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100 Years of earthquakes in the Pamir region as recorded in juniper wood: A case study of Tajikistan
Journal of Asian Earth Sciences 138 (2017) 173–185
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100 Years of earthquakes in the Pamir region as recorded in juniper wood: A case study of Tajikistan Piotr Owczarek a,⇑, Magdalena Opała-Owczarek b, Oimahmad Rahmonov c, Maciej Mendecki d a
Institute of Geography and Regional Development, University of Wroclaw, Uniwersytecki Sq. 1, 50-137 Wroclaw, Poland Department of Climatology, Faculty of Earth Sciences, University of Silesia in Katowice, ul. Bedzinska 60, 41-200 Sosnowiec, Poland c Department of Physical Geography, Faculty of Earth Sciences, University of Silesia in Katowice, ul. Bedzinska 60, 41-200 Sosnowiec, Poland d Department of Applied Geology, Faculty of Earth Sciences, University of Silesia in Katowice, ul. Bedzinska 60, 41-200 Sosnowiec, Poland b
a r t i c l e
i n f o
Article history: Received 31 October 2016 Received in revised form 21 January 2017 Accepted 5 February 2017 Available online 9 February 2017 Keywords: Central Asia Pamir region Earthquake Tree-ring Paleoseismology Juniperus
a b s t r a c t Active tectonics reflect high seismicity rates in the Pamir and surrounding areas. Long-lived trees growing in the western Pamir-Alay mountains are affected by ground accelerations and seismic-induced geomorphic processes. At a high-mountain site in the Pamir-Alay, 45 juniper trees were sampled in order to analyze the potential role of these trees in the reconstruction of disturbance events. Although the trees have a maximum age c. 500 years, only the period of the last 100 years was analyzed, aiming for the dendrochronological identification of earthquake events known by historic documents and measurements. Analysis mainly showed non-climatic growth suppression and releases. These signals were identified as the results of tectonic disturbances. Tree-ring data recorded many earthquakes from which the most distinct were: 1907, 1923, 1943, 1955, 1982, and 1998. The 1907 and 1923 large earthquakes took place after the vegetation period; therefore, growth reduction appeared in the following year. The 1943, 1955, 1982, and 1998 events occurred just before or during the vegetation period and therefore induced the immediate reduction of tree rings in the earthquake year. The identification of the characteristic growth ring patterns, which are a response to large earthquakes, will enable the reconstruction of these events in the past on the basis of centuries-long dendrochronological records. Ó 2017 Elsevier Ltd. All rights reserved.
1. Introduction High-mountain ecosystems belong among the most sensitive ecological systems and record various climatic and non-climatic factors (Schweingruber, 1996). Local site conditions, including, e.g., relief, tectonic and geomorphic deformations, soil and rock characteristics, exposition, and topoclimate, can influence the growth, vitality, and death of trees. In the trees growing in the mountains of Central Asia, due to differentiation of habitat conditions, we can find many signals connected with ecological, geomorphological, meteorological, and geophysical threats (e.g. Esper et al., 2002; Solomina, 2002; Opała et al., 2017). The westernmost part of the India-Eurasia plate collision zone is characterized by ongoing convergence that reaches 29 ± 1 mm/yr (Mohadjer et al., 2010; Ischuk et al., 2013). Active tectonics reflect high seismicity rates in the Pamir-Alay Mountains (Lukk et al.,
⇑ Corresponding author. E-mail addresses: [email protected] (P. Owczarek), [email protected] (M. Opała-Owczarek), [email protected] (O. Rahmonov), [email protected] (M. Mendecki). http://dx.doi.org/10.1016/j.jseaes.2017.02.011 1367-9120/Ó 2017 Elsevier Ltd. All rights reserved.
