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ScienceWeek
EARTH SCIENCE; EARTH'S CRUST AND EARTHQUAKES
The following points are made by Robert E. Holdsworth (Science 2004 303:181):
1) When Earth's crust deforms, earthquakes can be triggered by displacements along fault zones. Such fault zones can be sites of repeated deformation for hundreds of millions of years, suggesting persistent weakness relative to the adjacent crust. In addition, data on surface heat flow and stress orientations around crustal-scale fault zones such as the San Andreas fault in California indicate that they are much weaker than expected from laboratory models of friction. Yet despite 30 years of research, the causes of the inferred weakness -- and even the existence --remain controversial (1,2).
2) One problem is that much of the geophysical data cited as evidence for weakening lacks sufficient resolution to determine its underlying mechanical causes. Recently, additional insights have been gained from geological studies of ancient fault cores exposed at the surface, and from laboratory experiments on the deformation of materials that closely resemble fault rocks. These studies reveal that weakening originates at the scale of individual grains in mid-crustal fault networks whose natural interconnectivity then helps to transmit such effects to larger scales.
3) Two classes of continental faults are particularly likely to be weak over long time scales: low-angle normal faults (3) and reactivated faults (4). The ancient deep roots of these structures are often exhumed and exposed at the surface by major periods of tectonic activity. Assuming that the ancient examples are representative of what happens at depth today, direct study of weakening mechanisms operating within these faults may be possible, especially where they cut through the main load-bearing region between 5 and 15 km, close to the brittle-plastic transition (5). Ancient exposed mid-crustal fault cores preserve strikingly similar evidence for grain- and aggregate-scale weakening processes.
4) In the core of the fault where strain is high, "cataclastic" textures indicative of grain-scale brittle crushing are overprinted by and smeared out into low-temperature foliations or mineral alignments suggesting ductile flow. This results from pervasive fluid influx that alters the load-bearing mineral phases. The alteration produces fine-grained aggregates of weak, platy minerals (micas, clays) that align to form an interconnected network. Fluid influx also triggers widespread stress-induced dissolution-precipitation deformation mechanisms in the preexisting, finely crushed fault rocks. The permanent weakening effect caused by all these processes is vividly illustrated by the preferential localization of most subsequent displacements into the foliated fault cores.
References (abridged):
1. C. H. Scholz, Nature 381, 556 (1996)
2. M. D. Zoback, Nature 405, 31 (2000)
3. G. J. Axen, Geophys. Res. Lett. 26, 3693 (1999)
4. R. E. Holdsworth, M. Stewart, J. Imber, R. A. Strachan, Spec. Pub. Geol. Soc. London 184, 115 (2001)
5. D. L. Kohlstedt, B. Evans, S. J. Mackwell, J. Geophys. Res. 100, 17587 (1995)
Science http://www.sciencemag.org
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GEOPHYSICS: FAULTS, EARTHQUAKES, AND PLATE MOTION
The following points are made by Craig H. Jones (Science 2003 300:1105):
1) In early 1983, geologists confidently held that the main seismic hazard in California came from faults exposed at the surface: faults like the San Andreas that accommodate the Pacific plate sliding past the North American plate. But later that year, they were in for a surprise. The 1983 earthquake in Coalinga, California, was the first in a spate of earthquakes that showed considerable hazard from faults that were not slipping parallel to Pacific-North American plate motion.
2) Slip on these other faults, which in short order produced the Whittier Narrows, Northridge, and Loma Prieta earthquakes, shortens California nearly perpendicular to the big San Andreas fault. A more complicated case exists in Alaska, where large strike-slip ("sliding-past") faults exist inland of and east from the subduction zone that was responsible for the magnitude (M) = 9.2 "Good Friday" 1964 earthquake. One such fault, the Denali Fault, produced the M = 7.9 earthquake on 3 November 2002.
3) It is now clear that adjacent parallel faults slip in different directions at many plate boundaries where the two plates both move toward one another and slide past one another. Termed "slip partitioning", it is the tendency for deformation to divide the motion between two (or more) faults, with one accommodating the horizontal motion and the other the convergent motion.
Science http://www.sciencemag.org
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Notes:
Seismic studies indicate the interior of the Earth consists of three parts: a metallic core, a dense rocky mantle, and a thin low-density crust. The central part of the core is solid, but the outer part of the core is evidently liquid. The mantle, the layer of dense rock and metal oxides between the molten part of the core and the surface, has plastic properties (i.e., it is a solid capable of flow under pressure).
The term "lithosphere" refers to the outer layer of the Earth, comprising the crust and upper mantle, and extending to a depth of 50 to 70 kilometers. The traditional view of tectonics (changes in the structure of the Earth's crust) is that the lithosphere consists of a strong brittle layer overlying a weak ductile layer, the system producing two forms of deformation, namely, brittle fracture in the upper layer (accompanied by earthquakes), and aseismic (without earthquakes) ductile flow in the lower layer. The current consensus is that this view is generally correct but imprecise, since the accumulated evidence is now interpreted to indicate that frictional events along fault lines, rather than new fractures, are the causes of earthquakes.
