Leaky Mantle

A 4.1 magnitude earthquake that gently rattled parts of Southern California on April 23 went largely unnoticed by most residents.  Researchers at Scripps Institution of Oceanography at UC San Diego were closely monitoring groundwater during that quake to test a new instrument that detects liquids leaking from Earth’s mantle.

Scripps geochemist David Hilton uses the term “leaking mantle” to describe how volatile gases such as helium and carbon dioxide are transferred from the earth’s mantle to the surface passing through groundwater.  Scientists have long suspected that gas concentrations, and helium in particular, may vary during seismic activity but haven’t had the necessary tools to monitor continuously until now.

“How do these gases in groundwater respond to earthquakes?” asks Hilton. “This new approach has a lot of potential to answer definitive questions like this.”

Hilton and Scripps graduate student Peter Barry designed the prototype instrument to collect groundwater before, during, and after a tremor.  The researchers hooked the instrument into a groundwater source at a geothermal plant operated by the City of San Bernardino and began slowly drawing water from the ground.

Adapted from a similar concept used underwater, the instrument slowly draws 1 cubic centimeters of groundwater per day out of the earth and through a copper tube.  The instrument is equipped with 100 feet of copper tubing, enough to draw groundwater for a month or more at a time.

This tool offers new research opportunities for geochemists to analyze the earth’s fluids, as they move to the surface. Prior to developing this instrument last year, geochemists had limited options except to physically sample at the site immediately after an earthquake in order to quickly capture the chemical contents of groundwater before the levels returned to normal.

“It was impossible to sample every day,” said Hilton.

The researchers believe that this type of continuous monitoring near major faults has the potential to determine what, if any, role these fluids play during a seismic event.

Researchers are on the lookout for changes in gas chemistry, in particular helium, in response to an earthquake.

Helium-3 is an isotope that exists in the earth’s mantle and has been trapped there since the planet’s origin.  Helium-4, however, exists in the atmosphere and through radioactive decay. Scientists believe that variations in the ratio of helium-3 to helium-4 could provide a signal to where the fluids below the seismically active region came from.

This pilot study in San Bernadino confirmed that the instrument is effective in continuous sampling groundwater for one month or more.  The researchers hope to build additional instruments to monitor helium leaking to the surface through active faults in Costa Rica's Nicoya Peninsula.

“The initial hurdle was the successful deployment of the prototype instrument,” said Barry. “Now we know it works and we can move the science forward.”

-- Annie Reisewitz