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Figure 9. Main fault zones and surface ruptures of the Gobi-Altai range.
earthquakes. These are mostly short normal faults (less than 40
km) with throws of 2-3 m. The orientation of these ruptures are
irregular.
The northern and northeastern border of the Hangay block is
clearly defined with three earthquake ruptures:
The first recent earthquake occurred along northeast trending
Tsetserleg fault (Fig.2, [9]) of 9 July 1905 (N49.5°, E97.4°, M-
8.4 [Richter, 1958], M=7.8 [Khil'ko et al., 1985]), which created
surface ruptures over a distance of 130 km resulting in left-lateral
strike-slip displacement and a south face reverse scarps 1.5-2 m
high.
Two weeks later, the famous Bolnay ruptures (Fig.2,[10])
occurred along the Hangay fault (Fig.2,[6]). The magnitude, the
average displacement and dimensions of the surface rupture, and
the seismic movement of the Bolnay earthquake of July 23, 1905
(N49.2°, E96.8°; M=8.7 [Richter, 1958], and M=8.2 [Khil'ko et
al., 1985) make it one of the largest known recent earthquake in
continental regions. It created surface ruptures trending roughly
west-east over 375 km. The measured left-lateral strike-slip
displacement in the central part of the rupture is about 11 m and
decreases to both peripheries.
In the NE border of the Hangay block, the Mogod earthquake
(Fig.2, [17]) of 5 January 1967 (N48.22°, E102.9°; M=7.8)
created a right-lateral rupture that trends NS for 36 km and a SE
trending zone of reverse faulting for 9 km emanating from the
south end of the main rupture.
d) Surface Rupture in the Hóvsgól Region. There are only a few
known ruptures in this region, but surely due to dense forest
(taiga) and inaccessible terrain many ruptures remained
undetected.
The Büsiin Gol rupture (Fig.2, [8]) is located in the graben and
can be traced to over 20 km. Similarly, the Jara Gol rupture (Fig.2,
[7]), in the Darhad graben, forms a scarp 1-2.5 m high for almost
5 km. Both ruptures are characterized by normal faulting.
From the remotely sensed data available, four system of faults
active since the Holocene were mapped. They trend N-S, E-W,
NE-SW and NW-SE. The N-S faults such as Bij and Mogod have
small right-lateral displacement. Most of the E-W ruptures
including the Gobi-Altay fault, show sinistral strike-slip
displacement. The majority (about 75%) and the longest surface
ruptures occurred along the sub-latitudinal NW-SE trending faults.
These are characterized by dextral strike-slip displacement. Fewer
and shorter ruptures occurred along the submeridional and NE-SW
trending faults. The latter show sinistral strike-slip displacement.
From the characteristics of the four fault systems in the area it is
evident that the ruptures associated with earthquakes in this region
suggest that western Mongolia is undergoing conjugate strike-slip
deformation. At the same time, the region is undergoing NE-SW
shortening and NW-SE extension. The shortening can be seen as
a manifestation of the collision and further convergence of India
and Eurasia since Oligocene. The extension seems to mark a
continuum of compressional-extensional deformation which
makes transition from an area of largely crustal shortening in
Central Asia to stretching toward east as is illustrated for example,
in the Baikal Rift system, where crustal extension is dominant
(Baljinnyam at al., 1993). Fault plane solution of earthquakes
show east-trending reverse faulting in the Tianshan, strike-slip
faulting in Mongolia, and normal faulting in Baikal (Zhang,
1993), which are consistent with results of late Quaternary
deformation in this belt (Chen and Molnar, 1977; Tapponnier and
Molnar, 1979; Molnar and Deng, 1984; Nelson et al., 1987).
The blocks between the N-S and NW-SE are rotating in a
clockwise direction. Similarly, the blocks between the E-W and
NE-SW are rotating in an anticlockwise direction (see Fig.10).
The best illustration of this is evident from KOSMOS data of
Mountain Turgen (Fig.11 ) where the main drainage pattern of this
region shows the clockwise rotation. Here the valley of rivers and
seasonal streams have radial arch forms. The mountain is located
between the N-S Prealtai and the NW-SE Achitnuur faults.
Intemational Archives of Photogrammetry and Remote Sensing. Vol. XXXII, Part 7, Budapest, 1998 623
