[Skywise]

Although the quake was 875 km (543 miles) from Tokyo, and an astounding 678 km deep, it was still felt significantly in the Tokyo area.

What concerns me is, this is on the Izu Trench, which is the southern segment of the Japan Trench where the M9 Tohoku quake occurred recently, but on the other side of the Sagami Trough. The Sagami Trough forms a convergent boundary near Tokyo with the Okhotsk and Philippine Sea plates, and was the source of the M7.9 Great Kanto earthquake of 1923 which nearly destroyed Tokyo.

Could this M7.8 be a foreshock to a larger quake on this segment of the Izu Trench like there was for the Tohoku quake?

Could these large quakes on either side of the Sagami Trough place enough stress on the area to accelerate the time interval for the next great Tokyo quake?

A large quake in this area happens on average every 70 years, and it has now been 91.

At least Japan is very attuned with their seismic history and are very well prepared. The anniversary of the Kanto quake, September 1st, is now National Disaster Prevention Day in Japan. A day of remembrance and preparedness training and drills.

Brian

[Island Chris]

Brian,

As you know the M7.4 (?) foreshock for the Tokoku quake was shallow and consistent with thrusting on the subduction zone. Well agreed on seismotectonics and logic would say that such a quake would advance the time of a quake on the adjoining parts of the same fault, which is what happened. The M7.8 very deep quake is nothing like this. It was a stretching focal mechanisms and USGS suggested it was within the subducting slab (as probably all major to great very deep quakes are).

But, I came on Earthwaves for this post because of a shallow M6.2 quake today on the other side of the trench from the M7.8. But if the M7.8 was within the subducting Pacific Plate, this one was also within the same plate. And this one was also normal. I don't know whether static stress changes from the M7.8 could have advanced / triggered the 6.2. They are distant enough in both depth and horizontal distance one would think the effect would be weak.

There are other ways to trigger an earthquake than static stress, as we have discussed here. That includes seismic wave shaking. My gut feeling is that there is something else going on that is not known or not understood. Back when John Vidale was posting and there were a bunch of large quakes in a few days on both sides of Baja California, he, and us, brought up strain transients.

Brian, I know you know most or all of this, but others, including lurkers, may not.

Hey Duffy, one would wonder about your anomaly a few days before, but it seems really unlikely that such a deep 7.8 could affect the ionosphere in advance.

Chris

[Island Chris]

P.S.: I would be a tiny bit more concerned about the M5.3 on May 25 at 35 km depth just north of Tokyo. If there were some swarm of such quakes accompanied by some other information like a creep event (not known in this area??), then it would be interesting whether Japanese issued some sort of alert.

Chris

[Skywise]

(05-31-2015, 12:14 PM)Island Chris Wrote: The M7.8 very deep quake is nothing like this. It was a stretching focal mechanisms and USGS suggested it was within the subducting slab (as probably all major to great very deep quakes are).

My bad. You're correct. I didn't give enough thought to the depth. And, I failed to realize just how quickly the slab turns and dives downward. The quake was at 678km deep, but only about 270km from the trench. The slab has to dive pretty quick. I always thought the pictures of such quick diving were due to vertical exaggeration of the diagrams.

Then after doing a bit of digging I find an excellent cross section diagram of the Izu-Bonin-Mariana trench system on Wikipedia.


from: http://en.wikipedia.org/wiki/Izu-Bonin-Mariana_Arc

After a little georeferencing I found the quake occurred almost exactly on cross section B-B', and looking at it's chart the quakes at 140 longitude are indeed this deep. There seems to be a gap in depths in the middle range, so I wonder if these could also be due to phase changes of the minerals?

BTW, I estimate the vertical exaggeration in the above cross sections to be about 8.4 to 1. Even after compensating for that in image editing software, it is clear that the slab dives nearly vertically in some locations. Of course, the text accompanying the image on Wikipedia says exactly that.

Brian

[Duffy]

Hi Chris,

Yes, I agree, the M7.8 in Japan was to deep to have any detectable influence on the lower atmosphere. The two overnight observations I mentioned prior to posting on the 26th May, seemed to be affective over a particular band width, between 18.5 and 23.5 KHz. In both cases, the quakes that followed were 6+'s and no deeper than 12km, the anomaly I posted about affected frequencies between 24 and 25.5 KHz. As the 4.2 or 3.7 Welsh quake had occurred locally, at a much lower magnitude, I theorised weather there was a link due to ion density changes affecting different frequencies, dependant on distance from epicenter. More observations are needed to come to any real conclusion with this, but I'll keep all informed of any major overnight disruption.

Final note, Didn't bother seeing the movie, on reflection most of my posting was in poor taste, and I deserved a good stomping, I'm not an academic so having someone keep you in check is a good thing. As penance, I helped mother-in-law scrub the floors , no offence taken here, how goes it there ?.

Duffy,