Prediction of earthquake periodicity founders

In an number of well-studied areas of chronic seismicity it appears from historical records that earthquakes recur with regularity.  If that was so, it might be possible at least to prepare to throw many methods of detecting imminent movements at such areas, when they are “due” to go off.  The theory behind time-predictability is that earthquakes relieve tectonic stresses along faults, and that if the forces are maintained, stress builds up again, to be released after a roughly fixed time (the same might apply to volcanism where magma production stays constant).  A corollary is that high-magnitude events have longer periodicities than those lower on the Richter scale.  One of the best cases thought to support this view is a 25-km stretch of the San Andreas Fault near Parkfield in California.  The area has had  5 or 6 earthquakes greater than magnitude 6 since 1857, roughly every 22 years, the last being in 1966.  There ought to have been one in 1988, but the poor statistics give an uncertainty either way of 10 years.  By now there should have been a magnitude-6 event in the area, but it hasn’t happened.  Jessica Murray and Paul Segall of Stanford University have analyzed the physics of the last event, and of the period that followed it.  (Murray, J. & Segall, P 2002.  Testing time-predictable earthquake recurrence by direct measurement of strain accumulation and release.  Nature, v.  419, p. 287-291).

Their work involved using precise geodetic measurements obtained over the last four decades to assess the 1966 Parkfield earthquake’s size, which combines the movement then along the San Andreas Fault, the area involved in the slip and how “stiff” the crust is locally.  Comparing this with geodetic data since then suggests strongly that the strain released in 1966 must have recovered between 1973 and 1987.  They have shown that another Parkfield earthquake is long overdue.  Their method rigorously allows for the effects of movements along other nearby fault, and inherent unpredictability seems inescapable.  While other tests of the time-predictability principle, theoretically the most plausible approach, will continue, most devastating earthquakes continue to occur without forewarning.  That reflects the fact that there are only enough seismologists with fancy equipment to cover threatened areas in a few extremely rich countries.  Most people who live along active fault zones know whether or not high-magnitude earthquakes occur in their vicinity, yet will not have the privilege of scientists and equipment to provide warnings of this kind for a very long time, for simple economic reasons.  Perhaps some effort and funds should be diverted to providing warnings within days of a serious event, using less “robust” methods.

See also:  Stein, R.S. 2002.  Parkfield’s unfulfilled promise. Nature, v.  419, p. 257-258.

British Geological Survey sued over arsenic

The world’s largest ever class action has been launched in London against the British Geological Survey, over claims that it failed to spot arsenic contamination during a 1992water survey in Bangladesh. As many as 40 million Bengalis risk arsenic poisoning, following a major groundwater development programme in the 1970s and 80s.  Arsenic poisoning at non-fatal doses often shows first as water blisters on hands and shins. Long-term exposure via drinking-water causes cancer of the skin, lungs, urinary bladder, and kidney.

Aid agencies, led by UNICEF sank four million wells deep into alluvium, in the hope that groundwater use would alleviate the chronic problem of heavily polluted surface water in Bangladesh.  The arsenic is of natural origin, and stems from leaching of the toxic element from sulphide minerals by deep, reducing waters.  The case hinges on BGS’ failing to test for arsenic, which is easily detected using low-cost semi-quantitative methods, only 3 years after they had completed a comprehensive evaluation of groundwater quality in Britain that did include arsenic measurements.  Accusations of double standards have been flying.  However, UNICEF also failed to test for arsenic during the original drilling, because they did not expect to find it in the water.  World Health Organization guidelines are very clear that arsenic does pose a threat in groundwater, but most cases in the past have been associated with former mining areas.

Considerable work on measures to clean up well water has been conducted since the Bengal arsenic crisis surfaced.  Under oxidizing conditions, arsenic is adsorbed by ferric hydroxide, and a simple remedy is passing the water through iron wool or over ground-up rust or natural ochres.


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