Shock tactics: Dynamiting the San Andreas fault

It's 100 years since San Francisco was destroyed by a huge earthquake. Now, seismic experts are dynamiting the San Andreas fault. It sounds risky, but could save the city from the next 'big one'. Kate Ravilious reports

Tom Brocher likes his explosions. For the past three years, he has been making his way along the San Andreas earthquake fault in California, stopping every so often to drill a deep hole, stuff it with dynamite and then stand back as the rocks fly.

Detonating dynamite here might sound unwise, but Dr Brocher believes his explosions could provide the key to San Francisco residents surviving the next big quake. What's more, this technique could also be used to help reduce damage in other earthquake-prone areas of the world.

The San Andreas fault is an 800-mile-long crack in the earth's crust, where the Pacific Ocean plate grinds past the North American plate. It runs up the West Coast of the United States, passing close to Los Angeles and right through the city of San Francisco.

This fault has been responsible for some devastating earthquakes, including one measuring 7.9 on the Richter scale that razed San Francisco to the ground on 18 April 1906, and the magnitude 6.7 Loma Prieta earthquake that caused freeways and buildings to collapse in 1989. And the fault hasn't finished juddering yet; a study concluded that there is a 62 per cent chance of another major quake in the Bay area in the next 30 years.

So what can the people of San Francisco do? One of the biggest uncertainties of an earthquake is working out where the ground is going to rattle the most. The Loma Prieta quake in 1989 had its epicentre some distance south of San Francisco, in the Santa Cruz mountains, but some of the most extensively damaged structures were in the Marina district and in the city of Oakland, both about 65 miles north of the epicentre.

In these areas, many houses collapsed and concrete freeway viaducts (such as the Cypress Freeway structure in Oakland) crumpled, crushing cars and their occupants between the layers. Part of the reason for this destruction occurring many miles away is due to a phenomenon known as the Moho bounce, where seismic waves echo from the bottom of the earth's crust and reflect out to distant areas. "The Moho bounce can be a killer," Brocher says.

He and his colleagues from the United States Geological Survey are tackling this problem in a unique way. Using their underground blasts, they have created a detailed three-dimensional map of the underside of the earth's crust, which is helping them to predict which areas are most vulnerable to the Moho bounce.

"The Moho is what seismological geeks call the base of the crust," Brocher explains. It is where the rocks change from being brittle crust to the stronger, harder mantle. The depth of the Moho varies greatly, from about 12 miles near the Californian coast to about 25 miles underneath the mountains that lie inland. Like the earth's surface, the Moho is not smooth, but covered in lumps and bumps. And it is these underground undulations that make the pattern of earthquake waves so unpredictable and random.

By drilling holes to a depth of 50 metres or so and setting off explosions in them, Brocher and his team have managed to map out the shape of the Moho boundary. "The explosion creates a mini earthquake. We use seismic recorders to measure the arrivals of the waves and the Moho bounce," Brocher says.

A pattern of about 40 holes has enabled them to gather reflections from a 300-mile-long chunk along the San Andreas fault. Piecing together the reflection data has allowed them to visualise all the layers and fault lines in the crust, right down to its base.

Californian earthquakes tend to happen at a depth of six to 10 miles inside the crust. The waves ripple out in all directions, some going directly to the surface, some going sideways and others going down towards the Moho. "When they hit the base of the crust, there is lots of reflection," Brocher says.

Some of these reflected waves travel very quickly towards the surface, zooming along the "faster" rocks that lie deeper in the earth's crust. "At a radius of about 65 miles from the earthquake's epicentre, the reflected waves arrive at about the same time as the direct waves, making the earthquake bigger," Brocher says. This double whammy of waves causes the earth to shake for about twice as long as everywhere else, massively increasing the chances of buildings collapsing and bridges tumbling.

Part of the reason the Moho bounce from the Loma Prieta quake in 1989 was so destructive was because the epicentre was in a thick piece of crust beneath the mountains. As the waves travelled towards San Francisco and Oakland, the crust became thinner and the energy was focused into a smaller area. "The energy had less volume to spread out into because it was travelling into the thin end of a wedge," Brocher says. This effect, combined with the soft, wobbly sediments in this area, led to disaster.

No one knows exactly where the epicentre of the next big San Andreas earthquake will be, but having detailed three-dimensional maps of the crustal structure means Brocher and his team are one step ahead. Computer simulations help them to work out which areas are most likely to suffer extreme shaking.

Their simulations have confirmed that the Moho bounce was at least partly responsible for the severe damage in Oakland and San Francisco in 1989. "The collapse of the Cypress structure in Oakland and the damage in the Marina district were at about the right distance to be explained by the Moho bounce," Brocher says.

Next, Brocher plans to simulate earthquakes at other places on the fault to build up a picture of how and where the Moho bounce might cause most damage. Areas that are at high risk of extra wobble will be able to ensure that their buildings, pipelines and bridges are reinforced, significantly reducing the damage and danger of the quake.

It's not just San Francisco that stands to benefit from these underground maps. Many other earthquake-prone areas of the world could be made safer if they had a detailed Moho map and retrofitted buildings. The technique is being applied in other parts of the US, with seismologists blasting holes and mapping the inside of the crust in southern California and further up the West Coast near Seattle and Vancouver, and in southern Alaska.

Places that are likely to benefit most from Moho maps are those with the knobbliest bits of Moho underlying them. "The Moho varies quite a lot globally. It is quite flat under very old continental areas and much more bumpy near current plate boundaries," says Professor Ian Main, a seismologist at the University of Edinburgh. Countries such as Greece, Italy and Japan, which all teeter on the edge of a plate boundary, are likely to suffer from extra shaking as a result of the lumps and bumps in the crust underneath them.

But a Moho map alone cannot save lives. Good preparation and specially engineered structures are the key. Since the great earthquake of 1906, the population of San Francisco has more than doubled. Many new houses have been built, often on soft ground, close to the fault line. In some cases, houses even straddle the fault. Most of these dwellings were built before 1970, when stricter building codes came into force.

Now the local authorities have a tough job on their hands to make sure that everyone is prepared for the next big one. All across San Francisco, old buildings and bridges are being jacked up and strengthened, or rebuilt, to ensure that they can withstand the wobbles of the next quake.

Armed with their Moho map, the people of San Francisco are bracing themselves for the next shudder. "We are sitting on a tectonic time-bomb," says Professor David Schwartz, another geologist with the US Geological Survey. "It is a big challenge and a fight against time to retrofit all the structures."