Plate tectonics is the current consensus theory that the Earth's lithosphere is broken into fairly rigid plates, seven major plates and many smaller plates, and that convection within the underlying less rigid "asthenosphere" causes the plates (and the associated continents and crust) to move relative to each other, the movement manifested in continental drift and sea-floor spreading. "Continental drift" is the slow movement of the Earth's land masses, a shifting across the underlying molten material.
"Sea-floor spreading is the process whereby sea floor is continuously created as the crustal plates move apart and continuously destroyed where the plates push against each other.
A "strike-slip fault" is a movement parallel to the fault plane, and the San Andreas fault of California is of this type.
In this context, the term "subduction" refers to the process of underthrusting of the edge of a tectonic plate into the mantle underlying an adjacent plate.
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ON THE PREDICTABILITY OF EARTHQUAKES
The dominant cooling mechanism on Earth is "plate tectonics", which involves the movement of 8 large plates and a few dozen smaller plates, the motion driven from beneath by convection within the mantle. On Earth, plate tectonics concentrates most of the volcanoes, earthquakes, and other tectonic features along plate margins rather than scattering them evenly throughout the crust. The San Andreas fault in California, for example, is a strike-slip fault at the interface between the Pacific plate (an oceanic plate) and the North American plate (a continental plate).
Geophysical faulting is a break in rock structure that occurs when pressures in the planet's crust are strong enough to cause fracture and displacement. A strike-slip fault is a movement parallel to the fault plane, the two plates shifting tangentially in opposite directions along their interface. The 2 other major types of faults are 1) the "normal" fault, which consists of a simple vertical shifting at the interface, one plate moving up and the other down, and 2) the thrust fault, which involves the edge of one plate sliding over (overlapping) the edge of the adjacent plate.
The term "dilatancy" refers in general to an increase in the volume of a rock deformed by pressure, the increase in volume caused by the expansion and extension of small cracks within the rock. The effect can be detected in strained rocks just before an earthquake, and is the basis of one type of earthquake prediction.
The following points are made by C.G. Sammis and D. Sornette (Proc. Nat. Acad. Sci. 2002 99:2501):
1) Are earthquakes predictable? The answer, of course, depends on what is meant by a "prediction". In the broadest sense, the plate tectonics paradigm makes predictions. It predicts that earthquakes are far more likely to occur at the boundaries between plates than within their interiors. Actually, plate tectonics theory was in part based on this "in-sample" observation, which is verified continuously "out-of-sample." It also predicts an overall rate to the process. Averaged over time, the summed moments of the earthquakes is consistent with the relative motion between the plates determined from the analysis of magnetic anomalies, correcting for aseismic visco-plastic deformations.
2) The forecasting of individual large events has been more problematical. Although the paleoseismological dating of large prehistoric earthquakes has confirmed the plate tectonics hypothesis, the timing between individual events is extremely erratic. For example, the average recurrence interval for the last ten large earthquakes on the San Andreas Fault north of Los Angeles is approximately 132 years. Because the long-term slip rate on the Southern San Andreas fault is approximately 3 centimeters per year, this corresponds to an average displacement of approximately 4 meters per large earthquake -- a very reasonable value. The problem is that the intervals between events range from 44 to 332 years. This lack of quasiperiodicity in large events is also evident in other predictions, and observations have dimmed the hope that large earthquakes can be forecast based solely on the past history of large events on the same fault.
3) An alternative forecasting strategy is based on physical precursors observed to occur just before macroscopic failure in the laboratory. Most of these precursors are associated with microfracture damage and the associated dilatancy observed to precede the formation of a macroscopic shear failure of rock specimens under compressive loading. These laboratory observations have been incorporated into the "dilatancy-diffusion model" for earthquakes. However, the search for physical precursors before large earthquakes has been disappointing. The high hopes raised by the reports of Chinese success in using physical precursors to forecast the 1975 Haicheng earthquake have dissipated with the worldwide failure to produce additional valid predictions.
4) In the US, current work on earthquake prediction is primarily based on the search for precursors to large events in the seismicity itself. One motivation comes from a statistical physics interpretation of regional seismicity as being characteristic of a system at or near a statistically stationary dynamical critical point dubbed "self-organized criticality". Such a self-organized critical state is characterized by power law distributions of event sizes and long-range spatial correlation of fluctuations around the statistically stationary state. Because earthquakes are indeed characterized by several power laws, the application of this concept of self-organized criticality to earthquakes is now often taken for granted in the seismological community. However, the implication of this self-organized critical (statistically stationary) state for the predictability of large earthquakes remains controversial.
References (abridged):
1. Sieh, K. , Stuiver, M. & Brillinger, D. (1989) J. Geophys. Res. B 94, 603-623.
2. Bakun, W. H. & McEvilly, T. V. (1984) J. Geophys. Res. B 89, 3051-3058.
3. Nur, A. (1972) Bull. Seismol. Soc. Am. 62, 1217-22.
4. Whitcomb, J. H. , et al (1973) Science 180, 632-641.
5. Scholz, C. H. , Sykes, L. R. & Aggarwal, Y. P. (1973) Science 181, 803-809.
Proc. Nat. Acad. Sci. http://www.pnas.org
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