSAR data, the result will be useful in understanding tectonic process and seismic hazards of the region. We use ALOS PALSAR data from JAXA to derive SAR los interferograms associated with coseismic deformation of this earthquake. More than one evidence reveals that not only Greendale fault involved in this earthquake. Combining geological field survey observations with SAR displacement fringes, correlation, and range and azimuth offsets, we identify four faults slipped during coseismic rupture, of which seven segments are distinguished with various strikes and dip angles, as shown in the figure. Our inversion result shows that slip is concentrated in the upper 10 km depth. Slips along the Greendale fault (segments 1-4 in figure) are predominantly dextral with a maximum of up to 8 m. Fault segments 5 and 6 slipped reversely, with peaks of ~3 m and 3.8 m respectively. Slip on fault segment 7 is minor, no more than 1.8 m. We also compare the top 1 km slip along the Greendale fault with surface rupture distribution, and find very good agreement. The maximum surface slip is about 6 m, located about 26 km east of the west end of the fault surface rupture. The total seismic moment released equivalents to an Mw=7.1 event. Main features of InSAR data are well recovered, the residuals near the epicenter are less than 20 cm, confirming good data fitting of our fault slip model. The main data residuals are at footwall, possibly due to strong variation of deformation field revealed by complex interferogram fringes there. We also find that some displacements are not well explained near the east end of the Greendale fault, which could be caused by strong aftershocks occurred off the main fault. Based on our fault slip model we calculate the regional Coulomb stress change at 6 km depth assuming receiving fault parameters the same as that of the 2011 Mw 6.3 Christchurch earthquake, A relatively small Coulomb stress increase, of around 1 bar, is found in the hypocentral region of the 2011 Christchurch earthquake, about 30 km east of the Darfield earthquake. Details of Raw Data;