The risk of a major earthquake hitting California in the next 30 years has risen dramatically, US scientists said Tuesday, using improved forecasting techniques.

Earthquakes are notoriously hard to predict and while seismic activity in California has stayed mild in the last century, experts expect a big one to strike some time in the future -- they just don't know when.

"The likelihood that California will experience a magnitude 8 or larger earthquake in the next 30 years has increased from about 4.7 percent... to about 7.0 percent," said the US Geological Survey.

A report, known as the Third Uniform California Earthquake rupture Forecast, acknowledges the complex nature of fault lines and uses new methods to account for future risk.

"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.

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

While the risk of a big quake went up, that of a more moderate one actually declined since the last assessment in 2008.

"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," said the report.

Tom Jordan, director of the Southern California Earthquake Center and a co-author of the study, added: "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."