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Abstract The 2021 shallow plate‐boundary thrust‐faulting and 2023 outer rise normal‐faultingMW7.7 earthquakes southeast of the Loyalty Islands produced significant, well‐recorded tsunamis around the North and South Fiji Basins. The two earthquakes occurred in close proximity on opposing sides of the Southern Vanuatu Trench with similar seismic moments and east‐west rupture lengths but different faulting mechanisms. This provides a basis to examine tsunami sensitivity to source geometry and location for paths in the complex southwest Pacific region. Finite‐fault models of the source processes for both events were inverted from teleseismic body wave data with constraints from forward, nonhydrostatic modeling of regional tide gauge and seafloor pressure sensor recordings. The wave motions are reversed in sign, with a leading crest generated by 1.31 m uplift on the upper plate slope for the 2021 tsunami and a leading trough from 2.37 m subsidence on the subducting plate near the trench for the 2023 tsunami. The more recent outer rise normal faulting produces narrower seafloor deformation beneath deeper water resulting in shorter period tsunami waves that shoal and refract more effectively along seamounts and island chains to produce a more elaborate radiation pattern. The source location relative to seamounts and small islands in the near field influences the energy lobes and directionality of the far‐field tsunami to the north. In contrast, both events have very similar radiation patterns to the south due to absence of major bathymetric features immediately southward of the sources.
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Shallow Megathrust Rupture during the 10 February 2021 Mw 7.7 Southeast Loyalty Islands earthquake sequence
On 10 February 2021, an MW 7.7 thrust earthquake ruptured the megathrust along the southeast Loyalty Islands within the strong bend in the plate boundary between the Australian plate and the North Fiji Basin. The mainshock involved rupture with ~50 s duration, with pure thrust slip concentrated in an east-west trending slip patch with up to 4.2 m of slip extending from 10 to 25 km depth. Slip at depths <10 km depth is negligible on the curved fault surface, which conforms to the SLAB2 interface model. Static stress drop estimates are ~5.5 MPa, and the radiated energy is 2.38 x 1015 J, with moment-scaled value of 5.7 x 10-6. The relatively shallow rupture from 10-25 km was moderately efficient in generating tsunami, with waves amplitudes up to 20 cm recorded in New Caledonia, New Zealand, Kermadec, and Fiji. Numerous M5+ normal-faulting aftershocks occur south of the trench, indicating effective stress change transfer from the megathrust to the bending flange of Australian plate that is negotiating the bend in the trench. Highly productive sequences involving paired thrust and normal faulting have occurred repeatedly westward along the northwest-trending portion of the Loyalty Islands region, also indicating unusually efficient stress communication.
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- Award ID(s):
- 1802364
- PAR ID:
- 10330536
- Editor(s):
- Koper, Keith
- Date Published:
- Journal Name:
- The Seismic record
- Volume:
- 1
- Issue:
- 3
- ISSN:
- 2694-4006
- Page Range / eLocation ID:
- 154-163
- Format(s):
- Medium: X
- Sponsoring Org:
- National Science Foundation
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- Free Publicly Accessible Full Text
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Accepted Manuscript
- Journal Article:
- https://doi.org/10.1785/0320210035
