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Significant Earthquake and Tsunami Events

A tricky situation is unfolding in Japan. This happened several hours ago:



And JMA reportedly has issued a warning that chances for a megathrust quake near Tokyo and other areas near the submarine Nankai Trough are higher than usual.

There apparently has been only a half-meter tsunami thus far from today's event, and no Pacific-wide threat exists per the Tsunami Warning Center.

The "Big One" would be much more powerful.

Here's Wikipedia on the Nankai Trough threat (for more reliable information, there is plenty on Google Scholar but most of it will get very technical very quickly)
 
More news. Can't get GT to do it but per browser translation:

...During the press conference, Nao Hirata, chairman of the Nankai Trough Earthquake Evaluation Study Group, said that this earthquake is a "partial breach" of the "scenario" for the occurrence of a "Massive Nankai Trough Earthquake." Additionally, according to the Japan Meteorological Agency, although sudden changes were observed in a short period of time at strain observation points installed in Shikoku following this earthquake, no abnormal changes that could be a "precursor" to a massive earthquake have been observed since the earthquake.
Click to expand...

This layperson thinks what is meant by "breach" and "scenario" involves this:



Translation via X: "If there is an earthquake of magnitude 6.8 or higher within this yellow frame, the "Nankai Trough Earthquake Emergency Information (under investigation)" will automatically be displayed, but the epicenter in the Hyuga-Nada Sea mentioned earlier is quite on the edge of that.Types of information related to the Nankai Trough earthquake and conditions for its release (Japan Meteorological Agency)."

Looks like today's quake might just barely meet the criteria in terms of intensity and location, but meet them it did and so JMA pulled the trigger.

As mentioned in the Japan Times link above, this lower-level warning does not call for evacuation -- it sounds to me like a heads-up.
 
This was reposted by one of the knowledgeable people I've been following since the Noto quakes:



The link goes to a cabinet office page with lots of Nankai Trough quake information, per browser translation. Also, "An emergency notice "Massive Earthquake Warning" has been issued. What everyone needs to do is "reconfirm preparations." Please secure furniture, confirm evacuation destinations and evacuation routes, and check your stockpiles. The risk of a massive earthquake in the Nankai Trough is higher than usual, so please be serious about checking your supplies."
 
It feels kind of like a PDS watch, but for millions of people and a much wider area, yet I didn't think a thread would be appropriate because:



X translation: "'We have assessed that the likelihood of an earthquake occurring is several times higher than usual. Although several times is an extremely high probability in seismological terms, it is still a once in several hundred event. Therefore, we cannot say when or in which area it will occur. Please continue to make sure you are prepared for an earthquake as you have done up until now,' said Hirata Nao, chairman of the evaluation committee."

And then carry on with your life as normal, per some other sources.

Somehow.

Don't think very many people in the area will be sleeping tonight -- and most of them have to travel deep into a mega-city for work tomorrow, too. :(
 
No news updates; it's 4 a.m. over there, and perhaps this gentleman was tired:



X translation: "Disaster Desk Manager Shimakawa said, 'This may be a bit blunt, but we know there is a high probability that this information will be a false alarm. However, if a major earthquake were to occur without us being informed, people would say, 'I wish they had told us beforehand.' This information was launched to avoid that, so we want people to use it to their advantage and continue to spend the next week being safer.'"

It's such a tough situation for scientists and emergency managers because of the geological complexities. I think they did have to do this and not just because of the risk, however remote statistically, of a 2011-type event so close to Tokyo and the bay just funneling those tsunami in soon afterwards -- sobering thought. :(

There has also been a phenomenon just up the coast, offshore of Chiba Prefecture, called slow-slip seismicity. It's not unique to Japan: we have it in the Pacific Northwest; New Zealand's subduction-zone areas get them: etc.

As far as this layperson knows, SSE's (per Pacific NW Seismic Network and others) are not well understood. I've read that they might reduce stress on a plate boundary segment, making megathrust quakes less likely; that they might make the Big One more likely; and, of course, many that I let go because they were way above my head.

However, back in March of this year people studying the Chiba SSE's and others reportedly stated that one result of those quakes would likely be a M7-level quake in the Kanto region (greater Tokyo area) soon.

Instead, we've gotten one on the plate boundary.

What does it all mean?

??? but the experts who actually understand what is known of the issue's details are probably pulling all-nighters, too.

