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Abstract On 18 November 2022, a large earthquake struck offshore southern Sumatra, generating a tsunami with 25 cm peak amplitude recorded at tide gauge station SBLT. OurW‐phase solution indicates a shallow dip of 6.2°, compatible with long‐period surface wave radiation patterns. Inversion of teleseismic body waves indicates a shallow slip distribution extending from about 10 km deep to near the trench with maximum slip of ∼4.1 m and seismic moment of Nm (MW7.3). Joint modeling of seismic and tsunami data indicates a shallow rigidity of ∼23 GPa. We find a low moment‐scaled radiated energy of , similar to that of the 2010MW7.8 Mentawai event () and other tsunami earthquakes. These characteristics indicate that the 2022 event should be designated as a smaller moment magnitude tsunami earthquake compared to the other 12 well‐documented global occurrences since 1896. The 2022 event ruptured up‐dip of the 2007MW8.4 Bengkulu earthquake, demonstrating shallow seismogenic capability of a megathrust that had experienced both a deeper seismic event and adjacent shallow aseismic afterslip. We consider seismogenic behavior of shallow megathrusts and concern for future tsunami earthquakes in subduction zones globally, noting a correlation between tsunami earthquake occurrence and subducting seafloor covered with siliceous pelagic sediments. We suggest that the combination of pelagic clay and siliceous sediments and rough seafloor topography near the trench play important roles in controlling the genesis of tsunami earthquakes along Sumatra and other regions, rather than the subduction tectonic framework of accretionary or erosive margin.
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Tsunami Variability for the 2021 Megathrust and 2023 Outer Rise M W 7.7 Earthquakes Southeast of the Loyalty Islands
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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- Award ID(s):
- 1802364
- PAR ID:
- 10611468
- Publisher / Repository:
- American Geophysical Union
- Date Published:
- Journal Name:
- Journal of Geophysical Research: Oceans
- Volume:
- 130
- Issue:
- 3
- ISSN:
- 2169-9275
- Format(s):
- Medium: X
- Sponsoring Org:
- National Science Foundation
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Koper, Keith (Ed.)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.more » « less
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Abstract Models of near‐field tsunamis and an extreme hurricane provide further evidence for a great precolonial earthquake along the Puerto Rico Trench. The models are benchmarked to brain‐coral boulders and cobbles on Anegada, 125 km south of the trench. The models are screened by their success in flooding the mapped sites of these erratics, which were emplaced some six centuries ago. Among 25 tsunami scenarios, 19 have megathrust sources and the rest posit normal faulting on the outer rise. The modeled storm, the most extreme of 15 hurricanes of category 5, produces tsunami‐like bores from surf beat. In the tsunami scenarios, simulated flow depth is 1 m or more at all the clast sites, and 2 m or more at nearly all, given either a megathrust rupture 255 km long with 7.5 m of dip slip and M8.45, or an outer‐rise rupture 130 km long with 11.4 m of dip slip and M8.17. By contrast, many coral clasts lie beyond the reach of simulated flooding from the extreme hurricane. The tsunami screening may underestimate earthquake size by neglecting trees and shrubs that likely impeded both the simulated flows and the observed clasts; and it may overestimate earthquake size by leaving coastal sand barriers intact. The screening results broadly agree with those from previously published tsunami simulations. In either successful scenario, the average recurrence interval spans thousands of years, and flooding on the nearest Caribbean shores begins within a half‐hour.more » « less
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Abstract Oceanic transform faults connect spreading centers and are imprinted with previous tectonic events. However, their tectonic interactions are not well understood due to limited observations. The Discovery transform fault system at 4°S, East Pacific Rise (EPR), represents a young transform system, offering a unique opportunity to study the interplay between faulting and other tectonic events at an early phases of an oceanic transform system. Discovery regularly hostsM5–6 characteristic earthquakes, and the seafloor north of Discovery includes a 35 km‐long rift zone that records a complex history of rifting, faulting and volcanism, suggesting that the transform faults likely interact with regional tectonic activity. We apply a machine‐learning enabled workflow to locate 21,391 earthquakes recorded during a 1‐year ocean bottom seismometer experiment in 2008. Our results indicate that seismicity on the western Discovery fault is separated into seven patches with distinct aseismic and seismic slip modes. Additionally, we observe a patch of off‐fault seismicity near where seafloor abyssal hills intersect the rift zone. This seismicity may have been caused by varying opening rates as spreading rate decreases from north to south in the rift zone. Our findings suggest that the Discovery system is still evolving, and that system equilibrium has not been reached between rifting and faulting. These results reflect the complex yet rarely observed interactions between fault slip, plate rotation, and rifting which are likely ubiquitous at oceanic transform systems.more » « less
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Abstract The additional work of ploughing makes seamounts more resistant to subduction and more strongly coupled than smoother areas. Nevertheless, the idea that subducted seamounts are weakly coupled and slip aseismically has become dominant in the last decade. This idea is primarily based on the claim that a seamount being subducted in the southern Japan Trench behaves this way. The key element in this assertion is that largeM ∼ 7 earthquakes that abut the leading edge of the seamount require that the seamount be aseismically sliding to initiate them. More recent observations show instead that the surrounding region is aseismically sliding while the seamount acts as a stationary buttress. Here we re‐examine this case and model it with both weak and strong asperity assumptions. Our modeling results show that only a strong asperity model can produce this type of earthquake. Strong asperities also rupture the seamount in great earthquakes with long recurrence times. This provides the previously unknown source for a series of great tsunami earthquakes that have occurred along the southern Japan Trench, the most recent being the 1677 M8.3–8.6 Enpo Boso‐oki tsunami earthquake. The “weak asperity” hypothesis is thus found to be false in this foundational example.more » « less
- Free Publicly Accessible Full Text
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Accepted Manuscript
- Journal Article:
- https://doi.org/10.1029/2024JC021880
