| Teleseismic Earthquake Example
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Example seismograms from earthquake in Turkey
August 17, 1999, 00:01:47.8 - Magnitude 7.5
This destructive earthquake clearly illustrates the different arrival times of different seismic phases. The earthquake occurred at roughly 2 minutes after midnight on August 17th, Greenwich mean time. The first arriving phase is the P wave which arrives at roughly 11 minutes after the quake at 00:13. The S wave arrives after 21 minutes at 00:23 and the variety of surface waves (love and rayleigh) begin to arrive 34 minutes after the event at 00:36.
LCSN seismograms for the event are displayed below (vertical, north, east). All three types of waves are visible on each record. Comparing them reveals that the compressional P wave is best displayed on the vertical component, while the shear waves are most clear on the horizontal components (North and East). An event of this size generate massive surface waves which are visible on all components. For small earthquakes though, the vertical component is typically the strongest.
Closer observation reveals, more complicated phases as well such as the arrival at 00:28 which is the shear S wave bouncing once off the surface before reaching the seismometer. Note also the near-continuous jumble of vibrations that continue for hours after the event. These vibrations are the interaction of numerous surface waves with different speeds and directions (for example, the surface waves which travel the opposite direction around the earth to reach the seismometer). The clear sharp phases which arrive before the surface waves (such as P and S) are typically the most diagnostic.
