Structures and high-velocity frictional properties of the Pingxi fault zone in the Longmenshan fault system, Sichuan, China, activated during the 2008 Wenchuan earthquake

Abstract

Abstract This paper reports on the internal structures of the Pingxi fault zone near the northeastern end of the coseismic surface ruptures associated with the 2008 Wenchuan earthquake. The fault zone consists of a 50–80 cm wide fault core and several-meter wide fault breccia zones. The fault core contains yellowish gouge (YG), gray-blackish gouge (GBG), black gouge (BG) and gray-blackish fault breccia. A coseismic slip during the Wenchuan earthquake occurred within GBG in a slip zone of 24 ± 9 mm in width. High-velocity friction experiments were conducted at normal stresses σn of 0.45–2.50 MPa and slip rates v of 0.0014–2.09 m/s. YG and GBG exhibit dramatic slip-weakening at constant high slip rates that can be described as an exponential decay from peak friction coefficient μp to steady-state friction coefficient μss over a slip-weakening distance Dc. Specific fracture energy EG for YG and GBG was 0.37–2.02 MJ/m2 of the same order of magnitude as the fracture energy estimated for natural earthquakes. Empirical equations describing the changes in μp, μss and Dc in terms of σn and v indicate that EG decreases with increasing σn, so that higher-normal stress experiments will lead to EG lower than seismic values. Thus our experiments probably reproduced deeper seismic fault motion under dry conditions with respect to EG. Frictional behavior of BG is characterized by a sharp initial weakening, subsequent strengthening to the second peak friction, and slip weakening towards the steady-state friction. The initial weakening and strengthening seem to be caused by the formation of a very sharp slip zone (~ 5 μm in width) and subsequent formation of a widely deformed zone. A comparison of our data with reported results for other outcrops indicates that the high-velocity frictional properties of the Longmenshan fault system may be quite uniform. The observed dramatic high-velocity weakening must have promoted dynamic rupture propagation during the Wenchuan earthquake.