In order to determine the detailed spatial distribution of the different types of aftershocks, we evaluated Kagan’s angles (Kagan, 1991), which are the minimum rotation angles of the focal mechanisms relative to a reference mechanism corresponding to a typical interplate earthquake (strike ϕ = 195°, dip δ = 13°, rake λ = 90°) in this region (Figs. 2(e) and (f)). The results indicate that the Kagan’s angles of earthquakes occurring before the mainshock are mainly within 30 to 40°, especially for the earthquakes that occurred within 20 km from the plate boundary inferred from the depth distribution of the upper seismic plane along the subducting Pacific Plate (Hasegawa et al., 1994). On the other hand, the Kagan’s angles of the aftershocks are broadly distributed even for selected earthquakes within 20 km from the plate boundary, which also include normal-fault-type aftershocks near the trench and in the outer-rise. It is difficult to omit such normal-fault aftershocks and identify interplate aftershocks using only the Kagan’s angle and depth information. Therefore, we defined such interplate aftershocks by the following conditions: rake λ ≥ 0°, Kagan’s angle θ ≤ 35°, and centroid depth difference from the plate boundary δd ≤ 20 km. Based on their CMTs, 315 aftershocks were regarded as interplate earthquakes (Fig. 3(a)); deeper interplate earthquakes (centroid depth ≥ 40 km) were distributed off Iwate, Miyagi, Fukushima, and Ibaraki near the Pacific coast. On the other hand, shallower interplate earthquakes (centroid depth ≤ 40 km) occurred only off northern Iwate, Ibaraki, and Chiba. Note that no interplate earthquakes occurred in the region directly around the mainshock hypocenter.
The Geospatial Information Authority of Japan (2011) has estimated the spatial distribution of coseismic and post-seismic slip from GPS data. They showed that the large co-seismic slip (>8 m) area extended from off southern Iwate to off Fukushima and that its largest peak (>24 m) was located off Miyagi. The epicenter distribution of the 315 interplate aftershocks does not overlap with this large co-seismic slip area (Fig. 3 (a)). This suggests that the large coseismic slip area, which is thought to correspond to the mainshock asperity, can no longer slip in the form of aftershocks due to the large amount of the stress release during the mainshock rupture. On the other hand, interplate aftershocks occurred in the northern, southern, and deeper extensions of the large coseismic slip area. These surrounding areas, which probably did not have large amount of co-seismic slips, were primarily loaded by the coseismic slip of the mainshock asperity. In addition, a large amount of postseismic slip was estimated in the deeper extension of the large coseismic slip area off southern Iwate and Miyagi (Fig. 3(a)), which would also promote the occurrence of aftershocks along the plate boundary. Consequently, many interplate aftershocks may have occurred in this region.
Of the 1,028 aftershocks, 713 were identified as being non-interplate types. Instead, a variety of different focal mechanisms were found. In order to discuss the relationship between the coseismic stress change and the spatial distribution of these focal mechanisms, we plot the distribution of these non-interplate aftershocks in the hanging and foot walls in Fig. 3(b) and 3(c), respectively. In the hanging wall, normal faulting is predominant, although the T -axis directions of these aftershocks are scattered. In the foot wall, normal faulting with a T axis along the east-west direction is predominant and these aftershocks are distributed along the Japan Trench near the large coseismic slip area (Fig. 3(c)). These normal-fault aftershocks near the trench and in the outer-rise occurred mainly in the up-dip portion (eastern part) of the foot wall of the large coseismic slip area; seismicity in this area may have been activated by a tensional stress change caused by the thrust faulting of the mainshock. The down-dip portion (western part) of the hanging wall was also subjected to a tensional stress change as a result of the mainshock. Therefore, some of the normal-fault-type aftershocks in these regions, such as the shallow (~10 km) aftershocks near the Pacific coast of Fukushima, Ibaraki, and Chiba in Fig. 3(b), might have been activated by such a stress change. The western part of the foot wall is expected to have been subjected to a compressional stress change. Thrust aftershocks in this area that occurred within the subducting Pacific Plate (e.g. Mw 7.1 at 14:32 on April 7, off Miyagi in Fig. 3(c)) may also have been activated by this compressional stress change.