Measurements of spectral similarity for microearthquakes in western Nagano, Japan

We use P wave spectral ratios to estimate the seismic energy of 23 microearthquakes ( M w ≈ 0.9–2.7) recorded on a dense network of surface and borehole stations in western Nagano, Japan. These events were recorded at 45 surface stations and 2 borehole stations. The data set is unique in that most events have high signal‐to‐noise ratio at several surface stations and at the two borehole stations. The redundancy provided by the large number of stations recording these events allows us to assess the uncertainty of our results. We find that the seismic energy to moment ( E R / M 0 ) ratios from P waves for the events in our data set are reliable, that they vary between 1 × 10 −6 and 1 × 10 −4 , and that smaller events tend to have smaller values of E R / M 0 because they are relatively deficient in radiated energy above the corner frequency. We observe two spectral effects: a variation in corner frequency, which accounts for most of the variation in E R / M 0 , and steeper spectral falloff rates, which contribute to the small values of E R / M 0 for some of the smaller events. Our estimates of moment and corner frequency suggest that M 0 ∝ f c −(3+ e ) (ɛ > 0), and this implies that either the static stress drop or rupture velocity or both change with earthquake size.