- The earthquakes hammered a 3-mile stretch of the San Andreas fault
- Seventeen were stronger than 2.5 magnitude and six were stronger than 3.0
- It follows a string of ten tremors that struck Monterey County last week
- The largest, a 4.6-magnitude quake, was felt in San Francisco 90 miles away
Fresh fears have been raised that a huge earthquake is about to hit California after a swarm of recent tremors.
In the last week 134 earthquakes have hammered a three-mile stretch around Monterey County on the San Andreas fault.
Of those earthquakes, 17 were stronger than 2.5 magnitude and six of them were stronger than 3.0, with more tremors expected in the coming weeks, experts warn.
It follows fears raised last week that the 'Big One' is about to hit after a series of ten 'mini quakes' struck the same area.
The swarm included one 4.6-magnitude quake that was felt in San Francisco more than 90 miles (145 km) away.
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Since last week 134 earthquakes have hammered a three-mile stretch around Monterey County on the San Andreas fault which experts say is long overdue a 'Big One'. Last week it suffered a 4.6-magnitude quake (pictured)
'This one has been a quite productive aftershock sequence,' said Ole Kaven, a US Geological Survey (USGS) seismologist.
'We suspect there will be aftershocks in the 2 to 3 [magnitude] range for at least a few more weeks', he said.
There have not been any reports of injuries, writes San Francisco news outlet SFGate.
Last week's swarm hit California's Monterey County on Monday at 11:31am ET (4:31pm GMT) about 13 miles (20 km) northeast of Gonzales, near Salinas.
It dramatically increases the likelihood of a major quake in California, at least temporarily, experts claimed.
The initial 4.6-magnitude quake was followed by nine smaller aftershocks.
The largest of these measured magnitude 2.8, Annemarie Baltay, a seismologist with the US Geological Survey in Menlo Park, told SFGATE.
There were no reports of injuries or damage to buildings.
WHAT IS THE BIG ONE?
The 'Big One' is a hypothetical earthquake of magnitude 8 or greater that is expected to happen along the San Andreas fault.
Such a quake is expected to produce devastation to human civilisation within about 50-100 miles (80-160km) of the quake zone, especially in urban areas like Palm Springs, Los Angeles and San Francisco.
Contingency plans warn upward of 14,000 people could die in worst-case scenarios, with 30,000 injured, thousands left homeless and the region's economy setback for years, if not decades.
The quake happened at a depth of around 4 miles (6.5 km) on the infamous San Andreas Fault, close to a region where the Calaveras Fault branches off.
Experts have previously warned that any activity on the fault line is cause for concern.
'Any time there is significant seismic activity in the vicinity of the San Andreas fault, we seismologists get nervous,' Thomas Jordan, director of the Southern California Earthquake Centre, told the LA Times last year.
Of those earthquakes, 17 were stronger than 2.5 magnitude and six of them were stronger than 3.0, with more tremors expected in the coming weeks, experts warn. Pictured is a view of the San Andreas fault in the Carrizo Plain
WHAT QUAKE MAGNITUDES MEAN
Below a 3.0 magnitude: Earthquakes at this level are deemed level I in intensity. These are 'Not felt except by a very few under especially favorable conditions,' according to the USGS.
Magnitude 3.0 to 3.9: These quakes 'only felt by a few persons at rest, especially on upper floors of buildings,' or level III, which carry 'vibrations similar to the passing of a truck.'
Magnitude 4 to 4.9: These are widely felt but rarely cause serious damage.
Magnitude 5 to 5.9: The USGS says level VI intensity is where people start getting scared.
By level VII we start seeing 'considerable damage in poorly built or badly designed structures.'
Magnitude 6.0 to 6.9: This level quake can take down 'chimneys, factory stacks, columns, monuments' and walls
The official USGS forecast for California earthquakes now predicts a 16 per cent chance of an M7.5 quake or larger on this section of the fault within the next 30 years. Shown here is the chance of an earthquake across California over the next 30 years
'Because we recognise that the probability of having a large earthquake goes up.'
Despite this, Ms Baltay said the recent quakes are part of normal seismic activity and that there was no suggestion the tremors were signs of larger activity to come.
'This is really typical behaviour,' she said.
'It's as if someone put an oil can into the fault and lubricated it.'
Fears of California's 'Big One' were stirred in May when an expert warned that a destructive earthquake will hit the state 'imminently'.
Seismologist Dr Lucy Jones, from the US Geological Survey, warned in a dramatic speech that people need to act to protect themselves rather than ignoring the threat.
Dr Jones said people's decision not to accept it will only mean more suffer as scientists warn the 'Big One' is now overdue to hit California.
In a keynote speech to a meeting of the Japan Geoscience Union and American Geophysical Union, Dr Jones warned that the public are yet to accept the randomness of future earthquakes.
People tend to focus on earthquakes happening in the next 30 years but they should be preparing now, she warned.
Seismologist Lucy Jones from the US Geological Survey warned she is trying to make people accept the fact catastrophe is imminent and that they need to prepare themselves.
Dr Jones said our decision to not accept it will only mean more people suffer as scientists warn the 'Big One' is now overdue to hit California.
Dr Jones, who is from the US Geological Survey said there are three key reasons why the peril is so frightening - it cannot be seen, it is uncertain and it seems unknowable.
This means people bury their heads in the sand and pretend it won't happen.
The quake happened at a depth of around 4 miles (6.5km) on the infamous San Andreas Fault (pictured), close to a region where the Calaveras Fault branches off
Dr Jones said there are three key reasons why the peril is so frightening - it cannot be seen, it is uncertain and it seems unknowable.
This means people bury their heads in the sand and pretend it won't happen.
'We find patterns even when they're not real,' Dr Jones said.
She tweeted on 23 May; 'I'm not trying to terrify people. I'm trying to inspire action that will prevent our scenarios from coming true. It's in our power to change'.
CALIFORNIA AT RISK OF DEVASTATING MEGAQUAKE
A 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 per cent to about 7.0 per cent, they say.
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.'
ast week's swarm hit California's Monterey County on Monday at 11:31am ET (4:31pm GMT) about 13 miles (20 km) northeast of Gonzales, near Salinas
Her team published a scenario of a 7.8 earthquake on the San Andreas fault that could kill many people and devastate 15,000 buildings.
In 2011 a magnitude nine earthquake hit the east coast of Japan, killing around 20,000 people.
'The city leaders ignored protocol that said to move to higher ground and conducted their emergency meeting in the city hall', said Dr Jones.
'When the tsunami poured over the sea wall, they lost over 1,000 people, including most of their city government'.
PLANS FOR 'THE BIG ONE'
Federal, state and military officials have been working together to draft plans to be followed when the 'Big One' happens.
These contingency plans reflect deep anxiety about the potential gravity of the looming disaster: upward of 14,000 people dead in the worst-case scenarios, 30,000 injured, thousands left homeless and the region's economy setback for years, if not decades.
As a response, what planners envision is a deployment of civilian and military personnel and equipment that would eclipse the response to any natural disaster that has occurred so far in the US.
There would be waves of cargo planes, helicopters and ships, as well as tens of thousands of soldiers, emergency officials, mortuary teams, police officers, firefighters, engineers, medical personnel and other specialists.
'The response will be orders of magnitude larger than Hurricane Katrina or Super Storm Sandy,' said Lt. Col. Clayton Braun of the Washington State Army National Guard.