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Ground motion recording with peak ground acceleration (PGA) exceeding 1.43 g was observed at the Yamamoto (MYGH10) station during the MW 7.1 Fukushima earthquake off the east coast of Honshu, Japan, in February 2021. In this study, we investigated the site amplification effect and nonlinear site response at the MYGH10 site using the horizontal‐to‐vertical spectral ratio (HVSR) and surface‐to‐borehole spectral ratio (SBSR), respectively. The site transfer function of station MYGH10 was presented using HVSRs (HVSR on the ground surface) and SBSR at different frequency bands. The dominant contributing frequency band of the PGA was determined by increasing the high cutoff frequency of the bandpass filter when filtering the acceleration recording, which was used to interpret the anomaly of the PGA by combining it with the site characteristics. We found an obvious site effect below the downhole sensor through the HVSRb (HVSR on the downhole bedrock) and determined the influence of the frequency band of the downhole site on ground motion. In addition, substantial discrepancies in the frequency and amplitude between the spectral ratio curves for the Fukushima mainshock records and the weak historical motions were identified. The calculated values of the degree of nonlinearity suggested that a strong nonlinear site response occurred at station MYGH10. Finally, the recovery of the site after strong shaking was evaluated by using the average spectral ratio curves of several aftershock records. [ABSTRACT FROM AUTHOR]

Measurements of surface heat flux from boreholes of the NIED Hi‐net network of seismometers are used to determine temperature and shear stress on the plate interface beneath northern Honshu. At the maximum depth of thrust‐faulting earthquakes, about 60 km, the temperature is 660 ± 240°C and the average shear stress during slip on the interface is 100 ± 45 MPa. The equivalent quantities from the oceanic setting of the Kermadec trench are 630 ± 140°C and 115 ± 25 MPa. The shear stresses are substantially higher than commonly assumed in models of convergent plate interfaces, but are consistent with measurements made above other convergent plate interfaces where heat flux data are more sparse. They are also consistent with the level of shear stress required to explain measurements of pressure and temperature from high‐pressure‐low‐temperature terrains that experienced metamorphism on subduction interfaces, in particular those from the Cretaceous Sanbagawa belt of SW Japan. Plain Language Summary: The temperatures and stresses under which most of the world's great earthquakes nucleate are poorly known because they take place at depths of tens of kilometers, where conditions cannot be directly observed. Measurements of surface heat flux above plate interfaces provide the strongest indirect constraints, but those measurements are sparse. We use a dense set of heat flux measurements to determine shear stress and temperature on the plate interface of northern Honshu, on which the 2011 Tōhoku earthquake occurred. The shear stress, 100 ± 45 MPa, and temperature, 660 ± 240°C, are significantly higher than previous estimates, but are supported by analysis of heat flux above the Kermadec plate interface—the only location where a comparable density of heat flux measurements has been used to estimate shear stress. Shear stresses of ∼100 MPa can also account for the conditions of temperature and pressure recorded in rocks of the Sanbagawa belt of SW Japan, which are the exhumed remnants of an approximately 90‐million‐year‐old plate interface. This concordance supports arguments that the Sanbagawa, and similar terrains around the world, record physical and chemical conditions relevant to the generation of earthquakes on present‐day plate interfaces. Key Points: Surface heat flux measurements constrain temperature (660 ± 240°C) and shear stress (100 ± 45 MPa) at ∼60 km on the plate interface in N HonshuThese values are supported by estimates from dense heat flux measurements at the Kermadec plate boundary (630 ± 140°C) and (115 ± 25 MPa)Metamorphic temperatures in an exhumed plate interface, the Sanbagawa belt of SW Japan, are also explained by this level of shear stress [ABSTRACT FROM AUTHOR]