ScienceWeek

GEOPHYSICS: EARTHQUAKES AND MATERIAL RUPTURE

The following points are made by Mitiyasu Ohnaka (Science 2004 303:1788):

1) Rupture of materials is observed over a very broad range of size scales, from atoms to macroscopic objects. Indeed, we often come across the fracture of fragile bodies such as glass in everyday life. However, among the most well-known, large-scale examples are earthquakes, which are caused when a fault in the Earth ruptures. The brittle layer where earthquakes are generated and the individual faults therein are characterized by inhomogeneity. It is therefore critically important to unravel how and where an earthquake rupture nucleates in terms of the underlying physics and seismogenic fault structure. There needs to be an understanding of how a rupture develops spontaneously at accelerating speeds, finally reaching a steady high speed close to elastic-wave velocities. With this information, the hope is to form a comprehensive and integrated picture of the process leading up to a large earthquake in an environment conducive to seismic activity.

2) Xia et al (1) demonstrate how laboratory analysis can be used to understand some of the details of rupture in seismic events. In their work, Xia et al (1) used a unique, well-designed apparatus to observe the rupture that propagates along the frictionally held interfaces of a material under uniaxial compression, which simulates a preexisting fault in Earth's crust. A high-speed camera was used to capture images of the rupture zone after the rupture was explosively triggered. The stress on the sample was visually observed through circular polarizers and tracked during the evolution of the rupture. With this setup, they report a transition of rupture propagation at or below Rayleigh wave speeds to a much faster mode called "supershear propagation". But to understand how this can occur, the basic phenomenon of rupture needs to be understood.

3) Seismological observations and their analyses (2-4) commonly reveal that individual faults are heterogeneous, and include areas called "asperities" or "barriers". The presence of such asperities demonstrates that real faults comprise strong portions of high resistance to rupture growth, with the rest of the fault having low (or little) resistance to rupture growth. The resistance to rupture growth has a specific physical meaning in the framework of fracture mechanics: It is defined as the energy required for the rupture front to further grow. It has been shown that some of the asperities on an earthquake fault are strong enough to equal the strength of intact rock (5) (where shear fracture strength is the highest value of frictional strength).

4) Such strong portions of high resistance to rupture growth are required for an adequate elastic strain energy to build up in the elastic medium surrounding the fault as a driving force to bring about a large earthquake. It thus follows that the earthquake rupture is not a simple process of frictional slip failure, but a mixture of frictional slip failure and the fracture of initially intact rock such as an asperity fracture. Accordingly, the governing law for earthquake ruptures must be formulated as a unifying constitutive law (a relation between the shear traction and the slip displacement) that governs both frictional slip failure and the shear fracture of intact rock.

References (abridged):

1. K. Xia, A. J. Rozakis, H. Kanamori, Science 303, 1859 (2004)

2. K. Aki, J. Geophys. Res. Solid Earth 84, 6140 (1979)

3. H. Kanamori, G. S. Stewart, J. Geophys. Res. Solid Earth 83, 3427 (1978)

4. M. Bouchon, J. Geophys. Res. Solid Earth 102, 11731 (1997)

5. M. Ohnaka, J. Geophys. Res. Solid Earth 108 (B2), 2080, 10.1029/2000JB000123 (2003)

Science http://www.sciencemag.org

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GEOPHYSICS: ON RESISTANCE TO SLIP FAULTS DURING EARTHQUAKES

The following points are made by G. Di Toro et al (Nature 2004 427:436):

1) An important unsolved problem in earthquake mechanics is to determine the resistance to slip on faults in the Earth's crust during earthquakes(1). Knowledge of coseismic slip resistance is critical for understanding the magnitude of shear-stress reduction and hence the near-fault acceleration that can occur during earthquakes, which affects the amount of damage that earthquakes are capable of causing. In particular, a long-unresolved problem is the apparently low strength of major faults(2-5), which may be caused by low coseismic frictional resistance(3). The frictional properties of rocks at slip velocities up to 3 mm/s and for slip displacements characteristic of large earthquakes have been recently simulated under laboratory conditions.

2) There are significant experimental difficulties in determining the resistance to slip on faults during earthquakes. Laboratory experiments need to combine the high slip velocities (0.1 to 2 m/s), large slip displacements (0 to 10 m) and high normal stresses (>50 MPa) that might be necessary to activate dynamic fault weakening mechanisms operative during earthquakes. All existing laboratory friction data satisfy at most two of these three criteria. Values of the coefficient of friction for most rocks are relatively high over a wide range of normal stress; values of 0.6 to 0.85 are found when the slip velocity and displacement are 1 mm/s and <1 mm, respectively. This high friction at ambient normal stresses in the Earth's continental crust is consistent with the magnitudes of shear stresses measured in the crust.

