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ScienceWeek
SEISMOLOGY: ON PREDICTING THE FINAL SIZE OF EARTHQUAKES
The following points are made by Rachel Abercrombie (Nature 2005 438:171):
1) How does a seismic fault, initially essentially immobile, start to slip at speeds of meters per second as an earthquake rupture front runs along it at speeds of up to 3 kilometers per second? Does the eventual size of an earthquake depend on the nature of this process? Or do all earthquakes begin in the same way, with the extent of rupture determined by conditions along the fault? Such fundamental questions get seismologists talking, because knowing how earthquakes begin is an essential part of understanding and modelling the dynamics of earthquake rupture, and may allow an earthquake's course to be predicted. Research until now has been inconclusive, but results described by new work[1] imply that the final magnitude of an earthquake depends at least partially on what happens in its first few seconds. This timescale is equivalent to less than a tenth of the duration of the larger earthquakes in the study.
2) Research into the onset of earthquakes large and small has found that they often begin with small-amplitude shaking[2]. The interpretation of these initial "sub-events" remains controversial. One model has it that a small, isolated sub-event triggers a larger fault patch, which itself triggers further fault patches, and so on as long as sufficient energy is available. In this "cascade" model, the beginning of a large earthquake is no different from the beginning of a small earthquake: therefore, predicting the final magnitude from the first few seconds is impossible. An alternative model is that the small beginning is the last phase of some longer, slower, sub-seismic "nucleation" process. (Such a process has admittedly never been reliably observed[3].) In this case, the final magnitude would be related to the nature of the nucleation process, and seismograms of large earthquakes would look different from those of smaller earthquakes right from the start.
3) Early warning systems currently in operation in Japan, Taiwan and Mexico use observations of the earliest-arriving primary (P) waves to provide a few seconds' warning of subsequent large ground motion -- secondary (S) and surface waves -- produced by the same earthquake. In an earlier study[4], Allen and Kanamori investigated the first few seconds of earthquake seismograms in southern California. They found that the predominant period (a measure of the frequency) for the first 4 seconds of the P waves provides a good estimate of the size of earthquakes with a magnitude M of less than 6. The duration of such earthquakes, defined as the time during which the fault actually moves, is usually less than 4 seconds (the waves generated by an earthquake last for much longer than the earthquake itself). However, Allen and Kanamori's method[4] also predicted the approximate magnitude of three earthquakes of M greater than 6, and so an earthquake duration of more than 4 seconds. In other words, the final size of the earthquake could be predicted before the fault stopped moving.
4) Olson and Allen[1] set out to determine whether the final magnitude of the earthquake really does depend on the predominant period of the onset. They investigated the first few seconds of 71 earthquakes from California, Alaska, Japan and Taiwan, each recorded at multiple stations within 100 kilometers of the epicenters. Twenty-four of the earthquakes had a magnitude larger than 6, with durations of up to 70 seconds. Estimating the predominant period of the radiated seismic energy for each earthquake, the authors find that this value increases with magnitude for earthquakes of M between 3 and 8. This finding applies even to larger earthquakes in which the measurement is made after as little as a tenth of the earthquake's total duration -- suggesting that the final magnitude of an earthquake is indeed determined a very short time after onset.[5]
References (abridged):
1. Olson, E. L. & Allen, R. M. Nature 438, 212 215 (2005)
2. Ellsworth, W. L. & Beroza, G. C. Science 268, 851 855 (1995)
3. Bakun, W. H. et al. Nature 437, 969 974 (2005)
4. Allen, R. M. & Kanamori, H. Science 300, 786 789 (2003)
5. Nakamura, Y. in Proc. 13th World Conf. Earthquake Eng. Pap. No. 908 (2004)
Nature http://www.nature.com/nature
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Related Material:
GEOPHYSICS: ON MEGATHRUST EARTHQUAKES
The following points are made by A. Hirn and M. Laigle (Science 2004 305:1917):
1) During an earthquake, rupture propagates along the fault plane within a few tens of seconds. Much slower rupture, lasting for weeks or months, has recently been observed in slip transients or slow earthquakes (1,2). These events are also dubbed "silent earthquakes", because seismometers cannot sense any seismic waves during rupture. Silent earthquakes share their source region with that of low-frequency seismic waves (3-5), akin to the seismic tremor known to occur in volcanoes, where it is attributed to fluids trapped in cracks or conduits.
2) Silent earthquakes and seismic tremor do not cause strong, sudden ground motion, and are hence not considered hazardous. However, they occur in subduction zones where 90% of Earth's destructive seismic energy is released in large-magnitude (M greater than 7.0) megathrust earthquakes. Monitoring and interpreting such events may improve our understanding of the stress build-up in subduction zones and help in forecasting large future earthquakes. The documented examples of this activity are in regions where megathrust events are expected: the Nankai subduction zone in Japan and, most recently, the Cascadia subduction zone in the Pacific, off Washington state and western Canada.
3) In Japan, low-noise seismometer arrays have discovered deep nonvolcanic seismic tremor in the Nankai subduction zone, where at least nine great (M greater than 8.0) earthquake sequences have occurred in the historical record at intervals of one or two centuries, with devastating consequences. The tremor is attributed to water that has been liberated by metamorphism of the subducting Philippine sea plate and is trapped under the forearc crust (3). Intraslab earthquakes have been linked to such metamorphism. Seismic exploration has also elucidated the interplate fault region and its possible water content. For example, high pore-fluid pressure has been imaged in the Tokai segment and suggested as a cause of the silent earthquake detected there.
4) In the Cascadia subduction zone, a silent earthquake was detected (1) with space-geodetic, Global Positioning System (GPS) arrays, which sense the slow motion of Earth's surface over several hundred kilometers. Seismic tremor occurred in the same time span from sources in the region where the silent earthquake slip occurred. This activity, called "episodic tremor and slip" (ETS), was predicted to recur in Cascadia every 14 months, with the latest event predicted for July 2004. The expected ETS event was observed from 8 to 24 July, with the slip migrating northward from Puget Sound, Washington to Vancouver Island at the northern end of the Cascadia subduction zone. Two significant (M = 5.8 and 6.4) earthquakes were also detected off Vancouver Island. The event was preceded by another, unexpected episode of tremor and slip beginning in late April; this event may have moved southward into northern California and terminated at the southern end of the subducting slab.
References (abridged):
1. H. Dragert, K. Wang, T. S. James, Science 292, 1525 (2001)
2. S. Osawa et al., Science 298, 1009 (2002)
3. K. Obara, Science 296, 1679 (2002)
4. A. Katsumata, N. Kamaya, Geophys. Res. Lett. 10.1029/2002GL015981 (2003)
5. G. Rogers, H. Dragert, Science 300, 1942 (2003)
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.
ScienceWeek http://scienceweek.com
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