Glacial catastrophe of 20 September 2002 in North Osetia

Reconstruction of the Catastrophe Preparation Process

Figure 9

[17] Information of the anomalous reactivation of the Dzhimarai firn-field avalanches began to be reported at the end of July, 2002. Occasional tourists visiting this remote mountainous region photographed the disintegrating firn fields, the avalanches, and the alluvial fan of the collapsed material at the surface of the Kolka Glacier (Figure 9). These areas are known for their frequent and impressive avalanches, taking place at any time of the year. Accordingly, nobody could foresee any tragic end of these processes. It should be noted that the belt of the catastrophic shower, mentioned above, passed 2-3 km north and did not affect the glacial area of the Kolka Glacier. In order to answer the question: what might cause this menacing natural phenomenon, special-purpose work was carried out to reconstruct the factor of seismic effect in the area of Mountainous Osetia, using an analytical (calculation) method. As a result of this work, maps were plotted for each month of the 2002 year, showing the mark of the maximum seismic effect, which enabled the estimation of the factor discussed at any, even not easily accessible site.

[18] This work revealed that on 14 July, 2002, the rock mass of the Dzhimarai-Hoh peak had experienced shaking of intencity 5.2 on the MSK-64 scale. The epicenter of this earthquake was located 8 km west of the Dzhimarai-Hoh peak. In the initial KMV-2002 data sample this earthquake has a number of 244. Its main parameters are: No. 244 (Md = 3.88; K = 9.8; depth = 5.4 km). The same day witnessed two earthquakes more: No. 245 (Md = 3.11, K = 8.3) and No. 246 (Md = 3.20, K = 8.4).

Figure 10

Figure 11

[19] The earthquake No. 246 was recorded at a distance of 10 km northwest of the Dzimarai-Khokh peak. Chronologically associated with these earthquakes was the onset of landslide activity in the local area of the North shale depression which borders the Skalistyi Ridge in the south and is distinguished by a great number of landslide deformations because of plated shaly texture of the rocks. The width of this zone varies from 5 km to 7 km (Figures 10 and 11). As the additional information specifying the intensity of the seismic effect on the region of the glaciologic catastrophe source, it should be mentioned that:

[20] (1) The shaking corresponding to magnitude 5, communicated to the Dzhimarai-Hoh pyramid from the earthquake no. 244, had the following parameters: the acceleration (a) of ~0.28 m sec^-2, the displacement rate (v) of ~0.056 m sec ^-1, the absolute displacement (L) of ~0.04 m.

[21] (2) The effect of this intensity on the hanging firn fields of the "Kolka" glacial rock complex (the thickest and steeply dipping in the mountainous area discussed) can increase the momental value of their sliding down force almost by 9%, which will decrease their stability by the same value. Essentially, this effect can cause significant mechanical damages in the ice-firn massif and, eventually, its complete disintegration.

[22] Did the Dzhimarai-Hoh Massif experience any more powerful seismic effects during the previous 10-15 years? As follows from the data reported by Savich et al. [1996], the calculated shaking of the Dzhimarai-Hoh Massif from the Racha earthquake (MPSP = 6.6; 29.04.91) and from the Borisakh earthquake (MPSP = 6.6; 23.10.93) was as high as 5.6. No stronger seismic effects were recorded.