Frequency dispersion amplifies tsunamis caused by outer-rise normal faults

Tsunamis are typically modeled using long-wave approximations that ignore dispersive effects. This approach is common because most tsunamis have wavelengths much longer than ocean depths, making dispersion negligible. Prior research has shown that non-dispersive models often predict larger tsunami heights than dispersive ones, which is considered acceptable for hazard mapping. However, outer-rise earthquakes, which occur near subduction zones, generate unique tsunami characteristics. These events displace the seafloor downward in a localized area, potentially creating short-wavelength tsunamis. This gap motivated a closer examination of dispersive effects in outer-rise tsunami modeling. No prior work had resolved how frequency dispersion might amplify tsunami impacts in such cases. The 1933 Japan Trench earthquake provided a case study for this phenomenon. Observational data suggested discrepancies between non-dispersive predictions and actual inundation heights. This uncertainty drove the need to reassess modeling assumptions for outer-rise events.

This study aimed to evaluate the role of frequency dispersion in amplifying tsunamis caused by outer-rise normal faults. Outer-rise earthquakes differ from typical subduction zone events because they involve downward displacement of a narrow seafloor region. This displacement generates a tsunami with short wavelengths and a pulling-dominant mechanism, which may be more sensitive to dispersive effects. The researchers sought to determine whether dispersive modeling could better predict tsunami behavior in these scenarios. They focused on the 1933 Japan Trench earthquake as a test case. The goal was to compare dispersive and non-dispersive models to assess their accuracy in predicting inundation heights. The study also aimed to evaluate whether current hazard mapping practices adequately account for dispersion in outer-rise events. By addressing this gap, the authors hoped to improve tsunami risk assessments for similar geological settings.

The researchers used both dispersive and non-dispersive models to simulate the 1933 Japan Trench tsunami. Dispersive calculations employed Boussinesq equations, which account for frequency dispersion and short-wavelength effects. Non-dispersive models used long-wave approximations, which ignore dispersion. The study compared the simulated tsunami inundation heights from both approaches with observed data from the 1933 event. The team analyzed the spatial distribution of tsunami waves offshore and onshore. They evaluated how dispersive effects altered the wave deformation as it propagated. The comparison included quantitative assessments of inundation height differences between the two models. The researchers also examined whether dispersive modeling could better replicate observed tsunami patterns. This approach allowed them to determine the relative importance of frequency dispersion in outer-rise tsunami scenarios.

Dispersive calculations predicted significant frequency dispersion in the 1933 Japan Trench tsunami. The dispersive model simulated tsunami deformation offshore that led to higher inundation heights compared to non-dispersive calculations. The dispersive tsunami reached about 10% greater inundation heights than the non-dispersive model. This result suggests that dispersion amplifies tsunami impacts in outer-rise events. The dispersive model also better matched observed tsunami inundation heights than the non-dispersive model. The study found that the pulling-dominant mechanism of outer-rise earthquakes enhances the role of frequency dispersion. The short-wavelength nature of the tsunami generated by downward displacement increases the sensitivity to dispersive effects. These findings challenge the conventional assumption that dispersion is negligible in tsunami hazard mapping.

The authors conclude that dispersive calculations are essential for accurately modeling outer-rise tsunamis. The study found that frequency dispersion significantly amplifies tsunami inundation heights in these events. Dispersive models better replicate observed tsunami behavior than non-dispersive ones. The findings suggest that current hazard mapping practices may underestimate tsunami risks in outer-rise earthquake scenarios. The researchers propose that deterministic hazard maps should incorporate dispersive effects for outer-rise events. The study does not claim that dispersion is universally important for all tsunamis. The results are specific to outer-rise earthquakes with narrow seafloor displacement. The authors emphasize the need to reassess modeling assumptions for similar geological settings.