Deep Investigations of Outer‐Rise Tsunami Characteristics Using Well‐Mapped Normal Faults Along the Japan Trench

Abstract

To assess the risk of tsunamis from outer-rise earthquakes, we carried out tsunami simulations using 33 simple rectangular fault models with 60°dip angles based on marine seismic observations and surveys of the Japan Trench. The largest tsunami resulting from these models, produced by a M w 8.7 normal-faulting event on a fault 332 km long, had a maximum height of 27.0 m. We tested variations of the predictions due to the uncertainties in the assumed parameters. Because the actual dip angles of the Japan Trench outer-rise faults range from 45°to 75°, we calculated tsunamis from earthquakes on fault models with 45°, 60°, and 75°dip angles. We also tested a compound fault model with 75°dip in the upper half and 45°dip in the lower half. Rake angles were varied by ±15°. We also tested models consisting of small subfaults with dimensions of about 60 km, models using other earthquake scaling laws, models with heterogeneous slips, and models incorporating dispersive tsunami effects. Predicted tsunami heights changed by 10-15% for heterogeneous slips, up to 10% for varying dip angles, about 5-10% from considering tsunami dispersion, about 2% from varying rake angles, and about 1% from using the model with small subfaults. The use of different earthquake scaling laws changed predicted tsunami heights by about 50% on average for the 33 fault models. We emphasize that the earthquake scaling law used in tsunami predictions for outer-rise earthquakes should be chosen with great care. To improve early warning of shaking and tsunamis due to offshore earthquakes, a large-scale cabled observatory, the Seafloor Observation Network for Earthquakes and Tsunamis along the Japan Trench (S-net) was deployed in 2011 (NIED, 2019). S-net is equipped with about 150 ocean-bottom pressure gauges that can register the passage of tsunamis before their arrival at the coast and seven GPS-equipped buoys that can measure changes in sea level (Kato et al., 2005) have been installed less than 20 km off the coast. Real-time tsunami prediction systems have been developed that rely on offshore tsunami observation data. The tFISH method (Tsushima et al., 2009, 2012) estimates the initial sea surface deformation by inverting pressure data in real time. Maeda et al. (2015) and Wang et al. (2017) have applied data assimilation techniques to the pressure data to improve predictions of tsunami propagation.

Meteorological Tsunami Generation Due to Sea‐Surface Pressure Change: Three‐Dimensional Theory and Synthetics of Ocean‐Bottom Pressure Change

“…A dispersive‐tsunami amplification was also reported for normal fault earthquakes (Baba et al. 2020) and landslide tsunamis dominated by short‐wavelength waves (Baba et al., 2019).…”

“…It is interesting that the dispersion possibly enlarges the maximum tsunami height for the moving pressure change source, although it reduces the maximum sea-surface height of tsunamis arising from thrust fault earthquakes (e.g., Saito et al, 2014). A dispersive-tsunami amplification was also reported for normal fault earthquakes (Baba et al 2020) and landslide tsunamis dominated by short-wavelength waves (Baba et al, 2019).…”

Rupture processes of the 2021 and 2022 Fukushima-oki earthquakes: Adjacent events on the complex fault system in the subducting slab

“…Because faults in the outer-rise region reach a depth of 40 km from the surface and many faults are mapped over 300 km along the trench (e.g., Baba et al 2020), a similar fault system may exist outside the combined source region of the Fukushima-oki earthquakes. In the Japan Trench subduction zone, a neutral plane lies between the planes of the double seismic zone at a depth of 22 km from the plate boundary (Kita et al 2010).…”

Quantifying tsunami impact on industrial facilities and production capacity in ports: An application to Sendai Port, Japan

“…Another source of earthquake is an M7.7 earthquake off Fukushimaa local source for the Fukushima Prefecture [13]. Lastly, M8.7 outer-rise earthquakes along the eastern part of the Japan Trench have also been proposed as possible sources of earthquakes [14]. Tsunami inundation in the Sendai Port is modelled for each of these proposed scenarios.…”

“…Initial water level for tsunami generation was calculated based on [15], which assumes that the water level change is equal to the change in the seafloor due to the earthquake as calculated by the proposed fault parameters. Fault parameters for each earthquake scenario were obtained from [11][12][13][14]. The TUNAMI-N2 model was used to numerically simulate the tsunami [16].…”