Meanwhile, what strikes me as possibly the best summary of this alert's positive effects was expressed in an unofficial Japanese tweet that said, among other things (X translation): "So a week has little scientific meaning. If you are normally taking earthquake precautions, you can continue as usual. If you are not, please do so. This time. For now."

So -- wake-up call for some and reinforcement for other citizens that their earthquake preparation is a good move.

And, hopefully, a quiet week and on into the foreseeable future for everybody.

There's going to be another big one here eventually, but hopefully not for quite a while and in the meantime Japan (and a lot of volcanologists and emergency managers worldwide) have observed the first practical test of their planning, as well as more data to work within further research.

I hope that's all for now.
 
Last edited:
Posted about five hours ago:



X translation: "As you can see from yesterday's data from the Japan Meteorological Agency, there were 6 cases out of 1437 where a 7-day Mw 7.0 or higher was followed by a Mw 7.8 or higher within 50km, and in fact 4 of those occurred within 24 hours. 25 hours have already passed since yesterday's Hyuga-Nada M7.1. It is said that the probability has gone from 0.1% to 0.5%, but the latest probability can be said to have dropped to 0.167%."

And then the earth shook; according to that knowledgeable person it was in the general location of the Great Kanto Quake center. Fortunately, today's quake was "only" a 5-pointer and unrelated to yesterday's event according to multiple sources he quoted in another tweet, including JMA..
 
Per Google translation of what looked like the gist of JMA's press release (Japanese):

Detection time (time when the earthquake was first detected) 19:57, August 9

Occurrence time (time when the earthquake occurred) 19:57, August 9

Magnitude 5.3 (provisional value)

Location of occurrence Western Kanagawa Prefecture, depth 13km (provisional value; updated from preliminary value of depth about 10km)

Focal mechanism Reverse fault type with a pressure axis in the north-south direction (preliminary report)
Seismic intensity [Maximum seismic intensity 5-weak] Seismic intensity 5-weak was observed in Atsugi City, Nakai Town, Matsuda Town, and Kiyokawa Village in Kanagawa Prefecture, and seismic intensity 4 to 1 was observed in the Kanto, Chubu, and Kinki regions.
Seismic activity status (as of 21:30 on the 9th) Since the occurrence of this earthquake, one earthquake with seismic intensity 1 or higher has occurred (seismic intensity 1: 1 time)

Observation status of long-period seismic motion No areas were observed to be at level 1 or higher.

(Notes on disaster prevention)

There is no need to worry about a tsunami caused by this earthquake.

In areas where the shaking was strong, rockfalls and cliff collapses may be more likely to occur, so please be aware of future seismic activity.

(Outlook for future seismic activity)

In past cases, the rate of earthquakes of a similar magnitude occurring after a major earthquake has been 10-20%. Therefore, in areas where the shaking was strong, please be aware of earthquakes of a maximum intensity of 5-low for about a week after the earthquake. Large earthquakes are likely to occur in the next 2-3 days in particular.

For the full document, including charts and figures, please refer to the "Full document" below.
Click to expand...

They treat it as an ordinary earthquake. Some unofficial sources I follow suggest that it might have been in the descending plate.

As I learned from inquiring out of concern after the recent seismic swarm off Vancouver Island, megathrust quakes occur on the plate boundary.
 
Some of the responses to the warning that I've seen on X include bullet train slowing on certain routes (yesterday -- don't know if any changes are still in effect); closed beaches and cancelled events in places; hospitals facing the sea in one area moving patients to upper floors; and official notes like this:



X translation: "...Secure your furniture, check evacuation sites and routes, sleep in a place where you can escape immediately, and always carry emergency items with you. If you feel shaking or a tsunami warning is issued, be prepared to evacuate from the tsunami immediately....:

It's the first time for this Nankai Trough alert situation, and everybody must be just feeling their way through it.

Hope that's all it comes to!

What I haven't seen mentioned is a recent decision -- last year, I think, but would have to look it up -- regarding emergency response to another major hazard there: Godzilla a Mount Fuji eruption.

(Looks it up)

When experts realized in 2021 that Fujisan, whenever it does finally get around to waking up again, could possibly produce more lava than had been estimated by earlier studies, emergency plans reportedly were changed:

The plan calls for those living in areas where lava flows are expected to reach within 24 hours of the eruption to evacuate on foot to prevent traffic jams, although they may use vehicles when they voluntarily evacuate before the eruption.
Click to expand...

