Abstract
Weak materials in seismic slip zones are important in studies of earthquake mechanics. For instance, the exceptionally large slip during the 2011 Tohoku-Oki earthquake has been attributed to the presence of smectite in the fault zone. However, weak fault rocks cannot accumulate large amounts of elastic strain, which is thought to counter their ability to enhance seismic rupture. It is well known that if the permeability of a weak fault is low enough to allow friction-induced thermal pressurization of interstitial fluid, the fault strength decreases dramatically. However, whether intrinsic weakness of fault material or thermal pressurization more efficiently produces large slip on faults bearing weak materials has not been determined. To investigate the role of weak materials in earthquake rupture dynamics, we conducted friction experiments and dynamic rupture simulations using pure smectite and pure graphite to represent weak fault materials. Even when we assumed no thermal pressurization, simulated faults in both media were able to trigger large slip because their extremely low friction was insufficient to arrest the inertial motion of rupture propagating along the fault. We used similar rupture simulations to investigate the cause of the huge slip near the trench during the 2011 Tohoku-Oki earthquake and demonstrated that it can be attributed to thermal pressurization, although our findings suggest that the presence of smectite in the plate-boundary fault may also be required.
Highlights
Our knowledge of the frictional strength of faults, which is the predominant control of rupture dynamics and earthquake magnitude, has benefited from almost half a century of laboratory experiments on rocks[1,2]
We investigated the role of weak materials in earthquake rupture dynamics
To investigate a smectite-bearing slip zone we numerically modelled a plate-boundary thrust in the Japan Trench (Fig. 1a–d)
Summary
Weak materials in seismic slip zones are important in studies of earthquake mechanics. Our knowledge of the frictional strength of faults, which is the predominant control of rupture dynamics and earthquake magnitude, has benefited from almost half a century of laboratory experiments on rocks[1,2] Because weak materials such as phyllosilicates and graphite have low frictional resistance[3,4], their presence is thought to weaken faults in the brittle regime[5,6,7] and to account for fault creep such as that observed along the San Andreas Fault in California[6,7]. The surrounding host rocks showed a maze-like TEM pattern and no graphite XRD peak (Fig. 1j,k) Both graphite-bearing and smectite-bearing slip zones may play important roles during both subduction and inland earthquakes
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