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California’s long-term quake forecast

  • Written by  Tom Hart
  • Published in Tectonics
San Francisco’s Palace Hotel on fire during a major earthquake in 1906 San Francisco’s Palace Hotel on fire during a major earthquake in 1906 Everett Historical
11 Mar
2015
Outlook is for fewer small quakes, but an increased risk from the ‘Big One’

California’s long-term earthquake forecast, the first in seven years, has been released. The study, known as the Third Uniform California Earthquake Rupture forecast, teams the US Geological survey with three other state authorities on tectonics to work out the region’s risk from earthquakes.

The good news is that the risk from magnitude 6.7 earthquakes has reduced by around 30 per cent, and the frequency rate for these quakes has dropped from an average of one per 4.8 years to one per 6.3 year.

A 6.7 earthquake is the same magnitude as the 1994 Northridge quake that killed 57 people, injured more than 8,500, and cost California some $20billion in damages.

6990B6C47C9B11DAThe 1994 Northridge earthquake caused serious damage to California’s transport infrastructure (Image: USGS)

But the study also shows that the risk from the ‘Big One’ – a magnitude 8 or greater quake – has increased from 4.7 per cent to seven per cent over the next 30 years.

A 2008 USGS report, ShakeOut Scenario, outlined the possible impacts from a 7.8 magnitude earthquake in California. The study suggested the state could expect 10,000–100,000 individual landslides and 1,600 fires. Total losses to buildings alone were valued at $33billion , with a total economic cost reaching $213.3billion. Casualties were estimated at 1,800 deaths, and 50,000 injuries.

ShakeOut scenario gets the Hollywood treatment

‘There are dozens more actions and policies that could be undertaken at the individual and community levels to further reduce these losses,’ the report concluded.

It’s a warning some Californian cities have heeded. In San Francisco, the Advanced Community Disaster Resilience Project aims to make the city’s neighbourhoods better prepared for an earthquake. The city has also introduced legislation to earthquake-proof older buildings.

‘The new likelihoods are due to the inclusion of possible multi-fault ruptures, where earthquakes are no longer confined to separate, individual faults, but can occasionally rupture multiple faults simultaneously,’ says Ned Field, lead author on the Third Uniform report.

Two models are used to understand California’s complex fault system. The Earthquake Rupture Forecast indicates when and where there might be a slip on the state’s faults while the Ground Motion Prediction model estimates ground shaking at a given fault.

Read the full Third Uniform Earthquake Rupture Forecast.

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