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OFFICE OF EARTHQUAKE STUDIES U.S. GEOLOGICAL SURVEY, 345MIDDLEFIELD ROAD, MENLO PARK, CALIFORNIA 94025
SEISMOGRAPHIC STATION DEPARTMENT OF GEOLOGY AND GEOPHYSICS UNIVERSITY OF CALIFORNIA, BERKELEY, CALIFORNIA 94720
Abstract
Foreshock activity preceded both the ML = 5.5 June 28, 1966 Parkfield, and the ML = 5.7 August 1, 1975 Oroville, California earthquakes. The PZ source displacement amplitude spectrum (0.5 Hz < f < 10 Hz) recorded at Priest Valley, California (
= 25 km) for an ML = 2.6 immediate Parkfield foreshock appears distinctive, characterized by a corner frequency fo = 2.5 Hz and an 
–1 higher-frequency spectral dependence; spectra for another immediate Parkfield foreshock (ML = 3.1), two nearby Parkfield aftershocks (ML = 3.0 and 2.9), and three nearby "normal" Parkfield earthquakes (ML = 2.6, 2.6, and 3.1) are all higher frequency with fo 
10 Hz. PZ and SZ signals (0.4 Hz 
f 10 Hz) recorded at Whiskeytown, California ( = 150 km) for an immediate ML = 3.8 Oroville foreshock have similar corner frequencies (fo 
2 to 4 Hz) and marginally smaller high-frequency roll-off rates than the corresponding phases for four nearby 3.6 ML 4.1 Oroville aftershocks; an analogous comparison failed to discriminate an earlier (June 28, 1975) ML = 3.5 Oroville foreshock from the same five aftershocks. This exercise in attempted foreshock identification, using high-quality data and the two largest central California earthquakes in many years, points up the extreme subtlety of any unique properties in radiated body waves and the resulting difficulty in characterizing foreshocks.
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