Seismic Reflections

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To find out what’s shaking beneath the seafloor off southern California’s coast, USGS scientists turned on their hydrophones and used sound waves to “see” beneath the seafloor.

This article is part of the April-June 2021 issue of the Sound Waves newsletter.

People on the deck of a research vessel handling a piece of scientific equipment

Science crew work on deployment of seismic streamer on deck of R/V Robert Gordon Sproul. Green cable is the hydrophone streamer and a "bird" is being attached to control depth in the water.

On a recent 8-day research cruise in cooperation with the U.S. Bureau of Ocean Energy Management, a crew led by geophysicist Jared Kluesner and geologist Jamie Conrad (both with the USGS Pacific Coastal and Marine Science Center) used sound waves to map a network of submarine faults known as the Palos Verdes Fault zone (PVF).

Running roughly parallel to the California coast between Santa Catalina Island and the mainland, the PVF passes directly through the Port of Los Angeles before continuing north into Santa Monica Bay. Like the San Andreas fault located further inland, the PVF is a strike-slip fault, in which the plate boundaries move laterally against each other with relatively little vertical displacement. 

The PVF currently moves at a rate of about 3 millimeters a year. Magnitude 2 and 3 earthquakes associated with the fault occur frequently in the area with little consequence. Larger temblors, however, can trigger seafloor landslides that might displace huge volumes of water, creating tsunamis with the potential to cause damage in nearby coastal communities and especially in harbors.

Black and gray lines across the image dip down into a canyon area and interruptions in the lines indicate the Palos Verdes Fault

CHIRP sonar profile collected across the San Gabriel Canyon and Palos Verdes Fault zone offshore southern California. Dashed red line shows approximate location of imaged fault zone.

The fault also cuts through an active energy-producing area: four offshore oil platforms lie along the PVF, and where nearby venting gas bubbling up from the seafloor hint at other petroleum reserves. Back in the 1970s, oil exploration companies sent ships equipped with large seismic arrays to penetrate far below the seafloor, searching for those reserves with low-frequency sound.

a multicolored map shows many black lines covering an area offshore of the Port of Los Angeles, overlapping red lines

Colored slope map showing seismic-reflection track lines (black) and active faults (red): PVF - Palos Verdes fault zone; NIF - Newport-Inglewood fault zone; SPBF - San Pedro Basin fault; SDTF - San Diego Trough fault.

But for Kluesner, Conrad, and crew—who were trying to map where, exactly, the fault is most seismically active—those prior surveys offered only a low-resolution map of the PVF. To get more detail of the shallow subsurface, they needed to use higher frequency sound sources.

“The surveys in the 1970s used relatively low-frequency sound sources, which penetrate kilometers beneath the seafloor,” said Kluesner. “What we wanted to study was in tens of meters beneath the seafloor, so we used relatively higher-frequency sound to get very high-resolution imagery of the substrate.”

To do this, they employed new seismic reflection techniques designed to record the reflections of deformed strata just a few meters below the seafloor. These recordings, translated into profiles of stratigraphic layers, reveal the interbedded strata beneath the seafloor and show precisely where the two sides of the fault meet.

The team will use this high-resolution data to more accurately map the PVF along its submarine extent. By examining the layers of sediment bisected by the fault, the team can determine how fast and how often, on average, the fault moves, thus helping to understand the risk to coastal communities and offshore energy infrastructure.

“There are a number of factors that go into assessing the hazards of a fault,” Conrad said. “One is the area, or extent, of the fault, and its proximity to where people live. Another is the rate at which it is moving. And then there’s recurrence: ‘How often does this fault move, and when did it last shift?’ Seismic reflection allows us to piece together the history of this fault and improve our understanding of its hazards.” 

After mapping the PVF and identifying its most active areas, the team’s next step is to collect sediment core samples at those areas along the fault. Radiocarbon dates of these sediment samples will allow them to reconstruct the fault’s shifts over time and calculate its relative rate of movement.

 

 

sun setting over the ocean viewed from the back deck of a research vessel

Looking across the back deck/stern of R/V Robert Gordon Sproul. The wire going through the block in the A-frame leads to the CHIRP sonar fish towed in the water. Oil platforms are shown in the distance.

 

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