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I live in a generally geographically active area (Southern California, near the San Andreas fault, if that matters), that has had two relatively minor earthquakes that I have felt in the last week (in the 3-4 range on the Richter Scale, I believe), as well as frequently in the past. I am used to this, being born and raised in the region.

I was talking about these phenomena with a colleague who recently moved to the area from a non-earthquake prone place, and was not used to them. I realized that both of the earthquakes were late at night, when I was in bed, and searching my memory I can only recall encountering these minor earthquakes at similar times.

My question, therefore, is if there is any correlation between time of day and earthquakes? Or to put it another way, is my observation based on anything more than the fact that I am more likely to note and recall a minor earthquake what I am not engaged in another activity?

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  • $\begingroup$ (Mine isn't the same question at all, but figured I'd link here to Are there any seasonal or diurnal trends in earthquakes? since it also relates to possible time relationships) $\endgroup$ – JeopardyTempest Aug 29 '18 at 18:37
  • $\begingroup$ Some data points of uncertain value: I owned a condo in San Diego and the only earthquakes I felt were early in the morning between 4 AM and 6 AM. In Washington state, all three earthquakes were in the afternoon, between 2 PM and 4 PM. I'm sure there is actual data out there to show local time for global earthquakes. $\endgroup$ – Tim Nevins Aug 30 '18 at 16:00
  • $\begingroup$ emsc-csem.org/#2 would be a good place to start looking for patterns. When I lived in the Bay Area (SF, not LA), the local newspaper would print a list of earthquakes that occurred the previous day. The ones I recall occurred in the afternoon. I'm guessing your local/online paper might print something similar. $\endgroup$ – Barry Carter Sep 1 '18 at 1:16
  • $\begingroup$ I live in the same area and remember 'feelable' earthquakes at lunchtime, during work, middle of the night... $\endgroup$ – mkennedy Sep 1 '18 at 14:36
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As noted by Barry Carter, you could take a look at https://www.emsc-csem.org/Earthquake/Map/zoom.php?key=75&typ=NAM#5 to see the earthquakes around San Francisco, with a table below giving the time and date of the earthquake(s). The USGS can create a similar data-set (try there). I used that to extract the earthquakes of magnitude 2.5+ in the SF area over the last 4 years:

Earthquakes in the SF area of mag. 2.5+ per hour, for 4 years

Or the same data with a Kernel Density Estimator in a polar plot (the circle corresponds to 24 hours):

Earthquakes in the SF Area with a polar KDE plot

Based on these plots, I think that there is no specific trend suggesting that earthquakes are triggered at specific times.

Earthquakes tend to have their 'core' at depths of a few km's usually, which means that they are not very aware of day-night cycles. Apart from perhaps very minor influence of gravitational differences due to sun/moon locations (which vary over the course of a month), the earthquakes truly follow their own 'random schedule' with respect to our life-cycle!

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