Since the Great Tohoku Earthquake exactly one month ago, we have gone through an enormous amount of aftershocks. This website has a pretty neat interface for visualization. Unfortunately, the data stops on April 4, thus missing two of the largest aftershocks of magnitude more than 7 of last Thursday and today. Even so, it counts almost 900 individual aftershocks, more than 400 of which had a magnitude of higher than 5. These days, especially tonight, we are having what probably should be called aftershocks of aftershocks.
Based on my prior 2 1/2 years of experience of living in Japan, the last month seems to have brought several life-time supplies of earthquakes even for Japanese standards.
I come from continental Europe, most of which is not very seismically active at all. Before coming to Japan, I had witnessed a mere three earthquakes which were so minor that no Japanese would care to even mention them. Sometimes people from Europe ask me what an earthquake is like, and by now I feel I have acquired some competence to answer the question.
First, a word on seismic scales. The magnitude of an earthquake is usually given in the Richter scale. This scale quantifies the energy released by a quake. I’s important to understand that it is logarithmic. This means that a magnitude 9 earthquake is actually 10 times worse than a magnitude 8 one. To track the amount of shaking (and destruction) an earthquake causes, this number is not so useful. The effect varies greatly with the distances from the epicenter and the depth of the epicenter. The Japan Meteorological Society therefore uses a different scale, the so-called shindo scale (震度, shaking degree), which varies from place to place (the picture above gives the shindo magnitudes of today’s earthquake in color code).
Strength 1 is described as Felt by only some people indoors. In general, such an earthquake can only be felt by people sitting or lying down. I am often unsure in these cases whether there is a quake at all, so I make sure by checking whether the leaves of my house plants are swaying or not. Usually such quakes manifest themselves as a very gentle shaking which lasts for a few seconds.
Strength 2 is “Felt by most people indoors”. Sometimes of two people in the same apartment, one feels it, the other not. The one who didn’t feel it was probably standing up or walking around. If you’re sitting down, they’re hard to miss. Sometimes they come on with a bit of a stronger rattle.
At strength 3, the earthquake becomes really hard to miss and comes across as slightly obnoxious. Not scary, really, but as in “what’s up with all the shaking”. Things in the house start making noises. For some people, it’s the dishes. For us, it’s usually the sliding doors to the bedroom. In my experience, the stronger the earthquake, the longer it lasts. Strength 3 can last a good 10 seconds.
From strength 4 on, I start contemplating the possibility of maybe going under a desk or so. I’ve never actually done it yet. Modern Japanese buildings are quite well-made, and even when the sway and shake, you do not get the impression that anything could come loose. So usually I just stay alert and wait it out, which can take 20-30 seconds. Even though earthquakes often come on suddenly, longer ones tend to build up slowly in strength and then die down slowly. But there’s a lot of room for variation. Today’s earthquake was felt as a 4 at my workplace. Some people evacuated under tables (“Some people try to escape from danger.”) At home, a few small things had fallen (“Unstable ornaments fall occasionally”).
The Tohoku Earthquake registered as a strength 5-upper in the Tokyo region. The description about matches the state we found our flat in (“Most dishes in a cupboard and most books on a bookshelf fall.”) Since I was on a train in a tunnel, my own experience is not representative, I believe. Of course the train was shaking quite some, but then, train cars are made to absorb vibrations. Moreover, we did not really have a frame of reference in the tunnel. The Tohoku earthquake lasted almost three minutes. But it was very, very unusual in many respects.
Luckily, my own experience ends at strength five, and I can contain my curiosity for the higher levels.
Earthquakes consist of different waves which travel at different speeds. The waves arriving first are less destructive, and give warning systems time to kick in. The secondary waves are more destructive. Interestingly, it is sometimes possible to feel the first set of waves as well. For me, it only worked twice, when I was lying with my back flat on the floor. The first waves felt like short vibrations from below, which were followed after a few seconds by the big sideways ones one usually feels.
Even though today it doesn’t seem likely, I hope not to expand my experience with earthquake too much more in the future.