Our Apartment

Earthquake preparedness starts with the quality of the structures. So I found it interesting to look at the web site for our apartment. Part of one web page is reproduced above. One noticeable feature is the entry in the table: "Structure. Steel frame and reinforced concrete structure". That should perform ok in an earthquake. I don't know if this information is placed on the web by popular demand, or by some requirement. But it is valuable information. It should be this easy for everyone to find out about how their homes and offices are constructed.

How often do you feel an earthquake in Tokyo?

There is a rigorous way to answer this question. The plot below gives one answer.

This plot is based on a station in the K-net array of strong motion stations operated by the National Research Institute for Earth Science and Disaster Prevention (NIED). From their web pages (www.k-net.bosai.go.jp), one can obtain records of ground motion from any of their national network of over 1000 stations installed to record strong earthquake motions. The nearest station to our apartment is station TKY022. This station has been operating since May 2002. From the list of all the records at that station, and assuming that the station didn’t miss any earthquakes, I counted the number of times that different levels of ground motions have been exceeded. This plot shows the results. A station that has observed longer would give better statistics, and different stations feel different levels of shaking on average. But this plot is ok for an approximate answer to the question.

We have felt two earthquakes since we arrived, and they are included in this plot. The first one was on Sunday evening, August 9. It was 311 km from the station and 340 km deep with magnitude 6.9 (JMA). The second one was Tuesday morning, August 11. It was smaller (magnitude 6.6), closer (154 km), and shallower (20 km deep). The motions on August 11 were not as strong as on August 9. What this plot shows is that earthquakes stronger than the August 9 event have happened about once every two years, and that earthquakes stronger than the August 11 event have happened just under twice a year, on average. Shaking of 10 cm/s² is usually strong enough to feel, however, and that happens on average about 6 times per year. So we expect to feel some more earthquakes.