Twice every month, the sun and moon briefly line up, causing an extra little tug of gravity on the Earth. This pull creates a spring tide, or the period with the highest and lowest tides of every month. Now, new research by the USGS shows that the celestial alignment also pulls a little on California’s San Andreas fault, causing tiny tremors deep in the earth that give scientists a peak into the inner workings of the famous earthquake zone.

In 2008, researchers used extremely sensitive seismometers to note daily tremors deep under the earth’s crust below Parkfield, California, which lies on the San Andreas fault, writes Eric Hand for Science. These low frequency earthquakes, which are usually below magnitude 1, take place about 19 miles under the surface near the zone where the Earth's crust meets the mantle, reports Rosanna Xia at the Los Angeles Times. The researchers realized in 2013 that daily tides often triggered these little quakes.

USGS Geophysicist Nicholas van der Elst and his team took things one step further, combing through a catalog of over 4 million deep tremors recorded since 2008 and found that they are more likely to occur during the “waxing fortnightly tide” or the spring tide. Surprisingly, most quakes didn’t occur when the high tide reached its maximum height, but when the tide was growing and "was larger than the previous day’s tide by the greatest amount,” van der Elst tells Charles Q. Choi at LiveScience. The study was published this week in the Proceedings of the National Academy of Sciences.

“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,” van der Elst tells Xia. “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.”

These deep tremors aren’t an immediate threat to the surface. But they are important for the information they reveal about the structure of the San Andreas fault. The research shows a transition zone in the fault where continuous small slippage occurs compared to the upper zone, where infrequent slips lead to big quakes on the surface, Eliza Richardson, a seismologist at Pennsylvania State University, who was not involved in the study, tells Hand.

“They tell us that the fault continues down below where the regular or typical earthquakes stop on the San Andreas, about 10 or 12 km [about six to seven miles],” co-author and USGS sesimologist David Shelly tells Xia. “And they tell us a lot of things about that deep part of the fault that before, we had no idea existed at all.”

Shelly says the deep tremors act as little meters, recording how much the deep part of the fault is creeping, which transfers stress to shallower reaches of the fault. So far, the research has not linked the low-frequency tremors with an increased risk for quakes at the surface, but van der Elst hopes more research will show some connections.

“Every little thing we learn about the way faults work may ultimately contribute to a better understanding of the earthquake cycle and when and where big earthquakes are likely to happen,” he tells Choi. ‘The hope is that looking at low-frequency earthquakes that happen deep in the fault will ultimately shed light on how shallow parts of the fault accumulate stress.”