Integrated seismic source model of the 2021M7.1 Fukushima earthquake

Abstract

SUMMARYWe constructed an integrated rupture model of the 2021 Mw 7.1 Fukushima earthquake, an intraplate earthquake, by resolving both its spatiotemporal distribution of slip-rate and high-frequency (∼1 Hz) radiations. We analysed near-field seismic observations using a novel finite-fault inversion method that allows automatic parametrization and teleseismic data from multiple arrays using the MUSIC backprojection (BP) method that enhances imaging resolution. The inverted slip distribution obtained from waveforms filtered in the frequency band of 0.02–0.2 Hz showed that the kinematic rupture propagated along both the strike (∼35 km) and dip directions (∼85 km), and that the large-slip area was located southwest to the hypocentre with a maximum slip of ∼1.03 m. Overall, no obvious frequency-dependent rupture behaviours occurred during the rupture process due to the deep nucleation of the Fukushima earthquake on a heterogeneous fault where sizes of asperities do not monotonically increase with depth, which sheds light on understanding the rupture dynamics of intraplate earthquakes in subduction zones. Both the slip inversion and BP revealed the general rupture feature of this earthquake with southwestward and updip directivity. A comparison of BPs between multiple arrays indicates that the source–receiver geometry and the directivity effect of an earthquake may cause critical discrepancies in BPs of different arrays. From the temporal change of stress around the hypocentre of the 2021 Fukushima earthquake due to the 2011 Tohoku-Oki Mw 9.1 earthquake, the long-term dominance of viscoelastic relaxation increased the Coulomb failure function (CFF) by 0.3–0.7 MPa, indicating that the occurrence of the Fukushima earthquake has been likely promoted by the post-seismic deformation due to the Tohoku-Oki earthquake.