Hurricane Season & Microseisms

Microseisms are the dominant natural noise that we record on broad-band seismographs. This noise looks like continuous harmonic motion with varying patterns of amplitude modulation. The typical wave period of the harmonic motion is 6 seconds, though it can vary from 4 to 12 seconds.

"Hurricane Season" refers to an article in the EduQuakes column of the Seismological Research Letters (go to SSA link) by L.J. Ruff. This article refers to a research project by Richard Ensign (CSDM) that was originally published in the MichSeis newsletter (on-line via the MichSeis main page). The basic idea is to look at the averaged amplitude of the microseism noise over long periods of time (days), and compare that to the oceanic sources of the noise: storms at or near the margin of North America. Anyone can "play", see the EduQuakes column for web links to oceanographic data (e.g. Click here for current global wave heights, courtesy of the US Navy)

Hurricane Bonnie: August 1998

• For Hurricane track information, go to:
  National Hurricane Center, or
  Hurricane arhcive at Purdue
  Then compare to seismic data below:
• "Typical" seismogram at AAMN (plot gain is x32) with background noise before arrival of Hurricane Bonnie.
• AAMN seismogram (plot gain is x32, same as above) around the peak amplitude in the microseism storm.
  Note that the dominant period of the microseisms is about 5 seconds.
• AAMN seismogram (plot gain is x32, same as above) after Hurricane Bonnie hits the USA mainland coast.
• Graph of the 5-second microseism amplitude at MichSeis station AAMN, just north of Ann Arbor, MI
  This graph shows microseism amplitude over 11 days.
  Click for technical details of analysis

Hurricane Georges: September 1998

• For Hurricane track information, go to:
  National Hurricane Center, or
  Hurricane arhcive at Purdue
  Then compare to seismic data below:
• "Typical" seismogram at AAMN (plot gain is x32) on Sept. 21 & 21 hr, with background noise before arrival of Hurricane Georges.
• AAMN seismogram (plot gain is x32, same as above) around the peak amplitude in the microseism storm on Sept 25 & 9 hr.
  Note that the dominant period of the microseism maximum is about 8 seconds (compare to Hurricane Bonnie).
  But, even more puzzling is the location of Hurricane Georges during this microseism peak (look at track in the above Hurricane web sites).
• AAMN seismogram (plot gain is x32, same as above) on Sept. 29 & 18 hr during the secondary peak in the 5 second microseisms.
  Where is Hurricane Georges now?
• Graph of the 5-second and 8-second microseism amplitudes at MichSeis station AAMN, just north of Ann Arbor, MI
  This graph shows microseism amplitude over 11 days from Sept. 21 through October 1, 1998.
  Click for technical details of analysis
• After you have compared the track of Hurricane Georges (available from the above Hurricane web sites) with the microseism amplitude data, you might still have some questions about the association of hurricanes and microseisms.
  If so, you might be interested in this image (courtesy of the US Navy) of ocean wave heights from Sept 25 & O hr.