Internal Structures And High-velocity Frictional Properties Of Carbonate Fault In The Middle Segment Of Longmenshan Fault System

The Mw7.9Wenchuan earthquake of12thMay2008generated two parallelsurface ruptures along the Longmen Shan thrust belt in Sichuan Province, China, andthe surface rupture along the Yingxiu-Beichuan Fault is of a length of about250km.Based on rupture pattern and coseismic displacement, Yingxiu-Beichuan surfacerupture can be divided into two parts by Anxian transfer zone. One isYingxiu-Qingping fault characterized by right-lateral oblique faulting, and the other isBeichuan-Nanba fault characterized by thrust right-lateral faulting. Anxian transferzone separated Yingxiu-Qingping fault and Beichuan-Nanba fault with differentdynamic mechanisms, and it has a damping effect on surface rupture propagating andcan be considered as a tectonic boundary of surface ruptures. Thus it is important tostudy the coseismic behavior of this transfer zone, which can help to understanddifferent rupture properties between south and north part of Yingxiu-Beichuan faultduring the Wenchuan earthquake.The Xiaojiaqiao outcrop reported in this paper is a bedding-parallel fault inPermian limestone that moved by about0.5m during the Wenchuan earthquake. Thefault is located in Anxian transfer zone, about3km to the south from the middle partof Yingxiu-Beichuan fault. It is nearly optimally oriented for thrust motion with a lowangle of380, in contrast to high-angle coseismic faults at most places in Wenchuanearthquake. Surface outcrop and two shallow drilling studies reveal that the fault zoneis several centimeters wide at the most and the coseismic slip zone during Wenchuanearthquake is about1mm thick. Fault zone contains foliated cataclasite, fault breccia,black gouge and yellowish gouge. Many clasts of foliated cataclasite and black gougecontained in fault breccia indicate multiple slip events along this fault. But fossils onboth sides of fault do not indicate clear age differences and overall displacement alongthis fault should not be large.We also report high-velocity friction experiments conducted on yellowish gougefrom fault zone and limestone host rock using a rotary shear low to high-velocityfrictional testing apparatus, according to the normal stress and slip rate of the outcrop. Experimental results are as follows:(1) Dry experiments at normal stresses of0.4to1.8MPa and at slip rates of0.08to1.35m/s reveal dramatic slip weakening from thepeak friction coefficient of around0.6to very low steady-state friction coefficient of0.1~0.2.(2) Slip weakening distance decreased exponentially with the increasing ofnormal stresses and slip rates.(3) High velocity behaviors between yellowish gougeand limestone host rock are similar, with their peak coefficient and steady-statefrictional coefficient nearly the same.(4) Grain sizes decreased dramatically afterhigh velocity experiments, changing from initially homogeneous distribution to peaksof small grains.(5) Temperature goes up quickly in the high velocity experiments and(6) different units respected weakly and highly shear-deformed zones appeared afterexperiments.Slip weakening parameters of this carbonate fault are similar to those of clayeyfault gouge of Yingxiu-Beichuan fault at Hongkou outcrop and at Pingxi outcrop,indicating the coseismic properties of Yingxiu-Beichuan fault may be homogeneous.Specific fractured energy by Wenchuan earthquake is similar to Kobe earthquake andthe1997California earthquakes.