[J05-3] Crustal dynamics: Multidisciplinary approach to seismogenesis III

[J05-3-03] Three-dimensional seismic velocity structure beneath the northern South Island, New Zealand from dense seismic observation

^○Tomomi Okada¹, Yoshihisa Iio², Satoshi Matsumoto³, Stephen Bannister⁴, Shiro Ohmi², Masumi Yamada², Shintaro Horiuchi¹, Tsutomu Miura², Tadashi Sato¹, Jarg Pettinga⁵, Francesca Ghisetti⁶, Richard Sibson⁷ (1.Tohoku University, Japan, 2.Kyoto University, Japan, 3.Kyushu University, Japan, 4.GNS Science, New Zealand, 5.Univerisity of Canterbury, New Zealand, 6.Terra Geologica, New Zealand, 7.University of Otago, New Zealand)

The northern South Island of New Zealand straddles the plate boundary between the Australian and Pacific Plates and is a transition zone between the Hikurangi subduction margin and a continental transform system, with the Alpine Fault as its principal component. The Alpine Fault separates two distinct tectonic provinces. To the SE lies the Marlborough system of dextral strike-slip faults (MA faults), while to the NW lies an area of active compressional inversion where inherited normal faults are reactivated predominantly as steep reverse faults (e.g. Buller-Nelson area - BN faults). In this study, we image the seismic velocity structure beneath the shallow seismogenic zone for both the eastern MA faults and the western BN faults.
A temporary dense seismic observation network was deployed in the northern and central parts of the South Island, New Zealand. We use data from these temporary stations (51 stations), acquired from March 2011 to April 2015, together with data recorded by the New Zealand GeoNet network (22 stations). For the analysis we also include data from the aftershocks of the 2016 Kaikoura earthquake, recorded by GeoNet. Total number of located earthquakes is 5389. We employed the Double-Difference Tomography Method to relocate hypocenters and estimate the seismic velocity structure. The results from the seismic tomography show low-Vp, Vs and a high Vp/Vs zone beneath the epicenters in the NW South Island. The high Vp/Vs area seems to be continuously distributed from the upper surface of the subducted crust to the bottom of the shallow seismic active area in BN faults. We also see seismic low-velocity area in and beneath the hypocenter and the focal area of the 2016 M7.8 Kaikoura earthquake. These observations suggest that fluid from the subducting plate weakens the crust and promotes the occurrence of earthquakes in this area.