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1. A Compound Faulting Model for the 1975 Kalapana, Hawaii, Earthquake, Landslide, and Tsunami

   https://doi.org/10.1029/2021JB022488  

   Yamazaki, Yoshiki; Lay, Thorne; Cheung, Kwok Fai (November 2021, Journal of Geophysical Research: Solid Earth)

   Abstract The Kalapana, Hawaii,M_W7.7 earthquake on November 29, 1975 generated a local tsunami with at least 14.3 m runup on the southeast shore of Hawaii Island adjacent to Kilauea Volcano. This was the largest locally generated tsunami since the great 1868 Ka'u earthquake located along‐shore to the southwest. Well‐recorded tide gauge and runup observations provide a key benchmark for studies of statewide tsunami hazards from actively deforming southeast Hawaii Island. However, the source process of the earthquake remains controversial, with coastal landsliding and/or offshore normal or thrust faulting mechanisms having been proposed to reconcile features of seismic, geodetic, and tsunami observations. We utilize these diverse observations for the 1975 Kalapana earthquake to deduce a compound faulting model that accounts for the overall tsunamigenesis, involving both landslide block faulting along the shore and slip on the island basal décollement. Thrust slip of 4.5–8.0 m on the offshore décollement produces moderate near‐field runup but controls the far‐field tsunami. The slip distribution implies that residual strain energy was available for the May 4, 2018M_W7.2 thrust earthquake during the Kilauea‐East Rift Zone eruption. Local faulting below land contributes to geodetic and seismic observations, but is non‐tsunamigenic and not considered. Slip of 4–10 m on landslide‐like faults, which extend from the Hilina Fault Zone scarp to offshore shallowly dipping faults reaching near the seafloor, triples the near‐field tsunami runup. This compound model clarifies the roles of the faulting components in assessing tsunami hazards for the Hawaiian Islands. 

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2. Optimizing a Model of Coseismic Rupture for the 22 July 2020 M _W 7.8 Simeonof Earthquake by Exploiting Acute Sensitivity of Tsunami Excitation Across the Shelf Break

   https://doi.org/10.1029/2022JB024484  

   Bai, Yefei; Liu, Chengli; Lay, Thorne; Cheung, Kwok Fai; Ye, Lingling (July 2022, Journal of Geophysical Research: Solid Earth)

   Abstract The Shumagin seismic gap along the Alaska Peninsula experienced a major,M_W7.8, interplate thrust earthquake on 22 July 2020. Several available finite‐fault inversions indicate patchy slip of up to 4 m at 8–48 km depth. There are differences among the models in peak slip and absolute placement of slip on the plate boundary, resulting from differences in data distributions, model parameterizations, and inversion algorithms. Two representative slip models obtained from inversions of large seismic and geodetic data sets produce very different tsunami predictions at tide gauges and deep‐water pressure sensors (DART stations), despite having only secondary differences in slip distribution. This is found to be the result of the acute sensitivity of the tsunami excitation for rupture below the continental shelf in proximity to an abrupt shelf break. Iteratively perturbing seismic and geodetic inversions by constraining fault model extent along dip and strike, we obtain an optimal rupture model compatible with teleseismicPandSHwaves, regional three‐component broadband and strong‐motion seismic recordings, hr‐GNSS time series and static offsets, as well as tsunami recordings at DART stations and regional and remote tide gauges. Slip is tightly bounded between 25 and 40 km depth, the up‐dip limit of slip in the earthquake is resolved to be well‐inland of the shelf break, and the rupture extent along strike is well‐constrained. The coseismic slip increased Coulomb stress on the shallow plate boundary extending to the trench, but the frictional behavior of the megathrust below the continental slope remains uncertain. 

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3. Rupture Model for the 29 July 2021 M _W 8.2 Chignik, Alaska Earthquake Constrained by Seismic, Geodetic, and Tsunami Observations

   https://doi.org/10.1029/2021JB023676  

   Ye, Lingling; Bai, Yefei; Si, Daojun; Lay, Thorne; Cheung, Kwok Fai; Kanamori, Hiroo (July 2022, Journal of Geophysical Research: Solid Earth)

   Abstract A great earthquake struck the Semidi segment of the plate boundary along the Alaska Peninsula on 29 July 2021, re‐rupturing part of the 1938 rupture zone. The 2021M_W8.2 Chignik earthquake occurred just northeast of the 22 July 2020M_W7.8 Simeonof earthquake, with little slip overlap. Analysis of teleseismicPandSHwaves, regional Global Navigation Satellite System (GNSS) displacements, and near‐field and far‐field tsunami observations provides a good resolution of the 2021 rupture process. During ∼60‐s long faulting, the slip was nonuniformly distributed along the megathrust over depths from 32 to 40 km, with up to ∼12.9‐m slip in an ∼170‐km‐long patch. The 40–45 km down‐dip limit of slip is well constrained by GNSS observations along the Alaska Peninsula. Tsunami observations preclude significant slip from extending to depths <25 km, confining all coseismic slip to beneath the shallow continental shelf. Most aftershocks locate seaward of the large‐slip zones, with a concentration of activity up‐dip of the deeper southwestern slip zone. Some localized aftershock patches locate beneath the continental slope. The surface‐wave magnitudeM_Sof 8.1 for the 2021 earthquake is smaller thanM_S = 8.3–8.4 for the 1938 event. Seismic and tsunami data indicate that slip in 1938 was concentrated in the eastern region of its aftershock zone, extending beyond the Semidi Islands, where the 2021 event did not rupture. 

