A key question in earthquake seismology is whether earthquake sources in different tectonic regions are functionally equivalent in terms of radiated ground motion. This article compares the apparent earthquake source radiation for six different regions: Japan, Mexico, Turkey, California, British Columbia (B.C.) (western Canada), and eastern North America (ENA). The source, path, and site effects were consistently separated to the extent possible to allow interregional comparisons of apparent source radiation. Large Fourier spectra databases were created for both horizontal and vertical components for earthquakes with magnitude between 2.5 and 7.5. Regional parameters characterizing source, path, and site properties, including crustal and near-surface site amplifications, were investigated to isolate apparent source spectra.

Based on the vertical-component spectra at a reference distance of 1 km, a new magnitude scale m1 was defined from the spectral level at 1 Hz. The magnitude scale m1 is a natural complement to existing magnitude scales that measure the low-frequency and high-frequency spectral amplitudes (moment magnitude M and high-frequency mhf, respectively).

In all regions, the apparent source spectra for small to moderate earthquakes (M <6) show good agreement with the single-corner Brune (1970, 1971) point-source model (ω2 spectrum), suggesting a stress parameter of about 100 bars (10 MPa). A two-corner source model better matches the spectra for large-magnitude earthquakes (M ≥6), especially at intermediate frequencies. For small to moderate events, the two source models are nearly equal. Apparent source spectra from small to moderate earthquakes (e.g., m1 3.5-6.5) suggest a general similarity between the different tectonic regions studied. Although minor discrepancies appear in some cases, there are no noticeable regional characteristics. No evident depth effects were found on the apparent source spectra.

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