# Four Cities Forecast: Japan Update (March 13, 2011)

On July 30, this author published a blog on this web site giving the probabilities for major earthquakes having magnitudes M>6.7, within 12 months from July 30, 2010, within 150 miles, and for shallow depths of less than 30 km (most potentially destructive conditions), for each of four cities in Japan: Tokyo, Osaka, Niigata, and Sendai. At that time it was found that probabilities (chance of an earthquake) were the highest near Tokyo, with the second-highest chance near Sendai.

Following the M9.0 earthquake of March 11, 2011, off the coast of Sendai, we update the probabilities as shown below, for the same conditions. It can be seen that probabilities for all four cities have increased sharply, as a result of the heightened activity of small events over the entire island of Honshu. Probability values are now: Tokyo, 23.5%, up from 15.9%; Osaka, 12.3%, up from 10.5%; Niigata, 13%, up from 8.2%; and Sendai, 22.8%, up from 13.2%. The Sendai value may seem surprising, but is not -- very large aftershocks have a significant chance of occurring, as a result of the M9.0 mainshock.

Implications for the relatively near future are not good. As the technical analysis below indicates, it can be expected that an extremely large aftershock of roughly M8.0 can be expected, as well as about 7 aftershocks having magnitudes M>7, and 50 aftershocks having magnitudes M>6.0. Over the entire island of Honshu, down to depths of 50 km, there have so far been no M>7 aftershocks, and only 16 M>6 aftershocks. In fact the largest aftershock in this region had M=6.8. There was a large foreshock to the main event having M=7.3 at a depth of 32 km, according to the ANSS catalog.

It seems that the risk near Tokyo and Sendai has substantially increased, unfortunately, according to these calculations. Of course it should be remembered that probabilities are not the same as certainties, and should be regarded as only guidelines to possible future developments.

Synopsis of Technical Analysis. To compute the expected number of M~8 aftershocks, we use Bath's law, which states that the largest aftershock is typically 1 magnitude unit less than the magnitude of the mainshock. To compute the expected number of M7 and M6 aftershocks, we use the Gutenberg-Richter relation with a best fitting b-value of 0.85, found from performing a least-squares fit of the data from 1975-2011. We then compute the expected number of M>7 and M>6 earthquakes corresponding to 1 M=8.0 aftershock (considered as the "mainshock").

Below we show plots of the chance of a M>6.7 earthquake for each of the four cities, together with a table of probabilities, similar to what was shown in the blog of July 30, 2010.

Below is data in tabular form, similar to that presented on July 30, 2010.

## About OpenHazards Bloggers

**Steven Ward** is a Research Geophysicist at
the Institute of Geophysics and Planetary Physics, UC Santa Cruz. He specializes in the quantification and simulation of
natural hazards. **Read Steve's blog.**

**John Rundle** is a Distinguished Professor of Physics
and Geology at UC Davis and
the Executive Director of the APEC Collaboration for Earthquake Simulations. He
chaired the Board of Advisors for the Southern California Earthquake Center from 1994 to 1996. **Read John's blog.**

## Comments

Since the disastrous earthquake of last week I have been following the strength and frequency of the after shocks. By anybody’s measure the after shocks are still major as they are all mostly in the 5 plus range and some over 6.

Using data obtained from the USGS web site I decided to take a look at previous seismic activity in the area. The past 30 days was very easy to obtain for the region. What I found in the data I personally find of great concern and wonder.

Between February 15, 2011 and February 26, 2011, the region had a total of 8 quakes. After the 26

^{th}there is no activity until Wednesday March 9, 2011 when during that day there were 12 major quakes. The following day, the 10^{th}, there were 6. These were all in the two days prior to the magnitude 9 that hit on the 11^{th}.One would think that when an area gets 18 quakes in a 48 hour period after a fairly quiet time that there must be something going on. I understand that earthquake forecasting is not a science by any means but I think that kind of seismic activity should have been a concern to the people who deal with that field of study.

It is not my intent to form an opinion on what was done right or wrong in the tragedy only to make you aware what I learned from a little research. The dtata is the list of the quakes from 2/15 through 3/10 showing the time and magnitude. The near/off column is near or off report from Hondo Islands.

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