Frictional Properties Of Natural Fault Gouges From Drilled Cores In The Longmenshan Fault Zone Cutting Granitic Rock

Frictional constitutive relation of natural fault gouges has great importance in understanding the generation of natural earthquakes,strength of faults,and different slip behaviors of active faults.The 2008 Mw 7.9 Wenchuan earthquake occurred within the Longmenshan Fault Zone(LFZ).The Institute of Geology,China Earthquake Administration,started an integrated research,including outcrop studies and the Longmenshan Fault Shallow Drilling(LMFD)project In the latter project,some natural fault gouge samples were retrieved,for detailed mechanical researches.In this paper,we report quasi-static friction experiments performed on two natural fault gouge samples embedded in granitic wall-rock from drilled core by the LMFD project.One of the samples is the yellow-greenish gouge(YGG),which contains 5%quartz,30%feldspar,15%smectite,3%illite,and 47%chlorite.Compared with other natural fault gouge,the yellow-greenish gouge(YGG)is dominantly chlorite-rich.The other one is the grey-greenish gouge(GGG),which contains 10%quartzr 45%feldspar,8%calcite,3%smectite,15%kaolinite,and 19%chlorite.The grey-greenish gouge(GGG)is also chlorite-rich,but the chlorite content is much less than in YGG.To understand the frictional properties of the YGG and the GGG sample,experiments were performed at constant confining pressure of 130 MPa and constant pore pressure of 50 MPa,corresponding respectively to lithostatic(rock density?2450 kg/m3)and hydrostatic pressure(water density?1000 kg/m3)at 5 km depth environment of the Longmenshan fault zone(LFZ).To simulate various crustal depths in our experiments and to assess the effect of temperature on friction,we varied the temperature from 25?to 150 °C.Based on a heat flow of 55.3 mW/m2 for the Longmenshan region(Wang,1996),we estimated a one-dimensional geothermal gradient of?18.5 ?/km in the shallow portion.The temperature range of 50-150 ? applied in our experiments thus corresponds to 2-8 km depth range of the LFZ.The experiments aim to address the frictional slip behavior of the YGG and the GGG under shallow,upper crustal pressure,and temperature conditions.Compared with previous studies on natural gouges,our results show that the YGG is stronger and shows a steady state friction coefficient of 0.47—0.51.The GGG shows a steady state friction coefficient of 0.48-0.52,with an increasing trend as temperature increases.The YGG has dc values of 7-18?m?hile The GGG has a dc values of 14-24?m?Comparison with previous studies of natural gouge with similar content of non-mica phyllosilicates minerals shows that such phylosilicates are much weaker than quartz,feldspar,or calcite.The comparison also indicates a sequence of strengths of different clay non-mica phyllosilicates:chlorite>illite>smectite and chlorite>kaolinite.At temperatures up to 150 0C hence depths up to?8 km in the Longmenshan region the YGG and the GGG both shows stable velocity-strengthening behavior at shallow crustal conditions.The YGG has an a-b vahlue of +0.003-+0.0045,with an increasing trend with temperature rise.Similarly,the GGG has an a-b value of +0.003-+0.0052.Combined with the fact of strong direct velocity effect,i.e.,(a-b)/a>0.5,faults cutting the present clastic lithology up to-8 km depth in the Longmenshan Fault Zone(LFZ)are likely to offer a stable sliding resistance,damping co-seismic rupture propagating from below at not-too-high slip rates.However,as the fault gouge generally has low permeability,co-seismic weakening through thermal pressurization may occur at high slip rates(>0.05m/s),leading to additional hazards.