2016-07-18. Spring tides trigger tremors deep on California’s San Andreas fault.

posted Jul 24, 2016, 10:13 PM by Alan Gould
By Eric Hand, Science. For GSS Energy Flow chapter 2. Excerpt: ...Below the town of Parkfield, California, hundreds of thousands of slow microearthquakes called tremors go off routinely where Earth’s brittle crust gets weaker and softer. Now, scientists have shown that these tremors are triggered by the rhythmic pulsing of the tides: not just the twice-daily tides that occur as the moon revolves around Earth, but also the twice-monthly spring tides that occur when the sun and moon align and pull strongly on the planet. The finding gives scientists a new window into a deeper part of the San Andreas fault, and new insight into how stress builds up on small patches of the fault until they snap. ...as seismometers got more sensitive and were laid down in more places, scientists started to identify tremors in the lower crust. In these deeper regions, faults are weaker, and that means that tides can play a more important role. In 2012, scientists spotted deep tremors on the San Andreas fault below Parkfield that were tidally triggered, at the twice-a-day tidal peaks associated with the lunar day. In the new study, Van der Elst and his team found that bursts of tremors were also triggered during waxing of the twice-monthly spring tides, when the moon is aligned with the sun. Using a catalog of 4 million tremors that occurred between 2008 and 2015, they pinpointed the location and timing of the tremors in relation to the tides, they report today in the Proceedings of the National Academy of Sciences.  ...The tremors can’t predict the next “big one,” but in the long term, they could help scientists understand how big ones are set off....  http://www.sciencemag.org/news/2016/07/spring-tides-trigger-tremors-deep-california-s-san-andreas-fault