Hurricane Sandy over the Northeastern United States (Image: NOAA)
When the superstorm Sandy raged in New York last October, it brought not only destructive winds and floods with it. The earth itself shuddered under the force of wind and waves. Throughout the US, a network of seismometers recorded a significant increase in the quake signal during the storm, according to US researchers at the annual Salt Lake City Seismological Society of America meeting. The way of the storm could be understood so astonishingly exactly on the basis of its seismic effects. This finding could also help monitor future storms, researchers said. The Earthscope network, which spans from California to the east coast of the US, consists of around 500 mobile seismometers - gauges that register and record the horizontal and vertical vibrations of the subsurface. Unlike many other seismological measuring networks, however, they are not primarily for the monitoring of earthquakes, but for geological research. Because the sensitive instruments are to make the echo of distant quakes visible - the waves that spread from distant regions through the earth's mantle and then come back to the surface under North America. Similar to a computer tomography that makes the inside of the human body visible, the reflections and disturbances of the quake waves are supposed to provide information about the structure of the rock layers in the mantle.

Rash as in a quake of magnitude 2-3

As part of these investigations, Earthscope's seismometers were also on reception from October 18 to November 3, 2012 - at the time Hurricane Sandy moved north along the east coast of the US. The instruments registered meanwhile a whole series of small shocks, so-called microseisms. "These waves helped us track how the hurricane moved, " says Oner Sufri, a geologist at the University of Utah. Especially as the storm turned west-west and headed for New York, the seismometer rashes had become more intense. The registered vibrations corresponded approximately to those of an earthquake of magnitude 2 to 3.

"Many are unaware that seismic waves can be caused not only by earthquakes, but also by a whole host of other factors, " says Sufri. For example, explosions, the collapse of a mine during mining, even traffic or construction activities can cause the subsoil to vibrate, which can still be registered by sensitive measuring equipment miles away. The meteorite, which exploded on 15 February 2013 over the Russian Chelyabinsk, also generated rapidly spreading shockwaves. In contrast to such short-term events, a storm like Sandy leaves another, much longer-lasting signature in the seismic record, Sufri explains. A storm could release its energy for many hours. display

Standing waves make the ground vibrate

As the researchers report, Sandy's ground vibrations were not caused by the wind itself but by the waves generated by the hurricane. Normal sea waves do not reach very deep into the water, just a few meters below the surface of them is usually little more to feel. Unlike a storm like Sandy: Then waves collide with each other and rock so far that so-called standing waves arise. "The pressure of such standing waves is still high on the seabed and transmits there in the underground, " said Keith Koper, director of Seismografenstationen the University of Utah.

Especially many and strong waves of this type were created when Sandy changed his train track and swung to the west to the country. "The wind speeds did not increase, but the wave interactions intensified and significantly more seismic energy was generated, " says Koper. And not only that, but the frequency of the waves changed with Sandy's change of direction: "The periods were longer and so there were fewer heights and more bass in the seismic signal, " said the seismologist. For him and his colleagues, these signals came as if called - because with their help, they can now gain more knowledge about the underground structure under the North American continent.

But also in the monitoring of storms, networks such as the Earthscope could provide valuable help in the future. In the case of Sandy, the researchers only came across the telltale signals of the storm afterwards. In her view, seismometers like theirs could help in real time in the future to more accurately determine the orbit and energy of a storm. Because the vibrations could provide information that escaped the weather satellites.

Oner Sufri (University of Utah, Salt Lake City) et al., Seismological Society of America Annual Meeting © science.de - === Nadja Podbregar

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