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The world's largest and most destructive earthquakes are caused by subduction zones, where a slab of oceanic plate is pushed beneath another plate down into the mantle. Understanding and preparing for major earthquakes is dependent on reconstructing the geometry and stress conditions of the subducted slabs. Seismologists rely on the rare windows into the deeply buried slabs to see the infrequent but strong earthquakes that occur within them.
A research team led by the University of Tsukuba used data from a magnitude 7.3 earthquake that occurred off the northeastern tip of New Zealand's North Island on March 4, 2021.
The East Cape earthquake was likely caused by the location of the Kermadec Trench to the north and the Hikurangi Margin to the south. We used a novel method to investigate the geometry of the stress field and earthquake process.
Multiple episodes of rupture were revealed by this investigation. In a subduction zone, shallow (30 km) rupturing would typically be expected. The deep rupture occurred with compression parallel to the subduction trench.
Professor Yuji Yagi explains that there are two alternative or inter-related factors that may explain the East Cape earthquake. Subduction of a seamount or multiple seamounts along with the subducted slab could create local changes in the stress field. The Kermadec Trench's transition to the Hikurangi Margin's subducted oceanic crust could create local conditions that could cause the unusual faulting pattern.
Because of the rarity of deep earthquakes in this region, distinguishing between the two possibilities is difficult, and both factors might play a role in creating the stress field revealed by the East Cape earthquake. There may be more earthquakes off the northeast coast of New Zealand in the future.
Ryo Okuwaki and his team have published a paper about the evolution of a honeycombed slab during the East Cape earthquake in New Zealand. There is a DOI: 10.1029/2021GL095117.
The journal contains information about research.
A complex earthquake off New Zealand's East Cape was caused by a contorted oceanic plate.
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