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[1] Faulted coals from eastern Kentucky and Montana (USA), and the south Wales coalfield (UK) show faultrelated vitrinite reflectance anomalies interpreted as due to frictional heating during seismic slip. The vitrinite reflectance anomalies are converted to temperature using kinetic-based software (Easy%R) for post-seismic cooling rates. Shear stresses are calculated using Lachenbruch's [1986] piezometer, which depends only on the area under the thermal anomaly and fault displacement. For maximum displacements of 1 -3 meters, the minimum shear stresses are 13-64 MPa, and overlap with seismological estimates of stress drops during modern earthquakes. The calculated local mean normal stress is $100 MPa, resulting in coefficients of friction of 0.1-0.6 (thrust) and 0.2-0.4 (normal faults), which are lower than laboratory-based values.

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