This article from New Scientist gives a wider answer than those so 
far.

Terry Cannon

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Give the fault its due
                                              10 Oct 98


       The San Andreas may threaten California with death
       and destruction, but it is still the state's biggest
       blessing. Gabrielle Walker finds out why

       "IF YOU'D been standing here on 18 April 1906, you'd
       have seen that hill over there move about two metres to
       the right." Allan Lindh is pointing to a hillock a few
       hundred metres away. Now he gestures to the dry scrub
       beneath his feet. "This would have been moving back
       and forwards, up and down by metres. You'd have been
       on your knees, then flat out, yelling, trying to get up,
       wondering what in the world was going on."

       Lindh is describing the last really big quake to hit
       California. Up here on the crest of the Santa Cruz
       mountains, around 40 kilometres south of San Francisco,
       it's hard to imagine such sudden drama. The air is still,
       and all around the scene is reassuringly solid and
       unchanging. It's silent, but for a steady hum from the
       countless insects that live in the scrub. The only odd
       note is struck by a narrow green trench, luxuriant with
       vegetation, disappearing off to the southeast into the
       next valley.

       But it's that trench and the many others threading
       through this landscape that make all the difference.
       They are part of the San Andreas rift zone, and each
       fault trace grows lush and green where the rock has
       been pulverised by the fault motion into impervious clay
       that traps surface water. This is where the Pacific and
       North American plates grind against each other, crushing
       and folding the rocks, creating mountains, valleys and,
       of course, earthquakes. The Big One that struck in
       1906 ripped through here at around 2 kilometres a
       second after devastating San Francisco and taking
       hundreds of lives. And though a long period of calm
       followed, activity has been picking up again over the
       past 20 years. Nine years ago this week, the most
       recent serious quake hit near Loma Prieta, a squarish
       mountain just visible through the haze to the south,
       before setting off on its destructive path through cities,
       homes and lives.

       Californians spend every day not knowing whether an
       earthquake will strike. It seems like a terrible flaw in a
       beautiful and wealthy place. But that's not the way Lindh sees
       it. A seismologist from the US Geological Survey at Menlo Park,
       he has studied the fault system for the past 25 years.
       "Earthquakes are not an optional add-on sent by the devil to
       make life more difficult in California," he says. "They're an
       integral part of the system." Or, to put it another way, the
       same thing that brings the quakes also creates the beauty, the
       wealth and everything else that makes California unique.

       Lindh wants to give the San Andreas its due, and for
       years now he has been spreading his message on TV
       and radio shows and in local newspapers. Everyone
       knows the fault's destructive power, he says, but not
       everyone gives it credit for the good things it brings.
       Partly, he wants to use this message to try and
       encourage people to put more money into
       quake-proofing their buildings to reduce the death toll
       when the quakes do hit. Partly, it's a geologist's desire
       to redress the fault's bad press. And according to his
       calculations, it gives much more than it takes.

       The beaches, for instance. From here in the Santa Cruz
       Mountains, the road snakes down through redwood
       groves to San Gregorio beach. There you can walk for
       miles on the sand, between the Pacific Ocean and the
       high crumbling cliffs. The whole coast is made up of
       long stretches of beaches like this, broken by high
       mountains and capes jutting into the ocean. "They're
       not just boring, flat beaches that go on for ever," says
       Lindh. "There are wild places, with mountains above
       them, streams running down, great high points where
       you can look out over the ocean."

       That, he says, is a classic landscape produced by the
       geological process called transform faulting. As the
       plates grind past one another, irregularities in their
       curving fault lines catch and snag, pushing up great
       folds in the land. As a result, the cliffs are rising far
       faster than they can be eroded by the waves.

       The combination of mountains and shoreline is certainly
       spectacular, but the high ground created by the fault
       system is good for more than just the scenery. It also
       gathers the water that is vital for California's agriculture
       and industry. Moist air arriving from the ocean is forced to
       rise, cool and deposit its load into the mountain aquifers.
       Without the mountains, that precious moisture would be blown
       eastwards into the interior, leaving the coast arid. "The water
       that keeps California alive comes almost entirely from the
       mountains," says Lindh. "And they are a direct product of the
       San Andreas system."

