T017-0010
P-wave velocity structure of the northern Hikurangi margin from travel time tomography

Wednesday, 9 December 2020
Poster
Thomas Luckie1, Andrew Gase2, Katrina Jacobs3, Malcolm Charles Adan White1, Stuart A Henrys4, David A Okaya1, Harm J Van Avendonk5, Nathan L Bangs6, Daniel H N Barker4, Dan Bassett4, Shuichi Kodaira7, Ryuta Arai8, Gou Fujie9 and Yojiro Yamamoto9, (1)University of Southern California, Los Angeles, CA, United States, (2)University of Texas, Institute for Geophysics, Austin, TX, United States, (3)GNS Science-Institute of Geological and Nuclear Sciences Ltd, Lower Hutt, New Zealand, (4)GNS Science, Lower Hutt, New Zealand, (5)University of Texas at Austin, Institute for Geophysics, Austin, TX, United States, (6)Univ Texas, Institute for Geophysics, Austin, TX, United States, (7)Yokohama National University, Yokohama, Japan, (8)University of Hawaii at Manoa, Honolulu, HI, United States, (9)Japan Agency for Marine-Earth Science and Technology (JAMSTEC), Yokohama, Japan
Abstract:
The Seismogenesis at Hikurangi Integrated Research Experiment (SHIRE) carried out a wide-angle, onshore-offshore, reflection-refraction active-passive seismic survey from 2017 to 2019 around northern North Island, New Zealand to investigate the physical mechanisms that control slip behavior and uplift of the northern Hikurangi margin. This involved collecting seismic data along a 400 km trench-perpendicular transect which captures the subduction system from incoming plate to back arc rift. From the tomographic inversion of reflected and refracted phase travel times from active and passive sources, we constructed a P-wave velocity (Vp) model to depths of 50 km.

Vp in the incoming plate reveal a 10 km thick basement covered by 2 km of sediments and a smooth Vp gradient of 2.5-7.5 km/s. The accretionary prism is characterized by Vp of 2.5-3.5 km/s near the front and inner portions and Vp of 2.5-4.5 km/s in the middle of the prism. The plate interface steepens from a dip of 7.5° to 15° at 10 km depth. This increase in slab dip occurs near the downdip extent of a 2014 slow slip event and near the transition from a locked to creeping interface.

A decrease in Vp from west to east in the upper 15 km of the Australian plate correlates with Cretaceous units in the west and Neogene units in the east. Bounding the bottom of the Neogene units is a Cretaceous allochthon which has only been mapped at the surface. This Vp model provides insight into the depth and lateral extent of this allochthon. Consistent with previous studies of other regions of the margin, we observe anomalously slow Vp of 6.0-6.5 km/s in the lower Australian crust near the Moho-intersection. The Australian Moho intersects the plate interface at a depth of 29 km. The crust thins to the west through the rift, reaching a thickness of 22 km at the western edge of the model. The Vp gradient here is smooth, increasing from 2.5 to 7.0 km/s. A pocket of 6.0 km/s velocities near the eastern shoulder of the rift at a depth of 10 km aligns with the projected locations of several volcanoes, and a higher Vp of 7.0 km/s above the Moho on the western edge of the rift is observed.

These variations in P-wave velocity may have implications for sediment and fluid flux within the margin. Possible interpretations of the vertical and lateral heterogeneity, as well as apparent anomalies, will be presented.

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