USU geologists detail likely site of San Andreas Fault's next major quake

June 25, 2018

LOGAN, UTAH USA -- Back in 1905, the Colorado River, swollen with heavy rainfall and snowmelt, surged into a dry lake bed along California's San Andreas Fault and formed the Salton Sea. The flood waters submerged most of the small town of Salton, along with nearby tribal lands. The inundation also covered a key, seismically active stretch of the San Andreas Fault's southern tip in silt, hiding evidence of its potential volatility.

Utah State University geologist Susanne Jänecke began hypothesizing the location and geometry of the sediment-obscured fault zone more than a decade ago. After securing funding from the Southern California Earthquake Center in 2011, she, along with USU graduate student Dan Markowski and colleagues, embarked on the painstaking task of documenting the uplifted, highly folded and faulted area with geologic mapping and analysis.

The geologists' persistence revealed a nearly 15.5-mile-long, sheared zone with two, nearly parallel master faults and hundreds of smaller, rung-like cross faults. Dubbed the "Durmid Ladder" by the team, the well-organized structure could be the site of the region's next major earthquake. Jänecke, Markowski, USU colleague Jim Evans, Patricia Persaud of Louisiana State University and Miles Kenney of California's Kenney GeoScience, reported findings in the June 19, 2018, online issue of Lithosphere, a publication of the Geological Society of America.

The discovery of the Durmid Ladder reveals the southern tip of the San Andreas Fault changes fairly gradually into the ladder-like Brawley Seismic zone. The structure trends northwest, extending from the well-known main trace of the San Andreas Fault along the Salton Sea's northeastern shore, to the newly identified East Shoreline Fault Zone on the San Andreas' opposite edge.

"We now have critical evidence about the possible nucleation site of the next major earthquake on the San Andreas Fault," says Jänecke, professor in USU's Department of Geology. "That possible nucleation site was thought to be a small area near Bombay Beach, California, but our work suggests there may be an additional, longer 'fuse' south of the Durmid Ladder within the 37-mile-long Brawley Seismic zone."

Future earthquakes in that zone or near the San Andreas Fault could potentially trigger a cascade of earthquakes leading to the overdue major quake scientists expect along the southern San Andreas fault zone, she says.

"Fortunately, the northern continuation of the newly identified East Shoreline strand of the San Andreas Fault is farther away from major population centers than we first thought," Jänecke says. "The fault lies along the eastern edge of Coachella Valley. In addition, the broken rock throughout the ladder structure could damped ground-shaking associated with the next large earthquake."

On the other hand, she says the Durmid Ladder present an increase in the surface-rupture hazard in Durmid Hill and, if the Brawley Seismic Zone is involved, the next large earthquake might be slightly larger than scientists previously expected.

Among the tools Jänecke and her team used to identify the fault were high resolution aerial photography and false color imaging.

"Many months of fieldwork were critical to the research," she says. "We relied on this imagery to integrate the field study into our map of the complex ladder structure."

Geophysical imaging and drilling confirmed the northward extend and identified the tilted fault zone in the subsurface near Palm Springs.

"On the ground and to our eyes, all of the tan-colored sediment looks the same," Jänecke says. "But further analysis with digital imaging tools highlighted the slight color differences of distinctive marker units."

These markers, she says, allowed the team to recognize the hundreds of faults that displace the 3-0.2 million-year-old sedimentary rocks of the Durmid Ladder.

"The new maps and analysis revealed the ladder structure, which is a particular type of 'step-over,' where overlapping fault strands have many connecting cross faults," Jänecke says. "It's not clear now past earthquakes interacted with this structure and that makes its future behavior difficult to predict."

Until now, the main trace of the San Andreas Fault has been the only well-studied active fault this area, she says. "We need further study of the Durmid Ladder, the East Shoreline Fault and other fault zones of this area to identify the potential for surface-faulting hazards, ground sharing and cascading ruptures, to determine how to mitigate the risk posed by these important structures."
-end-


Utah State University

Related San Andreas Fault Articles from Brightsurf:

New fault zone measurements could help us to understand subduction earthquake
University of Tsukuba researchers have conducted detailed structural analyses of a fault zone in central Japan to identify the specific conditions that lead to devastating earthquake.

Ancient lake contributed to past San Andreas fault ruptures
he San Andreas fault, which runs along the western coast of North America and crosses dense population centers like Los Angeles, California, is one of the most-studied faults in North America because of its significant hazard risk.

Deep underground forces explain quakes on San Andreas Fault
Rock-melting forces occurring much deeper in the Earth than previously understood drive tremors along a segment of the San Andreas Fault near Parkfield, Calif., new USC research shows.

Signs of 1906 earthquake revealed in mapping of offshore northern San Andreas Fault
A new high-resolution map of a poorly known section of the northern San Andreas Fault reveals signs of the 1906 San Francisco earthquake, and may hold some clues as to how the fault could rupture in the future, according to a new study published in the Bulletin of the Seismological Society of America.

Geoscientists find unexpected 'deep creep' near San Andreas, San Jacinto faults
A new analysis of thousands of very small earthquakes in the San Bernardino basin suggests that the unusual deformation of some may be due to 'deep creep' 10 km below the Earth's surface, say geoscientists at UMass Amherst.

USU geologists detail likely site of San Andreas Fault's next major quake
Utah State University geologist Susanne Jänecke and colleagues identify the San Andreas Fault's 'Durmid Ladder' structure, a a nearly 15.5-mile-long, sheared zone with two, nearly parallel master faults and hundreds of smaller, rung-like cross faults that could be the site of the region's next major earthquake.

Site of the next major earthquake on the San Andreas Fault?
Many researchers hypothesize that the southern tip of the 1300-km-long San Andreas fault zone (SAFZ) could be the nucleation site of the next major earthquake on the fault, yet geoscientists cannot evaluate this hazard until the location and geometry of the fault zone is documented.

'Slow earthquakes' on San Andreas Fault increase risk of large quakes, say ASU scientists
A detailed study of the California fault has discovered a new kind of movement that isn't accounted for in earthquake forecasting.

Parkfield segment of San Andreas fault may host occasional large earthquakes
Although magnitude 6 earthquakes occur about every 25 years along the Parkfield Segment of the San Andreas Fault, geophysical data suggest that the seismic slip induced by those magnitude 6 earthquakes alone does not match the long-term slip rates on this part of the San Andreas fault, researchers report November 28 in the Bulletin of the Seismological Society of America (BSSA).

Fault system off San Diego, Orange, Los Angeles counties could produce magnitude 7.3 quake
The Newport-Inglewood and Rose Canyon faults had been considered separate systems but a new study shows that they are actually one continuous fault system running from San Diego Bay to Seal Beach in Orange County, then on land through the Los Angeles basin.

Read More: San Andreas Fault News and San Andreas Fault Current Events
Brightsurf.com is a participant in the Amazon Services LLC Associates Program, an affiliate advertising program designed to provide a means for sites to earn advertising fees by advertising and linking to Amazon.com.