San Francisco 'vulnerable to earthquakes' says expert

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The tremors, which began early on Tuesday local time and continued into Wednesday, were detected some 200 miles from Newport on Oregon’s central coast. Experts with the US Geological Survey (USGS) said they ranged in intensity between magnitude 3.5 and 5.8. The earthquakes erupted along one of the US’s busiest fault lines – fractures in the Earth’s crust where rocks on either side have slid past each other.

According to Harold Tobin, Director of the Pacific Northwest Seismic Network at the University of Washington, this specific area is known for its frequent seismicity.

He told CNN: “If you had asked me yesterday where on Earth would be most likely to produce a bunch of magnitude 5+ quakes in a single fay, this would have been high on my list.”

The tremors were too far off the coast to be felt and the US National Tsunami Warning Center has not issued any tsunami warnings since yesterday.

But the swarm was impressive nonetheless, as it contained at least nine tremors at or above magnitude 5.

The event has also sparked fears of a possible major quake brewing along the Cascadia Subduction Zone or megathrust that stretches from northern Vancouver Island in Canada to Northern California in the US.

One of the biggest earthquakes in US history is believed to have struck along the Cascadia zone in 1700, although there are no contemporary records of the disaster.

The USGS was asked by one Twitter user, Rogue Melanides, whether the swarm was linked to the megathrust “because that would be all kinds of not good”.

Luckily, the Pacific Northwest Seismic Network (PNSN) replied: “Not Cascadia!

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“These earthquakes are happening along the Blanco Transform Fault, which is about 200 miles further offshore from Cascadia.

“They are not likely to have an impact on the subduction zone fault.”

Dr Eric J Fielding, a geophysicist in Pasadena, California, has also said that although impressive, the earthquakes did not pose any danger.

The swarm struck along the Blanco Fracture Zone, which the expert said is not connected to the Cascadia megathrust.

He tweeted: “Quite an active swarm of earthquakes on the Blanco Fracture Zone about 400km/250 miles west of Oregon.

“Not a concern at this is far from the Cascadia subduction zone.

“Five magnitude 5 quakes in an hour is impressive though.”

Dr Lucy Jones, a seismologist and author based in Southern California, also assured magnitude 5 quakes in the region are common and “have never been followed by something on land”.

Earthquakes on a scale of magnitude 5 to 5.9 on the Richter scale are considered to be “moderate” and can cause damage to a building.

The USGS explained: “Magnitude is expressed in whole numbers and decimal fractions.

“For example, a magnitude 5.3 is a moderate earthquake, and a 6.3 is a strong earthquake.

“Because of the logarithmic basis of the scale, each whole number increase in magnitude represents a tenfold increase in measured amplitude as measured on a seismogram.”

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