Ancient Lake Contributed to Past San Andreas Fault Ruptures and Could
Help Explain Fault’s “Earthquake Drought”
Boulder, Colorado, USA: The 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. Based on its roughly 150-year
recurrence interval for magnitude 7.5 earthquakes and the fact that it’s
been over 300 years since that’s happened, the southern San Andreas fault
has long been called “overdue” for such an earthquake. For decades,
geologists have been wondering why it has been so long since a major
rupture has occurred. Now, some geophysicists think the “earthquake
drought” could be partially explained by lakes — or a lack thereof.
Today, at the Geological Society of America’s 2020 Annual Meeting, Ph.D.
student Ryley Hill will present new work using geophysical modeling to
quantify how the presence of a large lake overlying the fault could have
affected rupture timing on the southern San Andreas in the past. Hundreds
of years ago, a giant lake — Lake Cahuilla — in southern California and
northern Mexico covered swathes of the Mexicali, Imperial, and Coachella
Valleys, through which the southern San Andreas cuts. The lake served as a
key point for multiple Native American populations in the area, as
evidenced by archaeological remains of fish traps and campsites. It has
been slowly drying out since its most recent high water mark (between 1000
and 1500 CE). If the lake over the San Andreas has dried up and the weight
of its water was removed, could that help explain why the San Andreas fault
is in an earthquake drought?
Some researchers have already found a correlation between high water levels
on Lake Cahuilla and fault ruptures by studying a 1,000-year record of
earthquakes, written in disrupted layers of soils that are exposed in
deeply dug trenches in the Coachella Valley. Hill’s research builds on an
existing body of modeling but expands to incorporate this unique 1,000-year
record and focuses on improving one key factor: the complexity of water
pressures in rocks under the lake.
Hill is exploring the effects of a lake on a fault’s rupture timing, known
as lake loading. Lake loading on a fault is the cumulative effect
of two forces: the weight of the lake’s water and the way in which that
water creeps, or diffuses, into the ground under the lake. The
weight of the lake’s water pressing down on the ground increases the stress
put on the rocks underneath it, weakening them — including any faults that
are present. The deeper the lake, the more stress those rocks are under,
and the more likely the fault is to slip.
What’s more complicated is how the pressure of water in empty spaces in
soils and bedrock (porewater) changes over both time and space. “It’s not
that [water] lubricates the fault,” Hill explains. It’s more about one
force balancing another, making it easier or harder for the fault to give
way. “Imagine your hands stuck together, pressing in. If you try to slip
them side by side, they don’t want to slip very easily. But if you imagine
water between them, there’s a pressure that pushes [your hands] out —
that’s basically reducing the stress [on your hands], and they slip really
easily.” Together, these two forces create an overall amount of stress on
the fault. Once that stress builds up to a critical threshold, the fault
ruptures, and Los Angeles experiences “the Big One.”
Where previous modeling work focused on a fully drained state, with all of
the lake water having diffused straight down (and at a single time), Hill’s
model is more complex, incorporating different levels of porewater pressure
in the sediments and rocks underneath the lake and allowing pore pressures
to be directly affected by the stresses from the water mass. That, in turn,
affects the overall fault behavior.
While the work is ongoing, Hill says they’ve found two key responses. When
lake water is at its highest, it increases the stresses enough to push the
timeline for the fault reaching that critical stress point just over 25%
sooner. “The lake could modulate this [fault slip] rate just a little bit,”
Hill says. “That’s what we think maybe tipped the scales to cause the
[fault] failure.”
The overall effect of Lake Cahuilla drying up makes it harder for a fault
to rupture in his model, pointing to its potential relevance for the recent
quiet on the fault. But, Hill stresses, this influence pales in comparison
to continent-scale tectonic forces. “As pore pressures decrease,
technically, the bedrock gets stronger,” he says. “But how strong it’s
getting is all relevant to tectonically driven slip rates. They’re much,
much stronger.”
Session no. 36 – T94.
Induced and triggered earthquakes in the United States and Canada
Monday, 26 Oct.: 5:30 to 8:00 p.m. EDT
Presentation time: 6:05 to 6:20 p.m. EDT
Session Link:
https://gsa.confex.com/gsa/2020AM/meetingapp.cgi/Session/50065
Paper 148-9:
Can the lack of lake loading explain the earthquake drought on the
southern San Andreas Fault?
Abstract Link:
https://gsa.confex.com/gsa/2020AM/meetingapp.cgi/Paper/355082
Contact
: Ryley Hill, University of California San Diego, California, USA;
ryhill@ucsd.edu.
The Geological Society of America, founded in 1888, is a scientific society
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and social scientists, fosters public dialogue on geoscience issues, and
supports all levels of earth-science education.
https://www.geosociety.org
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