fmax

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Abstract
The title of this contribution refers to the high-frequency band-limitation of the radiated field of earthquakes. For close recording distances of ≃10 km, fmax for California earthquakes is generally found within the very narrow band 10 ≲ fmax ≲ 20 Hz, without regard to source strength or tectonic setting. Direct evidence for fmax (in the form of shear-wave acceleration spectral amplitudes) and indirect evidence as well (in the form of limiting spectral corner frequencies) have often been taken as manifestations of source properties, barrier end-zones, for example. This study concludes that fmax as actually observed is more likely to be a property of local site conditions, although this does not preclude source-controlled band-limitations at still higher frequencies. Observations of fmax for a single Oroville aftershock show a small but significant site-to-site variation that correlates well with gross surface geology; this variation is “normalized” with a demonstration that fmax for seven of the principal aftershocks (4.0 ≦ ML ≦ 4.9) at a single station has a distinctly smaller variation. Moreover, there seems to be nothing fundamental about the range 10 ≲ fmax ≲ 20 Hz so typical for California earthquakes; spectral shear-wave corner frequencies observed at very close distances (≲5 km) and for especially competent propagation paths are known to occupy the band 20 ≦ fo(S) ≦ 200 Hz.