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=====Monitoring Ground Deformation from the Wooded Island Earthquake Swarm at Hanford, Washington==== //Mika Thompson, ESS 590: Introduction to Radar Interferometry// ====Introduction and Background==== The Hanford Nuclear Reservation located on the Columbia River in Eastern Washington has been subject to seismic swarm activity for the past several decades. The site sits above the Columbia River basalt (CRB) flows that make up the Columbia Basin in southeastern Washington. It also falls within the region of the seismically active Yakima fold and thrust belt (YFTB) [//Blakely et al., 2012//]. A major swarm occurred between 1969 and 1970 followed by two smaller ones in 1975 and 1988 [//Wicks et al., 2011//]. The most recent event began in January of 2009 just south of Wooded Island (Fig. 1) and continued throughout the year. It is the first recording of clustered, shallow earthquakes in this area where geodetic measurements of surface deformation were available for analysis. Since then low-magnitude shallow events have continued to occur at the site. Earthquakes are major cause for concern in this area because of Hanford’s historical role in plutonium production for nuclear weapons during World War II. Plutonium manufacturing is an inefficient process that produces large amounts of solid and liquid waste [www.hanford.gov]. Until the Hanford reactors were mostly decommissioned in the 1980s, solid wastes were buried in pits or trenches on the Hanford Reservation and liquid wastes were poured on to the ground or stored underground in storage tanks [www.hanford.gov]. Ground deformation caused by earthquakes and aseismic slip on or near the reservation could potentially cause toxic waste leakage into the groundwater used by the homes and farms surrounding Hanford. In a previous InSAR study of the 2009 Wooded Island earthquake swarm by Wicks et al. [//2011//], surface deformation was identified on the nuclear reservation coincident with the earthquake swarm (Fig. 2). Wicks et al. [//2011//] observed ground displacement caused by ~1500 shallow earthquakes and aseismic slip between February 2009 and October 2009. They constructed a model of the fault environment, which consisted of a shallow thrust fault and a nearly horizontal fault that they theorized would fall within a thixotropic sediment layer between basalt flows. The thixotropic layer was proposed as an explanation for why the modeled geodetic moment of the planar fault was eight times greater than the cumulative seismic moment of the swarm. According to Wicks and his group, silt and clay interbedded between the CRB layers may have thixotropic properties that reduce the viscosity of the sediments when pressure is increased. When the sediment layers are stressed beyond their yield strength they behave like a lubricant and facilitate aseismic slip. Wicks’ 2011 paper also suggested that a fault gouge model similar to one developed by Amoruso et al. [//2004//] could be used to describe the aseismic slip along the bedding plane fault. The fault slip model, shown in Fig. 3 consists of a gouge between planar between fault layers filled with unconsolidated sediment with viscoplastic properties. A pressure pulse caused by seismicity or a breach in a permeability barrier at one end of the fault causes stress to increase on the fault. When the yield stress is exceeded the sediments in the gouge would begin to exhibit linear viscous fluid behavior, which would lead to ground displacement. Amoruso and Wicks’ fault models thoroughly describe a possible fault mechanism, however the origin of the pressure pulse is still inconclusive [//Wicks et al., 2011; Blakely et al., 2012; Gomberg et al., 2012//]. Several triggers have been suggested for faulting in this region. Gomberg et al. [//2012//] attempted to find a correlation between groundwater changes and increased seismicity rates by plotting seismic event locations over a map of ground water changes. They also looked for a relationship between irrigation of the farms surrounding Hanford and increased earthquake activity. Both analyses were inconclusive [//Gomberg et al., 2012//]. Aeromagnetic data was also used to try and find the cause of the swarm. A concealed portion of the Yakima Ridge anticline, a structure within the Yakima Fold and Thrust Belt, was found to extend beneath the location of the Wooded Island swarm [//Blakely et al., 2012//]. Blakely and his group determined that the northwest strike of this anticline ran parallel to the strike of the largest earthquake detected within the swarm, and also coincided with the ground deformation found in Wicks’ paper. He proposed that while fluids may play a role in the seismicity at the Hanford, regional scale horizontal compression was the driving force of the swarm activity. While there have been several papers dedicated to understanding the earthquake swarm activity and deformation in the Hanford area, the purpose of this study is focused on the evolution of the ground deformation on the Hanford Site. My intent is to also look for seasonal fluctuations in groundwater and irrigation practices at the farms in the region using InSAR methods. InSAR could reveal more subtle changes in ground displacement than the groundwater maps used in Gomberg’s 2012 paper. ====Methods==== SAR data collected for this project between 2009 and 2010 by the European Space Agency’s (ESA) Envisat over Eastern Washington to measure line-of-site ground displacement on the Hanford reservation. Because there were very few scenes available, I chose to generate interferograms from all SAR pairs with a perpendicular baseline less than 200 m (Table 1; Fig. 2; Fig. 4). The interferograms were processed using the open-source Repeat Orbit Interferometry Package (ROI_PAC) code and a digital elevation model (DEM) produced from the Shuttle Radar Topography Mission (SRTM) to correct for the topographic component in the SAR data. To aid in differentiating between atmospheric effects and ground deformation in the interferograms, I performed a query of earthquake locations near Wooded Island on the Pacific Northwest Seismic Network website. Kml files were generated with earthquake swarm magnitudes and locations that coincided with the master and slave dates of the interferograms. The earthquake locations were analyzed together with kmz files of my georeferenced interferograms in Google Earth. I used the ENVI program to make horizontal profiles through interferograms that showed signs of deformation. Radians of phase difference were converted to centimeters of vertical displacement using the equation {{ :mika:eqn.png?100 |}} where phi is the phase difference in radians, lambda = 5.6 cm is the Envisat pulse wavelength, and d is the vertical displacement in the line of sight. Though this isn’t an accurate measurement of ground displacement, it is sufficient to check for errors and the feasibility of the displacement values. ====Results==== ====Discussion==== ====Conclusions====

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