Alaska Tsunami on Oct. 19, 2020

 

We have simulated the tsunami generated from the Alaska earthquake (54.617°N, 159.635°W, depth = 35.4 km, M = 7.6 at 20:54:39 UTC according to USGS) on October 19, 2020 (Fig. 1). The assumed tsunami sources are located inside the aftershock area including the epicenter (Fig. 1).

Case 1 (NP1):
The fault length and width are 60 km × 30 km. The focal mechanism is strike:350º, dip:49º, slip:173º from the USGS's W-phase moment tensor solution (NP1). The top depth of the fault was assumed to 5 km. The average slip on the fault is 4 m. The seismic moment is 3.6 x 10²⁰ Nm (Mw = 7.6) assuming the rigidity of 5 x 10¹⁰ N/m².

Case 2 (NP2):
The fault length and width are 30 km × 20 km. The focal mechanism is strike:84º, dip:85º, slip:41º from the USGS's W-phase moment tensor solution (NP2). The top depth of the fault was assumed to 5 km. The average slip on the fault is 10 m. The seismic moment is 3.0 x 10²⁰ Nm (Mw = 7.6) assuming the rigidity of 5 x 10¹⁰ N/m².

As the initial condition for tsunami, static deformation of the seafloor is calculated for a rectangular fault model [Okada, 1985] using the source models. The used bathymetry data is the 15 arc-second grid data from GEBCO_2020, which was resampled to 12 arc-second grid data. To calculate tsunami propagation, the linear shallow-water, or long-wave, equations were numerically solved by using a finite-difference method [Satake, 1995]. We have downloaded the DART data and Tide gauge data from NOAA's and UNESCO/IOC's web sites, respectively, and compared the simulated tsunami waveforms and the observed ones (Fig. 2). We can see the tsunami propagation in the Pacific ocean (Fig. 3).

 

Fig.1 Tsunami Source Models of (left) Case 1 (NP1) and (right) Case 2(NP2)
The red lines indicate uplift with the contour interval of 0.2 m, while the blue dotted lines indicate subsidence with the contour interval of 0.2 m. Aftershocks determined by USGS are shown by red circles. The focal mechanism determined by USGS is also shown.

 

Fig.2 Maximum Height of Simulated Tsunami and Tsunami Waveforms
Solid lines in red indicate the observed tsunami waveforms. The blue and green lines are calculated tsunami waveforms for Case 2 and Case 1, respectively.

 

Fig.3 Tsunami Propagation for Case 2(NP2) (Click to start animation)
The red color means that the water surface is higher than normal sea level, while the blue means lower.

 

by Yushiro Fujii (IISEE, BRI) and Kenji Satake (ERI, Univ. of Tokyo)

References

Okada, Y. (1985), Surface Deformation Due to Shear and Tensile Faults in a Half-Space, Bull. Seismol. Soc. Am., 75, 1135-1154. Satake, K. (1995), Linear and Nonlinear Computations of the 1992 Nicaragua Earthquake Tsunami, Pure and Appl. Geophys., 144, 455-470. GEBCO Compilation Group (2020), GEBCO 2020 Grid (doi:10.5285/a29c5465-b138-234d-e053-6c86abc040b9)

Last Updated on 2020/10/27