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Surface waves and free oscillations
Abstract:
Marine microseisms are known to be the major source of seismic noise. Generally, ground motions in the frequency range between 0.05 and 1 Hz induced by ocean waves are referred to as microseisms. In this article we show that in addition to such microseisms, strong storms over the North Atlantic Ocean can also cause an increase of seismic noise at lower frequencies. As an example, a storm in 1999 October is analysed in detail. When the ocean waves caused by this storm hit the coastline, seismic Rayleigh waves with frequencies below 0.02 Hz were excited and could be observed globally. Using broad-band seismic networks in Germany and California as arrays, these Rayleigh waves can be traced back to the centre of the storm. Between 1999 and 2007, we identified more than 40 events with similar characteristics. Since it is expected that such storms also occur in other regions, it is likely that these storms together contribute significantly to the continuous excitation of the Earth's free oscillations, also known as the hum of the Earth.
