Stick-slips have been studied in the laboratory on granite, labrodorit
e and sandstone samples of two different sizes. Different roughness wa
s achieved on the sawcut surfaces by finishing them with different gri
nding compounds ranging from grit 40 to grit 1000. Stick-slips occurre
d as a result of 1) slowly increasing the shear and normal stresses, 2
) superimposing a sinusoidal stress modulation (0.1 and 10 Hz) on the
slowly increasing stresses, 3) triggering by a stress impulse when the
shear stress was well below the levels where stick-slips occurred wit
hout the impulse, and 4) foreshocks. Stick-slips triggered by impulses
or foreshocks occurred ''long'' after the beginning of the triggering
events, i.e., long in comparison with elastic wave travel times throu
gh the sample. All triggered events were very rich in high frequencies
(corner frequency of approximately 100 kHz). The untriggered stick-sl
ips did not contain much energy at the high frequencies (corner freque
ncy of approximately 10 kHz). The dynamic friction coefficients for th
e triggered stick-slips were smaller than for the untriggered events.
The ''long'' delay between the onset of the trigger and the stick-slip
, and the high frequencies may be a consequence of corrosion of asperi
ties. The ultimate triggering and the rate of corrosion are likely rel
ated to the interplay of the normal and shear stresses as they load an
d unload the fault surface. The consistent shape of the high frequency
spectra is probably due to sample resonances which are excited rather
than being characteristic of the details of the stick-slips. If these
laboratory observations are directly applicable to earthquake seismol
ogy, the spectra of earthquakes which were triggered by other earthqua
kes should be anomalously rich in high frequencies.