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
Just thinking out loud here, I am assuming that your data sample was UTC and therefore whilst a quake in Japan say might be 00:00 UTC it would be 09:00 in Japan (or however far ahead they are) and therefore I would imagine that the heat of the day would have little to do with this since in that scenario the high spot for Japan would be around 6 am UTC when you are showing a movement downwards in numbers. One would need to convert every quake into it’s local time equivalent for this to work I would have thought.
Like you I find the variability based on time of day strange. Perhaps some strange quirk of tidal movement? Don’t know.
Off courese, converting over five thousand quakes to actual some of day at the epicenter would be difficult. This makes them all equal and shows the strange distribution.
yours,
Bill
Over here in California’s Los Angeles Basin, we always get that 3pm breeze which will always cool off a heat spell. We also have an inversion layer (smog) that keeps us relatively on a moderate swing as far as temperature goes. You might want to try your “Earthquakes by Hour of the Day” survey according to regions of the world instead of Globally. You might get some surprising results. I wonder if the land masses around the Indian Ocean have more earthquakes than land masses around the North Atlantic, or the South Atlantic, or between Australia and New Zealand, etc.
I live in southern California for a number of years. I don’t think temerature has too much to do with earthquakes but I really do not know.
The regions of the world idea has merit, however, with over 5,000 in the population it will take more time than I currently have. I get over 500 emails per day.
Over 80% of global earthquake occur around the Pacific Ring of Fire, which includes the western part of South America, up the western edge of North America, southern Alaska and along the Aleutian Island chian west to the Kamchatka Peninsula in Russia, south down the Kirul Island, along the eastern coast of Japan, through the Philippines, then east to southeast through the Solomon Islands, Tonga, Vanuatu, the Sandwich Islands, Samoa; then south along the eastern side of New Zealand and then east along the southern South Pacific Ocean.
Therefore, their are not that many quakes, relatively speaking, in the North and South Atlantic; compared to the Pacific Tectonic Plate system. The North Atlantic includes Iceland, where the mid-atlantic spread fault zone appears on land.
yours,
Bill