How earthquake swarms arise
Model shows how fluids unlock faults to unleash earthquake swarms

Date:
September 25, 2020
Source:
Stanford University
Summary:
A new fault simulator maps out how interactions between pressure,
friction and fluids rising through a fault zone can lead to
slow-motion quakes and seismic swarms.



FULL STORY ========================================================================== Earthquakes can be abrupt bursts of home-crumbling, ground-buckling
energy when slices of the planet's crust long held in place by friction suddenly slip and lurch.


==========================================================================
"We typically think of the plates on either side of a fault moving,
deforming, building up stresses and then: Boom, an earthquake happens,"
said Stanford University geophysicist Eric Dunham.

But deeper down, these blocks of rock can slide steadily past one
another, creeping along cracks in Earth's crust at about the rate that
your fingernails grow.

A boundary exists between the lower, creeping part of the fault, and
the upper portion that may stand locked for centuries at a stretch. For decades, scientists have puzzled over what controls this boundary, its movements and its relationship with big earthquakes. Chief among the
unknowns is how fluid and pressure migrate along faults, and how that
causes faults to slip.

A new physics-based fault simulator developed by Dunham and colleagues
provides some answers. The model shows how fluids ascending by fits
and starts gradually weaken the fault. In the decades leading up to
big earthquakes, they seem to propel the boundary, or locking depth,
a mile or two upward.

Migrating swarms The research, published Sept. 24 in Nature
Communications, also suggests that as pulses of high-pressure fluids
draw closer to the surface, they can trigger earthquake swarms -- strings
of quakes clustered in a local area, usually over a week or so. Shaking
from these seismic swarms is often too subtle for people to notice, but
not always: A swarm near the southern end of the San Andreas Fault in California in August 2020, for example, produced a magnitude-4.6 quake
strong enough to rattle surrounding cities.



==========================================================================
Each of the earthquakes in a swarm has its own aftershock sequence,
as opposed to one large mainshock followed by many aftershocks. "An
earthquake swarm often involves migration of these events along a fault
in some direction, horizontally or vertically," explained Dunham, senior
author of the paper and an associate professor of geophysics at Stanford's School of Earth, Energy & Environmental Sciences (Stanford Earth).

The simulator maps out how this migration works. Whereas much of the
advanced earthquake modeling of the last 20 years has focused on the role
of friction in unlocking faults, the new work accounts for interactions
between fluid and pressure in the fault zone using a simplified, two-dimensional model of a fault that cuts vertically through Earth's
entire crust, similar to the San Andreas Fault in California.

"Through computational modeling, we were able to tease out some of
the root causes for fault behavior," said lead author Weiqiang Zhu, a
graduate student in geophysics at Stanford. "We found the ebb and flow
of pressure around a fault may play an even bigger role than friction
in dictating its strength." Underground valves Faults in Earth's crust
are always saturated with fluids -- mostly water, but water in a state
that blurs distinctions between liquid and gas. Some of these fluids
originate in Earth's belly and migrate upwards; some come from above
when rainfall seeps in or energy developers inject fluids as part of
oil, gas or geothermal projects. "Increases in the pressure of that
fluid can push out on the walls of the fault, and make it easier for
the fault to slide," Dunham said. "Or, if the pressure decreases, that
creates a suction that pulls the walls together and inhibits sliding."
For decades, studies of rocks unearthed from fault zones have revealed
telltale cracks, mineral-filled veins and other signs that pressure can fluctuate wildly during and between big quakes, leading geologists to
theorize that water and other fluids play an important role in triggering earthquakes and influencing when the biggest temblors strike. "The rocks themselves are telling us this is an important process," Dunham said.



==========================================================================
More recently, scientists have documented that fluid injection related
to energy operations can lead to earthquake swarms. Seismologists have
linked oil and gas wastewater disposal wells, for example, to a dramatic increase in earthquakes in parts of Oklahoma starting around 2009. And
they've found that earthquake swarms migrate along faults faster or
slower in different environments, whether it's underneath a volcano,
around a geothermal operation or within oil and gas reservoirs, possibly because of wide variation in fluid production rates, Dunham explained. But modeling had yet to untangle the web of physical mechanisms behind the
observed patterns.

Dunham and Zhu's work builds on a concept of faults as valves, which
geologists first put forth in the 1990s. "The idea is that fluids ascend
along faults intermittently, even if those fluids are being released or injected at a steady, constant rate," Dunham explained. In the decades
to thousands of years between large earthquakes, mineral deposition and
other chemical processes seal the fault zone.

With the fault valve closed, fluid accumulates and pressure builds,
weakening the fault and forcing it to slip. Sometimes this movement is
too slight to generate ground shaking, but it's enough to fracture the
rock and open the valve, allowing fluids to resume their ascent.

The new modeling shows for the first time that as these pulses travel
upward along the fault, they can create earthquake swarms. "The concept
of a fault valve, and intermittent release of fluids, is an old idea,"
Dunham said. "But the occurrence of earthquake swarms in our simulations
of fault valving was completely unexpected." Predictions, and their
limits The model makes quantitative predictions about how quickly a
pulse of high- pressure fluids migrates along the fault, opens up pores,
causes the fault to slip and triggers certain phenomena: changes in the
locking depth, in some cases, and imperceptibly slow fault movements or clusters of small earthquakes in others. Those predictions can then be
tested against the actual seismicity along a fault -- in other words,
when and where small or slow-motion earthquakes end up occurring.

For instance, one set of simulations, in which the fault was set to
seal up and halt fluid migration within three or four months, predicted
a little more than an inch of slip along the fault right around the
locking depth over the course of a year, with the cycle repeating
every few years. This particular simulation closely matches patterns
of so-called slow-slip events observed in New Zealand and Japan -- a
sign that the underlying processes and mathematical relationships built
into the algorithm are on target. Meanwhile, simulations with sealing
dragged out over years caused the locking depth to rise as pressure
pulses climbed upward.

Changes in the locking depth can be estimated from GPS measurements of the deformation of Earth's surface. Yet the technology is not an earthquake predictor, Dunham said. That would require more complete knowledge of
the processes that influence fault slip, as well as information about the particular fault's geometry, stress, rock composition and fluid pressure,
he explained, "at a level of detail that is simply impossible, given
that most of the action is happening many miles underground." Rather,
the model offers a way to understand processes: how changes in fluid
pressure cause faults to slip; how sliding and slip of a fault breaks
up the rock and makes it more permeable; and how that increased porosity
allows fluids to flow more easily.

In the future, this understanding could help to inform assessments of
risk related to injecting fluids into the Earth. According to Dunham,
"The lessons that we learn about how fluid flow couples with frictional
sliding are applicable to naturally occurring earthquakes as well as
induced earthquakes that are happening in oil and gas reservoirs."
This research was supported by the National Science Foundation and the
Southern California Earthquake Center.


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


========================================================================== Journal Reference:
1. Weiqiang Zhu, Kali L. Allison, Eric M. Dunham, Yuyun Yang. Fault
valving
and pore pressure evolution in simulations of earthquake sequences
and aseismic slip. Nature Communications, 2020; 11 (1) DOI:
10.1038/s41467- 020-18598-z ==========================================================================

Link to news story: https://www.sciencedaily.com/releases/2020/09/200925113640.htm

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