EXPERIMENTAL-EVIDENCE OF CONTACT LOSS DURING STICK-SLIP - POSSIBLE IMPLICATIONS FOR SEISMIC BEHAVIOR

We present results of an experimental study on the stick-slip phenomen on focused on detailed measurements of the relative normal displacemen t. This was done mainly to determine whether or not the sliding surfac es become separated during the slip phase as suggested in previous wor ks. Experiments were performed on PMMA as it has mechanical properties comparable to those of brittle rocks and has proved to be a good anal ogue for rocks in rupture mechanics experiments. Results show clearly that normal displacement occurs during and after the slip phase. The c haracterisation of the sliding surface enables us to compare the maxim um relative normal displacement during the slip phase with asperity he ight. In most cases, the maximum relative normal displacement was foun d to be higher than the average value of the peak-to-trough relief of the surface, showing that sliding surfaces were at least partially sep arated. The phenomenon generally consists of a monotonous slip opening phase followed by a monotonous slip closure phase. In some cases, the slip closure phase shows oscillations which have been interpreted as elastic bounces. This last phenomenon was observed irrespective of the experimental conditions with a probability ranging from 0.1 to 0.3 bu t no clear variations of this probability have been found as a functio n of normal stress and roughness. These observations may give insights for the explanations of earthquake-related problems such as the heat flow paradox, anomalous P-wave radiation or the inferred low average f riction coefficients in subduction zones. (C) 1998 Elsevier Science B. V. All rights reserved.