John Metcalfe was CityLab’s Bay Area bureau chief, covering climate change and the science of cities.
Earthquakes aren't the only thing to make the earth shiver.
East Coasters huddling in powerless homes as the winds outside raged weren't the only ones to feel the wrath of Superstorm Sandy. Seismic devices throughout the United States were also picking up on the low-pressure system's immensity – and some of these devices were located as far away as Seattle.
Earthquakes make the earth shiver. But on a lesser scale, so do strong tornadoes, construction work and you'd better believe that huge Russian meteor. These things generate smallish seismic waves called microseisms. When Sandy was approaching New England, it was throwing out such energetic microseisms that it was like the country was being rocked by 2 to 3-magnitude earthquakes.
That's the wobbly word from Oner Sufri and Keith Koper, scientists at the University of Utah who are getting ready to publish a paper about Sandy's seismicity. The men were able to detect the storm's geological footprint with the help of EarthScope, an ambitious endeavor to map what's under the surface of North America. EarthScope includes a network of 500 seismometers scattered around the states; by studying the data from Sandy's approximate lifetime of October 18 to November 3, the researchers got a pretty good idea of how the 1,000 mile-wide tempest was beating the Atlantic coast with fierce waves and causing the whole country to reverberate.
Here's how they explain it:
Normal ocean waves “decay with depth very quickly,” says Koper. But when Sandy turned, there was a sudden increase in waves hitting waves to create “standing waves” like those created when you throw two pebbles in a pond and the ripples intersect. “Pressure generated by standing waves remains significant at the seafloor,” he says.
“When Sandy made that turn to the northwest, although wind speeds didn’t get dramatically bigger, the seismic energy that was created got tremendously bigger because the ocean’s standing waves were larger from the wave-wave interaction,” he adds.
Not only did the seismic waves become more energetic, “but the periods got longer so, in a sense, the sound of those seismic waves got deeper – less treble, more bass – as the storm turned,” Koper says.
The researchers say that the ground-thrumming was greatest when Sandy made that calamitous turn toward New York and New Jersey, with powerful winds and a storm surge causing such a historic amount of damage that meteorologists have retired the hurricane's name forever.
Here's that shaking illustrated in a University of Utah video. The dots are seismometers – blue-green means low activity, yellow-orange-red means the party is going full-blast. Interestingly enough, this rock-deep technology might be of assistance in future hurricane forecasts, seeing as how it gives a second layer of data on where a storm is and how much energy it's packing:
Top photo of Sandy on October 30 by the NOAA/NASA GOES Project