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News & Current Events Apr 20, 2026 at 2:15 PM

Japan warns of slightly increased risk of mega-quake after a 7.7-magnitude one

Posted by Hrmbee

Earthquake sets off brief tsunami alert and a megaquake advisory in northern Japan
AP News
Earthquake sets off brief tsunami alert and a megaquake advisory in northern Japan
A 7.7 magnitude earthquake Monday off northern Japan sparked a short-lived tsunami alert and an advisory of a slightly higher risk of a possible megaquake.
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SpiderSlitScrotums Apr 20, 2026 +259
[About 5% of earthquakes](https://www.usgs.gov/faqs/what-probability-earthquake-a-foreshock-a-larger-earthquake) are foreshocks that precede a larger earthquake within a week, so it is always wise to be cautious for a little while after any significant earthquake, especially given that it may have weakened structures.
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sarhoshamiral Apr 20, 2026 -137
Be careful about implying causation vs timing. If you read all such studies, they actually state we are not sure if there is a relation. Because a large quake is less likely to happen, just timing of events would make it so that a large earthquake follows a smaller one some small percentage of the time. Think about two independent events where chance of one is 20% and the other is 1%. In long term, you will likely observe that the 1% event follows the 20% one 5% of the time.
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SpiderSlitScrotums Apr 20, 2026 +123
Perhaps, but I’m only quoting the USGS, who are the experts on this. They could be wrong, but this is the advice they put out.
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sarhoshamiral Apr 20, 2026 -121
They arent wrong but my point is it doesnt make a difference for risk assessment in practice. A big quake can happen anytime. I dont believe we have any evidence to suggest they occur more frequently after a small one in a measurable way.
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datsoar Apr 20, 2026 +64
The commenter never said they are more frequent. Just that it sometimes happens.
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GrinchWhoStoleEaster Apr 20, 2026 -91
In fact, there's every good reason to assume they DON'T. And Earthquake is a RELEASE of pressure, after all, not a building up of it. In theory, a quake should DELAY a larger quake the same way venting you pressure cooker delays its otherwise inevitable explosion.
-91
Romeo_Glacier Apr 20, 2026 +54
That isn’t how plate tectonics work at all. When pressure releases on one section of the plate it can increase pressure on others. There are no singular points of pressure. It builds up along the entire plate at varying amounts.
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Eggonioni Apr 21, 2026 +2
Sand a soft flat surface with a rough paper. Now look at all the varied ridges and dips that are existing after applying the sandpaper down. That's what you can expect the unpredictability of two incomprehensible gigantic masses of crust to look like between where they are interacting. There's no detailed telling how it looks down there, how could you dictate that the earthquake didn't suddenly create more intense roughness at the fault line or anywhere deeper after it occurred? That's why the caution is warranted, because we cannot see anything down there in granular-enough detail to call things safe until sufficient time has passed after large earthquake examples.
2
cteno4 Apr 20, 2026 +22
Although correlation ≠ causation, the correlation is *still there*, and it’s a strong one. The presence of a third factor (if there is one) does not eliminate the fact that the first two factors are related. This means that you should still be careful of the larger earthquake, and not dismiss it offhand.
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Hrmbee Apr 20, 2026 +126
>An earthquake registering a preliminary magnitude of 7.7 off northern Japan on Monday prompted a short-lived tsunami alert and the advisory of a higher risk of a possible mega-quake for coastal areas there. > >The Cabinet Office and the Japan Meteorological Agency said there was a 1% chance for a mega-quake, compared to a 0.1% chance during normal times, in the next week or so following the powerful quake near the Chishima and Japan trenches. > >Officials said the advisory was not a quake prediction but urged residents in 182 towns along the northeastern coasts to raise their preparedness while continuing their daily lives. From 0.1% chance to 1% chance is a pretty significant jump. Given the recent pattern of these larger quakes in this region, this is concerning.
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sun_not_cold Apr 20, 2026 +10
So every week they predict a 1/1000 chance of a mega-quake? Or roughly one every 19.2 years?
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Goldenrah Apr 20, 2026 +16
Considering Japan, that sounds about right. They have a huge earthquake every once in a while and multiple smaller ones every year.
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sarhoshamiral Apr 20, 2026 +22
In numbers wise yes but not in practice. You would still behave same, take same precautions etc.
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-TheExtraMile- Apr 20, 2026 +17
well if this starts with a 7.5 then let's hope there won't be a follow up...
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wyvernx02 Apr 20, 2026 +22
For those wondering, this is the section of plate boundary just to the north of the one that ruptured in 2011.
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Angrymilks Apr 20, 2026 +17
Amazing the number of earthquake experts suddenly appear from their foreign relations consulting and macroeconomic knowledge jobs.
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K1W1_S373N Apr 20, 2026 +7
Let’s put it into perspective here: “The Cabinet Office and the Japan Meteorological Agency said there was a 1% chance for a mega-quake…”
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SideburnSundays Apr 21, 2026 +5
1% is enough risk to make Japanese feel anxious.
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_mad_adventures Apr 21, 2026 +3
It makes me anxious and I live thousands of miles away on the northwest coast USA, in a tsunami zone. 
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Apoplegy Apr 20, 2026 +3
How big would the mega quake be?
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June_Fatality Apr 20, 2026 +5
Mega. Significantly bigger than large, and then larger than that.
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nilkski Apr 20, 2026 -2
Move this risk to Washington DC pls
-2
Starfox-sf Apr 20, 2026 +4
DC had an earthquake, in 2011 (I felt it couple of hundred miles away).
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June_Fatality Apr 20, 2026
Those demons clawed their way out of hell once, they'll just do it again.
0
muFUtaco Apr 20, 2026 -4
Expect a wobble. Don't know when, just "soon". Imbalanced planet (literally trillions of tons of former "pole ice" headed to the equator as melted seawater) and an altered shift in the relationship between the core (the armature) and the mantle (the stator). Wobble. You ever play with a gyroscope? Ever notice as it spins down it wobbles? Like that.
-4
DonnyTheWalrus Apr 21, 2026 +8
The mass of the earth is 6000000000000000000000000 kilograms. Any polar ice mass (which is not happening anyway, BTW, it's just replaced by other ocean water) is a rounding error. 
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YoghurtDull1466 Apr 20, 2026 -18
Washington is ready for the mega quake
-18
Drywesi Apr 20, 2026 +9
No they f****** are not Cascadia going is going to be a titanic disaster.
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YoghurtDull1466 Apr 20, 2026 -2
Hopefully it takes me out first for my ignorant comments
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