Relative moment tensors of thermally induced microcracks in salt rock

P-wave amplitudes were inverted to obtain relative moment tensors and the corresponding radiation patterns of thermally induced microcracks at 850 m depth in the salt mine Asse near Wolfenbuttel, Germany. Seismic waves of microcracks that occurred during a 3-week heating experiment and during the following 12 weeks of cooling were recorded with eight high-frequency piezoelectric sensors (5-100 kHz) positioned a few metres from the sources. About 42,000 microcracking events were automatically located during this observation period. Two microcrack clusters were studied that occurred immediately after switching off the heater power. The clusters were located a few centimetres from the heater borehole. The moment tensors of these clusters indicate dominant shear cracks with a strike-slip radiation pattern. The P-and T-axes directions appear to coincide with directions of measured maximum and minimum horizontal stresses. (C) 1998 Elsevier Science B.V. All rights reserved.