Geologists make new discoveries about the Newport-Inglewood Fault Zone in the Los Angeles Basin

June 29, 2015 by Julie Cohen, University of California - Santa Barbara
The Newport-Inglewood fault was responsible for the 4.9 magnitude Inglewood earthquake in 1920 and the 6.4 magnitude Long Beach earthquake in 1933. Credit: Sonia Fernandez

UC Santa Barbara geologist Jim Boles has found evidence of helium leakage from the Earth's mantle along a 30-mile stretch of the Newport-Inglewood Fault Zone in the Los Angeles Basin. Using samples of casing gas from two dozen oil wells ranging from LA's Westside to Newport Beach in Orange County, Boles discovered that more than one-third of the sites—some of the deepest ones—show evidence of high levels of helium-3 (3He).

Considered primordial, 3He is a vestige of the Big Bang. Its only terrestrial source is the mantle. Leakage of 3He suggests that the Newport-Inglewood fault is deeper than scientists previously thought. Boles's findings appear in Geochemistry, Geophysics, Geosystems (G-Cubed), an electronic journal of the American Geophysical Union and the Geochemical Society.

"The results are unexpected for the area, because the LA Basin is different from where most mantle helium anomalies occur," said Boles, professor emeritus in UCSB's Department of Earth Science. "The Newport-Inglewood fault appears to sit on a 30-million-year-old subduction zone, so it is surprising that it maintains a significant pathway through the crust."

When Boles and his co-authors analyzed the 24 gas samples, they found that high levels of 3He inversely correlate with carbon dioxide (CO2), which Boles noted acts as a carrier gas for 3He. An analysis showed that the CO2 was also from the mantle, confirming leakage from deep inside the Earth.

Blueschist found at the bottom of nearby deep wells indicates that the Newport-Inglewood fault is an ancient subduction zone—where two tectonic plates collide—even though its location is more than 40 miles west of the current plate boundary of the San Andreas Fault System. Found 20 miles down, blueschist is a metamorphic rock only revealed when regurgitated to the surface via geologic upheaval.

The cylinders Jim Boles used to gather casing gas samples from oil wells along the Newport-Inglewood fault, where he found evidence of helium-3. Credit: Sonia Fernandez

"About 30 million years ago, the Pacific plate was colliding with the North American plate, which created a subduction zone at the Newport-Inglewood fault," Boles explained. "Then somehow that intersection jumped clear over to the present San Andreas Fault, although how this occurred is really not known. This paper shows that the mantle is leaking more at the Newport-Inglewood fault zone than at the San Andreas Fault, which is a new discovery."

A geologic cross section of the Los Angeles Basin from the southwest to northeast. This profile intersects the Newport-Inglewood Fault Zone at Long Beach.

The study's findings contradict a scientific hypothesis that supports the existence of a major décollement—a low-angle thrust fault—below the surface of the LA Basin. "We show that the Newport-Inglewood fault is not only deep-seated but also directly or indirectly connected with the ," Boles said.

"If the décollement existed, it would have to cross the Newport-Inglewood , which isn't likely," he added. "Our findings indicate that the Newport-Inglewood is a lot more important than previously thought, but time will tell what the true importance of all this is."

Explore further: Magnitude-3.4 quake shakes buildings across Los Angeles

More information: Geochemistry, Geophysics, Geosystems, onlinelibrary.wiley.com/wol1/d … 015GC005951/abstract

Related Stories

Is there an ocean beneath our feet?

January 27, 2014

(Phys.org) —Scientists at the University of Liverpool have shown that deep sea fault zones could transport much larger amounts of water from the Earth's oceans to the upper mantle than previously thought.

Drilling into an active earthquake fault in New Zealand

September 24, 2014

Three University of Michigan geologists are participating in an international effort to drill nearly a mile beneath the surface of New Zealand this fall to bring back rock samples from an active fault known to generate major ...

Earthquake 'super-cycle' patterns on the Garlock fault

April 22, 2015

A new look at slip rate data and geologic evidence for ancient earthquakes on the central Garlock fault suggest that seismic activity along the fault may be controlled in part by "super-cycle" changes in strain that occur ...

Recommended for you

How the Elwha dam removals changed the river's mouth

January 19, 2018

For decades, resource managers agreed that removing the two dams on the Elwha River would be a big win for the watershed as a whole and, in particular, for its anadromous trout and salmon. The dams sat on the river for more ...

0 comments

Please sign in to add a comment. Registration is free, and takes less than a minute. Read more

Click here to reset your password.
Sign in to get notified via email when new comments are made.