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the hypocentral parameters of local earthquakes, even when they are recorded
by a small number of stations. These are the most frequent conditions
prevailing around the Strait of Messina.
The program employs the first arrivals of P and S waves, a model of
local crust consisting of four plane and parallel layers, and an algorithm
based on that of Geiger.
HYPOSTRE can determine simultaneously the four unknowns X, cp, h
and H (HYPOSTRE 4) and can calculate the epicentral coordinate separately
from focal depth (HYPOSTRE 2). In both cases the program follows
an iterative process. In particular, HYPOSTRE 2 shows convergence towards
a final solution, through a series of « damped » approximations.
The application of HYPOSTRE to about twenty earthquakes, which
have occured in the Strait of Messina and in the Gulf of Patti, demonstrates
a good convergence and a general stability of results.
Finally, comparison with analogous determinations — carried out by
other programs, with different crustal models and using a greater number
of recorded data — confirms the reliability of HYPOSTRE.
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