Frequency dispersion amplifies tsunamis caused by outer-rise normal faults

Abstract

Although tsunamis are dispersive water waves, hazard maps for earthquake-generated tsunamis neglect dispersive effects because the spatial dimensions of tsunamis are much greater than the water depth, and dispersive effects are generally small. Furthermore, calculations that include non-dispersive effects tend to predict higher tsunamis than ones that include dispersive effects. Although non-dispersive models may overestimate the tsunami height, this conservative approach is acceptable in disaster management, where the goal is to save lives and protect property. However, we demonstrate that offshore frequency dispersion amplifies tsunamis caused by outer-rise earthquakes, which displace the ocean bottom downward in a narrow area, generating a dispersive short-wavelength and pulling-dominant (water withdrawn) tsunami. We compared observational evidence and calculations of tsunami for a 1933 Mw 8.3 outer-rise earthquake along the Japan Trench. Dispersive (Boussinesq) calculations predicted significant frequency dispersion in the 1933 tsunami. The dispersive tsunami deformation offshore produced tsunami inundation heights that were about 10% larger than those predicted by non-dispersive (long-wave) calculations. The dispersive tsunami calculations simulated the observed tsunami inundation heights better than did the non-dispersive tsunami calculations. Contrary to conventional practice, we conclude that dispersive calculations are essential when preparing deterministic hazard maps for outer-rise tsunamis.

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

“…Glimsdal et al (2013) showed that dispersion effects may be expected for moderate-magnitude earthquakes. Accounting for dispersion effects can be important if the resulting series of excited oceanic waves locally interfere constructively and amplify, which has been observed in tsunami scenarios of the South China Sea (Ren et al, 2015) and outer-rise normal faults (Baba et al, 2021). Here, we do not detect significant differences in wave height or tsunami arrival times compared to our one-way linked scenarios, despite dispersion effects.…”

Evidence of northward dipping crustal layers underneath the eastern part of the Indo- Gangetic foreland basin, India: Implication for geodynamic evolution and seismogenesis

“…Such an interpretation suggests a strong mechanical coupling of the Indian plate beneath the MFT. Similar tectonic environment is in the case for the outer-rise earthquakes, like Showa-Sanriku (M W 8.0, 1934), Kuril (M W 8.3, 2007), and Samoa-Tonga (M W7.8, 2009), where the bending of the subducting plate is the primary reason for seismicity121 . The outer-rise earthquakes are generally shallow and involve normal faulting122 , which is…”