EMSC

The deformation in Mongolia up to Russia border is characterized by
large left lateral strkie slip faults ( LSSF) on EW direction and
right lateral strike slip fault (RSSF) on NNW-SSE.  This built a
parallelogram where each corner, or junction between RSSF and
LSSF, are associated with compressive deformation and reverse faults.
This can be observed at the junction between Altay and
Gobi Altay ranges and between Altay and the large fault north of Hangay
Dome (called Bolnay fault).

The large intracontinental event from the 27 september 2003, at 11h33 UT
(50 °N - 87.7°E - source EMSC) is located at the north
western part of the Altay Range (Russia - Mongolia border) (See seismic
map enclosed).  The Altay range, oriented NNW-SSE, is
characterized by RSSF  where the  largest known event, at its SW border,
occurred the 10 august 1931, (RSSF, Mw=8).  An
other magnitude 8 is suspected in the XVIII century at the eastern
border of Altay range.  At the north east of the range, occurred the 23
july 1905 the largest know intracontinental event with Mw=8.5 ( Bolnay
fault).  It was associated with LSSF and an average
displacement of 10 m along an EW fault of 500 km long.  The event of the
27 september occurred in the  region of the junction of
these RSSF and LSSF.  The north end of Altay range is characterised by a
change in the faults orientations.  They turn to the west
and the deformation is then associated with predominant reverse
component.  The event of 27 september 2003 occurred in this
region and the orientation of the aftershocks (EW) seems to accredit the
reverse component proposed by Harvard for the focal
mechanism.

This junction region is one of the most active regions of the
Mongol-Russia area as can be seen on the seismic map enclosed.  Several
moderate events have been observed recently in the region as in 1975,
1982, 1986, 1988, and 1995 with Mw between 5 and 5.5.  These events are
characterized by predominant reverse component.  This last event is
therefore the largest since 1957 -  not only in the Altay region but
also in the whole mongol region - when a magnitude 8.1 occurred in the
south Mongolia.

T. Dugarmaa, RCAG; A. Schlupp, LDG