Seismic anisotropy in the crust and mantle: a study at the western edge of the central volcanic region, New Zealand

Abstract

A main geological feature in the North Island of New Zealand is the Central Volcanic Region (CVR), which is a continental margin Arc System. It spans, wedge-like, from Mt. Ruapehu Volcano to White Island in the north-east and to East Coromandel Peninsula in the north-west. Tectonically the western border is less well defined than the eastern border, where it follows the active volcanic front. Measurements of seismic anisotropy across the western border were made to find out more about the crust and mantle structure and the relation between anisotropy and boundary deformation in this region, using data from the Central North Island Passive Seismic Experiment recorded during 2001. The Victoria University of Wellington component consisted of three broadband and six triggered short-period seismometers recording continuously. The broadband stations were positioned in a line across the western edge of the CVR with a station spacing of 10 km and 2 stations were moved after six months to get five stations in a line. The Institute of Geological and Nuclear Sciences provided data for selected teleseismic events for stations spanning the entire CVR. The technique of shear-wave splitting measurements was used to determine the orientation of the anisotropic system and thickness of the anisotropic layers. The results, working on the broadband teleseismic data, show that the splitting parameters change for a specific back-azimuth. Events from most back-azimuths show a consistent northeast-southwest, trench-parallel fast direction in New Zealand. But for most of New Zealand, events coming from a 80° back-azimuth yield an almost north-south fast direction, possibly explained by regional anisotropy in the D" layer. Splitting measurements of the local earthquakes yield a rotation of the fast direction from N-S for stations within to NE-SW for the stations west of the CVR. This is probably related to crustal stress changes at the western border of the CVR.