"24 hours" -- that's quite a long time compared to the minutes that would elapse between a megathrust quake and arrival of the first wave.

I even recall reading something around the same time advising Tokyo residents to shelter in place because it wouldn't be possible to evacuate the city in case of a major eruption, but I didn't save the links so please don't quote me on that. It needs to be verified or discounted.

Still, one of the major things on everybody's mind there right now must be, "Exactly how do we all 'evacuate from the tsunami immediately'?"

The good part of that concern is that millions of people and surely all government officials and scientists, are looking at it practically -- it is not an academic problem any more -- and thinking.

That could lead to innovation and solutions that would help in later emergencies of all sorts.

I like to think it will go that way, anyway. Fingers crossed.

Re: volcanoes and earthquakes -- I think Fuji, Aira (Sakurajima), and the Kirishimayama group in Aira's northern rim area are the main ones nearby and am working on a blog post about it for Sunday.

Short answer: As far as I know (which isn't very far but I've done some reading), some volcanologists don't see a link between earthquakes and eruptions, while others suspect that seismic waves from near or distant quakes might set off a volcano that was already primed.
 
There is forecast furor: for example, with the BBC quoting some people as saying things like that they were baffled by the advisory and one boffin as saying:

...
Robert Geller, professor emeritus of seismology at the University of Tokyo [told the BBC] “The issuance of the warning yesterday has almost nothing to do with science," ...
Click to expand...

Perhaps they misquoted him, but it made me run what looked like the gist of one of JMA's bulletins that day -- this one, I think (Japanese) -- through Google Translate. I think that they thought it out very carefully before issuing this historic forecast:

Nankai Trough Earthquake Emergency Information (Massive Earthquake Warning)

** Headline **
Today (8th) at around 16:43, a magnitude 7.1 earthquake occurred with its epicenter in the Hyuga-Nada Sea. As a result of examining the relationship between this earthquake and the Nankai Trough earthquake, it is believed that the possibility of a large-scale earthquake occurring in the hypothetical source area of the Nankai Trough earthquake is relatively higher than usual. Please take disaster prevention measures in accordance with future appeals from the government and local governments.

** Body **
Today (8th) at around 16:43, a magnitude 7.1 earthquake occurred with its epicenter in the Hyuga-Nada Sea. Seismic activity has remained active since then. In addition, step-like changes associated with this earthquake have been observed at strain observation points.
The Japan Meteorological Agency held an emergency evaluation committee on earthquakes along the Nankai Trough to examine the relationship between this earthquake and the Nankai Trough earthquake.
This earthquake was a reverse fault type with a pressure axis in the west-northwest and east-southeast direction, and was evaluated as a moment magnitude 7.0 earthquake caused by a part of the boundary between the land plate and the Philippine Sea plate in the assumed focal area of the Nankai Trough earthquake.

Statistical data on past large earthquakes around the world shows that of the 1,437 earthquakes with moment magnitude 7.0 or more that occurred between 1904 and 2014, there were six cases of earthquakes with moment magnitude 8 or more occurring in the same area within seven days of the first earthquake, and the frequency of occurrence thereafter decreases over time. This data includes a case where a moment magnitude 7 earthquake occurred two days before the 2011 Tohoku Pacific Ocean Earthquake (moment magnitude 9.0). In the world, the frequency of a moment magnitude 8 or higher earthquake occurring within seven days of a moment magnitude 7.0 or higher earthquake in the same area is about once in several hundred.

From these findings, it is believed that the possibility of a large-scale earthquake occurring in the expected focal area of the Nankai Trough earthquake is relatively higher than usual.

There is diversity in Nankai Trough earthquakes, and the focal area of a large-scale earthquake may be limited to the area around this earthquake, or it may extend to the entire Nankai Trough.

If the largest earthquake occurs, strong shaking is expected over a wide area from the Kanto region to the Kyushu region, and a large tsunami is expected along the Pacific coast from the Kanto region to the Okinawa region.

Please take disaster prevention measures in response to future calls from the government and local governments.

The Japan Meteorological Agency will continue to carefully monitor the progress of crustal activity along the Nankai Trough.

*The moment magnitude is a magnitude obtained by carefully examining the scale of the displacement of the earthquake source fault. The magnitude may differ from that reported by the Japan Meteorological Agency in earthquake information, etc.

*The evaluation committee is working together with the Tokai region earthquake disaster prevention measures strengthening regional assessment committee to conduct its review.

