Magnitude 4.1 Earthquake Near Jonesboro Arkansas
February 10, 2005
Origin Time: 08:04 am CST (14:04 UTC)

Download the files seismograms.pdf and map.pdf. The first file, seismograms.pdf contains the recordings for the Feb. 10, 2005 Arkansas earthquake. Data are from recording stations MPH (Memphis, TN), UALR (Little Rock, AR), and SLM (St. Louis, MO). The plot indicates the P wave and S wave arrival times for each station in UTC time (Coordinated Universal Time, formerly GMT). To calculate the earthquake location:

1) calculate S-P time:

MPH
14:05:17 - S
-14:05:07 - P
10 seconds
 UALR
14:05:52 - S
-14:05:27 - P
25 seconds
 SLM
14:06:18 - S
-14:05:40 - P
38 seconds

2) Take this time difference for each station and multiply by 8. The p waves travel at roughly 8 kilometers per second. This will give the distance in kilometers the quake occurred from each station.

MPH: 10 x 8 = 80 kilometers
UALR: 25 x 8 = 200 km
SLM: 38 x 8 = 304 km

3) To plot the location, refer to the station location map, map.pdf. Using the above distances as radii, you can draw the arc of a circle around each station. The map scale indicates 100 kilometers = 2.5 centimeters. Calibrate the compass and measure from each station point on the map. Where the arcs intersect is the earthquake location.

If you want to determine the "Origin Time" (actual time the earthquake occurred at the source), take the s-p times above and multiply by 1.37. Take this time and subtract that from the P arrival time to get the approximate origin time.

For example:

MPH: s-p = 10 seconds
10 x 1.37 = 13.7
P wave arrival time is 14:05:07
subtract 13.7 seconds from 14:05:07 to give
origin time of 14:04:53.3 UTC
For the location information and map for a comparison, refer to the website:
http://www.eas.slu.edu/Earthquake_Center/NEW/localmaps.html