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The spectrum of a P wave follows a characteristic shape, with a low-frequency plateau and a high-frequency rolloff. The intersection of spectral plateau and rolloff occurs at the corner frequency f_C, which scales inversely with the duration of fault rupture. The 1994 Northridge P-wave spectrum, computed from 180-s time windows at four GSN stations in Europe, has f_c ~ 0.1 Hz, which corresponds to a rupture duration near 10 s. By contrast, the spectrum of P-waves form the 2004 Sumatran earthquake, also using four GSN stations in Europe, has no clear low-frequency spectrum plateau, suggesting fault rupture longer than the 180-s duration of the ground motion analyzed.
Credits: Jeffrey Park, Yale University