Page last updated at 20:35 GMT, Monday, 20 October 2008 21:35 UK

America's 'most dangerous fault'

By Jonathan Amos
Science reporter, BBC News

Hayward City Hall (BBC)

It's a big white building on Mission Boulevard. You can't miss it; the Art Deco style is really striking. The grass is trimmed and it all looks perfectly inviting, except this is a lock-out.

The first Hayward City Hall in California has long been off-limits to occupants because its foundations sit right atop an earthquake fault and it's gradually splitting in two.

"Look up at the stairwell," says geologist Russ Graymer, as we peer through a window.

"There are huge cracks, several centimetres broad and many metres long - basically showing the evidence that this building is being torn in half."

The Hayward Fault is one of a network of cracks in the Earth's surface running through the San Francisco Bay Area. The San Andreas Fault is probably the best known, but right now the Hayward is the one everyone's talking about.

The records show that the past five large earthquakes on this fault have occurred on average about 140 years apart, and the last was - you've guessed it - 140 years ago. Tuesday is the anniversary.

At 0755 on the morning of 21 October, 1868, the Hayward broke with a Magnitude 6.8 quake.

The San Francisco Bay Area is crossed by a number of faults

The ground lurched some two metres and the cities of Oakland to the north and San Francisco to the west were shattered.

In the context of post-1850 quake history in the Bay Area, the tremor is rivalled in size only by the 1906 "Big One" in San Francisco (7.8), and the 6.9 event of Loma Prieta in 1989.

"The difference with Loma Prieta was that the maximum shaking was 50 miles to the south in the mountains," says Dr Graymer from the US Geological Survey (USGS).

"The squirrels and the redwoods took the brunt of the shaking in 1989. In contrast, this is an urban area and the folks who live here will take the brunt of it if the Hayward decides to go."

Some have now dubbed the Hayward "the most dangerous urban fault in America".

"It's the probability that the Hayward will generate a large earthquake in the next 30 years combined with the fact that it runs right through an urban area. These two facts taken together make it the most dangerous right now," says Dr Graymer.

The comparison is often made with Kobe, Japan, which suffered a Magnitude 6.9 earthquake in 1995.

Kobe, like Hayward and Oakland, sits on the east side of a bay - Osaka Bay - and the Nojima Fault running through Kobe mirrors the Hayward in type (strike-slip) and in length.

More than 5,000 people died in the 1995 Kobe event.

Offset kerb stone (BBC)
The Hayward Fault is creeping by as much as 5mm a year in some places

When scientists assessed the probabilities of a big quake occurring in the San Francisco Bay Area in the coming decades, they pointed to the Hayward and its northern extension, the Rodgers Creek Fault, as being the most likely locations for the tremor.

"There's a 63% chance of a Magnitude 6.7 or greater in the next 30 years across the entire Bay Area," says USGS seismologist Tom Brocher.

"That's roughly two out of three odds, and about half of that falls on the Hayward/Rodgers Creek Fault. That's why we're focussing on the Hayward and why we're so concerned about it."

The concern has been heightened in recent years by research which indicates that the Hayward and the parallel Calaveras Fault might actually be connected at depth, and, as such, could produce, in effect, a single, even more powerful event.

Between two and four million people would experience shaking strong enough to cause damage if the Hayward lets go in a similar fashion to 1868. Economic losses could exceed $200bn, says the USGS.

The apparent risks have authorities across the Bay racing hard to shore up local infrastructure. In excess of $30bn has been spent since Loma Prieta.

Roads, rail, bridges, buildings, hospitals, water and other utility systems are all being "retro-fitted", which involves suspect engineering being swapped out for structures thought more likely to ride out immense shaking.

"The local power company, Pacific Gas and Electric, has been replacing old cast-iron gas pipes with new flexible piping. A lot of work is being done but we have more to do," Dr Brocher tells BBC News.

Hayward is now on its third City Hall. While the first is not expected to survive a big quake, the latest incarnation will rock back and forth on a "pendulum" system fitted to its foundations. This should see it come through a Magnitude 7 event without major damage.

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The USGS has simulated the shaking effects of a M7.0 Hayward quake

Jonathan.Amos-INTERNET@bbc.co.uk


SEE ALSO
Pre-quake changes seen in rocks
09 Jul 08 |  Science & Environment
'Earthquake rocks' see daylight
05 Oct 07 |  Science & Environment
San Francisco faces big shaker
17 Apr 06 |  Science & Environment
Preparing for the next 'Big One'
16 Apr 06 |  Science & Environment
Mission inside the fault zone
04 Sep 04 |  Science & Environment

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