We describe a modification to the collapsing method, a technique that aims to find the simplest structures in a cloud of microearthquakes by utilizing the statistical uncertainties in the data. In the modified collapsing method, the movements of the locations are dependent on the shape of the distribution of the locations within the confidence ellipsoid, and not just the position of the center of gravity, as is the case in the original method. Additionally, whereas the original collapsing method implicitly assumes that all locations belong to point structures, in this modified version three types of structure are considered: point, line, and plane. Principal component analysis of the locations is used to evaluate to which type of structure each location most probably belongs. The modified technique has been applied to microseismic events associated with hydraulic stimulation at the Fenton Hill HDR Field, New Mexico, and known small-scale structures were imaged.

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