2011年東北地方太平洋沖地震の発生後に活発化した霞ヶ浦南端直下の正断層型地震活動

Abstract

We have investigated the characteristics of an activated earthquake sequence below the southern extremity of Lake Kasumigaura in the Kanto region, central Japan, after the 2011 Mw 9.0 Off the Pacific Coast of Tohoku Earthquake (Tohoku-oki earthquake). The earthquake locations of this sequence using one-dimensional velocity model suitable for the area reveal that the triggered earthquakes distribute at a depth from 13 to 20 km forming a 25 km-long in the NW-SE direction and 20-km-wide zone. The depth range indicates that they are shallow crustal earthquakes, which are uncommon in the area. The sequence seems to consist of multiple small-sized clusters rather than a clear planar fault plane structure. The double-difference relocation shows that the locations for the periods before and after the Tohoku-oki earthquake are basically complementary to each other. Focal mechanisms were determined from P-wave polarity data as well as body wave amplitudes for thirty-six earthquakes including one event that occurred before the Tohoku-oki earthquake. The pre-Tohoku-oki earthquake of the magnitude 1.9 has a reverse faulting mechanism with P-axis in the NW-SE direction. Conversely, the triggered earthquakes are primarily normal-faulting mechanisms with T-axes in the direction of WSW-ENE to NW-SE. The stress tensor inversion for the mechanisms of triggered earthquakes indicates that the area is characterized by a pure normal-faulting stress regime with the minimum compressive stress in the E-W direction subhorizontaly. The stress drop values estimated for three M3-class triggered earthquakes and their paired smaller earthquakes for empirical Green’s functions range approximately from 0.5 to 15 MPa, suggesting that there is no difference in the underlying physical process between these triggered earthquakes and natural tectonic ones. We computed the static stress changes due to the Tohoku-oki earthquake together with the Mj 7.6 aftershock offshore of Ibaraki so as to examine why such unusual normal-faulting earthquakes were activated in the area characterized by a reverse-faulting stress field. The analysis of Coulomb stress change (ΔCFF) shows that the induced stress favors the activation of normal-fault earthquakes of the present sequence. However, the induced stress cannot change the background reverse-faulting stress field into normal-faulting one by rotating the maximum compressive stress from horizontal to vertical, because both earthquakes produce a strike-slip faulting stress field in the area with the same maximum compressive stress direction as the background one. Therefore, we cannot attribute the activation of the seismicity to coseismic stress rotation. Instead, we suggest the existence of local stress heterogeneity that the pre-Tohoku-oki earthquake stress field in the area was a normal-faulting regime, which is also supported by the comparison of the induced stress magnitude with the stress drop values of triggered earthquakes. We thus conclude that the 2011 Tohoku-oki earthquake and the Mj 7.6 aftershock triggered the normal-faulting earthquake sequence in a limited area in combination with a locally formed pre-shock normal-faulting stress regime.