Person: Evans, Eileen Louise
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Evans
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Eileen Louise
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Evans, Eileen Louise
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Publication Geodetic Imaging of Coseismic Slip and Postseismic Afterslip: Sparsity Promoting Methods Applied to the Great Tohoku Earthquake(Wiley-Blackwell, 2012) Evans, Eileen Louise; Meade, BrendanGeodetic observations of surface displacements during and following earthquakes such as the March 11, 2011 great Tohoku earthquake can be used to constrain the spatial extent of coseismic slip and postseismic afterslip, and characterize the spectrum of earthquake cycle behaviors. Slip models are often regularized by assuming that slip on the fault varies smoothly in space, which may result in the artificial smearing of fault slip beyond physical boundaries. Alternatively, it may be desirable to estimate a slip distribution that is spatially compact and varies sharply. Here we show that sparsity promoting state vector regularization methods can be used to recover slip distributions with sharp boundaries, representing an alternative end-member result to very smooth slip distributions. Using onshore GPS observations at 298 stations during and in the ∼2 weeks following the Tohoku earthquake, we estimate a band of coseismic slip between 30 and 50 km depth extending 500 km along strike with a maximum slip of 64 m, corresponding to a minimum magnitude estimate of \(M_W = 8.8\). Our estimate of afterslip is located almost exclusively down-dip of the coseismic rupture, with a transition between 40 and 50 km depth and an equivalent moment magnitude \(M_W = 8.2\). This depth may be interpreted as coincident with the transition from velocity strengthening to velocity weakening frictional behavior, consistent with the upper limit of cold subduction estimates of the thermal structure of the Japan trench.Publication Geodetic Imaging of Fault System Activity(2014-06-06) Evans, Eileen Louise; Meade, Brendan J.; Ishii, Miaki; Rice, James; Shaw, JohnGeodetic observations provide kinematic constraints on the behavior of tectonically active fault systems. Estimates of earthquake cycle activity derived from these constraints may depend on modeling assumptions and/or regularization of a geodetic inverse problem, which is often poorly conditioned. Common model assumptions may affect kinematic solutions and conclusions about physical properties of faults and fault zones. For example, within a geometrically complex fault system, parameterization of nearby faults may affect slip estimates on an individual fault. In addition, fault slip models are often regularized by assuming that slip varies smoothly in space, which may artificially smear slip estimates beyond physical boundaries. As an alternative to smooth regularization, the applied mathematics field of compressed sensing provides a suite of methods for recovering sparse solutions. Applied to GPS observations of the 2011 Tohoku earthquake, compressed sensing algorithms enable imaging of spatially localized slip during and following the earthquake, and identification of a sharp boundary between coseismic and postseismic slip. Similar algorithms recover quantized solutions and may be applied to models of plate boundary deformation. Beginning with a dense array of tectonic micro-plates bounded by mapped faults in North America, these methods can be used to detect coherent motions of groups of micro-plates behaving as larger active blocks, effectively quantifying the complexity of North America plate boundary deformation. By improving our ability to identify and compare kinematic constraints on earthquake cycle processes, we are able to characterize the spectrum of earthquake cycle behaviors and gain a deeper understanding of earthquake phenomenology and physics.Publication Geodetic Constraints on San Francisco Bay Area Fault Slip Rates and Potential Seismogenic Asperities on the Partially Creeping Hayward Fault(Wiley-Blackwell, 2012) Evans, Eileen Louise; Loveless, John P.; Meade, BrendanThe Hayward fault in the San Francisco Bay Area (SFBA) is sometimes considered unusual among continental faults for exhibiting significant aseismic creep during the interseismic phase of the seismic cycle while also generating sufficient elastic strain to produce major earthquakes. Imaging the spatial variation in interseismic fault creep on the Hayward fault is complicated because of the interseismic strain accumulation associated with nearby faults in the SFBA, where the relative motion between the Pacific plate and the Sierra block is partitioned across closely spaced subparallel faults. To estimate spatially variable creep on the Hayward fault, we interpret geodetic observations with a three-dimensional kinematically consistent block model of the SFBA fault system. Resolution tests reveal that creep rate variations with a length scale of <15 km are poorly resolved below 7 km depth. In addition, creep at depth may be sensitive to assumptions about the kinematic consistency of fault slip rate models. Differential microplate motions result in a slip rate of 6.7 ± 0.8 mm/yr on the Hayward fault, and we image along-strike variations in slip deficit rate at ∼15 km length scales shallower than 7 km depth. Similar to previous studies, we identify a strongly coupled asperity with a slip deficit rate of up to 4 mm/yr on the central Hayward fault that is spatially correlated with the mapped surface trace of the 1868 \(M_W = 6.9–7.0\) Hayward earthquake and adjacent to gabbroic fault surfaces.