1995; Rautian and Leith, 2002; Schurr et al., 2014). This area, one of the most seismically active in Central Asia, is characterized by the occurrence of a large number of deep earthquakes that are disastrous to life and property (Evans et al., 2009). In the Pamir region, earthquakes larger than a magnitude of 5 were first been reported beginning in the 1860s (Musketov and Orlov, 1893; Shebalin and Kondorskaya, 1982). Older events can be reconstructed on the basis of historical sources, sedimentary sequences, and geomorphological analysis. One of the reliable proxy sources on earthquakes spanning from ancient to modern times are dendrochronological records (Jacoby, 1997, 2010). Strong earthquakes affect trees directly by not only causing growth-ring disturbances, but also tree mortality (Allen et al., 1999; Jacoby, 1997). Wood and Neumann (1931) presented criteria for the modified earthquake Mercalli intensity scale based on the effects of the shaking of trees, whereby broken stems indicate the VIII + quake intensity. Large-scale faulting can indirectly influence the growth disturbances as a result of mass movement or changes in the ground water level (Page, 1970; Jacoby et al., 1992). Displacements of the Earth’s crust combined with geomorphic processes on steep slopes affect the changes in the position of trees, and thus the
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changes in wood anatomy (Alestalo, 1971; Shroder, 1980; Stoffel and Bollschweiler, 2008; Stoffel et al., 2010; Stoffel and Corona, 2014). The earthquake magnitude, distance from the epicenter, and their frequency influence the degree of tree inclination, changes in crown shape, breakages, and uprooting. Allen et al. (1999) described earthquake-induced damage to mountain beech trees (Nothofagus solandri) (c. 24% tree mortality in the research plot) growing 10 km from the 1994 event epicenter in the Southern Alps, New Zealand. Kozaci (2012) also reported the decapitation of the crown of Scots pines (Pinus silvestris) growing on the North Anatolian faults as a result of a 1943 earthquake. The drastic growth reduction of conifer tree rings, observed from 1813, was noted in trees growing on the San Andreas Fault (Sheppard and Jacoby, 1989). These growth reductions are the results of 1812 ‘‘San Juan Capistrano” earthquake. With increasing distance from the earthquake epicenter the dendrochronological signal becomes weaker, but is still visible. Growth suppression as a result of ground shaking or growth releases after the death of neighboring
trees in the years following a disturbance is observed hundreds of km from the epicenter (Sheppard and Jacoby, 1989; Kitzberger et al., 1995; Jacoby, 1997). However, even though many studies have reported the use of tree rings as a proxy method for the reconstruction of past earthquake events in different seismic areas, there has been very little research conducted in the high mountains of Central Asia. The previous research has only concentrated on the northern Tian-Shan in Kazakhstan (Yadav and Kulieshius, 1992) and the south-western Karakoram in Pakistan (Bokhari et al., 2010, 2013). The presented study is the first-of-its-kind investigation into the Pamir-Alay Mountains, western Tajikistan, which deals with dendroseismology. In this paper, we explore the juniper tree as a source of information about seismic activity. The purpose of this investigation is: (1) to identify growth-ring disturbances connected with earthquake events from the beginning of 20th century; (2) to analyze the potential of the juniper tree in the reconstruction of disturbance events which took place before instrumental observations.
Fig. 1. (A) Major earthquake events between 1900 and 2014 in the north-western part of the Pamir region, black rectangle shows the study area; (B) detailed location of treering sampling site in the upper part of the Archa Maydan River Valley. Source: ANSS ComCat.
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2. Study area 2.1. General description The study area is located in the western part of the Pamir-Alay Mountains, which belongs to the extensive Pamir region (Fig. 1A). The Pamir-Alay Mountains, known as the Fann Mountains (Chimtarga peak 5489 m a.s.l.) in this area, consists of a typical highmountain relief characterized by steep slopes with numerous traces of mass movement activity. Small valley and cirque glaciers are situated in the highest part of the mountains. Large talus cones and landslides are typical geomorphic forms that are widespread throughout the mountains (Fig. 2A and B). Its deep valleys often have irregular longitudinal profiles with alternating wide and narrow zones, which are separated by Pleistocene frontal moraines (Fig. 2C and D). Trees were sampled in the upper part of the Archa Maydan River within the Zarafshan Range at the elevation 2150– 2250 m (Fig. 1B). This part of the Pamir-Alay Mountains, surrounded by desert plains, has a typical arid continental climate. The mean annual total precipitation is 271.3 mm with maximum in spring (March–May). The mean annual air temperature 6.6 °C (Iskanderkul Meteorological Station).
