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Time - term analysis using linear programming and its application to refraction data from the Queen Charlotte Islands Bird, David Neil


The Queen Charlotte transform fault zone forms the boundary between the Pacific and America plates north of Vancouver Island off western Canada. This boundary is of interest since there is direct contact between oceanic and continental crustal material along its length. There are also peculiarities at its southernmost portion where oblique subduction occurs and an enigmatic terrace within the fault zone replaces the continental shelf. To investigate the crustal structure in this region, an onshore-offshore seismic refraction experiment consisting of two profiles shot into three ocean bottom seismometers (CBS's) and seven land-based stations was undertaken in 1979. As a means of interpretating the areally distributted data, a new linear programming approach to time-term analysis is introduced. This new approach is more flexible than previous methods since there is a choice of solutions possible through a choice of objective functions. A "best fit" solution is found by minimizing the L[sub l]-norm of the misfit errors while upper and lower bounds for both the refractor velocity and delay-times are created by either maximizing or minimizing the sum of the time-terms. For all solutions the travel-time equations form the constraints of the problem. Various examples, both synthetic and real, are analyzed to exemplify the method. The areal data from the Queen Charlotte Islands are analyzed using the linear programming approach to time-term analysis. The data are subdivided by p value and range into four groups. Each is then analyzed separately and a four layer interpretation is made. The uppermost layer is assumed to be the sediments (V = 2.3 km/s) and is of constant thickness (1.4 km) to the west but of variable thickness within the fault zone. The second layer has a velocity of 5.2 km/s increasing to 5.8 km/s in the fault zone. This layer displays pronounced thickening beneath the terrace. It is considered as oceanic layer 2. The third layer (V = 6.5 km/s) lies above the upper mantle (V = 7.9 km/s assumed). There is evidence for the interface between these two layers to dip eastward at 15° beneath the fault zone. It is hypothesized that the increasing velocity of layer 2 and its thickening beneath the terrace is the result of compressional metamorphism caused by oblique subduction of the Pacific plate beneath the America plate. The dipping upper mantle is explained by the same mechanism.

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