Abstract

Kappa is a one‐parameter estimator of the spectral amplitude decay with frequency of a seismogram. Low values (∼5  ms) indicate limited attenuation of high‐frequency energy whereas higher values (∼40  ms) indicate high‐frequency energy has been removed. Kappa is often assumed to be a site term and used in seismic designs. We address two key questions about kappa: (1) how to identify source, path, and site contributions to kappa; and (2) can kappa estimates from smaller earthquakes, and more readily accessible weak‐motion recordings, be reasonably extrapolated to estimate kappa of larger earthquakes? The use of small earthquakes (ML<1) presents many challenges and requires new approaches. We develop estimates of kappa for seismograms from 1137 small earthquakes recorded by the ANZA seismic network in southern California, and compare these to results from the stronger recorded shaking generated by 43 ML>3.5 earthquakes inside the network. We find kappa from small earthquakes predicts the relative values of kappa for larger earthquakes (e.g., measurements at stations PFO and KNW are small compared with those at stations TRO and SND). For the SND and TRO data, however, kappa values from small earthquakes overpredict those from moderate and large earthquakes. Site effects are the most important contributor to kappa estimates, but the scatter within kappa measurements at a given station is likely caused by a significant contribution from near the source, perhaps related to near‐source scattering. Because of this source‐side variability, care is recommended in using individual small events as Green’s functions to study source‐time effects of moderate and large events.

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