Brent G. Delbridge

COSMO-SkyMed Spotlight Interferometry Over Rural Areas: The Slumgullion Landslide in Colorado, USA

In the last 7 years, spaceborne synthetic aperture radar (SAR) data with resolution of better than a meter acquired by satellites in spotlight mode offered an unprecedented improvement in SAR interferometry (InSAR). Most attention has been focused on monitoring urban areas and man-made infrastructure exploiting geometric accuracy, stability, and phase fidelity of the spotlight mode. In this paper, we explore the potential application of the COSMO-SkyMed® Spotlight mode to rural areas where decorrelation is substantial and rapidly increases with time. We focus on the rapid repeat times of as short as one day possible with the COSMO-SkyMed® constellation. We further present a qualitative analysis of spotlight interferometry over the Slumgullion landslide in southwest Colorado, which moves at rates of more than 1 cm/day.

Three-dimensional surface deformation derived from airborne interferometric UAVSAR: Application to the Slumgullion Landslide

Author(s): Delbridge, BG; Burgmann, R; Fielding, E; Hensley, S; Schulz, WH | Abstract: ©2016. American Geophysical Union. All Rights Reserved. In order to provide surface geodetic measurements with “landslide-wide” spatial coverage, we develop and validate a method for the characterization of 3-D surface deformation using the unique capabilities of the Uninhabited Aerial Vehicle Synthetic Aperture Radar (UAVSAR) airborne repeat-pass radar interferometry system. We apply our method at the well-studied Slumgullion Landslide, which is 3.9 km long and moves persistently at rates up to ∼2 cm/day. A comparison with concurrent GPS measurements validates this method and shows that it provides reliable and accurate 3-D surface deformation measurements. The UAVSAR-derived vector velocity field measurements accurately capture the sharp boundaries defining previously identified kinematic units and geomorphic domains within the landslide. We acquired data across the landslide during spring and summer and identify that the landslide moves more slowly during summer except at its head, presumably in response to spatiotemporal variations in snowmelt infiltration. In order to constrain the mechanics controlling landslide motion from surface velocity measurements, we present an inversion framework for the extraction of slide thickness and basal geometry from dense 3-D surface velocity fields. We find that the average depth of the Slumgullion Landslide is 7.5 m, several meters less than previous depth estimates. We show that by considering a viscoplastic rheology, we can derive tighter theoretical bounds on the rheological parameter relating mean horizontal flow rate to surface velocity. Using inclinometer data for slow-moving, clay-rich landslides across the globe, we find a consistent value for the rheological parameter of 0.85 ± 0.08.

Kinematics of the slumgullion landslide from UAVSAR derived interferograms

In order to measure the response of the Slumgullion landslide to hydraulic forcing, we utilize the unique capabilities of the NASA/JPLs UAVSAR airborne repeat-pass SAR interferometry system to provide surface geodetic measurements with “landslide-wide” spatial coverage. Unlike traditional space-based INSAR we are not restricted to fixed flight tracks or fixed repeat times, allowing for optimal imaging geometries and timing. We combine four look directions chosen based on the landslide geometry and invert for the full 3-D landslide-wide surface deformation. These observations complement ongoing GPS measurements and in-situ observations of pore-pressure and atmospheric parameters acquired by the U.S. Geological Survey.

Experimental measurements and bristle friction modeling of nonlinear hysteresis loops and harmonic generation in rock fractures

Frictional properties affect the propagation of high-amplitude seismic waves across rock fractures. Laboratory evidence suggests that these properties can be measured in active seismic surveys, potentially offering a route to characterizing friction in situ. We present experimental results from a subresonance torsional modulus and attenuation apparatus that utilizes micron-scale sinusoidal oscillations to probe the nonlinear stress-strain relation at a range of strain amplitudes and rates. Nonlinear effects are further quantified using harmonic distortion; however, time series data best illuminate underlying physical processes. The low-frequency stress-strain hysteretic loops show stiffening at the sinusoid’s static ends, but stiffening is reduced above a threshold frequency. This shape is determined by harmonic generation in the strain; the stress signal has no harmonics, confirming that the fractured sample is the source of the nonlinearity. These qualitative observations suggest the presence of rate-de...

Experimental evidence for dynamic friction on rock fractures from frequency-dependent nonlinear hysteresis and harmonic generation: Low-Frequency Torsional Dynamic Friction

Frictional properties affect the propagation of high amplitude seismic waves across rock fractures and faults. Laboratory evidence suggests that these properties can be measured in active seismic surveys, potentially offering a route to characterizing friction in situ. We present experimental results from a sub-resonance torsional modulus and attenuation apparatus that utilizes micron scale sinusoidal oscillations to probe the nonlinear stress-strain relation at a range of strain amplitudes and rates. Nonlinear effects are further quantified using harmonic distortion; however, time-series data best illuminates underlying physical processes. The low frequency stress-strain hysteretic loops show stiffening at the sinusoids static ends, but stiffening is reduced above a threshold frequency. This shape is determined by harmonic generation in the strain; the stress signal has no harmonics, confirming that the fractured sample is the source of the nonlinearity. These qualitative observations suggest the presence of rate dependent friction and are consistent between fractures in three different rock types. We propose that static friction at the low strain-rate part of the cycle, when given sufficient ‘healing’ time at low oscillation frequencies, causes this stiffening cusp shape in the hysteresis loop. While rate-and-state friction is commonly used to represent dynamic friction, it cannot capture static friction or negative slip velocities. So we implement another dynamic friction model, based on the work of Dahl, which describes this process and produces similar results. Since the two models have a similar form, parameterizations of field data could constraint fault model inputs, such as specific locations velocity strengthening or weakening properties.

Temporal variation of intermediate-depth earthquakes around the time of the M9.0 Tohoku-oki earthquake

Author(s): Delbridge, BG; Kita, S; Uchida, N; Johnson, CW; Matsuzawa, T; Burgmann, R | Abstract: ©2017. American Geophysical Union. All Rights Reserved. The temporal evolution of intermediate-depth seismicity before and after the 2011 M9.0 Tohoku-oki earthquake reveals interactions between plate interface slip and deformation in the subducting slab. We investigate seismicity rate changes in the upper and lower planes of the double seismic zone beneath northeast Japan using both a β statistic approach and a temporal epidemic type aftershock sequence model. We do not observe an anomalous precursory increase in intermediate-depth earthquake activity preceding the mainshock; however, following the mainshock, we observe a rate increase for the intermediate-depth earthquakes in the upper plane. The average ratio of upper plane to lower plane activity and the mean deep aseismic slip rate both increased by factor of 2. An increase of downdip compression in the slab resulting from coseismic and postseismic deformation enhanced seismicity in the upper plane, which is dominated by events accommodating downdip shortening from plate unbending.