Landslides triggered by distant earthquakes in Central Asia

Abstract:

Central Asia is particularly sensitive to the effects of natural and man-made climate change: the degradation of glaciers, landslides, the dying of the Aral sea and desertification. The effects, whether gradual or catastrophic, on the fragile economies of Central Asia countries, including Uzbekistan, can lead to the collapse of the socio-economic systems and infrastructures of these countries.

The area of the Republic of Uzbekistan is 450 thousand square kilometers of which 20% of total land is mountainous. Of that landslide prone zone, from 15,000 to 17,000 square kilometers is subject to the landslide disaster risk with a population of 3 million. The landslide (hazard) area includes more than 500 villages, 152 recreation facilities, more than 200 sites of roads and canals and more than 22 mines and water resource facilities. In Uzbekistan, from 100 to 300 active landslides (and avalanches) occur every year.

It is time to take a different view at the problem of seismic safety of mountain areas. It is important to pay more attention to the probability of secondary earthquake effects, such as rockslides, landslides, mudflows, and liquefaction of loess soils. It is necessary to amend seismic hazard maps with the probability estimate of dangerous slope processes and take it into account at the identification and calculation of seismic risks. It is especially relevant to mountain river valleys with the existing and planned cascades of hydro-technical facilities since even a minor earthquake may lead to substantial adverse effects.

In Central Asia, large landslides and mudslides that formed during the Sarez (1911), Faizabad (1943), Khait (1949) and other large earthquakes have been studied thoroughly. However the role of deep Pamir-Hindu Kush earthquakes in the formation of landslides, liquefaction, extrusion and other types was never considered, because the earthquakes occurred at a distance of over 500 km and were of low intensity (3-4 MSK intensity units). In Uzbekistan, the influence of local earthquakes in the formation of landslides has gained little attention, mostly because the studies focused on the role of rainfall and groundwater in the formation of new landslides.

The Pamir-Hindu Kush zone of deep earthquakes is a permanent seismic source. Here in the central part of the continent, within a fairly small area (about 60-70 km²), there is an extremely high level of seismicity at depths ranging from 70-300 km. Every year in this region, more than 200 earthquakes occur at depths of 200-250 km. Some of them reach magnitude 7 or more; the intensity of motions in Afghanistan is up to 9 MSK intensity units. In Central Asia, the intensity of the motions from these earthquakes is not more than 4-5 units, but they are characterized by a long duration and low-frequency spectrum of vibrations. Landslides, mudflows, and sinks caused by Hindu Kush earthquakes are hazardous due to their suddenness of formation, and many can form in different places at the same time.

For the Central Asian region the largest center of seismic activity is the zone of Pamir-Hindu Kush deep-earthquakes. Every year in this area occur about 200 earthquakes at depths of 180-250 km and 35-40% of it occurs in the spring. Some events are reaching M-7 and in the Central Asia territory, they produce ground motions such as 3-4 MSK units of intensity(Fig.1).

Among the four known intercontinental zones where earthquakes occur in intermediate focal depth (Burma, Romania, Spain and the Pamir-Hindu Kush), the latter is the most active and best covered by instrumental measurements and fairly well understood.

Comparative analysis of time synchrony of Pamir-Hindu Kush earthquakes and formation of large landslides in the period from 1969 to 2011 showed that more than 200 cases of landslides formed in South Kyrghystan, Uzbekistan and Tajikistan. Some Uzbekistan landslides are presented in Table 1.

Landscape sensitivity, in terms of the degree to which it can cope with these rates of change, should therefore be considered as a consequence of combined changes in the preparatory factors (e.g. precipitation events, antecedent groundwater conditions) and triggers (e.g seismic vibrations at this time). Relationships between rainfall patterns and slope instability are reported in the literature for a range of slope failure mechanisms and climates. These studies demonstrate the importance of considering the likely impact of future climate change on slope instability. However, triggers and antecedent rainfall thresholds are highly site-, region-and material-specific and therefore it is not possible to use studies reported in the literature as a guide to future behaviour of other landslides in regions that experience different climates and triggers.

