There is also a strong correlation between the phases of the moon and strong earthquakes. And a cool urban legend story to go along with it.
As the legend goes, during that same Sylmar earthquake there was a McDonnell Douglas aeronautical engineer living in the San Fernando Valley near the epicenter. Like most everyone else that morning, he also got tossed out of bed at 6:00am.
As he was picking himself up off the floor he supposedly glanced out his bedroom window and noticed that the moon was full. Being a good engineer this observation stuck in his mind. After a few days of mentally chewing on it, he called up a friend who was a geologist and asked if there might be any correlation.
Now it's a well known geologic fact that the earth itself is not perfectly solid, but rather quite elastic. In fact, the planet also experiences tides, just like the oceans. Except that the tidal deformation is only on the order of a couple of centimeters maximum.
So the geologist decided to plot the worldwide frequency of major seismic events (6.0+ on the Richter scale) against the lunar cycles. The plots matched almost perfectly. As well, a significant majority of those events had occurred within plus or minus two hours of dawn or dusk local time.
What was happening was that (picture this mentally) with the sun and moon on opposite sides of the earth, or both on the same side, the planet itself would be stretched oblong by slightly stronger parallel gravitational attraction. That stretching, or pulling apart, would be maximized at the point where the earth's rotation placed any fault lines in opposition at either the top or the bottom of the planet, if the sun and moon were horizontally oriented on either side. And that particular geometric orientation always occurs at sunrise or sunset. Or within plus or minus two hours of it.
That orientation, and the gravitational anomaly associated with it, is apparently just enough to reduce the static friction preventing rupture in those faults that are already near the breaking point. When those faults are already at their threshold, all it takes is a new or full moon orientation at near sunrise or sunset to be the straw that breaks the camel's back.
In both of the following cases the earthquakes occurred within one day of either a new or full moon, and within two hours of sunrise or sunset.
The Sylmar earthquake occurred on February 9, 1971 at 6:00:41 AM in the morning. Sunrise was at 6:44 AM. Here's a chart of the phases of the moon for February, 1971. The full moon was on February 10th, one day later:
The recent near-Los Angeles (main) seismic event occurred on March 29, 2014 at 9:09:42 PM in the evening. Sunset was at 7:12 PM. Here's a chart of the phases of the moon for March, 2014. The new moon was on March 30th, one day later:
So just by keeping an eye on the monthly phases of the moon, it's possible to predict (and live in fear of) periods of significantly higher risk for major earthquakes. My wife has heard me ask so often for years now "Was it a full moon?" every time there's an earthquake that she now just automatically includes that info when she reads about one and tells me.
And that brand new major 8.2 earthquake today off the coast of Chile? Well, today is only 2 days after the new moon, and it happened at 8:46:46 PM local time, when sunset was at 7:38 PM.
There you go...