Faulted coals from eastern Kentucky and Montana (USA), and the south Wales coalfield (UK) show fault-related 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.