Table 1 Comparison of time of Earthquakes and landslides

Time and intensity of earthquake					Time of landslide
Â	Year	Day/month	Ðœ	Н km	Day/month	Name	V,mln. m3
1	1969	5.03	5.9	208	5.03	Tally	0.4
2	Â	Â	Â	Â	5.03	Suffa	0.4
3	Â	Â	Â	Â	5.03	road Тashkent –Кokand	0.1
4	Â	10.03	5.1	201	10.03	Kattakishlok	0.8
5	Â	Â	Â	Â	10.03	Dumalak	2.5
6	Â	21.05	5.0	229	21.05	Chimgan	0,24
7	Â	Â	Â	Â	21.05	Sary-Cheku	0.35
8	Â	Â	Â	Â	21.05	Tanga -Topty	0.18
9	Â	10.06	5.4	203	10.06	Gulkam	2.0
10	Â	Â	5.2	213	10.06	Mazarsay	0.7
11	1972	22.02	5.3	212	0.2	Atchi	800
12	1976	27.11	6.1	190	27.11	Sary-Bulok	Â
13	1985	11.04	4.6	151	11.04	Rupat	Â
14	1991	23.03	5.0	214	23.03	Karadagana	0.42
15	1992	09.04	4.8	200	9.04	Sassyk-Bulak	0.63
16	Â	Â	Â	Â	9.04	Tutakata	0.5
17	Â	Â	Â	Â	9.04	Hummon	0.8
18	Â	20.04	4.5	204	20.04	Terakly	0.13
19	Â	21.05	5.0	35	21.05	Nondek	6,0
20	1993	15.05	5.0	211	15.02	Ingichka	1.2
21	Â	19.02	4.6	117	19.02	Madmon	0.75
22	Â	01.03	4.8	183	01.03	Madmon	0.36
23	Â	Â	Â	Â	01.03	Dehkanabad	0.49
24	Â	26.03	4.5	226	26.03	Uradarya	1.12
25	Â	25.03	5.3	93.5	25.03	Djauz-1	0.6
26	Â	12.04	4.5	121	13.04	Alikuzchi	0.6
27	Â	07.05	4.7	252	07.05	Kamar	60.0
28	Â	Â	Â	Â	07.05	Chakar	3.6
29	Â	Â	Â	Â	07.05	Suvlisay	0.19
30	Â	Â	Â	Â	07.05	Djauz -2	0.81
31	Â	Â	Â	Â	07.05	Turkishlak	0.15
32	1994	22.04	4.4	169,4	23.04	Tokberdy	0.07
33	1995	16.05	5.9	186	16.05	Naugarzan	20.0
34	1998	14.02	5.3	218	17.02	road Тashkent –Кokand	0.35
35	Â	21.03	6.0	227	21.03	Madmon -3	0.08
36	Â	Â	Â	Â	30.03	Shohkutan	2.0
37	Â	Â	Â	Â	31.03	Sangardak	0.8
38	Â	Â	Â	Â	28.03	Yakkarcha	0.4
39	Â	30.05	7.0	285	30.05	Tashbulok	0,36
40	2002	03.03	7,4	225	03.03	Baypaza	15.0
41	Â	30.01	4.5	93.8	31.01	Karakishlak	0.1
42	2004	12.03	5.8	218	12.03	Kushkul	0.57
43	Â	5.04	6.6	187	6.04	Sangenek	0.3
44	Â	23.04	4.7	190	23.04	Tokberdy -2	0.2
45	2005	25.02	6.1	114	25.02	Beshbulak	33.0
46	Â	11.03	4.4	98	12.03	Yakkarcha	0.2
47	Â	24.03	4.8	106	25.03	Okkul	3,5
48	Â	Â	Â	Â	25.03	Aulat	0,3
49	Â	05.04	4,8	124	5.04	Yakkarcha	0.02
50	Â	10.04	4,7	141	10.04	Dunangi	0.05
51	Â	15.04	4.3	216	15.04	Dovut	14.5
52	Â	16.04	4.3	234	17.04	Guldurama	0.4
53	2008	25.04	4.9	218	25.04	Khandiza	0.06
54	2009	17.04	4.5	85	19.04	autoroad Hisorak	4.0
55	2010	27.02	5.7	266	28.02	Madmon	0.07
56	Â	12.04	4.5	218	12.04	Karakishlak	0.35

The mechanism of the effects of climate change to the growth in the number of landslides at the turn of the twenty-first century is connected with the increased frequency of turnover of wet and dry years, number of years, when the amount of precipitation in preceding period between November and February was more than 550-600mm. In March - April heavy rainfalls fall more often around 30-40 mm, for a few hours with an intensity of 8-15 mm / hour. Increased cases where the value of rainfall for two - three days was 90-110 mm. This large volume of precipitation was significant enough to saturate the soil or weathered rock, and the higher water table thus contributed much to soil (debris) flows and made steep slopes potential to fail after earthquake shaking.

