The elusive role of a deep-seated low-angle normal fault activated by
the Mw 9.0 Tohoku-Oki megathrust in the triggering of a normal faulting
earthquake sequence in northeast Japan
Abstract
Although deep-seated blind normal faults are common in subduction
environments, their rheology, kinematics and interaction with the upper
crust are poorly constrained. A month-long shallow normal faulting
sequence in the Ibaraki-Fukushima prefectural border (IFPB), northeast
Japan, which followed the Mw9.0 Tohoku-Oki earthquake (TOE) and
culminated in the Mw6.7 Iwaki earthquake, provides a window into
megathrust-to-normal fault interaction. Stress change calculations
clearly indicate that the IFPB earthquake sequence cannot be explained
in terms of direct triggering by the TOE co- and post-seismic slip. In
quest for an alternative triggering mechanism, we analyzed post-TOE GNSS
data from eastern IFPB. A key step in this analysis is the removal of
the large-scale post TOE displacement field, after which a distinct
highly-localized strain along the coastline becomes apparent. The
accumulation of this strain was mostly aseismic, and migrated with time
prior to the Iwaki earthquake in a manner that correlates well with
post-TOE local seismicity. We attribute the pre-Iwaki earthquake strain
accumulation to aseismic slip along low-angle seaward dipping blind
normal fault, activated by the TOE. Stresses transferred by this slip
episode accelerated the failure along the IFPB shallow normal faults.
This indirect triggering of the Iwaki earthquake sequence by the TOE
highlights the complexity of stress transfers in subduction
environments.