The 1991 Hikurangi Margin Seismic Refraction Experiment

Abstract

The 1991 Hikurangi Margin Seismic Experiment was the first large scale refraction/wide-angle reflection study to be undertaken along the shallow portion of the Hikurangi subduction zone. Instruments and personnel were provided by DSIR Geology & Geophysics, Victoria University of Wellington, Leeds University, and the Canadian Geological Survey. The experiment was designed to study the lithospheric structure of the shallow subduction zone and involved a 300 km reversed profile between Palliser Bay and Hawke Bay. Eighty-six temporary seismographs were deployed to record six explosions: two 1000 kg offshore shots at each end of the profile and four 50 kg borehole shots along the profile. Seismic P-wave velocities near the surface were found to vary between 2.3 & 4.3 km/s along the profile, the latter being associated with the outcropping Jurassic greywacke at the southern end of the profile. Overall the greywacke velocities measured are somewhat lower than those observed to the west near Wellington, possibly reflecting their high fracturing and younger ages. Lower velocities of between 2.3 & 2.9 km/s were, as expected, measured over the sedimentary deposits associated with the Wairarapa Basin and at the north-eastern end of the profile in southern Hawke's Bay. Refractions from the top of the subducting Pacific plate were not observed along the profile but reflections interpreted as coming from the base of the subducted crust indicate average velocities of 6.8 to 7.3 km/s and ~ 6.7 km/s for the southern and north-eastern ends of the profile respectively. Reasons for not observing refractions from this layer may include it being too thin to produce first arrivals or that the arrivals are being delayed by a dipping structure or possibly an overlying low-velocity layer. Amplitude modelling of the recorded waveforms from the two offshore shots indicates that such a low-velocity layer may exist, at least for the southern end of the profile. Refractions from two upper-mantle layers were recorded along the profile. The shallower layer was found to lie at a depth of between 23 & 28 km, depending on the modelled presence or absence of a low-velocity layer in the overlying crust. The thickness of this ~ 8.2 km/s layer is modelled as being ~ 9 km. Below this layer, at between 30 & 36 km depth, lies an 8.7 ± 0.1 km/s layer. This velocity was determined from a time-term analysis of the shot recordings and those of shallow distant earthquakes recorded in the region. High apparent velocities measured for this layer along the profile (> 9 km/s) can be explained by the systematic delays caused by a crooked deployment line overlying a smoothly varying three-dimensional dipping structure. The high upper-mantle velocity that is observed along the profile is most likely due to high P-wave anisotropy (~ 12%) with the fast direction being along the strike of the subduction zone.