Seismic effect of the impact was determined by the parameters of amplitude, dominant frequency and duration of vibrations. The latter factor could be decisive for the stability of slopes in the wet spring season, but short-term impact, even with very high acceleration may be not dangerous. Therefore, drop-out of abnormally large amount of precipitation or severe earthquake in this region may not cause landslide, and may form several landslides, or even several hundreds. Much depends on whether the slope has reached a critical state of stability.

For the main part seismically generated landslides usually do not differ in their morphology and internal processes from those generated under non-seismic conditions. However, they tend to be more widespread and sudden. Almost every type of landslide is possible, including highly disaggregated and fast-moving falls; more coherent and slower-moving slumps, block slides, and earth slides; and lateral spreads and flows that involve partly to completely liquefied material Features of combination of two external spatial factors (atmospheric) rainfall and earthquakes on the time and place of formation of the local slope of the landslide have a very complex relationship. Since the seasonal conditions of moisture saturation of slopes can increase its susceptibility to seismic vibrations for the orders.

For example, three groups of landslides were considered. The first - massive landslides in wet years with frequent earthquakes. The second - mass manifestation of landslides in wet years, but with the lack of strong earthquakes. The third one - activation of man-made major landslides at earthquakes vibrations.

This study shows the relationship between the timing of large landslides and formation of mud flows in the mountainous areas of Central Asia to the timing of long-duration, low-frequency distant Pamir-Hindu Kush earthquakes. Fifty-six cases of landslide liquefaction, extrusion, and mud flows at the time of earthquakes were found in which there were with complex relationships between precipitation and earthquakes, in the time, place and mechanisms of the landslide development.

The main risk of landslides and mud flows caused by the Pamir-Hindu Kush earthquakes is in the suddenness of their formation, and it is very difficult to predict their place and time. As a result, it is suggested that agencies devote more attention and resources to early detection, warning, and loss prevention of landslide hazards associated with Pamir-Hindu Kush earthquakes.

Landslides are one of the most damaging collateral hazards associated with earthquakes. In fact, damage from triggered landslides and other ground failures has sometimes exceeded damage directly related to strong shaking and fault rupture. Seismically triggered landslides damage and destroy homes and other structures, block roads, sever pipelines and other utility lifelines, and block stream drainages. Predicting where and in what shaking conditions earthquakes are likely to trigger landslides is a key element in regional seismic hazard assessment.

Factors contributing to slope failure at a specific site are generally complex and difficult to assess with confidence; therefore, regional analysis of a large group of landslides triggered in a well-documented earthquakes is useful in estimating general conditions related to failure.

Landslides can occur during an earthquake where shaking reduces the strength of the slope. A preliminary comparative analysis of the synchronicity in time of deep foci Pamir - Hindu Kush earthquakes and the dates of formation of large landslides in the period from 1969 to 2011 showed that more than 100 cases of landslides formed in the south of Kyrgyzstan, Uzbekistan and Tajikistan. These earthquakes in the Central Asia territory induced low-frequency (1-5 Hz) prolonged (2-3 min) ground motions and in the spring time on the moist slopes causes processes of compaction, liquefaction and displacement of loess soils. Complex relationship of two spatial factors - precipitation and earthquake to origin time, place and mechanism of landslides, occurred in last years in Central Asia, are presented in examples. Seismically generated landslides usually do not differ in their morphology and internal processes from those generated under non-seismic conditions. However, they tend to be more widespread and sudden. Thus, even a small earthquake, although its consequences are not considered by building codes, can lead to adverse effects and have catastrophic consequences.

A relatively modest Gissar earthquake of 1989 with the magnitude of M=5.5 triggered the liquefaction of loess soils resulting in landslides and a huge (3.5 km) debris-flow on a slope with the gradient of only 5-6⁰ . This led to 274 human casualties.

The analysis of a specific site generally usually requires a probabilistic approach, but a deterministic check on the resulting decision is appropriate. Generally many tectonic faults and unidentified seismic sources contribute to the seismic hazard and risk at a site, and the integration of these through a probabilistic analysis provides the most insight.

These phenomena can lead to changing of earthquake hazard assessment results and constitutes a major portion of the seismic risk to the structures. Sometimes it required reconsideration of seismic zoning maps for providing seismic safety of constructions.