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4. Use of tsunami observations for resolving coseismic slip of recent and historic large submarine earthquakes

   https://doi.org/10.4401/ag-9142  

   Lay, Thorne; Romano, Fabrizio; Lorito, Stefano (January 2024, Annals of Geophysics)

   Tsunamis generated by seafloor displacements accompanying large submarine earthquakes provide sensitivity to absolute slip position and distribution for offshore faulting analogous to that of geodetic observations for landward faulting. Tsunami recordings at deep‐water and near‐shore ocean bottom pressure sensors and tide gauges, along with runup and inundation measurements, can now be reliably modeled using detailed bathymetric structures and robust numerical codes. As a result, tsunami observations now play an important role in quantifying coseismic slip distributions for large submarine earthquakes in subduction zones and other tectonic environments. Applications of joint modeling or inversion of seismic, geodetic and tsunami observations for recent major earthquakes are described, highlighting the specific contributions of the tsunami observations to source model resolution. Tsunami observations provide unique information on the up‐dip extent of earthquake coseismic slip on subduction zone megathrust faults and occurrence of near‐trench slip, which are usually not well constrained by seismic and land‐based geodetic signals. Tsunami signals also help to detect offshore slow slip that is not evident in seismic or land‐based geodetic data and to balance geophysical constraints on ruptures that extend from on‐shore to off‐shore. Tsunami runup measurements and stratigraphic deposits further provide unique constraints on large earthquake ruptures that occurred prior to modern geophysical instrumentation. 

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5. Linked and fully-coupled 3D earthquake dynamic rupture and tsunami modeling for the Húsavík-Flatey Fault Zone in North Iceland

   Kutschera, F; Gabriel, A-A; Wirp, SA; Li, B; Ulrich, T; Abril, C; Halldórsson, B (February 2024, Solid earth)

   Tsunamigenic earthquakes pose considerable risks, both economically and socially, yet earthquake and tsunami hazard assessments are typically conducted separately. Earthquakes associated with unexpected tsunamis, such as the 2018 Mw  7.5 strike-slip Sulawesi earthquake, emphasize the need to study the tsunami potential of active submarine faults in different tectonic settings. Here, we investigate physics-based scenarios combining simulations of 3D earthquake dynamic rupture and seismic wave propagation with tsunami generation and propagation. We present time-dependent modeling of one-way linked and 3D fully coupled earthquakes and tsunamis for the ∼ 100 km long Húsavík–Flatey Fault Zone (HFFZ) in North Iceland. Our analysis shows that the HFFZ has the potential to generate sizable tsunamis. The six dynamic rupture models sourcing our tsunami scenarios vary regarding hypocenter location, spatiotemporal evolution, fault slip, and fault structure complexity but coincide with historical earthquake magnitudes. Earthquake dynamic rupture scenarios on a less segmented fault system, particularly with a hypocenter location in the eastern part of the fault system, have a larger potential for local tsunami generation. Here, dynamically evolving large shallow fault slip (∼ 8 m), near-surface rake rotation (± 20∘), and significant coseismic vertical displacements of the local bathymetry (± 1 m) facilitate strike-slip faulting tsunami generation. We model tsunami crest to trough differences (total wave heights) of up to ∼ 0.9 m near the town Ólafsfjörður. In contrast, none of our scenarios endanger the town of Akureyri, which is shielded by multiple reflections within the narrow Eyjafjörður bay and by Hrísey island. We compare the modeled one-way linked tsunami waveforms with simulation results using a 3D fully coupled approach. We find good agreement in the tsunami arrival times and location of maximum tsunami heights. While seismic waves result in transient motions of the sea surface and affect the ocean response, they do not appear to contribute to tsunami generation. However, complex source effects arise in the fully coupled simulations, such as tsunami dispersion effects and the complex superposition of seismic and acoustic waves within the shallow continental shelf of North Iceland. We find that the vertical velocity amplitudes of near-source acoustic waves are unexpectedly high – larger than those corresponding to the actual tsunami – which may serve as a rapid indicator of surface dynamic rupture. Our results have important implications for understanding the tsunamigenic potential of strike-slip fault systems worldwide and the coseismic acoustic wave excitation during tsunami generation and may help to inform future tsunami early warning systems. 

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