This paper illustrates the use of near-source S-waves polarizations analyses
for constraining some rupture parameters of large
earthquakes, in the particular case of the Imperial Valley, 1979 mainshock.
The 10 SMA1 strong motion records closest to the source
have been studied in the 1-3 Hz frequency range.
The records were first corrected following
the analysis of
Zollo and Bernard (1989),
from some anisotropy which produced
strong S-splitting
at a few sites.
The rotation of the S
polarization with time at most of the sites was then interpreted in terms of the
northwestward propagation of the rupture. We assumed a constant focal mechanism on the
fault plane (vertical strike-slip).
The polarization data and the absolute times at some stations allowed
the identification of a set of subsources. Their location in space and time (assuming
a depth of 8 km) provided a mean rupture velocity of 3.1 km$/$s ($\pm$ 0.25 km$/$s),
i.e.,
90\% of the shear velocity. The
high frequency radiation (3 Hz) during most of the rupture
thus is very likely to have been generated about 1 s before the beginning of
the low-frequency radiation (0.5-1 Hz) studied by Archuleta (1984).
The final rupture model fits the S-polarization history well at most sites; it also constrains
the anisotropy characteristics of the sediments under some sites, in particular EM0,
where it is consistent with the alignment of vertical cracks in the NS stress field.
Applying such a method to other earthquakes requires a careful selection of the
distance and frequency range. In particular, the effect of perhaps pervasive-ubiquitous anisotropy
in the upper crust must be taken into account, which may
require the analysis of aftershock records at the same sites.