       The list continues. Not only does California have more
       than its share of water, it also has plenty of large,
       fertile valleys. And guess what-that's down to the San
       Andreas, too. "When you push mountains up, something
       else has to go down," says Lindh. In California's case,
       that's valleys like the Imperial Valley and the San
       Joaquin Valley, where most of the state's fruit and
       vegetables are grown. These vast trenches, filled with
       rich young soil that has eroded from the surrounding
       hills, bask in the long hot growing season of the desert
       climate, and are supplied with plentiful water trapped by
       the mountains. Then there are the friendly
       microclimates of the wine country in the north. Small
       wonder California's agriculture is so productive.

       And there's oil, too. It forms in places where in the past
       copious organic material has been deposited and buried. Then it
       must be carried downwards and cooked at temperatures and
       pressures that will turn it into petroleum. Finally, folding
       and faulting are needed to create traps where oil collects. All
       this, says Lindh, is just what happens in a tectonically active
       region. "They've taken an enormous amount of oil out of the Los
       Angeles Basin and it's all down to the faulting."

       Even San Francisco's extraordinary bay was formed by
       the fault system. This part of coastal California is a
       collage, made up of sundry pieces of crust that have
       been assembled by the plate motion over millions of
       years. The block beneath the bay is a piece of dense
       oceanic crust, forced downwards by the combined
       geometry of the San Andreas Fault to the west and the
       Hayward Fault to the east to create one of the most
       famous natural harbours in the world.

       It was thinking about agriculture and oil that set Lindh
       wondering just how many dollars the fault brings in. Add
       up the oil, agriculture, wine and tourism and Lindh
       reckons it earns the state a cool $10 billion to $20
       billion a year. Compare that with the cost of
       earthquakes from the faults. There is a really big
       quake once every 50 to 100 years, says Lindh, and he
       puts the cost at around $100 billion a time. So even at
       a conservative estimate, the fault is bringing five to ten
       times as much into the Californian economy as it takes out,
       Lindh reckons.

       Of course, earthquakes don't just cost money. They
       also take lives. But Lindh points out that it's bad
       buildings that kill, not the earthquakes themselves.
       Falling masonry from poorly constructed buildings is the
       biggest danger, and out here in the open, if an
       earthquake ripped by, the worst you'd have to worry
       about is the thorns. In fact, it's partly because of poorly
       designed and constructed buildings that he did his calculation.
       "It is California's poorest citizens who are often most at risk
       in a great earthquake," he says. "They live and work in the
       worst buildings, and share least in the state's bounty." More
       of the riches that the fault brings to the state should be
       funnelled back into making them safe, he believes. "It's up to
       us to share just a little of the wealth so everyone has a
       secure place to live and work."

       Lindh is also exasperated by Californians' habit of
       complaining about the dangers of living in an
       earthquake zone, without also acknowledging the
       benefits. "California and Japan-aren't they among the
       richest places in the world? Do you think it's coincidence that
       they are also two of the most tectonically active? I think more
       interesting people come to more interesting places."

       And he has a tale to back this claim. It concerns the
       Santa Clara Valley just down Page Mill Road on the
       other side of the Santa Cruz hills. Better known as
       Silicon Valley, the cradle of the modern computer
       industry, it is home to many of the world's top IT
       companies. But why is it here in California?

       It started, says Lindh, with Jane and Leland Stanford,
       who made their fortune in the 19th century building the
       first railroad to cross America. They built a vast country
       estate in a beautiful valley south of San Francisco, within
       striking distance of the ocean, yet protected by mountains from
       the fog that plagues San Francisco. When their only child died
       of typhoid, they converted their home into a university in his
       memory. By the end of the Second World War, Stanford University
       had become home to many applied materials scientists, and the
       conditions for the silicon revolution were ripe.

       "I think it's all because it was such a pretty place," says
       Lindh. "I'm sorry, but it just wouldn't have happened in Des
       Moines."

       Gabrielle Walker 
           From New Scientist magazine, vol 160 issue 2155,
                      10/10/1998, page 42

Terry Cannon
Social Science Department
Natural Resources Institute
University of Greenwich
Chatham Kent ME4 4TB

Tel 44 (0)20 8331 8944 
messages 44 (0)1634 883170
Fax 44 (0)1634 880066/77


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