A geodetic study of Otago: results of the central Otago deformation network 2004–2014

We have analysed 11 years of geodetic data from 30 stations distributed over the Otago Fault System in the South Island of New Zealand. Velocities were estimated from time series corrected for coseismic displacements from the 2004 Macquarie Island and 2007 George Sound earthquakes and the coseismic and the short term postseismic deformation associated with the 2009 Dusky Sound earthquake. By dividing the corrected time series in half we were able to demonstrate the existence of a systematic difference between the pre- and post-earthquake velocity fields, associated with a longer term viscoelastic transient related to the 2009 Dusky Sound earthquake. In the northern part of our study area, the geodetic strain rate data are consistent with elastic strain accumulation on the Alpine Fault while in the south and east, the strain rate tensors are consistent with the Otago Fault System. There is a significant change in orientation in the axis of contraction from east to west across the network that correlates with a transition between the Otago and Waihemo Fault Systems. We also demonstrate significant spatial variation in the rates of strain accumulation that may correlate with active and quiescent parts of the Otago Fault System. However these strain rates represent the average values for the 11 years that the COD network has been observed and may also be influenced by the longer term viscoelastic transient related to the Dusky Sound earthquake.

本研究对分布于新西兰南岛奥塔哥断层系统（Otago Fault System）的30个测站的11年大地测量数据开展了分析。研究人员对时间序列进行校正以剔除2004年麦夸里岛地震、2007年乔治湾地震的同震位移，以及2009年达斯基湾地震相关的同震与短期震后形变，并基于校正后的序列估算了地壳运动速率。通过将校正后的时间序列均分为前后两段，本研究证实震前与震后的地壳运动速率场存在系统性差异，该差异与2009年达斯基湾地震引发的长期黏弹性瞬态形变相关。在研究区域北部，大地测量应变率数据与阿尔卑斯断层（Alpine Fault）上的弹性应变积累特征相符；而在区域南部与东部，应变率张量则与奥塔哥断层系统的特征一致。整个测网范围内，收缩轴的方位由东向西发生显著变化，这一变化与奥塔哥断层系统与怀希莫断层系统（Waihemo Fault System）的过渡区域相匹配。本研究还发现，应变积累速率存在显著的空间异质性，该特征或与奥塔哥断层系统的活动段与静止段存在关联。不过上述应变率为COD观测网11年观测周期内的平均值，同时也可能受到达斯基湾地震引发的长期黏弹性瞬态形变的影响。