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US hit by 90 earthquakes on Tuesday

August 25, 2011

The earthquake felt across the east coast yesterday may have been a rare event – but there were 90 other shakes across the U.S. on the same day.
The biggest of those was a 5.3 in Colorado, but only a few of the 90 were felt by people and just 12 were above a magnitude of 3.0.

The 5.8 earthquake in Virginia that was felt in New York and Washington D.C. was so widespread because the tremors were shallow at between one and four miles deep.

Tuesday was a typical day for earthquakes in the U.S. in terms of numbers of tremors – but such a big earthquake on the east coast was very unusual.A small earthquake shook the San Francisco Bay area as well last night, striking at 11:36pm local time and centred six miles from Oakland.

‘It was a strange day,’ Rafael Abreu, of the U.S. Geological Survey, told the Los Angeles Times. ‘But Mother Nature teaches us something new every single day.’

The Virginia area was hit by its strongest quake since 1897, which resulted in buildings such as the Capitol and Pentagon being evacuated.

The quake’s severity was ‘highly unusual’, one Brown University professor said, although it was in an area known for smaller quakes and it spread because of accommodating terrain.

‘This is the largest earthquake by far that I am aware of occurring there in recent history,’ seismology professor Karen Fisher said.

The wide spread of the shockwaves was common for the east coast – which is largely due to the difference in the terrain, research seismologist Peggy Hellweg said in California.

‘(California’s) ground is all of this chopped-up stuff – like a pile of marbles,’ she said, which means that the waves from a quake don’t spread out that far.But in eastern America, ‘what you’ve got there is gorgeous bedrock (and) the waves propagate beautifully.’

In addition, there is a difference in plate tectonics because the west coast is over the boundary between the North American and the Pacific.
She said in the East, ‘There’s no driving engine in terms of the two plates sliding past each other – so that’s why it’s much more unusual.’

Small earthquakes have been happening in central Virginia since at least the 1770s, reported the U.S. Geological Survey.

Quakes in central and eastern parts of the U.S. are less frequent than those in the west but are normally felt over a larger region.

Earthquakes east of the Rocky Mountains can be felt over an area up to ten times larger than one of a similar strength in the west.

An earthquake of the 5.5 level can often be felt as far as 300 miles from where it happened and cause damaged up to 25 miles away.Quakes happen on faults within bedrock that are usually miles deep, reported the U.S. Geological Survey.

Bedrock beneath Virginia came together as continents collided to create a super-continent up to 500 million years ago.
This was what raised the Appalachian Mountains and the bedrock rifted apart to form northeastern America and Europe.

The nearest plate boundaries from the central Virginia seismic zone are in the centre of the Atlantic Ocean and Caribbean Sea.But few shakes in the seismic zone can be connected to named faults because many are deeply buried and undetected.

Therefore the best guide to earthquake hazards in this seismic zone is actually the earthquakes themselves.

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