Stormquakes: a new geophysical phenomenon

Image description: hurricane.

Researchers from Florida State University have discovered “stormquakes” – a new geophysical phenomenon formed when large storms interact with the ocean to cause a release of seismic waves that can travel for thousands of miles. 

These previously unknown seismic events can have the equivalent effect of earthquakes with magnitudes of over 3.5. It is hoped that this discovery could have a number of uses such as assisting studies that investigate the structure of the Earth.

As the name suggests, the newly coined term “stormquake” refers to tremors caused by storms such as hurricanes as they travel across the ocean. 

These tremors are coherent releases of energy and can continue to be a source of seismic waves for a relatively long time-period, from several hours to a few days.

A study published in the journal Geophysical Research Letters analysed data from 2006 to 2015 from the USArray; a project that collects seismic data from hundreds of sensors spread across the length and breadth of the North American continent. 14,077 stormquakes were detected during this period, well over a thousand per year.

Despite being so common, stormquakes have occurred undetected up until now because they were previously regarded as part of the background noise that these sensors produce. 

It was only by comparing the data to a record of storms or hurricanes during the same period that it became clear that stormquakes are in fact discrete, predictable events, separate from the background noise.

There are three ingredients needed to form a stormquake: large continental shelves, strong storms and ocean banks, which are flattened underwater hills. 

As all three of these conditions must be met, not every storm will create a stormquake.

For example, Hurricane Sandy, despite causing windspeeds of up to 90 miles per hour, produced no stormquakes when it struck the Caribbean and the eastern US coast in 2012. The geology of the path taken by Sandy was not right for generating the discrete bursts of energy needed to produce a stormquake. 

Stormquakes have occurred undetected because they were regarded as part of the background noise.

By contrast, Hurricane Bill created over 300 stormquakes in 2009 as it hit North America.

It is thought that the waves on the ocean surfaces that are produced as a storm passes over generate secondary waves that travel deep into the ocean itself. If this occurs in an area where the water is shallow enough, these secondary waves then strike the ocean floor like a hammer to produce the stormquake. In this manner, a seismic event can originate from within the ocean just like an ordinary earthquake can originate from within the Earth’s crust.

In the same way that there are “hotspots” around fault lines where earthquakes are more likely, researchers can use the topology of the ocean floor to predict where stormquakes could occur. 

For example, on the East coast of North America, stormquakes have been detected off the coasts of New England, Florida and in the Gulf of Mexico but are absent from New Jersey down to Georgia as well as being absent from the Mexican coast.

As this phenomenon has only just been described, it is not yet clear how significant a discovery it may turn out to be. They represent a signal amongst the noise (albeit a small one) and this could be used to provide information about the underlying structure of the Earth. Crucially, stormquakes might provide information for areas that are unaffected by earthquakes but are susceptible to stormquakes, such as New England. As with all scientific discoveries, it may take some time for the full impact of this new finding to be fully understood.

Image credit: US Coastguard