STRESS DROP ESTIMATES ON ANCIENT SEISMOGENIC FAULTS BASED ON FRICTIONAL HEATING OF COAL

Stress drops during earthquakes are normally determined solely from seismological data, but the estimates have large uncertainty. A new technique is presented whereby the stress drop during ancient earthquakes is inferred from vitrinite reflectance in faulted coals. Faulted coals from eastern Kentucky (Pennsylvanian), Montana (Cretaceous), and the south Wales coalfield (Pennsylvanian) 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 temperature gradients and heating rates consistent with a seismic source. New high velocity friction experiments indicate vitrinite reflectance responds rapidly to large temperature spikes. Shear stresses are calculated from the area under the thermal anomaly and fault displacement. For assumed and measured displacements of 1-3 meters, the estimated shear stresses are 13-64 MPa, which are somewhat higher than, but overlap with, seismological estimates (~0.3 to 30 MPa) during modern earthquakes. The calculated local mean normal stress is ~100 MPa at all three localities, resulting in calculated coefficients of friction of 0.2-0.4 for thrusts and 0.3-0.7 for normal faults. These values are lower than traditional (Byerlee) laboratory-based values, but are in agreement with recent high-velocity friction experiments.

Using this technique, it is difficult at present to rule out cumulative heat contributions from multiple displacement events on the same fault. In addition, the role of differential stress on vitrinite reflectance is somewhat unclear. Further examination of vitrinite reflectance values in faulted coals, however, is warranted as it has the potential to provide unique insight into earthquake dynamics.