Tests of Source-Parameter Inversion of the U.S. Geological Survey Intensities of the Whittier Narrows 1987 Earthquake

We validated the source inversions of the intensity data set of the Whit-tier NarrowsML5.9 1987 earthquake by comparing the source characteristics re-trieved by our inversion technique with those already known from instrumental mea-surements. In our grid-search technique the squared residuals (calculated minusobserved intensity at the sparse sites) are weighted by the Voronoi polygons tessel-lation, and then the sum of the weighted squared residuals is minimized. For com-parison, we also minimize the sum of the unweighted squared residuals. The inten-sities are calculated by our kinematicKFfunction. Twelve source parameters areinvolved in the inversions, the most sensitive of which are the epicentral coordinatesand the fault-plane solution. To find the minimum variance model, we explore thewhole space of the angular values that control the fault-plane solution and place somereasonable constraints on the other parameters. Two minimum variance models weredetermined by the inversions: one source that is in agreement with that already knownfrom instrumental measurements and one almost coinciding with its auxiliary planein the fault-plane solution. This finding confirms that, notwithstanding the introduc-tion of more source parameters, the nature of the problem of the inversion of analmost pure dip-slip earthquake, is still close to bimodality. It is demonstrated thatit is possible to get an approximate idea of the source of the studied earthquake byinverting the U.S. Geological Survey intensities. This result encourages us to validateour inversion technique with more well-documented earthquakes and to treat inten-sities of preinstrumental earthquakes, which are the principal target of our work.