Bend in an Offshore Fault Helps Shape the Rugged Terrain of California’s Big Sur Coast: New USGS Report on the San Gregorio-Hosgri Fault

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New report on how the bend in an offshore fault helps shape the Big Sur coast

Map shows the central part of California near Monterey, with onshore and offshore faults and features labeled.

Shaded-relief map of central California showing location of the Big Sur area (white dashed line). Red line shows the San Gregorio-Hosgri fault (SGHF) and the Big Sur Bend between Point Sur (PS) and Piedras Blancas (PB). Black lines show other faults. Figure 1 from new report; see caption for additional information.

This article is part of the August 2018 issue of the Sound Waves newsletter.  

A 65-mile-long stretch (the “Big Sur Bend”) of a long fault offshore of central California plays a major role in shaping the steep topography of the state’s Big Sur coast. A new report from the USGS, published last May in the journal Tectonics, explains this role. It also describes findings with implications for earthquake hazards the fault could trigger, including shaking, coastal landslides, and tsunamis.

The San Gregorio-Hosgri fault extends along the central California coast for about 250 miles. For most of its length, the fault lies beneath the Pacific Ocean, which is one reason why it is not so well known as its inland cousin, the San Andreas fault. Both faults are part of a broad zone separating the Pacific plate on the west from the North American continent on the east. Like the San Andreas, the San Gregorio-Hosgri fault is “right-lateral”: to an observer on one side of the fault, the other side would appear to move horizontally to the right.

In 2011, USGS scientists led by Sam Johnson mapped a poorly known stretch of the San Gregorio-Hosgri fault offshore of the Big Sur coast, between Piedras Blancas and Point Sur (see map). They used high-resolution tools to record seafloor depths, image sediment layers under the seafloor, and measure the amount of magnetic minerals in sub-seafloor rocks. Features they observed on and beneath the seafloor show that the fault slips vertically as well as horizontally in this region. The landward side of the fault here moves upward by about three-hundredths of an inch per year, as well as horizontally to the southeast by about a tenth of an inch per year. The vertical motion results from rocks on either side of the fault pushing against one another as well as sliding past.

The USGS scientists combined their 2011 results with data collected previously for the California Seafloor Mapping Program. They saw that the stretch of the fault they were investigating offshore of Big Sur forms a large bend between more northerly trending parts of the fault zone to the north and south (see map). They named this stretch the “Big Sur Bend.” Compressional force along the Big Sur Bend has helped push up the rugged Santa Lucia Range. The coast in this region has a narrower continental shelf, steeper terrain, and more frequent landslides than coastal areas to the north and south.

Cartoon looks at the seafloor at an angle to reveal a steep hillslope, indicative of a fault between tectonic plates.

Perspective view of part of the Big Sur Bend in the San Gregorio-Hosgri fault. The steep slope beside the fault results from uplift along the fault, which is part of the strike-slip fault system that forms the boundary in California between the Pacific and North American tectonic plates.

The 2011 mapping showed that the San Gregorio-Hosgri fault is continuous for at least 140 miles, and a major earthquake could potentially rupture that entire length. Such a rupture could generate a magnitude-7.8 earthquake, although the scientists note that no data exist to indicate whether such large earthquakes have occurred. The largest historical earthquake attributed to the San Gregorio-Hosgri fault was a 1926 magnitude-6.3 quake in central Monterey Bay. The largest earthquake on the fault in the Big Sur Bend area was a magnitude 6.2 event in 1952.

Earthquakes of such magnitude can trigger landslides, a major concern along the Big Sur coast, where winter rains already produce slope failures that periodically close the popular State Highway 1. “Large destructive landslides along the steep front of the Santa Lucia Range are common,” note the authors of the report, “and strong ground motions from large earthquakes will push many slopes beyond their stability thresholds.” Citing another danger, they write: “Submarine landslides along the steep shelf break and on the flanks of incised submarine canyons have the potential to generate small tsunamis.”

Read the new report for a detailed picture of factors that make the Big Sur coast both beautiful and hazardous.

The full citation is:

Johnson, S.Y., Watt, J.T., Hartwell, S.R., and Kluesner, J.W., 2018, Neotectonics of the Big Sur Bend, San Gregorio‐Hosgri fault system, central California: Tectonics, published online 31 May 2018, https://doi.org/10.1029/2017TC004724

USGS air photo of the Mud Creek landslide, taken on May 27, 2017

On May 20, 2017, after an unusually rainy winter, a massive landslide near Mud Creek on the Big Sur coast covered a quarter-mile stretch of California State Highway 1 with rocks and dirt more than 65 feet deep. The road re-opened 14 months later, on July 18, 2018.

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