Japan's tsunami exposes weakness as California awaits earthquake

This extraordinary footage of the tsunami hitting the coast of Japan on March 11 has been doing the viral rounds this week. It has been billed as the "most impressive tsunami video yet". I'm not sure that "impressive" is an appropriate word to use in a context where an estimated 21,000 people lost their lives. But the video is one of the more palatable if only because the cameraman survived and it apparently does not show lives being lost.

What it does show is the raw power of the series of tsunamis that grow stronger over almost 10 minutes, as the blackening ocean surges over the sea walls with sickening speed. It clearly demonstrates the power of a natural disaster of this kind as the surges sweep in, suck backward causing whirlpools before sweeping in again. It also illustrates how easy it would have been to have been caught out by the dangerous waves which seemed benign at first. Not everyone watching this close to the sea wall would have been as lucky as this cameraman.

Damage to California's coast estimated at $50m and the death toll (one near Crescent City) does not compare to Japan’s deadly earthquake and tsunami. But Californians were shaken, particularly in San Francisco, where six fault lines run through the city and its bay area, by this timely wake-up for the “Big One”.

Alarms sounded first in Crescent City, in far northern California, after the West Coast and Alaska Tsunami Warning Centre issued a red alert for the California coast around 5.45am UTC on Friday. Authorities responded quickly, sounding the tsunami siren across the fishing community and sending sheriffs door to door to help evacuate around 6,600 people.

Crescent City had learned the hard way about the devastating impact of tsunamis. In 1964, an earthquake in Alaska generated waves which wrecked Crescent City’s harbour and killed 11. On Friday, the first surges of around 3ft hit the California coast first at Crescent City at around 7.45am local time, rising to 8.1ft by 10.15am. A man attempting to photograph the surging waves near the Klamath river became California’s only casualty and four others had to be rescued from the water. At least 35 boats were damaged as 8ft walls of sea water surged into the harbour.

Further down the coast, residents in other towns were equally concerned as the surges travelled south. Pacifica and Half Moon Bay just south of San Francisco were under voluntary evacuation, while drivers clogged the roads out of Santa Cruz, 75 miles south of the city, as 6,000 people evacuated the popular surf spot and marina. Whirlpools gathered offshore and local television reports described surfers 300-400 ft out to sea who suddenly found themselves stranded on sand before the sea retreated in the first series of surges that lasted all day. Surges only a few feet high were powerful enough to tear boats from their moorings and cause an estimated $17m worth of damage.

San Franciscans adopted a typically casual attitude to the tsunami alert. Even as erratic waves started to hit Ocean Beach, the largest beach in San Francisco, at around 8.30am, onlookers, including children, were walking along the sand and swimming before surges grew stronger an hour later.

Surfers meanwhile made the most of a rare break at the foot of the Golden Gate Bridge, where surging waves flooded through the Marin headlands and into the bay.

The headlands which span the Golden Gate Bridge break the impact of tsunamis into San Francisco Bay and the worst effects were also mitigated by the tsunami effects arriving at low tide. Sausalito saw churning tidal action on Friday, and Tiburon residents noticed slow surging waves and wading birds unsettled as the waters rose. But it was still a far cry from the effects of the 1964 tsunami when boats were ripped from their moorings on the usually calm peninsula.

But the city’s shoreline is particularly at risk from high water levels – either from climate change or from tsunamis because large areas of the city centre are built on infill – much of it from rubbish dumped in the Bay until the 1980s.

The Pacific Institute estimates that 480,000 people, critical infrastructure, including San Francisco airport and nearly $100 billion in property along the California coast are at increased risk from flooding from a 1.4-metre sea-level rise.

But the high proportion of infill also means that it is prone to liquefaction from soft soils which amplify shaking – the deadly consequences of liquefactions were seen recently in Christchurch, New Zealand, and San Francisco’s last major earthquake, the Loma Prieta in 1989 which killed 57 and injured 3,757.

I now live a few miles away from the San Andreas fault that runs between Point Reyes and the mainland. Slippage along the San Andreas fault caused the Loma Prieta earthquake and the infamous 1906 earthquake which led to the destruction of the San Francisco.

The six fault lines in the Bay area have a 63% probability of creating a magnitude 6.7 or greater earthquake and the probability of a large earthquake on the San Andreas Fault in the next 30 years is about 21%, or about 1 out of 5.

Tom Brocher, US Geological Survey director of the earthquake science centre in Menlo Park, said I should not worry about the San Andreas fault. It’s the Hayward fault that keeps him awake at night.

“The last time we had a major earthquake on the Hayward fault was 142 years ago in 1868. And our work at USGS has shown that the average interval between Hayward’s earthquakes are somewhere between 140 and 150 years. We’re at a point now where we could have a magnitude 7 earthquake on the Hayward fault at any time and we’re looking at $200bn of damage. We’re in earthquake country here and people should be prepared for an earthquake at any time.”

But chances of a tsunami after an earthquake even within the bay are unlikely, he said. “To make a tsunami you have to push water up or down very quickly and in these faults the motion is side to side. Plus there’s not a lot of water in the bay the average depth is only about 8 feet. You could tip the whole bay over and it still would not make a very big tsunami. In 1906, the ‘tsunami’ was about four inches.”

Earthquakes 5,000 miles away from the coast of California may be liable to cause more damage than those closer to home, except for communities close that face the ocean. Damage from a 14-foot wall of water hitting San Francisco's peninusla which includes Silicon Valley and much of which is low-lying and unprotected by sea defences would have unimaginable consequences.

While San Francisco's city authorities puzzle over what to do about climate change adaptation, they should also be planning for the worst case scenario that may happen sooner than 2050.