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Validation of the Automatic Nonlinear Source Inversion of the U.S. Geological Survey Intensities of the Whittier Narrows 1987 Earthquake

The National Institute for Oceanography and for Experimental Geophysics (OGS)
Borgo Grotta Gigante, 42c
34010 Sgonico, Trieste, Italy
fgentile{at}ogs.trieste.it
lsirovich{at}ogs.trieste.it

Manuscript received 30 July 2003.

In this work, we validated the automatic inversion of the U.S. Geological Survey (USGS) intensity data set of the Whittier Narrows M_L 5.9, 1987 earthquake. This validation of our new technique was obtained by comparing the source inversion results with the principal source parameters coming from instrumental measurements independent of our study. To perform the inversion, first, we used a genetic algorithm (GA) with a population of 20,000 individuals (i.e., sources). Second, because the problem is bimodal, we also used a niching genetic algorithm, (NGA) with four demes of 2,000 individuals. This gave us almost the same results. The site intensities were calculated by our kinematic KF function. Twelve source parameters were involved in the inversions, the most sensitive of which are the epicentral coordinates and the fault-plane solution. Two minimum variance models were determined by both the GA and the NGA inversions: (1) one east-west trending dip-slip source, which is in agreement with that already known from instrumental measurements, and (2) one almost coinciding with its auxiliary plane in the fault-plane solution. These findings almost coincide with those produced by the grid-search inversion method, but the GA-NGA inversion is much faster and does not need strong constraints. This confirms that it is possible to get an approximate idea of the source of the studied earthquake also by automatically inverting the regional pattern of the USGS intensities. This result encourages us to validate our inversion technique with more well documented earthquakes and to treat intensities of preinstrumental earthquakes, which are the principal target of our work.

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