Recently, seismologists from the California Institute of Technology found that a global tsunami occurred in the Atlantic, Pacific, and Indian Oceans in August 2021, but could not find the cause for a while.
In 2004 there was a tsunami that spread across three oceans, with a clear origin in the Indian Ocean, but this time it was a little strange.
5 consecutive earthquakes
Based on monitoring data, scientists initially suspected that the tsunami was related to a 7.5-magnitude earthquake near the South Sandwich Islands in the South Atlantic, but the quake’s epicenter was at a depth of 47 kilometers, making it theoretically impossible to trigger a tsunami.
New research published in the journal Geophysical Research Letters suggests the causative earthquake that occurred near the South Sandwich Islands was not just one event, but actually five individual sub-quakes in a row, hiding a shallower magnitude 8.2 earthquake that resulted in the far-reaching tsunami.
After in-depth processing and analysis of the seismic wave data, scientists found that the earthquake was actually composed of 5 consecutive earthquakes, each only a few minutes apart. The third one reached a magnitude of 8.2, and the focal depth was only 15 kilometers, enough to cause a tsunami.
Monitoring system Did not capture the earthquake
However, the earthquake was ignored by the monitoring system.
The reason is that it combines the characteristics of two types of submarine earthquakes.
One is caused by the sudden rupture of the plate, the source of the earthquake is deep, the energy release is instantaneous, and the formed seismic wave is relatively short.
The other is caused by the slow sliding of two plates, the source is shallow, and the energy is released slowly over a long period of time.
This pivotal third earthquake, which lasted about 200 seconds, released 70% of the combined energy of the earthquake that set off the tsunami that hit three oceans.
It’s just that the seismic wave it generates has a long period and is not within the detection range of the existing early warning system, so only the obviously low measurement result of magnitude 7.5 was seen, and there was no tsunami warning.
