AN EXPERIMENTAL-STUDY OF TRIGGERED STICK-SLIP

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.