Ascending Rock: The Earth's Crust Also Has Its Own Tidal Movements

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For those whose livelihoods depend on the ocean, understanding how to interpret a tide table is essential. Coastal regions worldwide observe the sea level rising and falling several times daily. In some areas, like Canada's Minas Basin Inlet, the difference between low and high tides can reach an astonishing 53 feet (16 meters). Fishermen, divers, and ship captains must carefully consider these fluctuations, which is why governments provide tide prediction tables for various oceanic locations.

However, many are unaware that the solid ground we stand on also undergoes tidal movements. This phenomenon, referred to as "land tides," "crustal tides," "Earth tides," or "solid Earth tides," is driven by the same gravitational forces responsible for the more familiar oceanic tides.

Our Elastic Planet

Tides are complex phenomena, shaped by the interplay of multiple factors. The primary forces are the gravitational interactions between the sun, moon, and Earth. Surprisingly, the sun's influence on tides is weaker than the moon's, despite being 22 million times larger. This is due to the moon's proximity to Earth, making its gravitational pull about 2.2 times stronger on our planet's surface.

In most parts of the world, high ocean tides occur twice daily. One happens when the moon is directly overhead, and another occurs when the moon is on the opposite side of Earth. Low tides take place in the intervals between these points. (This unusual pattern is partly explained by the centrifugal force generated by Earth's rotation.)

Simultaneously, a comparable cycle occurs within the Earth's crust. The ground level subtly rises and falls daily, influenced by the moon's position. "This movement affects the entire solid Earth, not just the crust, but is most pronounced at the surface," explains Duncan Agnew via email. He adds, "The Earth is somewhat elastic." Agnew is a geophysicist at the Cecil H. and Ida M. Green Institute of Geophysics and Planetary Physics (IGPP) at Scripps Institution of Oceanography, University of California, San Diego.

While ocean tides are visible to the naked eye, detecting solid Earth tides requires specialized scientific equipment. During high tide, New York City can elevate by 14 inches (35.5 centimeters), and it descends by the same amount during low tide. However, pedestrians in Times Square or the Bronx Zoo remain oblivious to these changes, as everything—buildings, trees, streets, and people—moves in unison.

(Agnew notes that the "vertical motion at the surface" differs by location; some regions experience less pronounced bulging and sinking compared to New York. Other areas, however, exhibit more dramatic changes than the city.)

The Fortnightly Cycle

While we've emphasized the moon's influence on both solid Earth and oceanic tides, the sun also plays a crucial role. Coastal residents are familiar with how solar activity amplifies oceanic tides. When the sun and moon align, high tides become higher, and low tides become lower. Conversely, when these celestial bodies are at right angles, the result is lower high tides and higher low tides.

This cycle, known as the "fortnightly cycle," recurs every two weeks and impacts not only boaters but also solid Earth tides. Nicholas van der Elst of the U.S. Geological Survey led a 2016 study exploring the connection between this cycle, land tides, and seismic activity along California's San Andreas Fault.

"When the Earth's crust bends in response to tidal forces, it exerts stress on tectonic faults within the rock. If this tidal stress aligns with existing tectonic stress, it can trigger an earthquake," van der Elst explains via email.

In the 2016 study, van der Elst's team analyzed 81,000 San Andreas earthquakes. They discovered that the frequency of low-frequency earthquakes rises just before the fortnightly cycle reaches its solar/lunar alignment phase. However, Californians need not worry, as these quakes are too faint and occur too deep underground to cause significant surface damage.

Crustal tides are typically "far too minor to impact most faults," van der Elst notes. However, he has observed a "small but measurable effect in certain areas, particularly at mid-ocean ridges."

"There are also unique regions in the Earth's crust where faults are remarkably weak," he adds. "These areas are often found deep within subduction zone faults, such as those beneath Japan and the U.S. Pacific Northwest."

Deep beneath the Earth's surface, approximately 12 to 18 miles (20 to 30 kilometers) down, faults generate minor seismic tremors. "The tides can significantly influence these tremors, causing their rates to fluctuate by up to 30 percent in sync with tidal cycles," van der Elst explains. "However, these faint pseudo-earthquakes are completely imperceptible to humans and pose no danger."

Nevertheless, knowledge remains valuable.