M4.9 earthquake at the Cierro Prieto geothermal field,
Baja California, Mexico
A moderate earthquake in a huge geothermal energy field
((posted some days ago on their site, more about this
quake/area from 2 earthquake scientists))
KYLE BRADLEY AND JUDITH A HUBBARD
MAY 13, 2024
Para leer este post en español (traducido por Google),
haga clic aquí.
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On May 12, 2024 at 11:22 AM local time, a magnitude 4.9
earthquake struck just south of the USA-Mexico border,
northwest of Estación Delta, Mexico. The mainshock was
preceded by a small swarm of small events and followed
by a productive series of aftershocks, including a M4.6
and M4.2.
The earthquake produced shaking up to intensity VI near
the epicenter (strong) and was felt as far away as San
Diego. We are not aware of any reports of damage or
injuries. A ShakeAlert early warning was released,
alerting people further away from the epicenter seconds
to tens of seconds before the shaking arrived.
Figure 1: Earthquakes since May 12, 2024. Colored lines
are contours of shaking intensity estimated by the
USGS. Earthquakes are projected to the right onto a
timeline and shown below as a time series.
The low-lying Mexicali Valley is an important center of
agriculture, taking advantage of the huge amount of
fertile sediment that has been dumped by the Colorado
River at the north end of the Sea of Cortez, and
irrigation water from the river. This verdant flat
plain is deceptive - usually, such topographically
featureless areas are also tectonically quiet. But
along the west side of the valley, a blanket of young
river sediment largely obscures a fast-moving tectonic
plate boundary, where major strike-slip faults lurk at
depth and occasionally produce large earthquakes.
This already complex plate boundary extends northward
into the United States, where it breaks out into an
even more complex rash of parallel strike-slip faults,
each of which takes up some of the motion between the
Pacific and North American plates. A map and timeline
of the recorded seismicity in the area rewards some
careful inspection. The massive green event is the M7.1
2010 El Mayor-Cucapah earthquake, and will be discussed
later.
Figure 2: Earthquakes since 1995, colored by time.
At a more local level, we can see that the May 12
earthquake and most of its aftershocks struck within
the Cierro Prieto geothermal field, once one of the
largest producers of geothermal energy globally. The
geothermal field is easy to find in Google Earth due to
the presence of ponds where the salty wastewater is
sent to evaporate:
Figure 3: Google Earth satellite image of the Cerro
Prieto evaporation ponds. The Cierro Prieto volcano is
also visible at left.
Why is there such an important geothermal field out
among the fruits and vegetables? The answer, as usual,
is plate tectonics. Just to the south, the Gulf of
California grows slightly wider every year due to
active ocean floor spreading. The earliest geological
signs of continental breakup appeared between Cabo San
Lucas and Puerto Vallarta about 12 million years ago;
the rifting has since propagated northward. Over the
last ~6 million years, true oceanic spreading has
forced the peninsula of Baja California away from the
Mexican mainland with a dramatic sideways yank. The
main tectonic features can be seen on the map below,
which we modified from a very nice figure we found in a
report on a scientific ocean drilling project (Sawyer
et al., 2007).
Figure 4: Active tectonics of the Gulf of California,
modified from Figure 4 of Sawyer et al. (2007) - a
report on the endearingly titled COBBOOM (Continental
Breakup and Birth of Oceans Mission) project.
The tectonic behavior of the Gulf of California is
pretty fascinating. The rifting happened really fast
compared to most other continental rifts, which take a
lot longer to progress from early crustal extension to
actual oceanic spreading. Second, the total length of
the active strike-slip faults (between the spreading
ridges) is much larger than the total length of the
spreading ridges themselves. This means that most of
the earthquakes along this part of the plate boundary
are strike-slip events. The fast plate motions here,
and the geological youth of the rift, make it an
exciting plate to study continental breakup.
The northern edge and youngest part of this great rift
system lies just beneath Cierro Prieto, the focus
(ahem) of today’s post. There, two active strike-slip
faults come close to meeting, but are still separated
by a large rightward step of about 20 km. The Cierro
Prieto Fault enters this step from the south and
emerges as the Imperial Fault to the north. The fault
that hosted the May 12 mainshock is presumably one of
several normal faults that span the gap between the
Cierro Prieto and Imperial Faults. On May 24, 2006, a
M5.4 earthquake causing surface fracturing along the
Morelia Fault (MF in the figure below); this is
probably a good candidate for fault that produced the
May 12, 2024 earthquake.
Figure 5: Faults in the area of the Cerro Prieto
geothermal plant, from Suárez-Vidal et al., (2007). MF:
Morelia Fault; CPF: Cerro Prieto Fault; IF: Imperial
Fault.
