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The aim of this study is to point out the difficulties in the analytical
determination of hypocentral depth h and the consequent uncertainty in
The former attempt was realized at world-wide level (december 1964),
the latter in the Mediterranean Area (19 reigons, accordingly to the division
proposed by Flin, Engdal, Hill, 1974).
We considered hypocentral data of the earthquakes occurred in that
area during the 1964-1974 decade, with a great care to the h values and
their standard deviations.
First we pointed out the depth features of every region: afterwards
we looked into the standard deviations (E) and their respective h values
Data analysis suggested us to calculate a series of exponential functions
E = P h"- showing standard deviations as a function of focal depth;
every region is characterized by a and 3 coefficients calculated by a least squares fit.
We analysed results and also made an attempt to explain the standard
deviations scatter. We also made an attempt by only Italian earthquakes
data (ioining the four sections Northern, Central, Southern Italy, and
Sicily), and we calculated the t = f(h) function.
Both the world-wide analysis and the Mediterranean one pointed out
some interesting elements.
— Focal depth data, even if joined with their standard deviations,
are very unrealiable particularly for crustal earthquakes; in that case
standard deviations often are too large and focal depth lose their physical
— In the upper mantle the data are more reliable.
Obviously the choise of the travel-times used to calculate focal depth
influences data reliability, particularly in the most heterogeneous layers
in the crust.
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