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ScienceWeek
EARTH SCIENCE: ON THE GREAT LISBON EARTHQUAKE
The following points are made by Marc-Andre Gutscher (Science 2004 305:1247):
1) On 1 November 1755, as worshipers in Portugal and southwestern Spain were gathered for mass on All Saint's Day, a tremendous earthquake struck, toppling many churches and killing approximately 60,000 people (1,2). Many churchgoers were killed, sparking a lively debate among philosophers about divine justice. Recent studies have shed light on what caused the earthquake and what the seismic future of the region may be.
2) The Great Lisbon earthquake had an estimated magnitude (M) 8.7. It triggered a 5- to 10-m-high tsunami and caused many casualties in Europe and northwestern Morocco (2). In this region, the African plate pushes toward the northwest against southern Iberia at a rate of 4 mm/year. But the plate boundary off southern Iberia is not well defined (3), and the source of the Great Lisbon earthquake has remained elusive (2). Indeed, it has been difficult to find a simple plate-tectonic model that explains all geological observations in the region (4,5).
3) During the past 15 million years, crustal thinning and extension have produced a deep marine basin in the West Alboran Sea (western Mediterranean), while shortening and thrusting continued in the horseshoe-shaped Betic and Rif mountain belts(4,5). A popular model concluded that this region was a prime example of "delamination" (breaking off of a deep mantle root following continental collision) (4). However, new data increasingly support eastward subduction beneath the Straits of Gibraltar(5). Tomographic cross sections of the Earth show cold, dense material -- a slab of oceanic lithosphere -- descending from the surface to depths of nearly 700 km. The chemistry of 15-to 5-million-year-old volcanoes in the Alboran Sea shows that they were formed in an arc setting (like that of arcuate island chains in the West Pacific landward of the subduction zone).
4) Overall, the movement of tectonic blocks in the southern Iberia region is best explained by a model of slab retreat (roll-back) during subduction, causing extension in the region behind the subduction zone (5). The southeastern limit of deformation in this back-arc region appears to be a major north-east trending strike-slip fault across the West Alboran Sea. This fault emerges on land in northeast Morocco, right where the Al Hoceima earthquake (M = 6.3) struck on 24 February 2004, causing nearly 600 deaths.
5) One big question remains. Is the subduction system still active, and does it pose a seismic risk? New evidence supports continued activity. Numerous active mud volcanoes have been identified and sampled in the Gulf of Cadiz. These features indicate ongoing dewatering processes, which are widespread in accretionary wedges (compressed sediment piles formed at subduction zones, like piles of dirt in front of a bulldozer). Marine seismic data indicate active folding and thrusting of the youngest sediments (which are a few thousand years old) at the outermost edge of this accretionary wedge. Marine heat flow data are also indicative of active subduction.
References (abridged):
1. J. M. Martinez-Solares, A. Lopez, J. Mezcua, Tectonophysics 53, 301 (1979)
2. M. A. Baptista et al., J. Geodyn. 25, 159 (1998)
3. I. Jimenez-Munt, M. Fernandez, M. Torne, P. Bird, Earth Planet. Sci. Lett. 192, 175 (2001)
4. J. P. Platt, R. L. M. Vissers, Geology 17, 540 (1989)
5. L. Lonergan, N. White, Tectonics 16, 504 (1997)
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 by ScienceWeek:
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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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)
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