At 5.12am Pacific time on 18 April, 1906, an earthquake with a Modified Mercalli Intensity (MMI) scale of VII to IX swept throughout the entire San Francisco Bay area. The earthquake was felt over an area of nearly 321,000 square kilometres – from southern Oregon to south of Los Angeles and inland as far as central Nevada.
Survivor P Barrett recalled:
It was as if the earth was slipping gently from under our feet. Then came the sickening swaying of the earth that threw us flat upon our faces. We struggled in the street. We could not get on our feet. Then it seemed as though my head were split with the roar that crashed into my ears. Big buildings were crumbling as one might crush a biscuit in one’s hand. Ahead of me a great cornice crushed a man as if he were a maggot – a labourer in overalls on his way to the Union Iron Works with a dinner pail on his arm.
Fires broke out in the city almost immediately and burned for days, incinerating buildings across more than 500 blocks in the heart of the city. It is estimated more than 3000 deaths were caused directly or indirectly by the catastrophe, and in excess of 28,000 buildings were destroyed.
{{image2-a:r-w:300}}In ancient times, earthquakes were believed to be the work of gods, specifically Poseidon, the Olympian deity of the sea, earthquakes, storms and horses. Today we know better.
The current prevailing theory, in a nutshell, is that an earthquake is caused by a sudden slip along a geologic fault. Stresses in the earth’s crust push the sides of a fault together. Stress builds up and the rocks slip past one another. Energy is released in waves that travel through the crust and cause the shaking felt during an earthquake.
There is no doubt earthquakes can cause tremendous damage and loss of life. Ironically, earthquakes also create a major source of wealth – namely, gold.
Small cavities exist along fault zones deep within the earth’s crust. These cavities are filled with super-heated fluids rich in dissolved quartz, other minerals and metals. As rocks along a fault slip during a major earthquake, these tiny cavities expand, resulting in a rapid pressure drop.
The pressure drop is sufficiently large that super-heated liquid in the cavities instantly vaporises. The quartz, other associated minerals and metals (including gold) in the now supersaturated water crystallise in a process called flash deposition.
A single earthquake can produce an instant, albeit tiny, gold vein. However, there can be hundreds of thousands of tiny earthquakes (fault movements) every year in a single fault system. Over hundreds of thousands of years there is the potential to precipitate very large quantities of gold. Theoretically, typical rates of seismicity along the San Andreas Fault zone could generate a spider’s web of gold containing almost 100 tonnes of gold in less than 100,000 years.
For the exploration geologist, it is not about where the plate boundaries are now – it’s about where they were hundreds of millions of years ago. Knowing the location of ancient plate boundary faults helps geologists predict where regional mineralisation may occur today.
No amount of gold can make up for the loss of life, pain and suffering caused by earthquakes. However, the more scientists learn about earthquakes and the more seismic-aware society becomes, the better prepared people will be to minimise the effects of an earthquake.