Sunday, December 26, 2010

New experiment. Interesting science on this date.

I'm trying something new to get me writing more often.  This new experiment is to try to write something about some interesting science that occurred on the given date.  I want to post something every weekday, but suspect that may be too much.  We'll see.

26 December:

26 December 2004, the Sumatra-Andaman earthquake rocked Indonesia, Sumatra, India, Thailand, etc.  This quake had a seismic moment magnitude of between 9.1 and 9.3.  The resulting tsunami caused more than 283,000 deaths directly, though I suspect the full casualty count is closer to one million if you count the continuing problems associated with this earthquake and resulting tsunami.

The seismic moment magnitude is what scientists use as a replacement for the Richter magnitude.  This is a dimensionless number given by:

Mw = 2/3 log(M0) - 10.7.

The constants used are to give numbers consistent with the old Richter scale, so if you hear someone quote the magnitude "on the Richter scale", realize that it's similar but not really the same, and they're probably wrong--all earthquakes now are measured using the seismic moment magnitude scale.  An increase of one step in this scale corresponds to 10^1.5 = 32 times increase in amount of energy released.  Two steps corresponds to 10^3 = 1000 times increase in energy.

The Loma Prieta earthquake, in 1989, that affected the Bay area measured 6.9 on the moment magnitude scale.

6.9 to 9.3 is 2.4 steps or about a factor of 4,000 on this scale.
You can check this using WolframAlpha.
The Sumatra-Andaman earthquake had an equivalent energy release of 3.7x10^20 Joules.

The Loma Prieta earthquake had an equivalent energy release of 9.3x10^16 Joules.

That is, the Sumatra-Andaman earthquake was about 4,000 times as energetic as the Loma Prieta earthquake.

To get you thinking about the energy release of this earthquake, try this:  Little boy and Fat man, the nuclear bombs dropped on Japan during WWII had yields between 5x and 9x10^13 Joules.  The Loma Prieta earthquake was about 1,000 times as energetic as the bomb dropped on Nagasaki, and the Sumatra-Andaman earthquake was another 4,000 times as energetic as that.  Of course, the energy from the nuclear bomb was much more concentrated than the earthquakes'.

The Sumatra-Andaman is the earthquake that awakened the world to the need for tsunami warning systems.

The spacecraft Jason 1 detected the 1-meter trough-to-crest wave of the resulting tsunami 1500 km south of Sri Lanka.  That's right, the 3- to 11-meter tsunami that hit Sri Lanka was only about 1-meter trough-to-crest in the ocean.  In Thailand, the largest tsunami height was 19.6 meters, though most heights measured less than 10 meters.