Japan 8.9 quake - the source

On Friday, the morning after the devastating earthquake in Japan, I had my geophysics students compile some information about the quake.

Elvis Andino, Stephanie Douglas, and Andrew Tsang looked at the source parameter. They found:

The epicenter occurred at 38.322°N, 142.369°E on the Japan Trench subduction zone, where the Pacific plate is subducting beneath the Eurasian plate.  The epicenter was 373 km NE of Tokyo. The earthquake focus was about 24 km below the sea floor.

As shown below in the fault plane solution, the earthquake exhibited thrust faulting, consistent with its location along a subduction zone. The NE-SW strike of the fault plane is parallel with the orientation of the plate boundary shown above.

It had a magnitude of 8.9, putting it 5th on the USGS's list of largest earthquakes since 1900, right before the 2010 Chile earthquake,

Large foreshocks preceded the main shock over the previous two days, including a M7.2 quake about 40 km from the epicenter of the big earthquake.

Because Japan is partially surrounded by fault zones, it has had many earthquakes in its history. These have mostly been shallow quakes. The last major earthquake on the same fault was in 2005, a M7.0