The Slumgullion Natural Laboratory

THE SLUMGULLION NATURAL LABORATORY A. 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. B. 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. C. Example of probably Rayleigh were packets from similar sources that occurred throughout the experiment. D. Record of many signals with the same characteristics as tremor observed in plate-scale systems. Their durations last from several seconds to minutes. Although preliminary examination of envelope functions appears o 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.
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 com- parable richness of aseismic and seismic modes as crustal- and 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 inter- station 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.
</p><p>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 slide-quakes 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 pres- sure 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.
</p><p>Acknowledgements: This work was funded through the US Geological Venture Capital Program.</p>


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