Geologists detail likely site of San Andreas Fault's next major quake - Phys.org
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Geologists detail likely site of San Andreas
Fault's next major quake
26 June 2018, by Mary-Ann Muffoletto
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
Utah State University geologist Susanne Jänecke and
from the well-known main trace of the San Andreas
her team report findings about a newly identified,
possible nucleation zone on California’s San Andreas
Fault along the Salton Sea's northeastern shore, to
Fault. Credit: James P. Evans, USU the newly identified East Shoreline Fault Zone on
the San Andreas' opposite edge.
"We now have critical evidence about the possible
Back in 1905, the Colorado River, swollen with nucleation site of the next major earthquake on the
heavy rainfall and snowmelt, surged into a dry lake San Andreas Fault," says Jänecke, professor in
bed along California's San Andreas Fault and USU's Department of Geology. "That possible
formed the Salton Sea. The flood waters nucleation site was thought to be a small area near
submerged most of the small town of Salton, along Bombay Beach, California, but our work suggests
with nearby tribal lands. The inundation also there may be an additional, longer 'fuse' south of
covered a key, seismically active stretch of the San the Durmid Ladder within the 37-mile-long Brawley
Andreas Fault's southern tip in silt, hiding evidence Seismic zone."
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.
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integrate the field study into our map of the complex
ladder structure."
Utah State University Geology graduate student Dan
Markowski collects data along the newly identified
Durmid Ladder along California’s San Andreas Fault.
Credit: Susanne Jänecke/USU
Map illustrates the Durmid Ladder, a fault structure newly
identified by Utah State University geologists in
Future earthquakes in that zone or near the San California’s San Andreas Fault. BB=Bombay Beach;
Andreas Fault could potentially trigger a cascade of PS=Palm Springs. Credit: Susanne Jänecke/USU
earthquakes leading to the overdue major quake
scientists expect along the southern San Andreas
fault zone, she says.
Geophysical imaging and drilling confirmed the
"Fortunately, the northern continuation of the newly northward extend and identified the tilted fault zone
identified East Shoreline strand of the San Andreas in the subsurface near Palm Springs.
Fault is farther away from major population centers
than we first thought," Jänecke says. "The fault lies "On the ground and to our eyes, all of the tan-
along the eastern edge of Coachella Valley. In colored sediment looks the same," Jänecke says.
addition, the broken rock throughout the ladder "But further analysis with digital imaging tools
structure could damped ground-shaking associated highlighted the slight color differences of distinctive
with the next large earthquake." marker units."
On the other hand, she says the Durmid Ladder These markers, she says, allowed the team to
present an increase in the surface-rupture hazard recognize the hundreds of faults that displace the
in Durmid Hill and, if the Brawley Seismic Zone is 3-0.2 million-year-old sedimentary rocks of the
involved, the next large earthquake might be Durmid Ladder.
slightly larger than scientists previously expected.
"The new maps and analysis revealed the ladder
Among the tools Jänecke and her team used to structure, which is a particular type of 'step-over,'
identify the fault were high resolution aerial where overlapping fault strands have many
photography and false color imaging. connecting cross faults," Jänecke says. "It's not
clear now past earthquakes interacted with this
"Many months of fieldwork were critical to the structure and that makes its future behavior difficult
research," she says. "We relied on this imagery to to predict."
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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."
More information: Jänecke, Susanne U. and
Daniel K. Markowski, James P. Evans, Patricia
Persaud, Miles Kenney. "Durmid ladder structure
and its implications for the nucleation sites of the
next M >7.5 earthquake on the San Andreas fault
or Brawley seismic zone in southern California,"
Lithosphere. 19 June 2018. https://pubs.geoscience
world.org/gsa/lithosphere/article/533140/durmid-
ladder-structure-and-its-implications-for , DOI:
10.1130/L629.1
Provided by Utah State University
APA citation: Geologists detail likely site of San Andreas Fault's next major quake (2018, June 26)
retrieved 16 July 2019 from https://phys.org/news/2018-06-geologists-site-san-andreas-fault.html
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