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Magnitude 5.8 did happen off the coast of Japan. NHK reports that despite the JMA issuing an early warning alert, there was no strong shaking felt. Looking quite likely that quake alert was a false alarm, which has happened before.. 1/
In 2018, two small earthquakes happened at the same time, leading the system to believe a much larger earthquake was happening https://www.nikkei.com/article/DGXMZO25374280V00C18A1CC1000/
In August 2016, there was a heart-stopping warning (which didn't go to everyone's phones unlike today's) that indicated on some platforms like Yurekuru that a magnitude 9 quake had struck Tokyo Bay. People who got this alert were terrified. https://weathernews.jp/s/topics/201608/010145/
And in 2013 a magnitude 2.3 in Wakayama accidentally triggered a warning of a magnitude 7.8 quake in the area. Like today, the full warning system was triggered https://www.livemint.com/Politics/hE8GRNQHmR6TQWFtzymjRO/Japans-quake-warning-triggered-in-error-halting-bullet-tra.html
The situation is made "worse" now than in 2013 as everyone has smartphones, which all go off in a massive cacophony when the early warning is triggered.
Nonetheless, the early warning alert system is amazing technology and it literally saves lives.
Nonetheless, the early warning alert system is amazing technology and it literally saves lives.
No system like this can be 100% perfect, and it's designed to be overly sensitive - sending false positives but hopefully no false negatives, in the language of the day. But when unexpected things happen - such as 2 quakes at once - there's little can be done to remove the noise
The system is entirely automated. It triangulates shaking from multiple sensors (many of them out to sea) and tries to interpret the likely size of the quake from the p-waves (the less-damaging waves that precede the s-wave).
https://www.jma.go.jp/jma/en/Activities/eew1.html
https://www.jma.go.jp/jma/en/Activities/eew1.html
Since the time the system was set up in the 2000s, the JMA has acknowledged it's not perfect...
One common complaint I see is, it didn't go off when I felt <such-and-such a strong quake>.
The system is designed to go off when shaking of at least 5- on the shindo scale is expected in one area, as well as for areas expected to get shaking of shindo 4...
The system is designed to go off when shaking of at least 5- on the shindo scale is expected in one area, as well as for areas expected to get shaking of shindo 4...
Fortunately in Tokyo at least that doesn't often happen. There have only been a handful of times shaking of shindo 4 or more have been felt in the last five years (and long may that continue).
The Tokyo quake false alarm this morning was caused by the system miscalculating where the epicenter of the quake was. The early warning system thought it was 450 kilometers further away, turning a magnitude 5.8 quake into a much more significant 7.3. https://www.bloomberg.com/news/articles/2020-07-30/-overstated-magnitude-caused-quake-alert-that-set-tokyo-on-edge?sref=tXumBMhb
