Paris - The same powdery mineral used to dust baby's bottoms may be what keeps the capricious San Andreas fault in California from unleashing more major quakes, according to a study released on Tuesday.
The most active and probably the most studied fault it the world, the 1 300km San Andreas has long puzzled geologists.
Its northern and southern sections, which tend to "stick and slip", are prone to occasional but violent quakes, such as the one that killed 3 000 people in 1906 and reduced much of San Francisco to ashes.
But the central section tends to move in tiny increments, resulting in frequent and minor tremors.
This "aseismic creep" flies in the face of what was known about the fault, and the basic mechanics of tectonic masses butting up against each other, in this case Pacific and North American Plates.
A groundbreaking - in every sense of the word - study published in the British journal Nature, however, seems to have solved the mystery: the unexpected presence of talc, the softest of all minerals, three kilometres below the Earth's surface, may act to reduce friction between the continent-sized chunks of the Earth's crust.
"Talc's properties are likely to encourage slow, stable creep, inhibiting the build-up of elastic energy," thus preventing the faster sliding and unstable slip of a major earthquake, writes Christopher Wibberly in a commentary, also published in Nature.
A watery mineral composed of magnesium, silicon and oxygen, talc is not one of the major components of the Earth's crust.
But when present in sufficient quantity - such as detected by geologists Diane Moore and Michael Rymer at the San Andreas Observatory at Depth (SAFOD) near Parkfield - it can, at sufficiently high temperatures, forestall a dangerous build-up of pressure.
Talc has a hardness rating of "1", the lowest possible, while diamonds have a rating of "10". It is held together by weakly-bonded microscopic platelets that slide past one another, giving talc its soft, almost oily feel.
The mineral is used in the production of ceramics, paint, paper and a number of building materials, and is also present is some food and pharmaceutical products.
And then, of course, there's baby powder. - Sapa-AFP