ESR dating of fault gouge - review

The last activity of faults can be dated by the ESR dating of fault gouge. ESR dating of fault gouge is based on the measurements of the number of unpaired electrons (ESR center) in the quartz minerals. These ESR centers are generated by natural ionizing radiation from U, Th, and K and have accumulated over time. ESR signals of quartz are reset by frictional heating and lattice deformation during grain boundary frictional sliding at the time of fault movement. ESR signals then build up as a result of the formation of the unpaired electrons by natural radiation. Two parameters have to be determined; the equivalent dose (DE) and the dose rate (d). The ESR age is obtained by dividing equivalent dose by dose rate (ESR age = DE/d). Complete resetting of ESR signals will only occur when the fault plane has been subjected to normal stress over 3 MPa, and when there has been sufficient displacement. Fault gouges that are buried and later uplifted go through a stress history that can be explored by the effect of overburden, the maximum horizontal stress, and the angle between the strike of fault plane and the orientation of maximum horizontal stress. The depth of fault gouges at the time of complete resetting of ESR signals (over 3 MPa) can be estimated from the uplift rate and the ESR ages for sampling locations. Records of earthquake faulting events show that some parts of the fault cores or subsidiary faults were reactivated at the same time. In order to investigate the long-term pattern of fault activity, samples should be collected at each shear bands developed within the fault cores and subsidiary faults. ESR ages from both the fault cores and subsidiary faults (The Wangsukcheon fault zone, the Keumwang fault zone, the Yangsan fault zone, and the Ulsan fault zone) developed in the Korean peninsula range from 890 to 100 ka. The fault movements tend to show 1) temporal clustering into active and dormant periods, 2) migrate along the strike of the faults and 3) roam between widespread fault zones developed in the Korean peninsula similar to spatiotemporal occurrences of earthquakes in the interior of a tectonic plate.