3-DIMENSIONAL UPSILON (P), AND UPSILON (P) UPSILON (S) STRUCTURE OF THE HENGILL TRIPLE JUNCTION AND GEOTHERMAL AREA, ICELAND, AND THE REPEATABILITY OF TOMOGRAPHIC INVERSION/

We investigate the crustal structure of the Hengill triple junction in southwestern Iceland, ap plying tomographic methods to local earthqua ke data recorded in two held experiments with different network geomet ries and instrumentation. Data from the two experiments enable us to d erive three-dimensional models of the compressional-wave speed v(p), a nd the wave-speed ratio v(p)/v(s). Well resolved high-v(p) bodies corr elate,with sites of gabbroic intrusions. A small reduction in v(p)/v(s ) associated with the high-temperature part of the geothermal area is probably due to mineral alteration or supercritical fluids. The RMS di fference between the two v(p) models, about 0.26 km s(-1), indicates t he approximate repeatability that may de expected of good tomographic inversions.