Resetting of Quartz OSL (optically stimulated luminescence) Signals by Frictional Heating in Experimentally Sheared Gouges at Seismic Slip Rates.

Abstract:

Recent studies on natural and experimental seismic faults have revealed that frictional heating plays an important role in earthquake dynamics as well as in producing mineralogical and microstructural signatures of seismic faulting. Here, we report changes in OSL signals in quartz by frictional heating in experimental fault gouges. The gouges (80% of quartz and 20% of bentonite by weight) with a thickness of 1 mm were sheared between sandstone cylinders (diameter: 25 mm) at a normal stress of 1 MPa and slip rate of 1.31 m/s. The quartz grains from a sand dune on the western coast of South Korea were sieved to select size fractions between 90 and 250 µm. The equivalent dose (D_e) of the undeformed quartz grains was 8.0 ± 0.3 Gy. Upon displacement, the friction abruptly increases to the 1^st peak (with friction coefficient μ ≈ 0.75) followed by slip weakening. Then the fault zones show two more peak frictions (μ ≈ 0.53~0.75) and finally reach a steady-state friction (μ ≈ 0.2~0.35). The fault can be divided into three zones based grain size (thus slip rate); slip localization (SLZ), intermediate slip-rate (ISZ) and low slip-rate (LSZ) zones. SLZ develops adjacent to the moving side of the sandstone cylinder with P-foliation and shear band. The size of quartz (D_q) in ISZ and LSZ is 5-30 μm and 50-250 μm, respectively. SEM and TEM analyses indicate that the fault gouge of SLZ consists of subangular quartz clasts (D_q ≈ 3 μm) and matrix of nano-scale quartz, unidentified silicate minerals and amorphous material. The fault zones were sectioned into six layers (~160 µm thick for each layer) parallel to the fault zone boundary for OSL analyses. Quartz grains from all the layers except the one immediately adjacent to the stationary side of the sandstone cylinder show D_e of 'effectively' 0 Gy indicating a full resetting of OSL signals. The partial resetting of OSL signal in the layer adjacent to the stationary side of the cylinder indicates the temperature (T) there was not enough to empty the main OSL traps (i.e. 325 °C TL traps), which is supported by T (~250 °C) measured at the stationary side with thermocouples. Our results suggest that frictional heat during seismic events reset 'geologic clocks' of fault rocks and OSL signals in quartz from gouges in natural fault zones can be used to constrain the age of seismic events.