Using Fourier transform infrared spectroscopy (FTIR), we measured water contents of quartz and feldspar for four thin sections of felsic mylonite and two thin sections of banded granitic gneiss col- lected from a ductile shear zone of middle crust along the Red Rivers-Ailaoshan active fault. The ab- sorbance spectra and peak position suggest that water in quartz and feldspar of granitic gneiss and felsic mylonite occurs mainly as hydroxyl in crystal defect, but also contains inclusion water and grain boundary water. The water contents of minerals were calculated based on the absorbance spectra. Water content of feldspar in granitic gneiss is 0.05 wt%-0.15 wt%, and that of quartz 0.03 wt%-0.09 wt%. Water content of feldspar ribbon and quartz ribbon in felsic mylonite is 0.095 wt%-0.32 wt%, and those of fine-grained feldspar and quartz are 0.004 wt%-0.052 wt%. These data show that the water content of weakly deformed feldspar and quartz ribbons is much higher than that of strongly deformed fine-grained feldspar and quartz. This suggests that strong shear deformation leads to breakage of the structures of constitutional water, inclusion and grain boundary water in feldspar and quartz, and most of water in minerals of mylonite is released to the upper layer in the crust.
We relocated 2098 earthquakes that occurred in Beijing area between 1980 and 2000 using a double-dif- ference (DD) earthquake location algorithm, and obtained the high-precision relative locations of 1825 events. b values versus depth were investigated with the relocated hypocen- ters. The results show that the b values decrease with the increasing hypocentral depth systematically. A dramatic variation in b is observed around the depth of 8 km. It indi- cates that there are more smaller earthquakes at shallow depth (0―8 km), while more larger earthquakes at greater depth (8―25 km). The physical mechanism behind this phe- nomenon can be explained by the variations in material het- erogeneity and lithostatic stress condition. Large earth- quakes are more likely to nucleate at greater depth with more homogenous material and higher lithostatic stress. On the basis of the results, we suggest that future strong earth- quakes in Beijing area tend to occur below the depth of 8 km.
