Besides being a tragedy, as any loss of life is under such terrible circumstances, the Sasago Tunnel collapse is also worrying because of what it
While it's true that no earthquake activity near the collapse site has been reported internationally, the Japan Meteorological Agency (JMA) – which
officially does quake monitoring and reporting for Japan, has a report of a magnitude 2.9 event on Dec 1, 2012:
Earthquake Information (Information on seismic intensity at each site)
Issued at 12:05 JST 01 Dec 2012
Occurred at (JST)
12:00 JST 01 Dec 2012
You can access the above report on JMA's site at this link
. (Note: the page is
If you want to follow quakes in Japan, then JMA is the best resource as they record even the smaller ones that often don't make it onto the pages of
either (eg) the USGS or EMSC maps.
While a magnitude 2.9 quake is not large by any stretch, the event's epicenter was only 10 miles from where the tunnel collapse occurred. Viz this
image I've prepared:
– the big yellow line is the overall distance from the earthquake to the tunnel collapse location (reported as about 2km inside the tunnel on the
– the longer, thin red line from the quake to the tunnel entrance measures about 14.3 km (about 9 miles); the shorter one traces the line of the
tunnel to the collapse point.
Now, while such a small quake shouldn't collapse a major tunnel, especially in Japan where they have many tunnels and are very good at engineering
them, I'm wondering if this collapse is indicative of a tunnel that might have been weakened by previous events and failed due to the proverbial
straw breaking the camel's back, so to speak.
The other possibility relating to tectonic movement is that there might
episodic tremor and slip going on, which normally occurs at such a low
level that it's not noticeable except with special instruments – meaning not just the usual seismos. It's done using Very Large Arrays (VLAs).
This is how they track ETS in the PNW, for example.
In any case, an “unexplained” tunnel collapse in a known seismic region is always worth noting, just in case it might indicate something else is