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NOAA NCTR research product
Not an official forecast

The August 12, 2021, Sandwich Islands tsunami was the result of a complex seismic sequence of strong earthquakes in the Scotia Subduction Zone in the South Atlantic. The tsunami apparently was generated by the Mw 8.1 earthquake (58.451°S 25.327°W), at 2021-08-12 18:35:20 (UTC) (according to the USGS). This strong earthquake occurred just 3 minutes after and about 90 km to the south of the smaller Mw 7.5 foreshock (USGS). In the 24 hours following the mainshock, 61 aftershocks of Mw 4.5 or larger were detected, including three aftershocks greater than Mw 6 (USGS). Our analysis suggests that the tsunami was generated by the strongest Mw 8.1 earthquake. The tsunami was recorded at many coastal sea-level stations around the world (Figure 1). The largest recorded coastal tsunami amplitude was ~75 cm and was detected at the King Edward Point tide station 3 hours after the earthquake, while the locations in Alaska recorded tsunami signal over 15 cm after more than 24 hours of propagation from the source. To our knowledge, this was the first tsunami since the 2004 Indian Ocean tsunami that was recorded in three different oceans.

Figure 1. Tsunami model amplitudes and selected coastal sea-level records
Figure 1. Tsunami model amplitudes and selected coastal sea-level records
Click to see large image.

The closest DART system was over 7 hours of tsunami propagation away in a different ocean offshore Chile. Therefore, no DART data was available for tsunami source assessment. The tsunami model results are obtained using the pre-computed database of tsunami sources combination based on the seismic estimate of the earthquake source magnitude and location.

Model results shown below were created with the NOAA forecast method using the MOST model via inversion of the source with NOAA's TFS (Tsunami Forecast System).

The graphics display preliminary modeling analysis, showing qualitative and quantitative information about the tsunami, including tsunami wave interaction with ocean floor bathymetric features, and neighboring coastlines. Tsunami model amplitude information is shown color-coded according to the scale bar.

Modeling Results

Maximum wave amplitude distribution
Model amplitudes calculated with the MOST forecast model.
Click to see large image.
Propagation animation
Propagation animation
Click for animation.

Preliminary Model

The model of the August 12, 2021, Sandwich Islands tsunami was developed using global propagation model and high-resolution inundation forecast models that are available for several tide-gauge locations. The model of tsunami propagation across three different oceans is constructed from the pre-computed generation/propagation forecast database by considering seismic analysis of the source from USGS and the signature of the tsunami at selected tide gauges. Comparison of the measured tsunami recorded at tide gauges (black) with model results (red) is presented in the graphics.

The model results are preliminary. The tsunami source is estimated from seismic analysis and a limited number of tide gauge records. The discrepancies between model results and tide gauge observations are mostly due to 1) the limited knowledge of the earthquake/tsunami source configuration at this point, 2) the very long propagation distances that may amplify any errors in source estimation and 3) uncertain bathymetry data for locations outside of the U.S.

Disclaimer: The models on these pages show the results of ongoing research to enhance tsunami science and to improve NOAA operational tsunami forecasts. These products were developed during or shortly after the tsunami event, are intended for research use, and are not an official forecast. They should not be used as the basis of any public or private policy decisions. Please contact NCTR to find if there are more detailed follow-on analysis results.

Media Contacts

NOAA Public Affairs Monica Allen - 301-734-1123
NWS Public Affairs Susan Buchanan - 301-713-0622

When using information from this page, please credit NOAA / PMEL / Center for Tsunami Research

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