Seismic Anisotropy at the Hikurangi Subduction Margin
To determine the stress state in the southern North Island of New Zealand, we use shear wave splitting analysis to measure seismic anisotropy and infer the orientation of the maximum horizontal stress directions (Shmax) in the crust. We use data recorded by 44 temporary seismometers deployed as part of the Seismic Array Hikurangi Experiment, and from six permanent stations from the national GeoNet network. Using 425 local earthquake events recorded across the 50 stations we made 13,807 measurements of the two splitting parameters, φ (fast direction) and δt (delay time). These measurements are compared to SHmax directions obtained from previous focal mechanism studies (SfocalHmax), and stresses due to the weight of topography (SgravHmax). Generally there is good agreement between the alignment of SfocalHmax, SgravHmax, and the mean φ measured at each station. We also ﬁnd a∼ 90◦ change in the trend of φ in the Wairarapa region for stations across the Wairarapa Fault trace. Based on the variation of φ, we divide the study region into three regions (West, Basin, and East), whose bounds approximately coincide with the Wairarapa and Dry Creek faults. We ﬁnd the average φ of the West region average agrees with previous anisotropy studies, which were undertaken within the bounds of the West region on the Tararua array. Also, we use our delay time measurements to estimate a 3.7±1.2% strength of anisotropy in the overriding Australian Plate, which agrees with the 4% crustal anisotropy measured previously. There is close alignment of the region average φ of the West and East regions, which also agrees with the deep splitting measurements previously obtained. There is no signiﬁcant diﬀerence between the mean φ and Sgravhmax for the West and Basin regions; however, we ﬁnd a diﬀerence of 31± 19.5◦ for the East region. We argue that this diﬀerence is due to tectonic loading stresses being suﬃciently large in the East region to cause the total stress ﬁeld to deviate from the gravitational stress ﬁeld.