abstract

Crustal transfer-function ratios (0.033 to 3 Hz), using P-waves from earthquake sources in South America, Fiji-Tonga, and Japan recorded at Berkeley, California (BRK) are used to define composite transfer-function ratios representative of the crustal structure near BRK along the azimuth of the source epicenter. Observed ratios for events from the same azimuth are consistent while composite transfer-function ratios for different azimuths are strikingly different. The variation in the observed ratios with azimuth confirms the geological evidence for azimuthal variation in the crustal structure near BRK.

A measure of the contamination of the P-wave train indicates minimum SH generation within the crust near BRK for source azimuths (180 to 250°) near normal to the Pacific coast. Since P-waves incident at the base of a dipping structure generate SH motion at all azimuths except for updip or downdip approach, the observed minimum is consistent with downdip approach to crustal structure dipping toward the continent in central California.

The observed crustal transfer-function ratios are obtained by applying a “modified Hanning taper” to window the seismograms in the time domain. Contamination in the ratios from secondary arrivals of the pP and PcP phases is shown to be minimal and the ratios are independent of window length for window lengths greater than 20 sec.

The vertical P-wave travel times in the total crustal section and in the crustal surface layer can be determined from the crustal transfer-function ratio data with a minimum of computational analysis. The BRK data indicate the following vertical P-wave travel times:


 
  South America  Fiji-Tonga  Japan 
  Composite (sec)  Composite (sec)  Composite (sec) 

 
Total crustal section  3.4 ± 0.15  0.25 ± 0.03  3.85 ± 0.15 
Surface layer  0.25 ± 0.03  0.44 ± 0.05  0.31 ± 0.03 

 

 
  South America  Fiji-Tonga  Japan 
  Composite (sec)  Composite (sec)  Composite (sec) 

 
Total crustal section  3.4 ± 0.15  0.25 ± 0.03  3.85 ± 0.15 
Surface layer  0.25 ± 0.03  0.44 ± 0.05  0.31 ± 0.03 

 

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