Improving the Constraint on the <i>M</i> _w 7.1 2016 Off‐Fukushima Shallow Normal‐Faulting Earthquake With the High Azimuthal Coverage Tsunami Data From the S‐Net Wide and Dense Network: Implication for the Stress Regime in the Tohoku Overriding Plate

Tsunamis with amplitudes of up to 40 cm, related to the Mw 7.1 normal-faulting earthquake off Fukushima, Japan, on November 21, 2016, were clearly recorded by a new offshore wide and dense ocean-bottom pressure gauge network, S-net, with high azimuthal coverage located closer to the focal area. We processed the S-net data and found some stations included the tsunami-irrelevant drift and step signals. We analyzed the S-net data to infer the tsunami source distribution. A subsidence region with a narrow spatial extent (∼40 km) and a large peak (∼200 cm) was obtained. The other near-coastal waveforms not used for the inversion analysis were also reproduced very well. Our fault model suggests the maximum stress drop across the fault plane of >∼10 MPa and the average of 4.2 MPa, whereas the shear stress increase along the fault caused by the 2011 Tohoku earthquake was only ∼2 MPa. Past studies have suggested that horizontal compressional stress around this region switched to horizontal extensional stress after the Tohoku earthquake due to its stress perturbation. The present result, however, suggests that the horizontal extensional stress was locally predominant at the shallowest surface around the focal area even before 2011. The present study demonstrates that the S-net high-azimuthal-coverage pressure data provide a significant constraint on the fault modeling, which enables us to discuss the stress regime within the overriding plate at the offshore. Our analysis provides an implication for crustal stress states, which is important for understanding generation mechanisms of intraplate earthquakes.