Nicholas J. van der Elst from the US Geological Survey explained that "small and deep earthquakes within the fault" happen during the waxing of the tide.
The gravitational pull of the sun and the moon is responsible for a type of quake that is regularly felt along the San Andreas fault in California.
Low-frequency earthquakes are produced as a result of the gravitational pull of the sun and moon. The strongest tides occur when the moon and sun are aligned (spring tides), while the weakest tides occur when the sun and moon are perpendicular to each other with respect to Earth (neap tides). However, not only oceans and seas are affected by this phenomena but it also causes a compression and stretching of the Earth's crust.
But in recent years, the theory seems to have gained new traction.
"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", van der Elst said. They found that seismic activity synced up with phases of the "fortnightly tide". Instead, they peaked as the fortnightly tide was waxing, or strengthening. "The fault has to be very weak down there".
The scientists found that the number of low-frequency earthquakes did not increase during the strongest point of the fortnightly cycle. "So we now know the strength down there, as well as a way to estimate the actual rate of accumulation of stress on LFE patches". Researchers found that strands of the San Andreas Fault under the 45-mile long rift lake have have generated at least five 7.0 or larger quakes about every 180 years.
It's unclear whether or not low-frequency earthquakes could be used to predict aboveground earthquakes. "We're all waiting to see if the tremor pattern changes before or after a big natural disaster", Van der Elst says.
A valley filled with the swirled shapes of earth layers that were deformed by the collision of the Pacific and North American tectonic plates along the southern San Andreas Fault north of the Salton Sea is seen on July 3 near Mecca, California. But low-frequency earthquakes can be a proxy for understanding fault behavior from the ground up. 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.
As the surface of the Earth rises and falls with the tides, it influences activity at the faults, either promoting or inhibiting fault slip deep at the roots.
"It's nearly like having a lot of little creep meters embedded in the fault", co-author and USGS seismologist David Shelly told the Los Angeles Times. It offers a look at how the deeper parts of the faults work, and this could help seismologists with their predictions of when the bigger earthquakes and tremors might happen.