Small earthquake during President Obama's visit to Japan

We had a small earthquake here in Tokyo this morning. Actually very small, M=4.1 (JMA), 50 km deep with epicenter in Tokyo Bay, at 4:23 AM JST. It was strong enough to awaken my wife Gisela, but I slept through it. President Obama is here in Tokyo, and dominating the news. He might have felt it.

The following are images of the intensity map from the Japan Meteorological Agency web site, http://www.jma.go.jp/en/quake/. These show the instrumental intensities with three different scales.


According to the JMA web site as of 9:00 AM local time, the details are:

"Earthquake Information (Information about Seismic Intensity at each site)
Issued at 04:33 JST 14 Nov 2009

Occurred at (JST)	Latitude (degree)	Longitude (degree)	Depth	Magnitude	Region Name
04:23 JST 14 Nov 2009	35.4N	139.8E	50km	4.1	Tokyo-wan

More About the Hazard for Tokyo & Vicinity

My post on August 12 did not tell the whole story about the hazard for Tokyo. It suggests that shaking like what was felt on August 9 occurs every two years. But the real concern is the hazard from earthquakes that are much less frequent, but cause much larger levels of shaking.

One of the big concerns for Tokyo is what will happen when the Tokai seismic gap breaks. A seismic gap is, to put it simply, a part of an active fault that has not had a large earthquake for a long time.

I found the figure below on the web site prepared by Prof. Terry Tullis for an introductory class in Earth sciences at Brown University. This shows the parts of the subduction zone in southern Japan that have ruptured in earthquakes over the past 300 years. The entire zone ruptured in 1707. It ruptured completely again in two earthquakes on two days in 1854. Most of it ruptured again in earthquakes in 1944 and 1946. However, the 1944 earthquake left what looks on this map to be a short segment at its northern end unbroken.


What looks like a short segment on this map, labeled the Tokai Gap, is actually 100 to 150 km long. This is easily a big enough fault to cause an earthquake with magnitude close to 8.

Another web site that describes the Tokai Gap is:
http://cais.gsi.go.jp/Virtual_GSI/Seismology/Tokai_Slow/1t.html

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.

Preparedness and Resilience

We often urge people to be prepared for earthquakes.  That usually means having your home ready and supplies on hand, and knowing what to do before, during, and after the earthquake. There are many resources on the web that can help.  Here is one useful site, maintained by the US Geological Survey:  

http://earthquakes.usgs.gov/learning/preparedness.php 

Resilience goes beyond preparedness.  Resilience is measured by how quickly after an earthquake you are able to resume normal activities.  

Resilience is vital for business and communities.  Imagine a hardware store that is not resilient - not able to reopen right after an earthquake, at the very time that many people in the community need parts to repair earthquake damage.  Their lack of resilience not only hurts themselves, but also hurts the entire community that depends on them as they try to make repairs and get back to normal.    So we urge individuals, businesses, and communities to aim for a high level of resilience to earthquakes and other disasters.

The US Geological Survey web site has some links that emphasize steps that go beyond preparedness and help to be more resilient.

Why this blog?

I started this while I was visiting the Earthquake Research Institute at the University of Tokyo. One goal of this visit is to see what I can learn about earthquake preparedness and earthquake resilience, while in Japan, that might help in my home state of Nevada. My goal is to keep posts here simple to understand discussions of earthquake hazards and how to increase resiliance.

Original posted July 7, 2009

Revised March 30, 2011