Evidence for strain build up along faults in southern California

In the centenary year of the 1906 San Francisco earthquake a lot of attention has been paid to the northern part of the infamous San Andreas Fault. That avoids the fact the its southerly extension to the south-east of Los Angeles has not ruptured in a devastating way for at least 250 years. Faults break after protracted build-up of elastic strain. Such strains are detectable using data from spaceborne radar systems. These have been available since 1992 from the European Space Agency ERS-1 and ERS-2 satellites. A sequence of data sets provides information about the annual rate of deformation (Fialko Y. 2006. Interseismic strain accumulation and the earthquake potential on the southern San Andreas fault system. Nature, v. 441, p. 968-971). Fialko shows that the parallel San Andreas and San Jacinto faults near the Salton Sea are building up strain at about 3 cm per year, so that about 7 to 10 metres will have accumulated since the last major earthquake in that part of the system. This exceeds the largest known seismic movement on the system, thereby suggesting that Los Angeles is likely to experience a ‘big one’ shortly.

Supervolcanoes

Outside of a major meteorite impact, the greatest danger posed by geological processes is a monster volcanic eruption. As well as the close-by effects of massive debris avalanches and ash falls, explosive eruptions blast sulphur gases into the stratosphere where they reside for a long time as sulphuric acid aerosols. Clouds of these tiny particles reflect a proportion of solar radiation back into space and so cause global cooling. The eruptions of Pinatubo and Krakatau in recent historic times did just that, as have several others with more devastating global effects such as famine. Yet these are tiny compared with eruptions known from the recent geological past that are marked by ash deposits over vast areas. About 71 ka ago, Toba in Indonesia  blasted out a 30 by 100 km caldera and its ash extends across much of south Asia and surrounding ocean floors. Genetic evidence from human Y-chromosomes suggests a massive decline in human numbers at the time, to create an evolutionary bottleneck. This near-extinction may have been connected in some way to eruption of the Toba supervolcano. Such events are a more likely risk than impacts, and a recent review of research into them highlights those that are well-known (Bindeman, I.N. 2006. The secrets of supervolcanoes. Scientific American, v. 294 (June 2006 issue), p. 26-33). The western USA has two potential threats: calderas in Yellowstone National Park and Long Valley. Between 760 and 640 ka both exploded to blanket the whole southern USA and northern Mexico with around 1000 cubic kilometres of ash. Bindeman’s own research sheds light on the details of magma evolution during such eruptions using isotopic signals in zircons contained within ash deposits..

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