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A new report from the U.S. Geological Survey has warned the risk of 'the big one' hitting California has increased dramatically.

Researchers analysed the latest data from the state's complex system of active geological faults, as well as new methods for translating these data into earthquake likelihoods.

The estimate for the likelihood that California will experience a magnitude 8 or larger earthquake in the next 30 years has increased from about 4.7% to about 7.0%, they say.

In the new study, the estimate for the likelihood that California will experience a magnitude 8 or larger earthquake in the next 30 years has increased from about 4.7% for UCERF2 to about 7.0% for UCERF3.

 'We are fortunate that seismic activity in California has been relatively low over the past century,' said Tom Jordan, Director of the Southern California Earthquake Center and a co-author of the study.

'But we know that tectonic forces are continually tightening the springs of the San Andreas fault system, making big quakes inevitable.

'The UCERF3 model provides our leaders and the public with improved information about what to expect, so that we can better prepare.'

The Third Uniform California Earthquake Rupture Forecast, or UCERF3, improves upon previous models by incorporating the latest data on the state's complex system of active geological faults, as well as new methods for translating these data into earthquake likelihoods.

The study confirms many previous findings, sheds new light on how the future earthquakes will likely be distributed across the state and estimates how big those earthquakes might be.

Compared to the previous assessment issued in 2008, UCERF2, the estimated rate of earthquakes around magnitude 6.7, the size of the destructive 1994 Northridge earthquake, has gone down by about 30 percent. 

The expected frequency of such events statewide has dropped from an average of one per 4.8 years to about one per 6.3 years.

However, in the new study, the estimate for the likelihood that California will experience a magnitude 8 or larger earthquake in the next 30 years has increased from about 4.7% for UCERF2 to about 7.0% for UCERF3.

'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,' said lead author and USGS scientist Ned Field. 

THE 1906 EARTHQUAKE

The M 7.8 San Francisco earthquake of 19O6 struck the coast at 5.12 am on 18 April 18. Devastating fires lasting several days broke out in the city. 

As a result about 3,OOO people died and over 8O percent of San Francisco was destroyed.

The earthquake and resulting fire are remembered as one of the worst natural disasters in the history of the US alongside the Galveston Hurricane of 19OO and Hurricane Katrina in 2OO5.

People walk through the rubble following an earthquake in San Francisco on April 18, 1906. On April 17, 1906, San Francisco was cosmopolitan enough to host Enrico Caruso in 'Carmen' and so financially flushed it ranked fourth among American cities in raising money to help victims of a volcano in Italy. A day later, San Francisco was pleading for help itself after a giant earthquake struck along the San Andreas fault.

'This is a significant advancement in terms of representing a broader range of earthquakes throughout California's complex fault system.'

Two kinds of scientific models are used to inform decisions of how to safeguard against earthquake losses: an Earthquake Rupture Forecast, which indicates where and when the Earth might slip along the state's many faults, and a Ground Motion Prediction model, which estimates the ground shaking given one of the fault ruptures. 

The UCERF3 model is of the first kind, and is the latest earthquake-rupture forecast for California. It was developed and reviewed by dozens of leading scientific experts from the fields of seismology, geology, geodesy, paleoseismology, earthquake physics and earthquake engineering.

Lucy Jones, a USGS seismologist and Los angeles Mayor Eric Garcetti's adviser on earthquakes, tweeted Tuesday about the randomness of big quakes. 

'This new science doesn't change the bottom line for emergency managers,' she wrote. 

'Which one happens in our lifetimes is a random subset.'