Ground shaking caused by Hurricane Ivan, as seen by the South Carolina Earth Physics Project seismograph at Scott’s Branch High School, South Carolina on September 16th, 2004.
Hurricanes can easily be recorded by seismographs, and before the days of satellite images, seismographs were sometimes used to track the storms. Ocean waves generated by the hurricane cause the most shaking, but shaking trees and buildings also contribute to the seismic waves. 24 hours of ground movement is plotted in the figure (each line is 2 hours long) and a gradual decrease in the amplitude over the course of the day is seen. This time period corresponds to the time when the winds from Hurricane Ivan dropped from 135 to less than 50 miles per hour. The signals shown on the plot are referred to by seismologists as microseismic noise and are always visible to some degree on global seismographs. The amplitude varies depending on the weather and the distance a seismic station is from the coast.
(Photo by South Carolina Earth Physics Project) |
