New constraints on tectonic contraction across Los Angeles after
removal of groundwater pumping effects
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The squeezing of Los Angeles; a tale of earthquakes and water
US Geological Survey and University of California Los Angeles geophysicists
made two significant findings about how and why the ground moves
beneath Los Angeles. The researchers combined satellite radar images
and Global Position System (GPS) observations to measure more accurately
than ever before the tectonic squeezing of the region that is responsible
for earthquakes like the 1987 M=6 Whittier Narrows and M=6.7 Northridge
shocks. In the process, they discovered that large regions of metropolitan
Los Angeles are rising and falling by up to 11 cm (4.3 in) every
year with a large portion of the city of Santa Ana sinking at a
rate of 12 mm (0.5 in) per year.
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Satellite radar
interferogram draped over the topography of metropolitan Los
Angeles, California. The repeating color bands of the large
oval show about two inches (54 mm) of subsidence throughout
the basin between May and September, 1999 (175 days). The
Newport-Inglewood fault is the straight line located just
inside the coast line. The small isolated 'bull's eye' feature
North of Palos Verdes and East of downtown LA is from pumping
activity in the Inglewood oilfield.
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The satellite radar images show that the 40-km (25mi) long Santa
Ana basin (which includes the cites of Santa Ana, Anaheim, Garden
Grove, Norwalk, Downey, Lakewood, and Fountain Valley) rises every
winter and then drops during the summer. This ‘breathing’
of the earth’s surface is not caused by the tectonic forces
that squeeze the Los Angeles basin, rather it is caused by the widespread
pumping and recharge of groundwater, the basin reacting like a giant
sponge that is annually drenched and drained. The magnitude and
extent of these seasonal motions are a product of Los Angeles’ great thirst for water; they are unprecedented, and have not been
observed elsewhere in the world.
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image to enlarge it.
Schematic block
diagram showing the association of GPS site motions with blind
thrust faults in metropolitan Los Angeles. The blind thrust
faults are in bold orange lines and other active faults are
thin orange lines. Large earthquakes are shown as orange stars
with the date and magnitude. The large white arrows show the
direction of shortening across Los Angeles with the expected
motion from the strike-slip faults removed. The size of those
arrows does not relate to any particular scale. Figure modified
from Fuis et al., 2001 (Geology).
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Since the risks for earthquakes in Los Angeles are greater than
any other city in the United States, determining which faults are
moving and how they move is an essential step in assessing earthquake
hazards. After the Whittier Narrows and Northridge earthquakes revealed
that blind thrust faults threaten metropolitan Los Angeles, an array
of 250 continuously recording GPS stations (SCIGN) was deployed
to detect and monitor the motion associated with the movement of
the blind thrust(inclined faults that do not reach the surface)
and the surface faults. Gerald Bawden and his colleagues analyzed
data from this network and found that groundwater, oil, and natural
gas pumping produce motions at some of the GPS sites that obscure
and in some cases mimic the tectonic signals expected from the blind
thrust faults. The satellite radar imagery allows them to discriminate
sites that contain only the tectonic signal from those that have
a combination of tectonic and human-induce motions. By utilizing
only the GPS sites that contain tectonic signal, they determined
that the tectonic squeezing (4.4±0.8 mm/yr: 0.17±0.03
in/yr) across Los Angeles will likely produce earthquakes on the
either the blind Elysian Park or Puente Hills thrust faults systems.
The seasonal rising and falling of the Los Angeles area is caused
by the increasing demand for water throughout the region. To meet
the growing need for water during the hot summer months, water districts
in southern California pump water into the ground throughout the
year, and then when the demand for water is highest (between May
and September), water is pumped out of these large underground ‘reservoirs’.
The repeating pattern of pumping and recharge produces the seasonal
motion that is evident in the satellite imagery and GPS observations.
Repeatedly lowering the water table each summer to new lower levels
is causing the Santa Ana basin to sink, with about 84 mm (3.3 in)
of subsidence near Santa Ana between 1993 and 1999.
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GPS site motion
across Los Angeles that is produced by tectonic contraction.
Here we highlighted the motion that is not associated with
the surface faults. This displacement (black arrows) show
that tectonic forces are squeezing the Los Angeles basin.
And indeed, this squeezing is likely accommodated by the thrust
faults hidden beneath the metropolitan area.
The red regions
delimit areas where the ground has sunk over the years. The
blue patches point to where the ground rose. This intriguing
pattern is due to human activity: the pumping and injection
of ground water and oil extraction.
The dotted lines
are the surface projections of the Elysian Park and Compton
thrust faults.
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