U.S. Geological Survey Professional Paper 1769, Chapter 7
This chapter describes a two-step technique for determining earthquake hypocenters at Augustine Volcano. The algorithm, which was originally developed in the mid-1970s, was designed both to overcome limitations in the standard earthquake-location programs available at the time and to take advantage of the detailed seismic-velocity information obtained at Augustine Volcano. Hypocenters are calculated on the basis of a two-dimensional (2D) ray-tracing procedure that accounts for in plane lateral discontinuities within the seismic velocity structure. This algorithm calculates the minimum P- and S-wave travel time between theoretical grid points embedded in the velocity structure to each station in the seismic network. Station corrections that account for the differences between the model and actual velocity structure are derived from a time-term analysis of the 1975 active-source seismic experiment. Each relocated hypocenter is assigned to the grid point with the lowest rms residual between observed and calculated arrival times. Statistical techniques are used to assess the effect of random errors in P-wave-arrival determination on hypocentral location. These tests suggest that the 2D ray-tracing procedure presented here is able to resolve earthquake hypocenter depths to within 0.25 km between the volcano’s summit and sea level and within 0.5 km from sea level to depths of 2 km below sea level.
Software (in Appendix 1)
This appendix contains copies of the FORTRAN programs TRAVEL and NORM configured for the 1975, 1985–1986 and 2005–2006 seismic networks at Augustine Volcano. It also has a copy of FASTM2 that works with networks of all three configurations. Note that station names must match 1975 network names.
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Lalla, D.J., and Power, J.A. 2010, A two-step procedure for calculating earthquake hypocenters at Augustine Volcano, chapter 7 of Power, J.A., Coombs, M.L., and Freymueller, J.T., eds., The 2006 eruption of Augustine Volcano, Alaska: U.S. Geological Survey Professional Paper 1769, p. 129-142 and software.