New analysis of seismic data shows that the sun and the moon could be behind mysterious low-frequency earthquakes occurring in and around the tumultuous San Andreas fault line. According to research published in the Proceedings of the National Academy of Sciences journal, the gravitational pulls of our moon and the sun are the cause of regular seismic disturbances.
Discovering the cause of these small quakes gives new insights into both the state of the fault and what lies at the very base of the fault some 20 miles (32 km) below the surface. Most earthquakes only occur in the top 6-7 miles (10-12 km) of the earth’s surface. These deeper earthquakes provide a source of constant data about the state of the fault.
The scientists in this study examined records of over 80,000 earthquakes that occurred between 2008 and 2015. After plotting all of the data, the researchers noticed a regular increase in seismic activity aligning with the two-week tidal cycle referred to as the "fortnightly tide."
The finding is remarkable given the orientation of the fault and the ocean; the fault is not directly parallel or adjacent to the ocean, meaning tidal forces aren’t at their strongest at the point where they interact with the fault line. This implies that rather than the tide itself, the gravitational forces of the moon are acting on the fault.
Nicholas van der Elst, a geophysicist with the U.S. Geological Survey and lead author of this study, stated in a Phys,org release he was surprised by the finding:
It's kind of crazy, right? That the moon, when it's pulling in the same direction that the fault is slipping, causes the fault to slip more - and faster. What it shows is that the fault is super weak - much weaker than we would expect - given that there's 20 miles of rock sitting on top of it.
The “super weak” fault line should be of grave concern to the millions of California residents who live within its vicinity. A 2006 Nature article predicted that the current stress on the San Andreas fault is strong enough to produce a 7.0 magnitude earthquake in the vicinity of Los Angeles much sooner than geologists previously believed.