Seafloor Faults off Southern California Active

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Scientists did not collect highly detailed seafloor maps and seismic data beyond 25 miles off the coast of Southern California until 2008. The thinking was that faults beyond that distance would not affect U.S. coastlines, which turned out to be incorrect. Technology to map the seafloor in great detail was also very costly until the late 1990s. USGS geologist Jamie Conrad points out that even as recently as 2009, offshore fault maps of Southern California did not show that the San Diego Trough fault—one of the longest in Southern California—connects to the San Pedro fault near Santa Monica Bay. But these faults do connect. These earlier maps also showed that the Palos Verdes fault, well known because it runs partly onshore, connects with the Coronado Bank fault. But these faults do not connect. Longer faults boost the threat of larger earthquakes along this highly populated coastline, including Conrad’s childhood home in the Palos Verdes Hills.

Issue

The San Andreas fault is perhaps the best-known fault in Southern California. It’s part of a much larger system of faults generated by the Pacific and North American tectonic plates grinding past each other at a rate of about 50 millimeters a year. Other faults extending throughout Southern California also enable this boundary to move. Up to 20 percent of the movement occurs on offshore faults within 75 miles of the coast. 

Some notable earthquakes in Southern California occurred on these seafloor faults. The 1933 magnitude 6.4 Long Beach earthquake on the Newport-Inglewood fault killed 115 people and caused \$40 million in damage. In 1986, the magnitude 5.4 Oceanside earthquake on an unknown segment of the San Diego Trough fault, caused at least 29 injuries, 1 death, and \$1 million in damage. And the 1951 magnitude 5.9 San Clemente Island earthquake on the San Clemente fault caused some damage in Santa Barbara and Ventura counties.

Estimating earthquake risk depends on fault characteristics, such as length; slip rate (the average distance a fault moves each year); how slip varies along segments of the fault; the average time between earthquakes; the last time the fault moved; and how it links to other active faults. These characteristics are challenging to determine for faults lying beneath a few thousand meters of seawater. Until recently, seafloor maps covering large parts of Southern California lacked adequate resolution to uncover the crucial details of fault movement and seafloor warping associated with these submerged faults. 

Earthquake probability forecasts didn’t even include faults located entirely offshore until 2014, despite the potential for these faults to generate destructive earthquakes. These forecasts help organizations set earthquake insurance rates, design building codes, and prepare for disasters in California.

What the USGS is doing

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The USGS began studying faults offshore Southern California in much greater detail in 1999 by collecting new seismic data: two-dimensional views that revealed faults and distorted sediment layers beneath the seafloor. Seven years later, higher resolution seismic surveys filled in missing data from key sections of some faults. Those data came from a new instrument towed just a few meters below the surface that sends out pulses of sound that penetrate the seafloor and reflect off deeper sediment and rock layers. This minisparker uses an electric spark as the sound source, and one hydrophone (underwater microphone) to record the echoes. 

By emitting fan-shaped pulses of sound that bounced off the bottom, multibeam sonar could map swaths of the ocean floor in enough detail to pick up faults and the contorted environment around them. By collaborating with other agencies in 1999, the USGS used this technique to map the ocean floor off Los Angeles and San Diego in much higher resolution than ever before.

Using deep-water coring devices in 2003 and 2009, USGS scientists collected samples of seafloor sediment next to the faults to calculate the fault’s age.

In 2007, the USGS began collaborating with the Monterey Bay Aquarium Research Institute (MBARI) to survey the seafloor. Researchers used MBARI’s Autonomous Underwater Vehicle (AUV), to fly about 50 meters above the ocean bottom. This robotic submarine mapped the seafloor using multibeam sonar to reveal features as small as 1 meter. In contrast, a ship-mounted instrument operating hundreds of meters above the bottom can typically distinguish features 15 to 20 meters across. The AUV surveyed carefully selected features, such as seafloor channels displaced by faults, which provided scientists with clues to a fault’s slip rate. The AUV also carried a chirp system, which sends out a high-frequency "chirp" sound and listens with a small collection of hydrophones. It collects data beneath the seafloor, revealing layers as thin as 2 to 3 centimeters.

From 2010 to 2011, the USGS collected multibeam bathymetry, in depths of 100 to 800 meters, off the coast from Dana Point to La Jolla, one of the last areas left in Southern California to be mapped in high resolution.

What the USGS has learned

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The USGS created a new offshore fault map for Southern California. The map shows an active connection between the San Pedro Basin fault and the San Diego Trough fault, previously thought to be separate faults. This continuous fault extends 260 kilometers, from offshore Santa Monica into Mexico, and is one of the longest faults in Southern California. In general, the longer a fault, the greater its earthquake potential.

Calculating a slip rate, or how much a fault moves over time, is critical when determining the fault’s earthquake hazard. Using the new high-resolution seafloor maps and seismic data, USGS scientists determined a slip rate for the first time for the San Diego Trough fault, and an underwater section of the Palos Verdes fault that differed from the onshore rates, indicating that the slip rate may change along the fault. The Palos Verdes fault runs through the top of an underwater landslide dated at 31,000 years old. Measuring how much the seafloor shifted from this clear boundary helped researchers determine the age of the fault to be 3-5 million years old.

Near the epicenter of the 1986 Oceanside earthquake, USGS scientists identified a 5-kilometer-long stepover (or short gap) in the San Diego Trough fault. Seafloor displacement along this stepover is different from regular strike-slip fault movement, which could explain some unusual characteristics of that earthquake, such as the long aftershocks that are atypical for this earthquake’s magnitude.

New seafloor data from 2008-2011 illuminate an obscure group of faults that parallel the coast about 15 to 20 kilometers from San Onofre, which may interact with the Newport-Inglewood Fault.

Detailed views of the seafloor have greatly enhanced our understanding of active faults off Southern California, and improved hazard assessments for the region. For example, this information contributed to an update of the seismic hazard assessment of the San Onofre Nuclear Generating Station in 2014, and the 2014 National Seismic Hazard Mapping Project, which included faults located entirely offshore for the first time.