Earthquake offshore of the Sanriku Coast

We just had a good sized shallow compressional earthquake in the northern part of the 2011 Tohoku-Oki earthquake slip region. The USGS web page lists the magnitude at Mw 6.7. Here is a great summary of the tectonics of this region. Based on the magnitude, this earthquake is not likely to generate a tsunami that would affect the continental US. The initial magnitude was 6.9.
There are several report updates, listed here:

• 2015.02.16 M 6.7 Japan Sanriku Coast – Main Report
• 2015.02.16 M 6.7 Japan Sanriku Coast Update #1
• 2015.02.16 M 6.7 Japan Sanriku Coast Update #2
• 2015.02.20 M 6.7 Japan Sanriku Coast Update #3
• 2015.02.21 M 6.7 Japan Sanriku Coast Update #4
• 2015.02.25 M 6.3 Japan Sanriku Coast Update #5

Here is a map that shows the earthquake as it relates to Japan and the Japan trench. I have labeled the different tectonic plates and the relative plate motions. While the plate boundaries between the Pacific-Okhotsk, Pacific/Philippine sea, Philippine Sea/Eurasia plates are well known, the plate boundary between the Okhotsk/Eurasia plates is lesser well known nor understood.


This map shows the region along with the historic epicenters. I have plotted a moment tensor from the 2001 earthquake, as well as the moment tensor from today’s earthquake.


This map shows the slab contours (the depths to the top of the downgoing Pacific plate). Today’s earthquake has an hypocentral depth of 10 km. In the epicentral location, the slab is probably at about 75 km depth, so today’s earthquake is clearly in the accretionary prism of the upper plate. UPDATE! The depth has been updated to 23.3 km (the 10 km was an automatic depth). So, now we may interpret this as an interface earthquake (on the subduction zone fault). See my updated post here.


Also, here is a short explanation (from the USGS) of the graphical solutions for focal mechanisms. While moment tensors (shown in my interpretation figure above) and focal mechanisms are determined differently, their graphic representation of the deformation from earthquakes is the same.