‘Large-Scale Movement’ on San Andreas Fault, Prompting Fears of Major Earthquake

There is consensus among scientists that a magnitude seven or eight earthquake is overdue on the 800-mile fault that runs through California, because there has not been a significant one since the 7.9 Big One that hit California in 1857, 159 years ago.

Earthquake expert Trevor Nace said: “In earthquake prediction, geologists work in probability distributions rather than absolute terms. Consensus is growing, however, of the likelihood of a large and devastating earthquake to strike California.”

News of the movement detected has led to fears strain in the fault line could be building for another severe tremor.

It was already feared a big one could be brewing after a series of magnitude three and above earthquakes stuck in California in the past fortnight.

The latest came yesterday when a shallow magnitude three earthquake was reported three miles from Big Bear Lake, according to the US Geological Survey, at 9.21pm local time.

This graphic shows areas of uplift and subsidence along the fault.
This graphic shows areas of uplift and subsidence along the fault.

It is feared when it goes, around 140,000 people could be killed and many more injured, and it will be the US’s worst natural disaster since Hurricane Katrina.

But the new study, published in the journal Nature Geosciences, could help scientists use smaller lobes of vertical motion to predict when more significant ruptures are likely to happen, possibly giving more warning.

For the study, geologists analysed GPS data to reveal new areas of motion around the San Andreas Fault System.

The top diagram shows the movement, with areas of uplift in red and those of subsidence in blue.

Using data collected by the EarthScope Plate Boundary Observatory’s GPS array, researchers identified 125-mile-wide “lobes” of uplift and subsidence.

Over the last several years, the lobes, which straddle the fault line, have hosted a few millimeters of annual movement.

Computer models simulating the San Andreas Fault System have predicted such crustal movement, but the actual areas of motion hadn’t been physically identified until now.

Researchers used advanced statistical modeling to identify the movement among the inevitable statistical noise that comes with monitoring minute movements in the Earth’s crust.

Samuel Howell, a researcher at the University of Hawaii at Manoa, and lead author of the study, said: “While the San Andreas GPS data has been publicly available for more than a decade, the vertical component of the measurements had largely been ignored in tectonic investigations because of difficulties in interpreting the noisy data.

“Using this technique, we were able to break down the noisy signals to isolate a simple vertical motion pattern that curiously straddled the San Andreas fault.”

The validity of the vertical patterns was confirmed by the fact that similar motions were predicted by an earthquake model designed by researchers at the University of Hawaii’s School of Ocean and Earth Science and Technology.

Researcher Bridget Smith-Konter said: “We were surprised and thrilled when this statistical method produced a coherent velocity field similar to the one predicted by our physical earthquake cycle models.

“The powerful combination of a priori model predictions and a unique analysis of vertical GPS data led us to confirm that the buildup of century-long earthquake cycle forces within the crust are a dominant source of the observed vertical motion signal.” WHEN NOT IF: ‘Large-scale movement’ detected on San Andreas Fault, prompting fears of major quake.

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