Earthquake potential under Ports of LB and LA higher than previously believed: Recent study suggests Wilmington Fault could actively generate large quake.

The Southern California temblors a couple of months ago may have us on slightly higher alert. But a new study suggests we might need to be even more prepared for a Long Beach-area earthquake.

Franklin Wolfe, a researcher at Harvard University’s Earth and Planetary Sciences department, led a study on the Wilmington Fault, a seismic fault line that cuts through the Ports of Long Beach and Los Angeles. Using 2D and 3D seismic-reflection surveying and petroleum, water-well and groundwater-aquifer data, the researchers concluded that the “blind-thrust” fault is active rather than dormant as previously believed.

Furthermore, the fault’s size suggests it can generate 6.3- to 6.4-magnitude earthquakes and that linking to nearby faults of Huntington Beach, Torrance and Compton can potentially result in earthquakes greater than 7.4 in magnitude on the Richter scale.

“We define the Wilmington blind thrust as a tectonically active fault capable of generating large, damaging earthquakes,” the researchers assert. “These earthquakes would directly impact the overlying Ports of Los Angeles and Long Beach, as well as the broader Los Angeles metropolitan area.”

Published last month in the Bulletin of the Seismological Society of America, the study recommends that the Wilmington Fault should be included in local assessments of earthquake hazards.

“The Wilmington blind-thrust offers an extreme example of the ambiguity in hazard assessments,” the researchers determined. “The fault’s linkage to other structures in the region, and location within a deep sedimentary basin, pose the threat of large-magnitude events with significant ground-shaking.”

A “blind” fault is hidden beneath the earth’s surface, making it difficult to observe, notes Maya Wei-Haas of National Geographic in her recent article on the findings.

“While scientists have long known the [Wilmington] fault is present– stretching 12.4 miles under southern Los Angeles into San Pedro Bay– it was presumed to have sat quiet for millions of years,” she says.

The study’s researchers also describe blind-thrust faults as significantly challenging because they do not extend up to the surface and can be overlooked.

“The concealed nature of these faults, combined with a lack of established standards for defining their paleo-earthquake activity, has led to them being underrepresented in most regional seismic-hazard assessments,” they say.

Using energy waves similar to an ultrasound, researchers estimate that the Wilmington Fault has moved an average of .16 millimeters per year over the past 500,000 years and likely causes an earthquake every few thousand years. By comparison, the San Andreas Fault moves approximately 30 to 50 millimeters each year, with an average of 150 years between earthquakes, Wei-Haas notes.

Though it’s slow-moving, as a “thrust” fault, the Wilmington Fault would produce more of a vertical shove than a horizontal back-and-forth motion that causes buildings to sway, Wei-Haas describes, comparing it to the intense impacts of the magnitude-6.7 Northridge quake that killed more than 60 people and injured thousands in 1994.
Using 3D fault-models allowed the researchers to calculate the size of the Wilmington Fault and its neighboring underground structures to determine maximum magnitudes of earthquakes it could generate.

“Ruptures on the Wilmington blind-thrust could generate [magnitude] 6.3–6.4 earthquakes,” they estimate.

But if a rupture involved both the Wilmington and Compton faults, it could cause an earthquake of greater magnitude than 7.2, they say. And if the Wilmington and Newport-Inglewood faults interacted, then the combined thrust and “strike-slip” ruptures could result in a quake greater than 7.4, as in the magnitude-7.9 Alaska earthquake of 2002.

“Given its activity and earthquake potential, the Wilmington blind-thrust represents a significant deterministic seismic hazard in Southern California,” the researchers warn. “Even moderate-magnitude earthquakes could cause considerable damage, given the dense population in the Los Angeles metropolitan region (about 19 million) and the fault’s location directly beneath the Ports of Los Angeles and Long Beach that facilitate shipments that value more than $460 billion per year.”

The Wilmington Fault’s links to other structures deep in the sedimentary basin may even result in soil liquefaction and possible tsunamis, the researchers predict.

Duane Kenagy, the Port of Long Beach’s capital-programs executive in charge of its $4-billion infrastructure-improvement program, told the Signal Tribune in an interview Wednesday that the port is prepared to withstand the effects of earthquakes, including tsunamis.

“In an extreme tsunami event, you could have water breach our structures,” Kenagy said. “But they’ve been designed to minimize [those] risks.”

The Port employs state-of-the-art seismic design in its infrastructure to address risks associated with earthquakes, Kenagy said. The capital-improvement initiative he heads includes the new Gerald Desmond Bridge– a major connector for the port– which has seismic dampers that act like giant shock absorbers during an earthquake.

The bridge itself is designed to tolerate earthquakes that occur within the design-life of the Port, but also withstand collapse if an even larger quake occurs within 1,000 years, Kenagy said. Caltrans will eventually take over bridge operations and has its own earthquake-response plan, he added.

“When we design facilities here in the Port for earthquakes, we look at a full spectrum of different earthquakes and different faults and try and figure out which ones would have the largest ground acceleration locally right at that specific site,” Kenagy said.

The Port also has contingency plans for different sizes of earthquakes and sends out teams of engineers after such events to ensure all facilities are still safe and operations can continue.

“We’re as prepared as we can be for major earthquakes,” he said.

Kenagy added that the Port is not only interested in uninterrupted trade operations, but also remaining operational to help the public in the event of a disaster.

“The ports are vital facilities that are really important for any recovery that needs to take place in the event of a major earthquake and major damage,” he said.

Learning about a potential increased threat from the Wilmington Fault under the Port of Long Beach didn’t seem to bother Kenagy.

“Earthquakes are an ever-evolving area of study,” he said. “We try to stay right at the state-of-the-art in everything we do.”