The same phenomenon that creates ocean tides is responsible for certain small “low-frequency” San Andreas fault earthquakes, according to new research. In a study recently published in Proceedings of the National Academy of Sciences of the United States of America, four scientists from the U.S. Geological Survey (USGS) and the Los Alamos National Laboratory found during one tidal cycle phase that small earthquakes happen more than during other phases. The information won’t help scientists predict earthquakes just yet, but will give them more insight into deeper, formerly inaccessible areas of the fault.
Like ocean tides, the Earth’s surface undulates during the tidal cycle. Albeit a well known occurrence, the new study shows low-frequency earthquakes are “most likely to occur” during a certain part of the tidal cycle: the “waxing fortnightly tide.”
Low frequency earthquakes usually measure less than one on the Richter scale and happen nine to 19 miles underground. The study’s authors scrutinized 81,000 of these little earthquakes on the San Andreas fault between 2008 and 2015.
Lead author Nicholas van der Elst of the U.S. Geological Survey told the Los Angeles Times, “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.”
According to another study author, David Shelly, the small San Andreas fault earthquakes “tell us a lot of things about the deep part of the fault that before, we had no idea existed at all.” Scientists can now keep track of how the fault moves. They don’t yet know if this new information will afford them a “warning” before a big earthquake occurs, and intend to keep monitoring the fault.
Via the Los Angeles Times