Florida’s Shakin’, Shakin’, Shakin’…Not


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Seismic history of Florida

This map illustrates Florida’s earthquakes, 1973 – 2012. Or perhaps it would be more accurate to state that it shows Florida’s lack of tremors during this period. Image by USGS.

Ah, Florida. Land of oranges and rich, single princesses on the hunt; of white beaches regularly redesigned by hurricanes; of golf courses patrolled by alligators; of acres of retirement parks full of happy campers singing ‘The Old Folks at Home’; of Everglades and, bizarrely, a (not officially recognised) state tartan in red, white and blue.

One thing is missing from this list. No-one in their right mind could claim that Florida is the earthquake state.

In fact a look quick look shows the map of the state’s recent seismic history to be almost embarrassing in its blankness and one questions the value of the Earthquake Seismograph Station at the University of Florida which:

became operational [in October 1977]. No local events were recorded until November, 1977 when slight shock was recorded north over peninsular Florida. This tremor was not large enough to be felt, but was recorded as Richter magnitude 0.8. At this writing (December, 1981) no additional tremors have been recorded. However, with the advent of continuous seismic recording for Florida, continuous updating of this record may now be accomplished.”

Not much has happened since 1981, either. Take a look at the map for yourself…

Florida’s Tectonic Setting and Geology

This inactivity in a dynamic world begs a question: Is the Sunshine State suspiciously quiet? Well, a look at the planet’s earthquake zones suggests not. Most earth tremors are associated with the margins of the Earth’s tectonic plates and Florida is securely distant from any of these; the closest is the transform boundary between the North American and Caribbean plates, almost 800 miles to the south.

Florida is hundreds of miles from the nearest plate boundary. Image by USGS.

Areas distant from plate boundaries are not, of course, immune to earthquakes, either natural or anthropogenic (see below).

Smaller tremors regularly occur in otherwise stable areas as a result of movement along existing faults — a mechanism which accounts, for example, for pretty much all of the United Kingdom’s earthquakes. Yet compared to Florida the otherwise-tectonically-calm UK is a hotbed of tremblors, about as stable as a jelly at a child’s birthday party.

Why is that? Well, in a bizarre coincidence, the Retirement Home State is, geologically at least, very young. The older the bedrock, the more likely there are to be faults, legacy of millions of years of continental breakup and reassembly.

Most of Florida, although its bedrock is of the order of 200 million years old, was laid down between 56 and 23 million years ago. With no major continental movements, there’s been little opportunity for faults to develop.

Florida’s Earthquakes

Against this background you might be surprised to hear that Florida, like every other teenager, has had its geological temper tantrums. The most disturbing of these were in 1780 in the northwest of the state and in 1879 near St Augustine.

Both of these events predate the development of seismography, so their size can only be estimated from contemporary written sources, but both have been assigned an intensity value of VI — broadly, around M5-M5.5. There’s nothing to suggest the causes of these tremors but the likelihood is that they were the result of normal movement on existing faults.

There have been other minor tremors too. As the USGS notes (clearly trying not to sensationalise):

On June 20, 1893, Jacksonville experienced a slight shock, apparently local, that lasted about 10 seconds. Another minor earthquake shook Jacksonville at 11:15 a.m., October 31, 1900. It caused no damage… On November 18, 1952, a slight tremor was felt by many at Quincy, a small town about 20 miles northwest of Tallahassee. Windows and doors rattled, but no serious effects were noted. One source notes, “The shock interfered with writing of a parking ticket.” It didn’t say in what way.”

Florida’s problems, seismologically at least, are more likely to come from its noisy neighbours. The state felt shaking from the largest earthquake in North Carolina in 1886 and from shocks in Cuba a few years earlier.

Not in my Backyard

Just as Nature intended – Florida is not just earthquake-free but fracking-free, too. Image by NASA

Much has been written, not least on Decoded Science, about human-induced seismicity and it’s a hot topic of conversation across the US. The processes of mining, and of fracking and wastewater injection, have been linked to earthquakes elsewhere, in the latter case as a result of fluid injection reactivating ancient faults.

Florida, as we’ve seen, doesn’t have that many ancient faults to be reactivated. Nor (at present) does it have any fracking – and the mining activity in the area is shallow and largely related to extraction of sand and gravel.

This doesn’t mean that there’s no seismic activity as result of human activity, however, because quarrying involves dynamite, and dynamite is pretty impressive stuff.

That said, the seismic activity attributed to blasting is limited and debatable. The USGS comments that:

Three Florida shocks of doubtful seismic origin rumbled through the Everglades – La Belle – Fort Myers area in July 1930, Tampa in December 1940, and the Miami – Everglades – Fort Myers area in January 1942. Most authorities attribute these incidents to blasting, but a few contend they were seismic.”

A Land Far, Far Away: Tsunami Jon

When the Earth moves in one place, it can be felt in another. In all probability the most serious threat to Florida as a result of an earthquake doesn’t come from within the the state itself but from elsewhere. Nor does it come directly from an earthquake but from the sea.

Meet philanthropist Jon Plotkin (and his grandson, and a fish) who ‘won’ this article in the Decoded online auction of 2014.

The eastern edge of the Caribbean plate is marked by the Lesser Antilles subduction zone, thought by some scientists to be capable of generating a megathrust earthquake of M8-M8.5. Earthquakes of this size can produce major tsunamis which pose a risk to coasts many thousands of miles away. Even here, the risks to Florida are pretty small as the path of any such tsunami would be expected to have its greatest impact on coasts on the other side of the Atlantic.

If such a wave, which we call Tsunami Jon in honour of this article’s sponsor, is to strike in Florida, it’s most likely to be caused by a landslide rather than an earthquake. There’s massive speculation that the collapse of Mount Teide, in the Canaries, is imminent: but Floridians can probably rest secure.

A BBC report quoted research scientist Russell Wynn as noting that the risk has been massively overhyped, noting that he “says it means there is a lot less to worry about if a landslide is triggered.
‘If you take a brick and drop it in a bath you’re going to generate quite a big splash. But if you break the brick up into 10 pieces and drop them in one by one you’re going to get 10 much smaller splashes’.

The Right Attitude to Earthquakes

According to the USGS, Florida is one of only four states that had no earthquakes between 1975 and 1995 (the others were Iowa, North Dakota, and Wisconsin). The geological youth of the state and its sheltered position in relation to other earthquake and tsunamigenic zones means that the seismic hazard of the state is limited, although not negligible.

My advice Floridians is to rest easy. You’re more likely to be at risk from a murderous librarian.

NOTE: This article is published courtesy of Jon Plotkin, successful bidder in the Decoded fundraising auction for an earthquake article on an area of his choice. Jon chose Florida. Thanks, Jon!

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