The Seismogenesis at Hikurangi Integrated Research Experiment (SHIRE) is an interdisciplinary geophysical and geological study of the convergent margin of northeastern New Zealand. At this subduction zone, the oceanic Hikurangi Plateau slips beneath the continental crust of North Island, New Zealand. Compared to other subduction zones around the world, the Hikurangi margin has a relatively shallow forearc, and from south to north there is a large contrast in the nature of slip along the plate boundary. In the south, convergence between the Pacific and Australian plates appears to cause elastic loading of the megathrust fault, and this elastic energy may be released during earthquakes. In the north, the stress at the plate boundary is occasionally released in slow-slip events, the equivalent of earthquakes that occur over the course of several weeks. The northern Hikurangi margin has also recorded tsunami earthquakes, which require rupture of faults near the seafloor. The primary purpose of the SHIRE project is to understand the geological conditions that causes the along-strike change in the nature of slip at the plate boundary. Secondary goals of the project are the cause of uplift in the adjacent Raukumara Peninsula, and the focusing of mantle melts in the northern Taupo volcanic zone. To obtain much needed constraints on the crustal structure of the Hikurangi margin, the team of scientists gathered marine seismic reflection and refraction data in October 2017, and land seismic data in March 2019.

With the new SHIRE geophysical data sets we investigate whether the along-strike change from a locked plate boundary at the southern Hikurangi margin and a slowly slipping plate boundary at the northern Hikurangi margin was the result of 1) northward increasing roughness of the subduction basement, 2) greater fluid pressures along fault zones of the northern Hikurangi margin, or 3) a difference in the frictional properties of the rocks along the plate boundary. By observing the seismic wave speeds and reflectivity, we can interpret the geometry of subducting seamounts of the Hikurangi Plateau, the sediment stratigraphy of the downgoing plate, faulting and deformation of the forearc prism to assess the causes of the along-strike change in megathrust slip behavior.

The seismic reflection lines across the northern Hikurangi margin show that the downgoing Hikurangi Plateau is laterally heterogeneous, with seamounts, thick deposits of volcaniclastic sediment, calcareous sediments and mudstones. This variety of inputs to the subduction zone has led to strong along-strike variations in the degree of faulting and compaction of the frontal prism. Farther landward, low seismic wave speeds on the northern Hikurangi margin provide evidence of weak sediments in the upper kilometer beneath the seafloor, which helps explain the occurrence of tsunami earthquakes here. In contrast, the southern Hikurangi margin has a larger accretionary prism, and the stratigraphy of the incoming Pacific plate is much more uniform. The SHIRE seismic reflection data show that Mesozoic clastic sediments blanket the volcanic crust, seamounts, and volcaniclastic sediments of the subducting Hikurangi Plateau. This area is coincident with the locked portion of the subduction megathrust along the southern Hikurangi margin, and the blanket of sediment may promote locking.

The SHIRE team also produced seismic velocity and seismic reflectivity images of the northern Taupo volcanic zone in the Bay of Plenty. The SHIRE results indicate that the rift is just 40 km wide, and the focusing of extensional strain coincides with the presence of magmatic sills at depths between 4 km and 15 km in the crust. The cause and extent of mantle melting and crustal melt plumbing are important topics for geohazards, because this volcanic system hosts New Zealand’s most active volcano on White Island, which erupted in 2019.

The SHIRE project provided training in marine seismic data acquisition and processing for 11 participating students on the R/V Marcus Langeth and R/V Tangaroa. Many of these students came from universities that were otherwise not involved in SHIRE, and they were selected to join the marine seismic experiment based on their experience and motivation to participate. Two graduate students at UT Austin used data from SHIRE for their dissertation. The results of SHIRE provide an important framework for the interpretation of other geophysical data and a drilling expedition that aim to improve our understanding of seismic hazards on the New Zealand margin.

Last Modified: 11/22/2021
Modified by: Harm J Van Avendonk