3) However, several mechanisms have been proposed that could lower shear resistance during fast coseismic slip, such as shear melting, pore fluid pressurization(3), normal interface vibrations, acoustic fluidization, and elastohydrodynamic lubrication. Given all of these potential dynamic weakening mechanisms, it seems quite plausible that resistance during earthquake slip might be lower than implied by the high values of friction measured at slow slip velocities. Nevertheless, these mechanisms and/or their applicability to earthquakes are still poorly understood; thus resistance to slip during earthquakes is still unknown.

4) In summary: The authors report data on quartz rocks that indicate an extraordinary progressive decrease in frictional resistance with increasing slip velocity above 1 mm/s. This reduction extrapolates to zero friction at seismic slip rates of 1 m/s, and appears to be due to the formation of a thin layer of silica gel on the fault surface: it may explain the low strength of major faults during earthquakes.

References (abridged):

1. Kanamori, H. Mechanics of earthquakes. Annu. Rev. Earth Planet. Sci. 22, 207-237 (1994)

2. Brune, J. N., Henyey, T. L. & Roy, R. F. Heat flow, stress, and rate of slip along the San Andreas fault, California. J. Geophys. Res. 74, 3821-3827 (1969)

3. Lachenbruch, A. H. Frictional heating, fluid pressure, and the resistance to fault motion. J. Geophys. Res. 85, 6249-6272 (1980)

4. Mount, V. S. & Suppe, J. State of stress near the San Andreas fault: Implications for wrench tectonics. Geology 15, 1143-1146 (1987)

5. Zoback, M. D. et al. New evidence on the state of stress of the San Andreas fault system. Science 238, 1105-1111 (1987)

Nature http://www.nature.com/nature

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GEOPHYSICS: EARTHQUAKES AND TECTONIC FAULT DYNAMICS

The following points are made by Chris Marone (Nature 2004 427:405):

1) For several decades now, geophysicists have been trying to understand why the energy budget for tectonic faulting does not seem to add up. The problem is that faults appear to be more slippery -- less constrained by friction -- than has been predicted by laboratory and theoretical work. The measurements of rock friction described by Di Toro and colleagues(1), may put things on firmer ground. They demonstrate that friction of quartz-rich rock is indeed high at low slip rates, consistent with previous studies, but that under certain conditions it drops dramatically as slip velocity approaches a few millimeters per second.

2) Conventional wisdom is that missing expenses are the cause of the imbalance in the faulting budget. On the income side of this budget are the driving forces of plate tectonics and the elastic energy stored in Earth's crust. Laboratory measurements of fault-zone friction indicate that the frictional stress during faulting near Earth's surface should be of the order of 50 to 100 megapascals. This implies that substantial frictional heat is produced during faulting, because the other main energy expenses -- radiation of seismic waves, and the creation of surface area from the production and comminution of "wear material" -- are thought to account for only a small fraction of energy dissipation. The problem is that the expected frictional heat is missing(2,3).

3) To study the strength of frictional contact between rock surfaces, Di Toro et al(1) used an apparatus that applies rotary shear to the samples. The apparatus allows only comparatively small unidirectional movement, so the authors sheared samples back and forth to achieve the large net displacements that occur at earthquake faults. As in previous experiments in geophysical rock mechanics, they sheared samples under the high stresses expected to apply at tectonic faults.

4) Consistent with existing data, Di Toro et al(1) found that the coefficient of friction was 0.6-0.7 at low sliding velocities (up to 1 mm/s). However, their experiments show that the coefficient for novaculite -- a rock composed of silicon dioxide (quartz) --decreases dramatically to values as low as 0.2 when the shearing velocity exceeds 1 to 10 mm/s. This effect is transient. On returning to lower sliding velocity, the coefficient of friction returns to high values. Identical experiments on samples of granite did not show the same reduced friction (or "weakening") at high speed. So the authors suggest that the weakening mechanism is related to the formation of a thin layer of silica gel, which acts as grease between the surfaces.(4,5)

References (abridged):

1. Di Toro, G., Goldsby, D. L. & Tullis, T. E. Nature 427, 436 439 (2004)

2. Saffer, D. M., Bekins, B. A. & Hickman, S. J. Geophys. Res. 108, doi:10.1029/2002JB001849 (2003)

3. Scholz, C. H. Geology 28, 163 166 (2000)

4. Spray, J. G. J. Geophys. Res. 98, 8053 8068 (1993)

5. Mair, K. & Marone, C. J. Geophys. Res. 104, 28899 28914 (1999)

Nature http://www.nature.com/nature

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