66,725 Earthquakes from 1973 - 2011 with color indicating magnitude
Time and tide wait for no one. Add to that: earthquakes. I live in the San Francisco Bay Area, a.k.a. “earthquake country”, in a small house built in the 1950s before earthquake building codes had been created. Within the next 30 years, the USGS tells us we can expect a “big one” in the East Bay right along the fault where I dwell.
So here’s my question: If there’s a 30-year window for the next big one to occur, can I at least know the most likely time of day? This is not a crazy question. The time of day is really just a way of expressing where the sun is located with respect to your geographical location. The moon is responsible for sweeping the tides all around the Earth. So it seems reasonable to think that the moon or the sun may at least be an influence in “Earth tides” which might act as a trigger for earthquakes. Here’s a quick sketch showing my hypothesis:
This question turned out to be fairly straight-forward to answer and I’ll cut to the chase and say, no, not really. There are some hours that are a teeny bit more earthquake prone than others, but the variations proved to be statistically insignificant.
The Method
I downloaded all earthquakes magnitude 5 or greater from 1973 through mid-2011 from the USGS Global Earthquake Search website. This gave me a list of 66,725 earthquakes — a reasonable sized dataset. I mapped the positions and color-coded magnitudes of all 66,725 earthquakes (green = mag 5.0 up through red = mag 9.1), shown at the top of this blog entry.
It’s an interesting fact that these earthquakes span a time period of 338,117 hours which implies a chance of 20% for an earthquake (mag 5 or greater) during any hour. The chance during any hour for a magnitude 6 or greater earthquake drops to only 1.6%. By the time you get to a magnitude 7 or greater it’s much less than 1%/hr.
The next step was to calculate the right ascension of the sun and moon and translate the longitudinal position of each earthquake into right ascension to match. Below shows an illustration of what I’m describing.
The important thing to note is that the latitude of the sun, moon and earthquake can all be different and I am calculating the difference with respect solely to longitude. This is because I’m wondering about the most earthquake-prone time of day so the longitudes are the relevant quantities rather than the latitudes.
After these coordinates had been calculated for all 66,725 earthquakes, I found the difference of the position of the sun/moon with respect to each earthquake in terms of right ascension. Following that, I grouped the differences by “relative hour” (by which I mean the relative position as described above) and graphed the resulting histogram.
Position of the Sun vs. Earthquake Time
The histogram of the earthquakes relative to the position of the sun looked like this.
The blue bars show what the histogram should look like if there was an equal probability of having an earthquake regardless of the position of the sun and the red shows the actual data. You can see the bars are very close in size. There’s a small peak around the 17th hour. But is it statistically significant?
The mean number of earthquakes during any given relative hour was 2780 earthquakes. The standard deviation was +/- 60 earthquakes. The number of earthquakes in the 17th relative hour was 2902 earthquakes — just outside two standard deviations which shows it to be an outlier by 2 earthquakes. Not a strong outlier! In fact, the p-value using the Watson U-Squared test is a paltry 0.48 which is well outside the threshold for being a significant result. Translation: not publishable!
Position of the Moon vs. Earthquake Time
Since it is the moon, not the sun that is primarily responsible for sweeping the ocean tides around the Earth, perhaps I am looking at the wrong entity (actually, the sun is responsible for a smaller, secondary ocean tide, but the magnitude pales in comparison to the moon’s effect). The time of day has nothing to do with the moon’s position so if there worked out to be a correlation then you’d need to consult moon charts every day!
I redid my previous analysis, grouping earthquakes by position relative to the moon. The following histogram was the result:
As can easily be seen with the eye, the deviation of the data from the mean is even smaller this time (+/- 53). In fact, there were no points outside two standard deviations and the p-value was 0.33. Again, statistically insignificant!
Conclusion
In short, there’s no particular hour to be extra wary of earthquakes. Unfortunately, I’ll just have to settle for the USGS’s “sometime in the next 30 years”.
Data extracted from:
- Earthquake data from the USGS website which lists earthquake worldwide by year: http://earthquake.usgs.gov/earthquakes/eqarchives/year/
- To specify exactly which earthquakes you are interested from which database, etc (where this dataset came from): http://earthquake.usgs.gov/earthquakes/eqarchives/epic/epic_global.php
- Planet positions calculated via WolframAlpha: http://www.wolframalpha.com/
References:
- General information on the Earth: http://en.wikipedia.org/wiki/Earth
-Lyndie Chiou
An algorithm from researchers at Cornell has managed to data-mine the underlying laws of physics in just under one day.
In another example of data-mining, researchers were able to answer one part of a hieroglyphic mystery that has perplexed archeologists to this day. The Indus Script from 4,000 years ago has remained undeciphereable. Some linguists have insisted that it is no language at all, but merely political ciphers (like the Democtratic donkey or Republican elephant) that were important in that day. The problem is that the longest chain of Indus Script contains only 27 characters, making it extremely difficult to crack. A group of researchers has now managed to “prove” that it is a real language by showing that the entropy level of the order of the Indus characters is very similar to human language. 
In other fascinating news, did you know that you can’t tie your shoelaces? If you don’t believe me, visit