February 21, 2012 - Earthquakes by Time of Day: I downloaded all earthquakes >= to 6.0 magnitude, from 1973 to 2012, from the USGS website. I then categorized them by time of day. The results are presented in the graph above. The axis at the bottom (X) reflects the time of day, e.g., ’0AM is between Midnight and 59 minutes and 59 seconds, ’1AM is between 1:00AM and 1:59AM, etc. The left axes (Y) are the number of earthquakes. The gold line running through the data is a linear best fit line.
What struck me as odd was the significant variance between hours of the day and earthquakes given that the sample size was so large (N = 5,329). The lows at 9AM (203), 4PM (202) and 10PM (201) are almost two standard deviations away from the average, which means that they are 95% outside a normal curve and very unusual. The highs at 2PM (246), 6PM (242) and 11PM (240) are also statistically significant, because they are greater than two standard deviations away from the mean.
The number of earthquakes between 1AM and 3AM are fairly high and fall off to a low at 9AM. The quantity then increases from 9AM and peaks at 2PM. From 2PM, it plunges and then forms a rather chaotic pattern of highs and lows.
Heating and cooling of the outer crust of the earth changes from the night to the day. Cold tends to contract and heat expands. On average, if around 6AM is sunrise then about 2PM would represent the higher heat of mid-day and perhaps explain the high of 246 quakes at 2PM? Then there is a drop-off to 4PM and another peak around sunset at 6PM.
However, the large variability during the night or day time is not explained and I am perplexed as to why this occurs. With a sample this large, I would expect a normal distribution! (Credit: Data – USGS, Narrative – W. G. Foster)
The Master of Disaster