Detailed spectral analysis of two small New York state earthquakes

A statistically complete methodology for the analysis of SMA-1 data, including a direct inversion of the amplitude spectra, is developed and applied to the records of two small earthquakes ( M L = 1.5 in Attica and M L = 2.2 in Blue Mountain Lake) obtained at epicentral distances of ≈1 km. Using ray dependent corner frequency-radius relations, the fault radii are determined to be a = 80 ± 15 m and 25 ± 10 m. The stress drops vary by an order of magnitude; Δσ = 6 ± 4 bars and 85 ± 60 bars. Using hydrofracture data to estimate σ ¯ , the average stress, the seismic efficiency of the Attica event is determined to be ≈0.08. Because of a high estimate of the fracture energy associated with its rupture, the Blue Mountain Lake event is interpreted as having fractured fresh rock. The stopping character of the two events, inferred from their spectral shapes, is shown to differ considerably. These differences are used to contrast the stress and frictional regimes of the two source regions.