** Next announcement scheduled **

From now on, the status of crustal activity will be announced in the "Nankai Trough Earthquake Related Commentary Information".

**
(Reference) Types of information related to Nankai Trough earthquakes **

Nankai Trough Earthquake Emergency Information]
Information release conditions:
○ When an abnormal phenomenon is observed along the Nankai Trough, and an investigation is started or ongoing to determine whether the phenomenon is related to a large earthquake along the Nankai Trough
○ When announcing the results of an investigation into the observed abnormal phenomenon
Keywords to be added to the information name:
○ "Under investigation" When an "Evaluation and Review Meeting on Earthquakes Along the Nankai Trough" is held temporarily due to any of the following reasons
・An earthquake of magnitude 6.8 or greater occurs in the monitored area※1
・Significant changes are observed at one or more strain gauges, along with changes thought to be related to them at multiple other observation points, and there is a possibility that unusual slow slip is occurring at the plate boundary within the expected epicenter, and other changes are observed on the strain gauge that are deemed necessary to consider the relationship with the Nankai Trough earthquake
・Other phenomena that may indicate a change in the fixation state of the plate boundary within the expected epicenter are observed, and other phenomena that are deemed necessary to consider the relationship with the Nankai Trough earthquake are observed
○ "Massive Earthquake Alert" If an earthquake with a moment magnitude of 8.0 or more is deemed to have occurred at the plate boundary within the expected epicenter
○ "Massive earthquake warning"
・If an earthquake with a moment magnitude of 7.0 or more is deemed to have occurred within the monitoring area※1 (excluding cases that fall under the mega-earthquake warning)
・If an unusual slow slip is deemed to have occurred at the plate boundary within the expected epicenter
○ "Investigation ended"
If the phenomenon is deemed to be neither (Massive earthquake warning) nor (Massive earthquake warning)
※1 The expected epicenter of the Nankai Trough and the area up to about 50 km outside the trench axis of the expected epicenter
※2 Excluding earthquakes with deep epicenters due to the subduction of the Pacific Plate

[Nankai Trough Earthquake-Related Commentary Information]

Conditions for information release:

○ When announcing the progress of the situation after announcing the results of an investigation into an observed abnormal phenomenon

○ When announcing the results of an investigation at a regular meeting of the "Evaluation and Review Committee for Earthquakes Along the Nankai Trough"

(However, excluding the announcement of emergency information on Nankai Trough earthquakes)

*If the necessary disaster prevention measures have already been taken, the start of the investigation and the results of the investigation may be announced in the Nankai Trough Earthquake-Related Commentary Information.
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The BBC article buries it but does report that "anecdotally" the alert worked, giving examples.

And today one of the knowledgeable Japanese I follow tweeted ("Tohoku" refers to the 2011 event):



X translation: "It's pretty scary if it's like this from morning to night
1f628.svg


However, a magnitude 7 plate boundary earthquake in the Nankai Trough probably occurs once every 20 years (the last time was 28 years ago), so I personally think that a one-week warning once every 20 years is within the acceptable range
1f613.svg

In Tohoku, a massive earthquake of magnitude 9 occurred two days after the magnitude 7 earthquake."
1f4a6.svg
 
A major holiday weekend (Obon) at a beach in Japan this week:



X translation: "Even though it's a long weekend, there's no one at the beach. Beaches are closed due to the risk of a major earthquake, and some people question this https://asahi.com/articles/ASS8B2D1FS8BPXLB001M.html?ref=tw_asahi

The Shirarahama beach in Shirahama Town, Wakayama Prefecture, which had been closed and swimming prohibited for about a week in response to the Nankai Trough Earthquake Emergency Information (Warning of a Major Earthquake), was deserted on the 10th, with not a single person there, including swimmers."
 
By the way, with this hanging over everyone, also northern Japan is about to experience a typhoon that may bring record rains and associated damage, per this thread (Japanese) by an NHK disaster reporter on X.

All they need now is a Fuji eruption to go full "Japan Sinks" (which is an excellent work that I just heard of and read, available as an eBook and only incidentally a disaster story -- but an extremely good disaster story, too).
 
So far, so good. Part of this bulletin (Japanese, PDF), via GT with emphasis added:

Nankai Trough Earthquake Related Information (No. 2)

** Headline **

A magnitude 7.1 earthquake occurred at approximately 16:43 on August 8th, with its epicenter in the Hyuga-Nada Sea. As a result of this earthquake, it was thought that the possibility of a large-scale earthquake occurring in the expected epicenter of the Nankai Trough earthquake was relatively higher than usual, so an emergency Nankai Trough earthquake information (giant earthquake warning) was issued at 19:15 on August 8th. Please continue to take disaster prevention measures in accordance with calls from the government, local governments, etc.

