JAMES KING-HOLMES / SCIENCE PHOTO LIBRARY JAMES KING-HOLMES / SCIENCE PHOTO LIBRARY
Sonic event seismograms. Recordings, by 2 British Geological Survey stations in the Scottish Lownet network, of the sonic event on 20 February 2001, over Montrose, Scotland. Vertical ground movement speeds were recorded by sensitive seismographs. The movement was produced as a sonic boom (from an aircraft or meteor) hit the ground. The shockwaves spread outwards through the ground at around two kilometres a second. The event occurred at 09:37 UTC (GMT). The Dundee seismogram (top) recorded a strong signal about 21 seconds before the weaker signal at the Logiealmond site (bottom). This is consistent with the extra distance the Logiealmond detector is from Montrose (around 43 kilometres).
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