The Importance of Time in Seismology

In seismology, accurate time is very important.  Shock waves travel thousands of miles per hour and can cover vast distances in a short period of time.  However, the ways of reporting time can be confusing.  Hope this helps:

Military Time

Midnight is  00:00 hours
Noon is  12:00 hours
11:59 pm is  23:59 hours
4:32 am is  04:32 hours
4:32 pm is   16:32 hours

It is pretty simple, just add 12:00 to all time between 1:00 pm and 11:59 pm.  The am and pm are always dropped and the colon is sometimes dropped.  In seismology, time may be expressed to the thousandth of a second.  For example, an earthquake could be reported as occurring at 14:20:55.783 or around 2:21 pm.

UTC

All earthquake times are reported as UTC.  UTC is an abbreviation for Coordinated Universal Time or Universal Time Constant which are similar to Greenwich Mean Time (GMT) which is no longer used in seismology.  In Cleveland, Ohio when we are on Eastern Standard Time (EST), UTC is five hours ahead; for Eastern Daylight Saving Time (EDT), UTC is four hours ahead.

For example, the seismograph in Cleveland has registered shock waves from an earthquake.  The first wave is detected in Cleveland at 11:31:40 EST.  A check of the National Earthquake Information Service Website shows an earthquake in Peru at 16:24:07 UTC.  To convert UTC to EST, subtract 5 hours.  Now the time of the Peruvian earthquake is 11:24:07 EST.  This is still 7 minutes and 33 seconds earlier than the time recorded in Cleveland. Traveling thousands of miles per hour, it takes about 7 or 8 minutes for the P-wave to reach Cleveland from Peru, this at first appears to be unbelievably fast.  Yet we would determine that the Museum seismometer detected the Peruvian earthquake, and the P-wave traveled from Peru to Cleveland in 7 minutes and 33 seconds.