Geologist explains some more science behind Japanese earthquake & tsunami


Fabrizio Bensch / Reuters

I've been reading about some pretty astounding impacts of last week's 8.9 earthquake. For instance, there's the fact that the quake not only shook Japan, it moved the country. Specifically, pushed it Westward by about 8 feet. The US Geological Survey confirmed this to, as well as the fact that the quake shifted Earth's axis by 6.5 inches. And then, there's the news from a lab at NASA's Jet Propulsion Laboratory in Pasadena, Calif., where geophysicist Richard Gross has found that the quake also sped up Earth's spin, just a little a bit. Days are now 1.8 microseconds shorter, by his estimation.

How does all of this make sense? I've got a better understanding now, having read this great Scientific American guest blog post by geologist and blogger Chris Rowan. He covers all sorts of good information about the earthquake and the tsunami. From how we can tell that Japan moved, and why … to what's going on with the changing numbers on how powerful this quake really was.

GPS stations in Japan – installed to measure the slow build-up of elastic strain in the crust between big earthquakes – show most of Eastern Honshu moving several metres to the east as a few centuries worth of that elastic strain – which pushes the crust in Japan westwards and upwards – was released over the space of a few minutes.

If you're wondering why there is some still some confusion over exactly how large this earthquake was, it's because – rather counter-intuitively – measuring the magnitude of large earthquakes is actually more difficult than it is for smaller earthquakes. To estimate earthquake magnitudes, you look at the amplitude of the seismic waves it generates: the larger the amplitude of the waves, the larger the magnitude of the earthquake that produced them. However, in very large earthquakes, this relationship starts to break down, at least for the frequencies of seismic waves that are generally used to produce the quick magnitude estimates: they 'saturate', or stop increasing in amplitude as the earthquake magnitude does. This means that the magnitude estimates for the largest earthquakes will be somewhat underestimated until seismologists look at lower frequency seismic waves, which are less susceptible to this saturation effect.