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====Introduction==== Interferometric synthetic aperture radar (InSAR) data collected over winters are used to determine if pre-failure movements of landslides are detectable along the North Fork Stillaguamish River Valley, Washington. Large landsliding is common to the study region and poses considerable human and societal risk. C-band ENVISAT and ERS2 data are found to provide insufficient coherence for interferometry at the study site. L-band data, being less sensitive to small vegetation changes between scenes, produced one coherent interferogram from the four scenes collected, with polarization and temporal decorrelation errors preventing further interferogram creation. This work indicates that for consecutive, like-polarized, ALOS L-band interferograms, landslide detection may be possible near or at the study site, but additional geometric constraints limit the applicability of this approach. ===Research Statement=== Large precipitation-induced landslides pose a significant risk to human safety and existing infrastructure in the state of Washington, as shown in the recent March 22, 2013 Steelhead Haven landslide tragedy. Previous field-based studies (e.g. Petley et al., 2002; Bozzano et al., 2014) have shown landslide pre-failure movement and velocity time histories are able to predict time of failure. Building on these relationships, Mazzanti et al. (2012) present a framework in which landslide failure could be predicted using pre-failure displacement relationships using InSAR data. Preliminary results are shown here for detecting pre-failure movements of known landslides in western Washington to explore interferometry as a means of landslide early warning or detection. Active slides along the North Fork Stillaguamish River valley were explored during the winter prior to known landsliding dates in 2006 and 2011. ===Motivation of Work=== Studies of past landsliding and susceptibility to future landsliding have shown the extreme pervasiveness of these events locally (Haugerud, 2014; Miller, 1999). On March 27, 2013 The Whidbey Island slide displaced ~150,000m3, reactivating an older landslide complex and endangered 20 homes (DNR, 2013). The more recent, March 22, 2014, Steelhead Haven slide displaced 7.6 million cubic meters of material, buried 600m of highway, caused an estimated $50M in damages and took the lives of 43 individuals (Keaton et al., 2014). Steelhead Haven was a continuation of frequent landsliding at the Hazel site near Oso, Washington, where slides in 2006, 1988, and 1967 have all partially dammed the North Fork Stillaguamish. Current annual precipitation over the western half of Washington is approximately 1.25m, falling mostly from October through March, and is expected to rise statewide in the coming decades as a result of climate change (Elsner et al., 2010). In our current climatic regime precipitation-induced landslides present a significant risk in our region, and increases in precipitation intensity or duration as a result of climate change will only increase this risk. ===Background=== ==Landsliding== In this work the movements of large, deep-seated, landslides are targeted. Deep-seated slides or slumps in sands and fine-grained material tend to have a strong rotational component of motion (see Figure 1). Overall motion in the upper portion of the slide, the zone of depletion, is predominantly downward, with a small translational component. Movement in the lower zone of accumulation will be a combination of upward and lateral motion. A slowly developing, or pre-failure creeping, deep rotational landslide should produce a distinct pattern of line of sight (LOS) deformation in InSAR scenes, roughly shaped as a rounded hourglass, with range increases in the upper portion and a more diffuse range decreasing zone below.\\ {{:alex:main:slide_img.png?direct&500|}}\\ Figure 1, Diagram of slump-earth flow failure, modified from Varnes (1973). Image via http://www.for.gov.bc.ca/hfd/pubs/docs/sil/Sil411/A4110025.htm\\ Field observations have noted that rather than the rigid body failure idealized in most models, large deep-seated landslides are characterized by slow, increasing strain rates up to a threshold point, at which rapid failure occurs (e.g. Mazzanti et al., 2012; Crosta and Agliardi, 2003; Bozzano et al., 2011). This strain-acceleration behavior is well demonstrated in Figure 2 from Petley et al. (2002), which plots inclinometer data at four points along a rotational failure plane. These test data show a strong ground deformation signal, with steady displacements of ~7.5cm/mo at the toe of the slope (point 10) over the last 400 days prior to failure. \\ {{:alex:petley_fig.jpg?direct&800|}}\\ Figure 2. Inclinometer data showing cumulative displacement along the Selborne landslide test site, cumulative displacement in cm. From Petely et al. (2002) \\ Interferometry has been used in a wide range of locations recently to identify new landslides or monitor deformation of known slides (e.g. Roering et al., 2009; Zhao et al., 2012; Liu et al., 2011; Jebur et al., 2013). In most studies ALOS data, collected using 24cm L-band wavelengths, is preferred to 5.6cm C-band (ERS1/2, ENVISAT, RADARSAT-1/2) systems given its relative insensitivity to vegetation and perpendicular baseline decorrelation. ALOS derived landslide mapping and deformation monitoring has been shown to be an effective technique from sparsely vegetated scrublands in northern California (Roering et al., 2009) to Malaysian tropical forests (Jebur et al., 2013). Additionally, Liu et al. (2011) have successfully shown an implementation of ENVISAT C-band data along the densely vegetated and steeply sloped riverbanks of the Yangtze to track deformation of two landslides using a persistent scatterer technique. ==Site History and Geology== In this work an actively landsliding portion of the North Fork Stillaguamish River (NFSRV), located along SR530 in Washington (Figure 3), is considered. The North Fork Stillaguamish River has incised steep hillslopes in glacial-fluvial outwash, till, and glacial-lacustrine deposits (Keaton et al., 2014; Troost and Booth, 2008) that have been host to many historic and recent landslides (Haugerud, 2014; Miller, 1999). \\ {{:alex:main:studysite.jpg?direct|}}\\ Figure 3, Location of study in the context of Washington State On January 25, 2006 the Hazel landslide occurred just above the Steelhead Haven community. Regular failures have occurred on this slope, most recently this past March. A partially denuded slope, clear-cut land above, and homes below provide potential reflectors for SAR. In February 2006 cracks were observed just downriver of Oso at the Skaglund Hill site, leading to a $13.3M WSDOT remediation project to prevent losses to the road and utility lines located at the toe of the slope. Reactivation of movement at Skaglund hill occurred in early 2011 (Haugerud, 2014). Within the past half-century six significant movements have occurred along the North Fork Stillaguamish River valley study site (Table 1). Smaller movements in this time period are likely, but were not recorded by the Department of Natural Resources (DNR, 2014) or impact infrastructure managed by the Washington State Dept. of Transportation (WSDOT, 2013). {{:alex:main:slide_table.jpg?direct|}}\\ Table 1, Significant Landslides in the North Fork Stillaguamish River Valley since 1967 ====Methodology==== 14 SAR scenes were collected across three winters for this work, summarized in Table 2. During the winter of 2005-06, prior to initial movements at Skaglund hill and the Hazel failure, four consecutive ENVISAT scenes are available along the descending track 156, frame 2637. In the winter of 2006-07, post failure and collected for use as a control year, six ERS2 scenes along track 156, frame 2637 were collected. Four ascending ALOS scenes from path 217, frame 960 were used from the winter of 2010-11 to capture reactivation movements at Skaglund hill. \\ {{:alex:main:table2.jpg?nolink&500|}}\\ Table 2, Available SAR scenes. Note the 12.13.2010 scene from ALOS was only available at full resolution for HV-polarization. To improve the likelihood of developing coherent interferograms initial perpendicular baseline limits of 200m for ERS2 and ENVISAT data and 1000m for ALOS data were set. Due to large baselines in the available ENVISAT data scenes with perpendicular baselines less than 1000m were processed. Time between data acquisitions was limited to consecutive scenes, i.e. Δt = 34 - 35 days for ERS2 and ENVISAT data, Δt = 45 – 46 days for ALOS, except in the case of one ALOS interferogram, which was processed at Δt = 92 days. Enforcing the above restrictions, only seven interferometric pairs (Table 3) remained. The Repeat Orbit Interferometry Package (ROI_Pac) (Rosen et al., 2004) was used to process interferograms at 4 range looks. 1-arcsecond SRTM Digital Elevation Model (DEM) data was used to remove the topographic component of phase information during processing. Seven interferograms were able to be developed and are discussed below. As shown in the results and Appendix A: Additional Interferograms, baseline restrictions were not lifted in this work to create more interferograms given the poor coherence developed by small baseline pairs.\\ {{:alex:main:table3.jpg?nolink&600|}}\\ Table 3, Potential interferogram pairs in this work. Black: acceptable pairs, Grey: pairs with unacceptable perpendicular baselines, Red: cross-polarized images due to 12.13.10 data\\ ====Results==== ===C-Band Data=== Processing of ERS2 and ENVISAT C-band data yielded entirely (Appendix A) or mostly (Figure 4) decorrelated interferograms. All interferograms included here are filtered, wrapped, imaged sampled at 4 range looks to ~30m resolution. None of the interferograms processed in this work were capable of unwrapping within the ROI_Pac software.\\ {{:alex:main:envi_fig.jpg?direct&1000|ENVISAT Interferogram for 11-4-05 to 12-9-05}}\\ Figure 4, ENVISAT data collected on 11-04-05 and 12-09-05, is the most coherent C-band interferogram available from this work. Urban areas, such as the I-5 corridor in the left of the image are reasonably coherent but no useful data is available along the North Fork Stillaguamish River valley (Oso to Darrington in the Figure.) ===L-Band Data=== ====Conclusions==== ====References==== ====Appendix A====

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