A 7.1 earthquake happened on Aug. 8 in the southern region of the Nankai Trough.

That's a sizeable earthquake, but apparently it's only a prelude of what is to come.

Japan issues first-ever alert over risk of Nankai Trough megaquake

From a seismological perspective, the probability of a large quake in the Nankai Trough has become several times higher, the agency said. Such quakes happen roughly once every 100 years, but the exact timing cannot be predicted. The most recent one took place on Dec. 21, 1946, which was measured between 8.1 and 8.4 on the moment magnitude scale.

According to the government’s damage assessments published in 2012, a tsunami caused by a Nankai Trough megaquake could result in up to 224,000 deaths, which can be reduced by roughly 80% with sufficient evacuation efforts immediately after the earthquake.

Another article:

Japan sees higher-than-usual risk of megaquake off Pacific Coast

In the worst-case scenario, a powerful temblor could shake a wide area of Japan -- from the Kanto region centering on Tokyo to the southwestern Kyushu region -- and high tsunami waves could engulf the coastal areas of Kanto to Okinawa, according to the Japan Meteorological Agency.

The Japanese government has predicted there is a 70 to 80 percent chance of a magnitude 8 to 9 quake occurring along the Nankai Trough within the next 30 years, with a 2012 estimate showing that the number of deaths could reach up to 323,000.

The Kanto Region includes Tokyo, Yokohama, and, well, us.

A 70-80 percent chance of an 8 to 9 quake in the next 30 years? That's an unsettling prediction.

When the quake hit yesterday, which we did not feel, I wasn't worried because it was well south of us. Now that I've learned more about the Nankai Trough and that it reaches to just south of us, I'm taking notice. A quake in the northern part of the trough could result in high water in our neighborhood, literally. Our house is at sea level.

Note that the Nankai Trough alert is showing up on the weather page, along with the continuing high temperatures.

5.3 in Kanagawa.

I've read that a major quake near Fuji could set it off. This is getting closer to that.

EDIT: We were out walking and didn't feel it, but I did get an alert on my phone.

One observation today - the Tokyo Bay was oddly choppy today, almost like when there's a low pressure system in the winter, but today we're under high pressure with very low wind, maybe 5kt this morning when I first noticed it. It didn't fit. And I've been watching the water in this bay pretty much every day for close to four years, so I'm pretty attuned to it.

We just went for a walk and it was the same. Choppy. And this was before the little 5.2 that hit just a while ago.

OK, this is a stretch, but could ocean floor energy release, even perhaps micro, very small movements, immeasurable movements, cause this? Or even no movement, but energy release of another variety? Something is causing this choppy water.

EDIT: Before the 7.1 yesterday, the water was smooth, very smooth and calm.

(C)ould ocean floor energy release, even perhaps micro, very small movements, immeasurable movements, cause this? Or even no movement, but energy release of another variety? Something is causing this choppy water.

EDIT: Before the 7.1 yesterday, the water was smooth, very smooth and calm.

All we can do is record observations.

One thing that we have observed is that the notion that a mid to large (4-6) sized earthquake 'releases the pressure' is crap. More often it seems to presage an active period - setting off a chain reaction.

It's interesting that prediction has advanced to the level of a 'trough warning' - i've not seen that before.

I was mistaken regarding the location of quakes that can trigger Fuji. Apparently a quake in 1707 on the Nankai Trough was the last one to trigger a Mt. Fuji eruption.

In 1707, a rupture along its entire 600km length caused the second-biggest earthquake ever recorded in Japan and was followed by the eruption of Mount Fuji. Source.