Spectral scaling of earthquakes in the Miramichi region of New Brunswick

Seismograms of 12 earthquakes in the Miramichi region of New Brunswick were analyzed to determine source scaling relations. The events ranged in magnitude from 3.3 to 5.8 (m_bLg). P-wave spectra between 1 and 20 Hz were calculated using digital records from station RSNY (Δ = 6°). Spectral ratios were then formed using the main shock as a reference event. Because the travel paths were virtually identical for all of the signals, the spectral ratios yield the scaling of the sources as a function of frequency. The spectral ratio curves show a smooth progression with event size, with their separation decreasing toward higher frequencies. These data were compared with theoretical curves for source models whose displacement spectra roll off as ω⁻² and ω⁻³ above the corner frequencies. The data strongly favor the ω⁻² models; the ω⁻³ models predict greater variation across the 1- to 20-Hz band than is observed. Comparison of the P-wave scaling at 15 Hz with the Rayleigh wave scaling at a period of 8 sec indicates that stress drop may increase with moment for these events, although not as fast as Nuttli (1983) has suggested for intraplate earthquakes.