A Mw 6.9 aftershock shook the Iwaki region of the coast of Japan on November 22, 2016. Considered an aftershock, since it was within 2 rupture lengths of the 2011 Great East Japan earthquake that itself ruptured a 300 km stretch of seafloor, this is just the latest shaker of the hundreds of quakes >Mw 4 that have occurred since March 11th, 5 years ago. While on the human timescale, there has been enough time for many structures to be rebuilt and life to return to normal for many, geologically speaking the M9 quake is still reasonably fresh. While aftershocks DO get more spaced out in time since the main shock, they do not necessarily become weaker and so this is unlikely to be the last tremor of this magnitude in the area.
The shaker woke many in the region at 5:59 am and evacuations quickly began from the coast to higher ground with the memories of the March 11th quake all too fresh in local memory. This time, most headed the warnings and only a handful of injuries have been reported.
Tsunami warnings were issued and 1 m waves were recorded at Fukushima Daiichi at 6:31 am, just 30 minutes after shaking began. Two minutes later, a similar height wave struck Fukushima Daini, a sister power plant located 12 km to the south of Daiichi that was not actively producing power at the time. While neither tsunami caused damage to these facilities, the ground motion of the quake did sloshing in the spent fuel pools. Enough motion was detected at the spent fuel pool of reactor #3 at the Fukushima Daini plant to trigger a sensor that caused an automatic shutdown of coolant pumps to this pool at 6:10 am. TEPCO workers were sent to assess the situation and were able to determine that no damage was sustained. The pump was restarted ~90 min later at 7:49 am. TEPCO reported the temperatures regularly throughout the shutdown to NHK reporters and stated that the pool rose 0.8 C during the 90 min shutdown from 27.9 C to 28.7 C due to decay heat from the stored nuclear fuel. For reference they noted that safe operating temperature of the cooling pumps is 65 C. Given the rate of observed temperature increase, it would have taken 6-7 days before the situation came to an emergency status. Thankfully, the system restarted without a hitch.
Initial inspections, on Nov 21st, were unable to locate any damages to the reactor buildings at either power plant, the decomissioning work undergoing at Fukushima Daiichi, or the farm of contaminated water storage tanks that carpet the grounds at Fukushima Daiichi. Sensors around Fukushima and Miyagi Prefectures did not detect any increase to local radiation levels.
Tsunami washed ashore in multiple prefectures with the highest wave recorded in Sendai Port at 1.4 m at 8:03 am local time. Fortunately, the largest tsunami waves arrived before the local high tide that ~10:30 am and little damages were reported. Tsunami energy reflected and refracted around the coastal basins for some time though, as you can see in the tide gauge record above. Had the tsunami come ashore at high tide, more damage may have occurred even from the modest wave.
This event is a good reminder to those living in earthquake prone regions that aftershocks are not limited to a certain timeframe after the main shock and are not limited in amplitude. While generally on alert for earth motions in Japan, the memories of March 11, 2011 will continue to be refreshed with each additional tremor.
A selection of resources regarding this, and other November 2016 earthquakes and tsunami:
Japan – Mw 6.9 – Nov 22
- A look at Japan’s earthquake early warning system in effect during Nov 22, 2016 quake
- Change in stress field as a result of Nov 22, 2016
- Japan Meteorological Agency: 2011 Great East Japan Earthquake Portal
- More than 9,500 aftershocks logged since mega-quake
- USGS Earthquake Event
El Salvador – Mw 7.0 – Nov 24
New Zealand – Mw 7.8 – Nov 13