Why is this area so geothermally productive? Within the
stepover, the continental crust has been thinned out by
continued northwest-southeast stretching, with
sediments collecting in the resulting sag. These
sediments are the host rock for the geothermal fluids
at depth. Basaltic magma is also being constantly
intruded into the shallow crust beneath the basin,
providing a source of intense heat. This magmatism is
represented at the surface by a single small volcanic
cone located just west of the geothermal field.
Figure 6: A geological cross section across the Cerro
Prieto geothermal field, from Sarychikhina et al.
(2011), following Lira (2005). Modified to add labels
for fault types: N = normal, SS = strike-slip.
One geological interpretation of this fault stepover is
that it represents the early phase of the birth of a
new segment of the Sea of Cortez spreading ridge. This
is supported by the fact that when these normal faults
slip, the resulting direction of crustal extension is
nicely aligned with the northwest-southeast separation
between North America and the Pacific Plate. You don’t
have to be a really big, impressive fault to be a plate
boundary!
If we give this system a few million years, we should
eventually expect to see a transition from stretching
of the continental crust to actual oceanic spreading.
The crustal stretching decompresses and melts the
mantle beneath the stepover, creating the basaltic
magmas that intrude into the crust (and rarely erupt).
While the recent May 12 earthquake was pretty small,
much larger earthquakes have occurred nearby. On April
4, 2010, the M7.2 El Mayor-Cucapah earthquake was
produced by rupture of a previously unmapped strike-
slip fault located just west of Cierro Prieto. The town
of Estación Delta was particularly damaged by this
earthquake. This was one of the largest on-land North
American earthquakes of the instrumental era, was
preceded by a long and interesting foreshock sequence,
and was also followed by a long period of continued
aseismic slip on the fault; as a result, many studies
of this earthquake and its geological aftermath have
been published. (Google Scholar returns 2,780 results!)
As soon as the earth stopped trembling in 2010, people
started to wonder whether the Cierro Prieto geothermal
plant had caused, or at least had an effect on, the
earthquake.
Why would a geothermal field affect nearby faults? At
Cierro Prieto, it mostly boils down (ahem) to stress
changes caused by extracting and reinjecting
groundwater.
Pressurized water at depth slightly pushes apart the
grains of the sedimentary rock. When this water is
removed, the pore space collapses and the rocks
compact. This causes the ground surface above to drop
downward. Ground subsidence has been nicely monitored
at Cierro Prieto using both traditional ground surveys
and satellite radar measurements:
Figure 7: Figure 2d of Samsonov et al. (2017), showing
satellite measurements of land surface subsidence at
Cerro Prieto. Each full cycle of colors indicates 100
mm of displacement. There are about 5 cycles between
the reference level and the most highly subsided point
- a total of almost half a meter!
This deformation inevitably causes changes in crustal
stress surrounding the geothermal field, which can
affect nearby faults. Some faults might be pushed
closer toward failure, and some might be become less
likely to fail, depending on their location and
orientation.
The extracted water also has to go somewhere. Much of
it is evaporated into the air in those giant ponds, but
some of it is actually pumped back into the ground.
This pumping also causes stress changes in the crust.
Wastewater injection is common in oil and gas fields,
and is a significant cause of human-induced earthquakes
in those settings.
So, does this geothermal plant have an effect on large
earthquakes? The current scientific consensus seems to
be a resounding… “maybe”. There are apparently no
smoking gun correlations, where an influence is obvious
from simple inspection of earthquake timings versus
extraction periods. So, studies have had to develop
numerical models that try to relate water extraction to
the earthquake cycle in a more complicated way. Thus
far, it seems that some kind of influence can’t be
confidently excluded, but it also hasn’t been reliably
demonstrated.
However, it is important to remember that the
geothermal plant is located where it is because the
tectonics are so active. It is fundamentally
unsurprising that these kinds of earthquakes will occur
near Cierro Prieto - and so the M4.9 that occurred on
May 12 is exactly what we would expect to occur in this
area, perhaps once a decade or so. This expected
volcanic-tectonic activity stands in contrast with
anthropogenic earthquakes in other places like Texas,
Oklahoma, or Poland - which are far from plate
boundaries, where the natural rate of earthquakes is
much lower.
It would even be geologically unsurprising, although
pretty exciting and a bit scary, if a new volcano
erupted near Cerro Prieto. If you want to see what a
scenario like that actually looks like, an important
geothermal plant in south Iceland is currently
threatened by the ongoing eruptions from the recently
re-awakened Mid-Atlantic spreading ridge!
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