Geotimes - August 2002 - Low tide blues

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Marine Geophysics
Song of the low tide blues

Scientists have long suspected that many undersea earthquakes are linked to tides, but connections have been difficult to prove due to a lack of long-term data sets. Recently, studies on a seafloor volcano along the Juan de Fuca Ridge in the Pacific Ocean show a correlation between microearthquakes on the ocean floor and ocean tides, report Maya Tolstoy of Columbia University’s Lamont-Doherty Earth Observatory and researchers at the Scripps Institute of Oceanography.

Tolstoy and her colleagues recorded 402 microearthquakes over two-months and found that earthquakes tend to occur at tidal lows, when the weight of water is at its least. They also discovered that harmonic tremors, which occur as superheated liquid travels through cracks, follow a tidal schedule. These findings, published in June issue of Geology, imply that the seafloor crust “breathes” with ocean tides, since the release of seismic energy and fluid flow take place on a periodic tidal basis.

Maya Tolstoy and colleague Mike Perfit investigate a sonar map of a possible seafloor eruption site at the East Pacific Rise near the equator. They are looking for evidence of fresh lava flows based on the presence of seismic activity picked up by hydroacoustic monitoring in the area. The picture was taken aboard the RV Melville on the AHA-NEMO Leg 2 cruise in the spring of 2000. Courtesy of M. Tolstoy

The team did not find a correlation between earthquakes and earth tides, caused by the deformation of Earth’s crust by the gravitational attraction of the moon and sun. This implies that ocean tides, which precede earth tides by two hours, play a more significant role in the day-to-day spurring of earthquakes than crustal deformation.

“Tolstoy’s article shows striking evidence of the great effect of tidal forces, especially the ocean’s load over the seafloor, on earthquakes related to volcanism,” says Junzo Kasahara, a marine seismologist at the Earthquake Research Institute in Tokyo, Japan, who postulated tidal effects on seafloor earthquakes in the 2001 Earth-Science Reviews. “We had not found this evidence before. Since ocean loading is a main effect influencing the seafloor crust, the correlation of earthquakes to ocean tides is reasonable.”

The team proposed a working hypothesis on why low tides appear to trigger some earthquakes. “The mid-ocean ridge setting is dominated by normal faults,” explains Tolstoy. “Pushing down on this geometry makes the faults more likely to lock. Releasing some of this force at tidal lows makes it easier for the faults to slip, causing earthquakes. In addition, lowering the overlying weight may cause gases in pores to expand, increasing pore pressure and helping to induce slip,” she says. “We also know from measurements at hydrothermal vents and from the tremor we observed on this experiment that there are changes in the fluid flow through the crust with the tides. These variations in the rate of flow may also be influencing the stress on the cracks which they flow through, leading to earthquakes.”

Further evidence is needed before this hypothesis can be verified, says coauthor John Orcutt from the Scripps Institute of Oceanography. “Long-term observations in locations like Juan de Fuca will allow us to make the measurements of seismicity, subseafloor pressure and fluid flow needed to support the hypothesis. If the fault zones are sufficiently permeable so pore fluid pressures don’t exceed pressure from overlying material, the proposed mechanism simply won’t work.”

The results are particularly exciting because they may suggest a link between the physics and biology of the seafloor system, where organisms thrive in extreme environments absent of sunlight. “This tidal influence on earthquakes may also mean that there is a tidal influence on the nutrient supply to the micro and macro biology associated with the mid-ocean ridge system. The earthquakes will open new cracks and allow more nutrients to be extracted from the rocks,” Tolstoy says.
This summer, Tolstoy will be part of an expedition led by Stacy Kim, a biologist from California State’s Moss Landing Marine Labs, to look for this tidal influence on larval release at the seafloor. “I am also looking for evidence of tidal triggering in other data sets we have from different locations, such as earthquakes on the Mid-Atlantic Ridge.”

Neeta Bijoor
Geotimes contributing writer

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