(1) Geophysical Institute, University of Alaska, Fairbanks 903 Koyukuk Dr. Fairbanks, AK 99775, USA e-mail:jjalaska@giseis.alaska.edu (corresponding author).

Conference Proceedings of the 2004 IAVCEI General Assembly. Pucón, Chile, November 15-19, 2004.

Earthquakes located at Veniaminof volcano using a sparse network of 9 stations have typical location errors of 1 km horizontally and 4 km vertically. A straightforward method is applied to constrain these locations. We performed systematic searches for the Vp/Vs ratio that minimized formal errors. A value of Vp/Vs = 1.82 was found to minimize the average root-mean-square residuals in travel times. Relocation of earthquakes using this Vp/Vs value reduced the average epicentral location errors to < 1 km. We next used a combination of Wadati and Riznichenko diagrams to estimate origin times, Vp/ Vs ratios, and hypocenters, as well as their corresponding uncertainties. We found a systematic decrease of Vp/Vs ratio with depth beneath Mt. Veniaminof and significant differences in Vp/Vs between relatively shallow (< 10 km) and relatively deep (> 30 km) earthquakes. Intermediate-depth earthquakes show scatter in their Vp/Vs values and no differences are resolved with respect to other depths. Hypocenters of individual earthquakes are constrained to within a few hundred meters, with average uncertainty of 1.2 km. The location and media parameters found by this approach are largely independent of any velocity model and allow us to resolve several first-order velocity variations near Mt. Veniaminof. For example, a N-S elongated region of low Vp/Vs ratios is found to be sub-parallel to the alignment of SE-NW trending quaternary vents. We conclude that simple methods can be effectively used to constrain the locations of earthquakes at volcanoes where the small number of seismograph stations puts challenging limitations for research.