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S‐wave energy estimation of small‐earthquakes in the western Nagano region, Japan

We estimate the S‐wave energy radiated by nine small earthquakes (MW 1.2–2.4) that occurred in the western Nagano region, Japan. First, we obtain the S‐wave energy radiated by 156 earthquakes (MW 0.8–2.6) below 60 Hz in the frequency domain. We separate the radiated energy spectra and the site amplification functions using a least squares inversion method including corrections for the S‐wave radiation pattern at the source and the attenuation along the path. Then, we assume an ω2‐model to determine the corner frequencies for the nine earthquakes by using an empirical Green's function (EGF) method to remove the site and attenuation effects. Using the obtained corner frequencies, we then calculate radiated energy using these source models above 60 Hz. Our result indicates that the ratio of the seismic moment and S‐wave energy is between 8 × 10−6 and 7 × 10−5.