Here is a map showing the depth contours from Hayes et al. (2012). The depth contours are at 20 km intervals, the eastern most contour is 20 km. The USGS hypocentral depth is 23.3 km, which is between 20 and 40, consistent with this Hayes et al. (2012) slab model. The slab contours (the depth to the top of the down going Pacific plate, i.e. the subduction zone fault interface) are located using seismicity. Seismicity has considerable spatial variability, so the slab depths are not too well constrained in this way. Therefore it is reasonable to interpret this earthquake as an interface (subduction zone fault) earthquake (rather than a slab or crustal earthquake, not on the subduction zone fault).
There is an article online that quotes Japan Meteorological Agency (JMA) seismologist Yasuhiro Yoshida. Yoshida tells reporters that “This quake is an aftershock of the 2011 quake that hit the Tohoku region”
There are several different ways to estimate how long after an earthquake we might expect aftershocks. They each rely on unknowns, so include considerable uncertainty. The two main “laws” are Omori’s Law and the Gutenberg–Richter [relation] Law. The b-value controls the decay of aftershocks. In most cases, we do not have enough seismologic data to have a good estimate for the b-value of any given fault (One needs many many earthquake cycles for a fault to know this constant. Paleoseismology is perhaps the single most important way to evaluate the b-value, since our seismologic data are only about a century old, far too young to capture the seismologic variation along any given fault.).
Here is a plot of the aftershock decay (G-R Law) using different b-values, from this source.
Here is a list of foreshocks and aftershocks associated with the 2011 Tohoku-Oki M 9.0 earthquake. This is the fourth largest earthquake ever recorded with modern seismometers. It has been almost 1.5 years since we had an aftershock near the magnitude of today’s earthquake.
- Hayes, G. P., D. J. Wald, and R. L. Johnson (2012), Slab1.0: A three-dimensional model of global subduction zone geometries, J. Geophys. Res., 117, B01302, doi:10.1029/2011JB008524.