May 2008 GEOLOGY media highlights

The sea-level fingerprint of the 8.2 ka climate event
Roblyn Kendall et al.; Jerry X. Mitrovica, University of Toronto, Physics, 60 St. George St., Toronto, ON M5S 1A7, Canada. Pages 423-426.

8200 years ago, Earth experienced a cooling event that was the most abrupt, widespread climate instability of the last 10,000 years. There is general agreement that the event was triggered by the release of fresh water into the North Atlantic associated with the catastrophic drainage of glacial Lakes Agassiz and Ojibway, with a resulting disruption in ocean circulation. The event has received significant attention because it occurred during a period in which the climate was otherwise similar to the present-day; therefore, it may provide clues as to how sensitive the modern ocean/climate system is to the melting of ice reservoirs associated with global warming. Unfortunately, the volume of the ancient Lakes Agassiz and Ojibway is uncertain. Could sea-level records provide a robust measure of this volume? Kendall et al. show that the drainage of these lakes produced a highly distinct fingerprint in sea level. For example, their analysis demonstrates that sites relatively close to the lakes, including the west and Gulf coasts of the United States, had small signals due to the lake release, and thus may not be optimal field sites for constraining the outflow volume. Other sites, such as the east coast of South America and western Africa, have significantly larger signals associated with the lake release, and are thus better choices in this regard.

Stress-forecasting (not predicting) earthquakes: A paradigm shift?
Stuart Crampin et al., Professor of Seismic Anisotropy, Shear-Wave Analysis Group, School of GeoSciences, The University of Edinburgh, Grant Institute, West Mains Road, Edinburgh EH9 3JW, Scotland, UK. Pages 427-430.

A paradigm shift has been suggested before earthquakes can be predicted. Seismic shear-wave splitting monitors fluid/rock deformation of the most compliant elements of in situ rock: fluid-saturated stress-aligned microcracks in almost all crustal rocks. Because rocks are vulnerable to shear stress, stress before large earthquakes has to accumulate over enormous volumes of rock, in order for shear-wave splitting to monitor the effects on the microcracks at substantial distances from impending epicenters. The paradigm shift is to ignore the source zone that is locally heterogeneous and complicated by initial conditions and instead monitor stress accumulation away from the source. Using small earthquakes as the source of shear waves, characteristic effects have been observed with hindsight before 14 earthquakes worldwide (M 1.7 to M 7.7), and on one occasion, when changes were recognized early enough, the time, magnitude, and fault-break of an M 5 earthquake in southwest Iceland were successfully stress-forecast in a narrow time-magnitude window. The key effect is that the various effects of the observed deformation are logarithmically proportional (self-similar) to earthquake magnitudes, just like the Gutenberg-Richter relationship. Using controlled-source cross-hole seismics, Crampin et al. suggest techniques or routine forecasting.

Review abstracts for these articles at http://www.gsajournals.org/perlserv/?request=index-html&issn=0091-7613.

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