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The Slumgullion Natural Laboratory
Joan Gomberg (US Geological Survey), Paul Bodin (University of Washington), Bill Schulz (US Geological Survey), Jason
Kean (US Geological Survey)
Observational advances continue to reveal diversity in
the seismic signals associated with fault slip. A particularly
rich example are episodes of slow fault slip near major
plate boundaries that manifest as geodetically observed
aseismic deformation abetted by a new family of seismic
signals (named ‘episodic tremor and slip’ or ETS). While
the driving forces and scales differ, there are striking parallels
between some observations and models of ETS and
of landslide behaviors. To explore common features and
the underlying processes we are studying the Slumgullion
landslide in southwest Colorado, and an ideal natural laboratory
for observing fault slip and associated phenomena.
Unlike crustal- or plate-scale studies significant deformation
can be measured within a single field season, because
the Slumgullion moves at average rates of up to 2 cm/
day. We completed a field experiment on the Slumgullion
to test several hypotheses, particularly that slip along the
basal surface and side-bounding faults occurs with comparable
richness of aseismic and seismic modes as crustaland
plate-scale boundaries. From August 18-26, 2009 we
monitored the seismic radiation with 88 short-period vertical
seismometers recorded on “Texan” seismographs from
the IRIS PASSCAL facility. The seismographs, with interstation
spacings of 25-50 m, recorded continuously at 250
samples per second. In addition, we recorded deformation
on several extensometers that continuously measure slip
across one of the two lateral faults bounding the landslide,
and tracked displacements of 29 sites on and off the slide
with an automated total-station and differential GPS. More
observations came from 2 borehole-mounted piezometers
and a meteorological station.
The seismic data contain an abundance of network-wide
coherent signals with an amazing variety of characteristics.
We observed impulsive earthquakes with clear P,S, and surface
wave phases. There are also “repeaters”, or multiplets of slidequakes
with very similar waveforms. There are episodes of
tremor-like radiation coherent across our network. Noteably, a
diurnal variation in the slide velocity tracks atmospheric pressure
fluctuations, which correlates with the rates of repeating
harmonic seismic signals. This correlation and our analyses of
the wavefield associated with these events leads us to suggest
that the signals are trapped waves generated at a ‘sticky spot’
within the side-bounding strike-slip fault.
Acknowledgements: This work was funded through the US
Geological Venture Capital Program.
3150 Elevation (m) 3200
Networks{
Seismic & Geodetic
Weather
Station
50 m
50 m
Continuous
Pressure Sensor
& Extensometer
Topographic map of the narrowest and fastest section of the Slumgullion
Topographic map of the narrowest and fastest section of the Slumgullion landslide,
showing landslide, the locations showing of the the locations seismographs of the (labeled), seismographs weather (labeled), station, and weather pressure
sensor, station, all of and which pressure recorded sensor, continuously. all of which The recorded outer-most continuously. lines of seismic The stations
lie outside outer-most the active lines slide, of seismic which stations is bounded lie by outside strike the slip active faults slide, nearest which the seismic
stations bounded numbered by strike 03 and slip 02. faults R00 is nearest a few the meters seismic from stations the nearest numbered road. ‘Geodetic’ 03
stations and correspond 02. R00 is to a prisms few meters that serve from Slide? the as targets Quakes nearest for road. the robotic ‘Geodetic’ total stations
and 3
extensometers correspond (2 not to prisms shown that are located serve as above targets and for below the the robotic networks). total station
and 3 extensometers (2 not shown are located above and below the networks).
B. C.
A.
Slide? Quakes
The relatively long duration waveforms
of events in this vigorous
sequence, which lasted several tens of
minutes, suggest the sources are
similar but not identical in location,
mechanism, and size.
Waveforms from a single, tiny quake
show clear P and S arrivals & suggest
an impulsive shear source a few
hundred meters from station C03. This
quake is part of a sequence containing
tens of events within a few minutes.
Seconds from 08/26/2009 04:19:23.20
Example of probable Rayleigh wave
packets from similar sources that
occurred throughtout the experiment.
Seconds from 08/26/2009 04:19:23.20
The relatively long duration waveforms
Waveforms from a single, tiny quake
D.
of events in this vigorous Tremor show clear P and S arrivals & suggest
E. Harmonic Sources
sequence, which lasted several tens of
minutes, suggest the sources are
similar but not identical in location,
mechanism, and size.
Tremor
an impulsive shear source a few
hundred meters from station C03. This
quake is part of a sequence containing
tens of events within a few minutes.
Example of probable Rayleigh wave
packets from similar sources that
occurred throughtout the experiment.
Harmonic Sources
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Seconds from 08/25/2009 17:10:46.73
We recorded many signals with the same characteristics as
We recorded hundreds of events
A. The tremor relatively observed long in plate-scale duration systems. waveforms Their durations of events last in this with vigorous waveforms sequence, similar to these which
from several seconds to minutes. Although preliminary examination
of envelope functions appears to show promise (e.g. enve-
largest amplitude signals are shown
transient harmonic signals. The
lasted several tens of minutes, suggest the sources are similar but not identical in
location, lopes mechanism, correlate visually and at multiple size. B. stations), Waveforms we have yet from to a single, orange tiny quake and are recorded show clear at P
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Seconds from 08/25/2009 17:10:46.73
and S arrivals quantitatively & suggest analyze the an tremor impulsive signals. shear source a few hundred stations near meters the side-bounding from station
We recorded many signals with the same characteristics as
We
fault.
recorded hundreds of events
C03. This tremor quake observed is part in plate-scale of a sequence systems. Their containing durations last tens of events with waveforms within similar a few to minutes. these
C. Example from several of probable seconds to minutes. Rayleigh Although wave preliminary packets examination
of envelope the experiment. functions appears D. to Record show promise of many (e.g. enve-
signals with largest the amplitude same characteristics
signals are shown
from similar transient sources harmonic that signals. occurred The
throughtout
lopes correlate visually multiple stations), we have yet to
in orange and are recorded at
as tremor quantitatively observed analyze in the plate-scale tremor signals. systems. Their durations stations last from near the several side-bounding seconds
to minutes. Although preliminary examination of envelope fault. functions appears to show
promise (e.g. envelopes correlate visually at multiple stations), we have yet to quantitatively
analyze the tremor signals. E. Record of hundreds of events with waveforms
similar to these transient harmonic signals. The largest amplitude signals are shown
in orange and are recorded at stations near the side-bounding fault.
A03
A05
A07
A11
B02
B06
C03
C05
C07
D02
D06
E03
E05
E07
F02
F06
G03
A03
G05
A05
G07
A07
H02
A11
H06
B02
I03
B06
I05
C03
I07
C05
J02
C07
J04
D02
J06
D06
J08
E03
K03
E05
K05
E07
K07
F02
L02
F06
L04
G03
M03
G05
M05
G07
M07
H02
N02
H06
N04
I03
O03
I05
O05
I07
P02
J02
P04
J04
R00
J06
J08
K03
K05
K07
L02
L04
M03
M05
M07
N02
N04
O03
O05
P02
P04
R00
2010 IRIS Core Programs Proposal | Volume II | Episodic Tremor and Slip, Triggered Earthquakes | II-85