Contorted oceanic plate caused complex quake off New Zealand's East Cape


Date:
December 27, 2021
Source:
University of Tsukuba
Summary:
Researchers used a novel finite-fault inversion method with
seismometer data from around the world to investigate a deep
intraslab earthquake that occurred on March 4, 2021, off the
northeastern tip of New Zealand's North Island. This imaging
revealed complex rupture geometry that included shallow faulting
with trench-perpendicular extension and unusual deep faulting with
trench-parallel compression, possibly caused by seamount subduction
and/or the transition between the Kermedec Trench and Hikurangi
Margin, with different thicknesses of subducting oceanic crust.



FULL STORY ========================================================================== Subduction zones, where a slab of oceanic plate is pushed beneath another tectonic plate down into the mantle, cause the world's largest and most destructive earthquakes. Reconstructing the geometry and stress conditions
of the subducted slabs at subduction zones is crucial to understanding
and preparing for major earthquakes. However, the tremendous depths of
these slabs make this challenging -- seismologists rely mainly on the
rare windows into these deeply buried slabs provided by the infrequent but strong earthquakes, termed intraslab earthquakes, that occur within them.


==========================================================================
In a new study published in Geophysical Research Letters, a research
team led by the University of Tsukuba used seismic data generated by a magnitude 7.3 earthquake that occurred off the northeasternmost tip of
New Zealand's North Island on March 4, 2021, detected by seismometers
around the world, to investigate the particularly unusual geometry and
stress states of the subducted slab deep below the surface in this region.

"The 2021 East Cape earthquake showed a complex rupture process, likely
because of its location at the boundary between the Kermadec Trench to
the north and the Hikurangi Margin to the south," lead author of the
study Assistant Professor Ryo Okuwaki explains. "To investigate the
geometry of the stress field and earthquake rupture process, we used
a novel finite-fault inversion technique that required no pre-existing knowledge of the area's faults." This investigation revealed multiple
episodes of rupture, generated by both compression and extension in the subsurface at different depths. These episodes included shallow (~30 km) rupture due to extension perpendicular to the trench as would typically
be expected in a subduction zone. Unexpectedly, however, the deep (~70
km) rupture occurred with compression parallel to the subduction trench.

"Two alternative or inter-related factors may explain the unique rupture geometry of the 2021 East Cape earthquake," senior author Professor Yuji
Yagi explains. "First, subduction of a seamount or multiple seamounts
along with the subducted slab could contort the slab and create local
changes in the stress field. Second, the transition from the Kermadec
Trench to the Hikurangi Margin, where the subducted oceanic crust is considerably thicker, could create the local conditions responsible for
the unusual faulting pattern." Because of the rarity of deep intraslab earthquakes in this region, distinguishing between these two possibilities
is currently challenging, and indeed both factors might play significant
roles in creating the complex stress field revealed by the East Cape earthquake. Additional earthquakes off the northeast coast of New Zealand
in the future may shed further light on this deep tectonic mystery.

This work was supported by the Grant-in-Aid for Scientific Research
(C) 19K04030.

========================================================================== Story Source: Materials provided by University_of_Tsukuba. Note: Content
may be edited for style and length.


========================================================================== Journal Reference:
1. Ryo Okuwaki, Stephen P. Hicks, Timothy J. Craig, Wenyuan Fan, Saskia
Goes, Tim J. Wright, Yuji Yagi. Illuminating a Contorted Slab With
a Complex Intraslab Rupture Evolution During the 2021 Mw 7.3 East
Cape, New Zealand Earthquake. Geophysical Research Letters, 2021;
48 (24) DOI: 10.1029/2021GL095117 ==========================================================================

Link to news story: https://www.sciencedaily.com/releases/2021/12/211227154349.htm

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