A fault that juts beneath the surface of Los Angeles County’s South Bay and into the ocean, once thought to be sleeping, is actually awake and active, spelling the potential of a destructive earthquake for the region’s busy ports and metropolitan areas, a new study suggests.

For years, researchers have believed the Wilmington blind-thrust fault has been dormant for millions of years.

However, researchers found that the fault line, which was the subject of the study published in August by the Bulletin of the Seismological Society of America, is indeed active and may on its own generate quakes as large as magnitude 6.4. And if the Wilmington interacts with other nearby faults in Torrance, Compton and Huntington Beach, the researchers warn that the effect could be an even larger, more disastrous 7.4 magnitude quake.

“Even moderate magnitude earthquakes could cause considerable damage, given the dense population in the Los Angeles metropolitan region and the fault’s location directly beneath the Ports of Los Angeles and Long Beach that facilitate shipments that value more than $460 billion/yr,” researchers wrote in their analysis, also mentioning the potential for tsunamis to come from the temblors.

The Wilmington fault is considered “blind” because unlike its more known counterpart, the San Andreas fault, the Wilmington lies beneath the earth’s surface, making it difficult to study.

Researchers determined the fault was active by analyzing the rocks below the earth’s surface, petroleum and water wells; and studying recent mapping of groundwater aquifers in the South Bay. They found that rock structures beneath the surface were deformed and shaped in a way that showed recent activity.

Such a discovery is not surprising, Zachary E. Ross, California Institute of Technology assistant professor of geophysics at the school’s Seismological Laboratory said.

“It is generally recognized that there are faults all over Southern California, and that our knowledge of them is still incomplete,” he said in an email. “When new faults are identified they can be incorporated into seismic hazard analyses.”

Ross said he has not had experience with the Wilmington fault and it does not directly involve his work.

The Wilmington fault’s structure is similar to the source of the 1994 Northridge earthquake, researchers learned. That 6.7 quake killed 57 people, injured thousands more and caused $20 billion in estimated damage.

Despite its similarities, the Wilmington fault is slow, rupturing every 3,000 to 4,000 years, moving at a rate of 0.16 millimeter per year, according to the study. Its speed is far slower than other blind-thrust faults in the region, such as the blind-thrust fault that caused the Northridge quake, which moves at about 7 milimeters per year.

Researchers were sure to underline the Wilmington fault lines’ configuration, which sits below the busy Los Angeles and Long Beach ports, the two largest ports in the United States, and hub for the movement of billions of dollars worth in goods, leaving the potential for serious losses.

In the study’s conclusion, researchers urged the public to consider the Wilmington fault line and what it may mean for the region.

“Thus, consideration of this fault in seismic hazard assessments is important for efforts to ensure the safety and resilience of the Los Angeles metropolitan and harbor regions,” researchers wrote in the study’s conclusion.

Staff writer Richard K. De Atley contributed to this report.