A new study, led by the US Geological Survey (USGS) scientists revealed the gravitational pull of Sun and Moon, responsible for inducing the tides, can also trigger special types of earthquakes on the well-known San Andreas fault.
Researchers discovered the small, deep tremors, known as low-frequency earthquakes on the Parkfield section in California, some 10 years ago. The San Andreas fault releases tectonic energy from the northern to the southern portion at that location.
The scientists surveyed data from 81 000 earthquakes of the same type in the period between 2008 and 2015 along the Parkfield section and then compared them to the data representing the fortnightly tide, a two-week tidal cycle. The comparison revealed the tremors will probably occur during the time when the tide rose at the fastest pace, the waxing period.
“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,” said Nicholas van der Elst, a USGS geophysicist and lead author of the study.
The strength of occurring tides depends on the relative location of the Sun and Moon in respect of each other. Earth tides are at its maximum when they are aligned and weakest when they are perpendicular. Some faults are more sensitive to the tides than others, and the response also depends on the faults' characteristics, such as its orientation or the proximity to the planet's crust.
The San Andreas fault is not oriented in a way which would make it susceptible to the full tidal strength, and that means it's quite amazing it produces the response tremors, at all.
Low-frequency earthquakes, named after the rumbling sound they produce, are of magnitudes lower than 1.0, located between 15 and 30 km (9 and 19 miles) below the surface, close to where the crust transitions to the mantle. These tremors are important because they are capable of providing the scientists valuable information about the deeper parts of the fault, that cannot be accessed in other way
“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]. And they tell us a lot of things about that deep part of the fault that before, we had no idea existed at all,” said David Shelly, a USGS seismologist.
“It's almost like having a lot of little creep meters embedded in the fault. We can use these low-frequency earthquakes as measurements of, at least in a relative sense, how much slip is happening at each little spot on the deep part of the fault where we see these events. When we don’t see them, we don’t know what’s happening; we don’t know whether it’s slipping silently or not slipping at all,” he explained.
Research results showed that more tremors were produced when the tidal stress was largest, as if the fault has an earthquake budget: “If you used them up yesterday, you don’t have as many to trigger today. By actually measuring that, we get an estimate of what that stress budget is,” explained Van der Elst.
The study provided the scientists with a tool to measure the recharge time of the fault along some locations.
“Scientifically, it's really cool, because we don't have any other way to directly estimate that number – the rate at which stress is accumulating on the fault. This is another study that's adding to our knowledge of how faults work in this transition. We don't quite know yet what it's going to mean in the long term, whether it'll result in some sort of warning that an earthquake is coming. We're going to have to monitor it for a lot longer.”
- "Fortnightly modulation of San Andreas tremor and low-frequency earthquakes" – Nicholas J. van der Elsta, Andrew A. Delorey, David R. Shelly, and Paul A. Johnson – Proceedings of the National Academy of Sciences of the United States of America (2016) – doi: 10.1073/pnas.1524316113
Featured image: San Andreas fault in the Coachella Valley, from Keys View, February 12, 2014. Image credit: NPS/Robb Hannawacker/Joshua Tree National Park (Flickr-CC)
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