Springer Online Journal Archives 1860-2000
Abstract Strong motion (SM) data of six Mexican subduction zone earthquakes (6.4≤M S≤8.1) recorded near the epicentral zone are analyzed to estimate their far-field source acceleration spectra at higher frequencies (f≥0.3 Hz). Apart from the usual corrections such as geometrical spreading (1/R), average radiation pattern (0.6), free surface amplification (a factor of 2), and equal partitioning of the energy into two orthogonal horizontal components (a factor of 1/ $$\sqrt 2 $$ ), the observed spectra are corrected for a frequency dependentQ(Q=100f), a site dependent filter (e −πkf ), and amplification ofS waves near the surface (a factor of about 2 atf≥2Hz). We takeR as the average distance from the rupture area to the site. If we model the high frequency plateau (f≥1 Hz) of the source spectra, by a point source ω−2-model, and interpret them in terms of Brune's model we obtain δσ between 50 and 100 bars for all earthquakes. The low-frequency broadband teleseismicP wave spectra, corrected witht *=1.0 s, agrees within a factor of two with SM source spectra near 1 Hz. The ω−2-model is inadequate to explain the observed source spectra in a broad frequency range; these resemble spectra given byGusev (1983) with some differences. SM source acceleration spectra require significant corrections to explain observed spectra and RMS acceleration (arms) (a) at farther coastal sites for extended sources due to directivity effect and (b) at inland sites (100≤R≤200 km) because of unaccounted path and site amplification and/or invalidity of body-wave approximation. The observed spectra and arms at these sites are significantly greater than the predicted values from the estimated source spectra.
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