sciencetech-blog

comments

Researchers have revealed that four heavily populated areas in San Francisco are in danger of an imminent major earthquake.

They all lie in the area of the bay on the San Andreas Fault system and have stored enough energy to make a disaster overdue - with three nearing or past their average recurrence interval.

Experts say the new analysis reveals a quake would cause massive damage, and affect up to five million people in the San Francisco area.

Rodgers Creek, northern Calaveras, southern Green Valley and southern Hayward named as highest risk areas. All lie in the area of the bay on the San Andreas Fault system. The red and yellow lines show the fault lines.

When they hit two are expected to measure over 7 on the Richter Scale - meaning they are likely to cause serious damage over larger areas.

Some sections of the notorious line are locked - when the most damaging tremors occur. 

This prevents 'creep' where the slow release of strain in the uppermost part of the Earth's crust leads to thousands of tiny micro-quakes.

Instead much greater stress is released in only seconds - resulting in carnage. 

Enough creep on a fault can diminish the potential size of its next earthquake rupture.

Dr James Lienkaemper, a geophysicist at US Geological Survey, said: 'The extent of fault creep - and therefore locking - controls the size and timing of large earthquakes on the Northern San Andreas Fault system.

'The extent of creep on some fault sections is not yet well determined making our first priority to study the urban sections of the San Andreas which is directly beneath millions of Bay Area residents.'

So his researchers estimated how much creep occurs on each section of the fault system - enough can diminish the potential size of its next earthquake rupture.

Understanding the extent of creep directly impacts seismic hazard assessments for the region. 

The San Andreas system in Northern California consists of five major branches with an overall length of about 1,25O miles.

Sixty per cent of the energy is released through fault creep - ranging from O.1 to 25.1 mm (.OO4 to one inch) - per year, and about 28 per cent remains locked at depth, according to the research.

Skater Gabe Haugen (R) of Napa took advantage of a buckled street after a 6.1 magnitude earthquake hit the San Francisco Bay Area, in Napa, California, USA, 24 August 2014.Experts now say the areas could be hit by more.

Monitoring of creep on Bay Area faults has expanded in recent years - including measurements made by the San Francisco State University Creep Project and recently expanded GPS station networks.

These provide the primary data which the researchers used to calculate the average depth of creep for each fault segment.

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.

Where available details of past ruptures of individual faults allowed the researchers to work out recurrence rates and the probable timing and size of future earthquakes.

Three creeping faults have large locked areas - less than 1 mm or.O4 inches of creep per year - that have not ruptured in a major earthquake of at least magnitude 6.7 since the reporting of them by local inhabitants.

These were named as Rodgers Creek, northern Calaveras and southern Green Valley.

The southern Hayward fault - which produced a magnitude 6.8 earthquake in 1868 - is now approaching its average recurrence time based on paleoseismic studies.

'A large earthquake on the Hayward Fault would affect the entire San Francisco Bay Area — over 5 million people,' says Greg Beroza, the Wayne Loel Professor of Earth Sciences in the Department of Geophysics at Stanford University, who was no associated with the study. 

'Damage would likely be most severe near the fault in the East Bay, but there are areas removed from the fault, including the entire periphery of San Francisco Bay, and the Sacramento Delta, that would be very strongly shaken as well.' 

The study said three faults appear to be nearing or have exceeded their mean recurrence time and have accumulated sufficient strain to produce large earthquakes - the Hayward (M 6.8), Rodgers Creek (M 7.1) and Green Valley (M 7.1).

Dr Lienkaemper said: 'The San Andreas Fault and its two other large branches - the Hayward and Northern Calaveras - have been quiet for decades. 

'This study offers a good reminder to prepare today for the next major earthquake.'