** Body of the text **

A magnitude 7.1 (moment magnitude 7.0) earthquake occurred at approximately 16:43 on August 8th, with its epicenter in the Hyuga-Nada Sea. Seismic activity has continued to be active near the epicenter of this earthquake. The number of earthquakes (preliminary figures) that occurred in the expected focal area of the Nankai Trough earthquake (including the area surrounding the focal area of the earthquake on August 8th) with a seismic intensity of 1 or higher from 16:00 on the 8th to 14:00 today (10th) are as follows:

8th 16:00 to 24:00 8 times (1 seismic intensity 6-low, 2 seismic intensity, 1 seismic intensity)
9th 00:00 to 24:00 8 times (1 seismic intensity 3, 2 seismic intensity 2, 1 seismic intensity)
10th 00:00 to 14:00 1 time (1 seismic intensity 1)

In addition, at the strain observation point, a step-like change associated with a magnitude 7.1 earthquake has been observed, but no changes other than those normally observed after an earthquake have been observed so far.

As a result of examining the relationship between the earthquake on August 8 and the Nankai Trough earthquake, it was determined that the possibility of a large-scale earthquake occurring in the expected focal area of the Nankai Trough earthquake was relatively higher than usual, and therefore a Nankai Trough Earthquake Emergency Information (Massive Earthquake Warning) was issued at 19:15 on August 8.

The government is calling on areas where disaster prevention measures for the Nankai Trough earthquake are being promoted to reconfirm their earthquake preparedness on a daily basis and to be prepared to evacuate immediately if they feel shaking for one week after the earthquake on the 8th. Please continue to take disaster prevention measures in accordance with the calls from the government and local governments.

The Japan Meteorological Agency will continue to carefully monitor the progress of crustal activity along the Nankai Trough.

*The moment magnitude is the magnitude obtained by carefully examining the magnitude of the displacement of the earthquake source fault. The magnitude may differ from the magnitude announced by the Japan Meteorological Agency in earthquake information, etc. ＊＊

Next announcement ＊＊

We will continue to announce the status of crustal activity in the "Nankai Trough Earthquake-related Commentary Information". The next announcement is scheduled for around 15:30 on the 11th.

In addition, we will announce any new changes observed at any time.
Click to expand...

There also appear to be respectable aftershocks listed from the recent and unrelated 5-pointer on land.
 
JMA is issuing daily reports, and per today's (Japanese, PDF), aftershocks still occur at the epicenter, along with what sound to me like SSE's on the Kii Peninsula, but they have not seen any unusual activity nor any indications that the Nankai system is unlocking.

I'll just follow up here if there is a major change.
 
We're so focused on a possible Nankai Trough disaster that it's easy to overlook just how powerful a 7-pointer can be.

Per Facebook translation of this post , repair work in Italy on telemetry instruments at Stromboli after the recent paroxysm had to be delayed three hours because teleseismic waves from the Japan quake obscured data from the seismological equipment INGV uses to catch signs of impending trouble ASAP whenever anyone is on the volcano.

Per Japan media reports, there were no fatalities from the 7-pointer, but several people were injured, two seriously, and it sounds like quite a few structures sustained damage.
 
Not hazardous in this particular case but definitely significant: took a year for them to figure out what it was!

 
This quake isn't of significant magnitude itself but it's always intriguing when seismologists say that a sequence in an occasionally powerful seismic zone is ongoing -- for multiple strong M4's, that's something to watch, just in case.

It's an incredibly complex geological area (to this layperson, anyway) -- I think this is in the ECSZ-Walker Lane (jargon alert). Anyway, if you're in the general Fresno-Las Vegas-Bakersfield area and felt the shaking from any of these 4-pointers, the USGS would like to hear from you -- your experiences could help them better understand the intricate tectonics here.

 
I was surprised to learn, during my (ultimately unsuccessful) undergrad geology time in upstate New York during the mid-1980s, that northeastern North America is capable of some respectable shakes, rattles, and rolls, like this one:



The thick, colder rock crust here ensured a broader area of felt quakes than we get in the West, just as with the many times stronger New Madrid quakes in the 1800s.
 
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