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