New model may improve San Francisco Bay Area, U.S., seismic hazard maps


Date:
February 25, 2022
Source:
Stanford University
Summary:
Using the Santa Cruz Mountains as a natural laboratory, researchers
have built a 3D tectonic model that clarifies the link between
earthquakes and mountain building along the San Andreas fault for
the first time. The findings may be used to improve seismic hazard
maps of the Bay Area.



FULL STORY ==========================================================================
The Santa Cruz Mountains define the geography of the Bay Area south of San Francisco, protecting the peninsula from the Pacific Ocean's cold marine
layer and forming the region's notorious microclimates. The range also represents the perils of living in Silicon Valley: earthquakes along
the San Andreas fault.


==========================================================================
In bursts that last seconds to minutes, earthquakes have moved the
region's surface meters at a time. But researchers have never been able
to reconcile the quick release of the Earth's stress and the bending
of the Earth's crust over years with the formation of mountain ranges
over millions of years. Now, by combining geological, geophysical,
geochemical and satellite data, geologists have created a 3D tectonic
model that resolves these timescales.

The research, which appears in Science Advances Feb. 25, reveals that
more mountain building happens in the period between large earthquakes
along the San Andreas Fault, rather than during the quakes themselves. The findings may be used to improve local seismic hazard maps.

"This project focused on linking ground motions associated with
earthquakes with the uplift of mountain ranges over millions of years
to paint a full picture of what the hazard might actually look like in
the Bay Area," said lead study author Curtis Baden, a PhD student in
geological sciences at Stanford University's School of Earth, Energy & Environmental Sciences (Stanford Earth).

Bending and breaking Geologists estimate the Santa Cruz Mountains started
to uplift from sea level about four million years ago, forming as the
result of compression around a bend in the San Andreas fault. The fault
marks the boundary between the Pacific Plate and the North American Plate, which shift past each other horizontally in a strike-slip motion.



========================================================================== Measurements of deformation -- changes in the shapes of the rocks --
have shown that Earth's surface warps and stretches around the San
Andreas fault during and in between earthquakes, and behaves much like
an elastic band over seconds, years and even decades. But that classic
approach cannot align with geologic observational data because it doesn't
allow the rocks to yield or break from the stress of the warping and stretching, as they eventually would in nature - - an effect that has
been observed in Earth's mountain ranges.

"If you try to treat the Earth like an elastic band and drive it forward
too far, you're going to exceed its strength and it's not going to behave
like an elastic anymore -- it's going to start to yield, it's going to
start to break," said senior study author George Hilley, a professor of geological sciences at Stanford Earth. "That effect of breaking is common
to almost every plate boundary, but it's seldom addressed in a consistent
way that allows you to get from earthquakes to the long-term effects."
By simply allowing the rocks to break in their model, the study authors
have illuminated how earthquake-related ground motions and ground motions
in between earthquakes build mountains over millions of years. The results
were surprising: While the geosciences community conceives of earthquakes
as the primary drivers of mountain-building processes, the simulation
showed most uplift has occurred in the period between earthquakes.

"The conventional wisdom is that permanent uplift of the rock actually
happens as the result of the immense force of the earthquake," Hilley
said. "This argues that the earthquake itself is actually relieving the
stress that is built up, to some degree." A neighborhood laboratory
Because the Santa Cruz Mountains neighbor several research institutions, including Stanford, the University of California, Berkeley, and the
United States Geological Survey (USGS), scientists have gathered an
immense amount of information about the mountain range over the course
of more than 100 years.



========================================================================== Efforts to collect geological and geophysical data were especially
spurred by major recent events like the 1989 Loma Prieta earthquake and
the 1906 San Francisco earthquake, but the formation of the Santa Cruz Mountains likely spanned hundreds of thousands of smaller earthquakes
over millions of years, according to the researchers.

The study authors compiled the existing suite of observations, and also collected new geochemical data by measuring Helium gas trapped within
crystals contained in rocks of the mountains to estimate how fast these
rocks are coming to the surface from thousands of feet below. They
then compared these datasets with model predictions to identify how
earthquakes relate to uplift and erosion of the mountain range. The
process took years of specifying material properties to reflect the
complexity that nature requires.

Seismic implications The researchers ran their simulation from when the
Santa Cruz Mountains started to uplift four million years ago until
present day to understand how the evolution of topography near the
San Andreas fault through time influences recent and potential future earthquakes.

"Currently, seismic hazard assessments in the San Francisco Bay area are largely based on the timing of earthquakes spanning the last few hundred
years and recent crustal motions," Baden said. "This work shows that
careful geologic studies, which measure mountain-building processes over
much longer timescales than individual earthquakes, can also inform these assessments." The scientists are currently working on a companion paper detailing how hazard risk maps could be improved using this new model.

"We now have a way forward in terms of actually having a viable set
of mechanisms for explaining the differences between estimates at
different time scales," Hilley said. "The more we can get everything
to fit together, the more defensible our hazard assessments can be."
Study co-authors include David Shuster and Roland Bu"rgmann of UC
Berkeley; Felipe Aron of the Research Center for Integrated Disaster
Risk Management (CIGIDEN) and Pontificia Universidad Cato?lica de Chile;
and Julie Fosdick of the University of Connecticut. Aron and Fosdick
were affiliated with Stanford when they conducted research for the study.

This study was supported by NSF Career Grant EAR-TECT-1108 105581, Fondo
de Financiamiento de Centros de Investigacio'n en A'reas Prioritarias ANID/FONDAP/ 15110017-Chile (CIGIDEN) and the Ann and Gordon Getty
Foundation.

========================================================================== Story Source: Materials provided by Stanford_University. Original
written by Danielle Torrent Tucker. Note: Content may be edited for
style and length.


========================================================================== Journal Reference:
1. Curtis W. Baden et al. Bridging earthquakes and mountain building
in the
Santa Cruz Mountains, CA. Science Advances, 2022 DOI: 10.1126/
sciadv.abi6031 ==========================================================================

Link to news story: https://www.sciencedaily.com/releases/2022/02/220225142112.htm

--- up 11 weeks, 6 days, 7 hours, 13 minutes
* Origin: -=> Castle Rock BBS <=- Now Husky HPT Powered! (1:317/3)