Near-Field Imaging of Near-Identical Mw5.9 Earthquakes in the Crust of Ibaraki, Japan

Understanding the physical mechanisms of intraplate earthquakes requires a better understanding of the earthquake cycle and aseismic slip. A potential short-term repetition of intraplate earthquakes has been documented from interferometric synthetic aperture radar (InSAR) observations of two M_w5.9 events in northern Ibaraki Prefecture, Japan, which occurred on 19 March 2011 and 28 December 2016. However, our understanding based on surface geodetic measurements remains limited, particularly in terms of spatial resolution at depth. Here, we used the near-field seismic waveform data to complement InSAR data, thereby improving the spatial resolution of the coseismic slip distributions. First, we confirmed that the aftershocks of both events occurred along a common fault area. We then conducted a joint inversion that successfully reconciled near-identical surface displacements with the different seismic waveforms. Our results indicate that while the 2011 event mainly ruptured just updip of the hypocenter at 6 km depth, the 2016 rupture initiated deeper, at 10 km, and propagated to the shallow, northern side, causing large slip near the 2011 rupture area. The slip distributions of two events were complementary in the deeper portion (3 < z < 6 km), whereas those on the shallower side (z < 3 km) largely overlapped, resulting in nearly identical surface displacements. Small earthquakes rarely occur shallower than 3 km, suggesting that this shallow segment releases accumulated strain aseismically. The two events repeatedly triggered seismic slip on this typically aseismic shallow segment, possibly due to dynamic stress changes.