Boga-Bogd Massif reaches an elevation of 3590 m and is almost 
unscalable. 
Fig.6 Shows another interesting example observed on KOSMOS 
imagery. The centre of the image is approximately 44.5°N, 
97.5°E. The KOSMOS data shows the presence of an extensive 
active fault (L1-L2) oriented NW-SE. This major fault marks the 
southern edge of the Gobi-Altai range. Although, there are no 
known earthquakes along this fault, its Holocene activity is rather 
obvious. 
The Edrengiin Nuruu in the Centre of the image might have 
splitted from Gobi-Altai at an earlier stage of tectonic activity. 
In the southern edge of the image, clear straight strike-slip active 
faults, known for their recent surface ruptures are evident. These 
faults are a continuation of the Tien Shan range of mountains in 
Xinjian and is known as the Gobi-Tien Shan. Clear left-lateral 
offset are evident within this fault in the south east. Within the SW 
corner of the image, thrust faults were mapped from the valleys. 
c) Structures cutting the Cenozoic Depressions. Freshness and 
continuity of geomorphic expression in space strongly suggest a 
surface rupture created during one event or multiple events closely 
spaced in time. Cenozoic terrigeneous sediments crop out in local 
places in thé north with grey, dark grey, light grey tones. In most 
cases, the common features are tiny dendritic drainage patterns. 
Such sediments are associated with the mountain massifs as relicts 
of earlier sedimentation basins in the depression between the 
mountain ranges. 
Various other geomorphic features cutting the Cenozoic 
depressions are also evident in western Mongolia. These include: 
dip-slip faults and strike-slip faults (having similar and dissimilar 
surface expression), small height fault scarps (created abruptly by 
one seismic event), fault scarps, fault line scarps, triangular facets, 
uphill facing scarps, linear fault valleys, fault angle valleys 
(halfgraben), shutter-ridges (blockage of valley/stream by lateral 
displacement of topography along strike-slip fault), abrupt change 
in topographic slope angle along fault trace, pressure ridge (up 
bulge of landsurface accompanied by folding and reverse faulting), 
and sagpond. The regional and to a certain extent the local 
manifestations of active faults in this respect is best understood by 
using remotely sensed data subsequently followed by conventional 
field mapping techniques. 
Seismogenic structures, covered by Cenozoic deposits of the 
intermontane depressions, are for example, distinctly displayed on 
Figs.3a,b. Most of these depressions are bounded by gently 
rolling overlapping alluvial fans. The surfaces are characterized by 
parallel seasonal streams (ravines) defining a striped texture, but 
in the area of dislocation this parallel-striped texture is displaced. 
In addition often springs, sag ponds and behaded streams have 
formed along these faults, and they too appear very distinct on the 
remotely sensed data (see KOSMOS image on Fig.3a). The 
lineaments marked by numerous headless valleys, offsetting of 
streams and abrupt changes in gradients of streams (i.e., alignment 
of nick points) indicating a neotectonic fault. 
According to Bayasgalan & Galsan, 1993, the Sharga-Nuur 
surface ruptures (46.50°N, 95.50°E and M=6.5), formed near the 
maximum curvature, to the north of the Shargyn fault, were 
forecasted with the help of aerospace imagery, subsequently 
established by fieldwork before the Hasagt Hairhan earthquake of 
15 December, 1988. 
The centre of the image in Fig.7 is roughly 43°N, 101°E and is 
situated at the border with China. The fault seen at the centre, to 
the south of the anticlinal fold is an uplifted and exposed Tertiary 
clastic sediments. Field evidence reveals the fault to be a thrust 
fault. Offset drainage, surface ruptures, truncated fan and behaded 
drainages are clearly visible on Fig.8. Its light tone clearly 
manifests the very active nature of the fault. The Gobi-Tien Shan 
fault seen to the north of the image closes to its eastern end. Clear 
left-lateral offsets are evident within this fault with thrust faulting 
evident along the valleys. 
The playa-lake deposits are characterized by light (almost white) 
tone. Some of these deposits form narrow, slightly curved and 
elongated depressions along the lowermost parts of the basins. 
Seasonally these depressions are filled by water. 
4.2 Historical Records of Earthquakes 
Most of the strong earthquakes (M>8.0) in the territory of western 
Mongolia belonging to the deep-seated faults occurred along 
suture zones of different orogenic zones (Baikalian, Caledonian 
and Hercynian zones). Only in this century, eleven earthquakes of 
magnitude (M)26.7 have occurred, forming surface ruptures of 
various depths and lengths. Most of these earthquakes took place 
within the active faults. Several of these faults and displaced 
features including the known faults are plotted from the 
KOSMOS, Landsat TM and SPOT images. The locations of major 
earthquakes are derived from Bayasgalan & Galsan (1993). These 
are portrayed in Figs.3,9, and there is no question but that at least 
through most of western Mongolia, the faults has had both vertical 
and right-lateral slip in Quaternary time. Numerous closed 
depressions preserved along their traces suggest that some of this 
movements have been Holocene in age. 
4.2.1 Surface Ruptures of Strong Earthquakes. These are 
categorized here to include: 
a) The Mongolian-Altay 
b) The Gobi-Altay 
c) The Hangay, and 
d) The Hóvsgól ranges. 
3) Surface ruptures of the mongolian altay range. The 
Mongolian Altay (Fig.2) have a wedge shape in plan view 
broadening N-NW direction. Because of this, the faults diverge 
NW-NNW direction. Many large surface ruptures are described in 
literatures but detailed account is only available for the surface 
ruptures along the deep-seated Hovd fault (Chihteyn and Ar 
Hótól), the Fu-yun earthquake of 10 August 1931, the Üüreg Nuur 
earthquake of 15 May, 1970 as well as the ruptures Bij, Bulgan 
and Sagsay (Fig.2). 
The strike-slip faults are particularly clear on the KOSMOS 
satellite data. Rivers follow most of them. Geomorphic field 
evidence of several active faults reveals both right-lateral strike- 
slip and trust or reverse components of slip with the Mongolian 
Altay range. Although some stream valleys seem to be displaced, 
no estimates of the total amounts of slip could be given. 
Unfortunately, the lack of Mesozoic and Tertiary rocks within the 
Mongolian Altay of westernmost Mongolia is likely to make it 
difficult to distinguish the amounts of Cenozoic and older 
displacements. Maximum reported stream offsets along the Hovd 
fault (Fig.2) are within 3.5 to 6 km (Khil'ko et al., 1985), but 
clearly larger cumulative offsets cannot be disproved easily. 
Stream offsets of a few kilometres are clear on the mesoscale 
images (KOSMOS, Landsat TM). In addition, segments of some 
of these strike-slip faults show evidence of very recent surface 
ruptures. 
620 International Archives of Photogrammetry and Remote Sensing. Vol. XXXII, Part 7, Budapest, 1998 
  
  
  
  
  
  
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