There continue to be aftershocks in the M 4-5 range. These are aftershocks and triggered earthquakes (if they are generally on different faults) from the Mw 6.7 earthquake. Here is my page on this mainshock and here is the USGS page for the mainshock. Here is the USGS page for a triggered earthquake (unlikely on the same fault), a M 5.8 earthquake.
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 showing the most recent assemblage of these aftershocks.
This M 5.8 earthquake is located closer to land, but it is a much smaller earthquake. These two factors combine to have similar shaking intensity at the coast. Each step in magnitude (e.g. from M = 5 to M = 6) the energy released (strain) is 33 times more. So a M = 6 earthquake releases 33 times more energy than a M = 6.
Here is a map for the larger earthquake.
Here is a map for the smaller earthquake.
Here is the attenuation plot for the larger earthquake. Note how these reports match well with the western US attenuation (GMPE) model.
Here is the attenuation plot for the smaller earthquake.
Further demonstrating how magnitude and distance relate between these two different earthquakes are the pager model results. These are modeled estimates of damage to people and their belongings (roads, houses, etc.).
This is the pager report for the larger earthquake.
This is the pager plot for the smaller earthquake. Note how the probability estimates for damage are similar.