2.2. Geological background The Pamir region is composed of continental crust of about 70 km in thickness (Chen and Molnar, 1981; Holt and Wallace, 1990) and three major tectonostratigraphic units can be distinguished there. The region is divided into the North, Central, and
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South Pamir (Schurr et al., 2014). Furthermore, the Pamir syntax (a convergence of the Pamir ranges towards a single point) is surrounded by sedimentation basins from the west (the Tadjik-Afghan Basin), the east (the Tarim Basin), and the south (Indus Valley). The Pamir is bordered from the north by the Tian-Shan unit (Burtman and Molnar, 1993). The sedimentation and rock origins of the Pamir region stem from Carboniferous until Neogene and are related to various geological processes such as basin deposition (terrigenous and marine), volcanic activity, and the accretion of subduction complexes. The collision between the Indian and Eurasian plates in the Cainozoic Era caused deformations in the Pamir region and formed many systems of faults and folds (Chen and Molnar, 1981; Holt and Wallace, 1990; Burtman and Molnar, 1993; Ducea et al., 2003; Schurr et al., 2014). The External Pamir complex comprises the Pamir-Alay region and the eastern part of the Tadjik Depression (Tadjik-Afghan Basin), and it changes gradually. The geological structures mainly include Upper Palaeozoic and, to a lesser extent, Carboniferous, Cretaceous, and Neogene rocks. It consists mainly of thick terrigenous sandstone but also marine deposits as schists, limestone, marls, and clay. During the Neocene, when the collision between India and Eurasia had started, the massif was isolated and a very thick sequence of non-marine sediment was accumulated within the Tadjik Basin (Burtman and Molnar, 1993; Nicolaev, 2002; Ducea et al., 2003). 2.3. Seismicity Seismic and neo-tectonics studies pointed out that the Pamir region moves northward en bloc and collides with the Tian-Shan,
Fig. 2. The main relief features of the western Pamir-Alay: (A) different generations of talus cones; (B) large inactive landslide (dashed line) that developed on the right slope of the U-shaped valley; (C) large frontal moraine dissected by outflow from glacier lakes, upper part of the Fann Mountains; (D) U-shape valley filled by glacial, glacifluvial and slope deposits, the arrow indicate location of the frontal moraine presented in (C).
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Fig. 3. Shaded map of Central Asia region with marked main fault and thrust systems, compiled with the Vs30 hazard map. Abbreviations are as follows: MAT – Main Alay Thrust, PTS – Pamir Thrust System, SKF - Sarez-Karakul Fault, DKF - Daravaz-Karakul Fault, CBF - Central Badakhshan Fault, SMTS - Sarez-Murghab Thrust System, HF – Herat Fault, MKT – Main Karakoram Thrust, FBFS – Fergana Basin Fault System, TSFS – Tian-Shan Fault System and KF – Karakoram Fault (after Burtman and Molnar, 1993; Nicolaev, 2002; Ducea et al., 2003; Mohadjer et al., 2010, 2016, simplified). Source: Global Vs30 Map Server.
which is reflected by high seismicity along this frontal Pamir thrust system (PTS) and the Main Alay thrust system (MAT) (Schurr et al., 2014) (Fig. 3). The western Pamir shows higher seismic deformation rates, explained by the ongoing collapse of the western margin of the Pamir Plateau and the westward extrusion of Pamir rocks into the Tadjik Depression. Further to the south the PTS connects to the system of faults in the Hindu Kush where the meridian Daravaz-Karakul (DKF) and Central Badakhshan (CBF) faults can be distinguished (Fig. 3). The CBF extends in latitudinal directions as the Herat Fault (HR), while the southern and eastern Pamir is separated from the Kunlun Shan by the Karakoram faults and thrust system (e.g., the Main Karakoram Thrust (MKT) and the Karakoram Fault (KF)) (Fig. 3). Within the Pamir, the SarezKarakul Fault (SKF) system separates the mostly undeformed high plateau of the eastern Pamir from the seismically active, highrelief, and deeply incised western Pamir. The SKF connects with a large fault complex called the Sarez-Murghab Thrust System (SMTS). What’s more, on the border with the Tarim Basin the seismicity also shows active deformation at the eastern end of the PTS (Fig. 3) (Mohadjer et al., 2010). The seismic hazard for Central Asia as well as for Tajikistan is caused by the tectonic stresses associated with the Indian subcontinent continuing to move northward against Eurasia. This moving process manifests as a large number of earthquakes (King et al., 1999). The seismic events are mainly focused in the vicinity of the Pamir thrust system and Central Badakhshan fault (Fig. 3). However, the seismic event locations appear around the Pamir region, and one event occurred very close to the study area (M = 5, 1982-05-14) (Fig. 1, Table 1). Part of the earthquake events exceed P 6.5 magnitude. A seismic hazard map based on the Vs30
estimation for the Pamir region is shown in Fig. 3. The Vs30 parameter is the most important parameter in the classification of the soil in response to the average shear-wave velocity in the topmost 30 m of sediments or rock. This quantity is also used in the building code Eurocode 8 (EC8) to classify sites according to soil type into five major categories (A, B, C, D and E), and two specific geoengineering categories that correspond to very loose or liquefiable material (Roser and Gosar, 2010). The study area, according to United States Geological Survey (Global Vs30 Map Server), is characterized as a B-type soil which then can be described as deposits of very dense sand, gravel, or very stiff clay (eroded rocks), with Vs P 760 m/s and < 6 m of soil above rock, characterized by a gradual increase in the mechanical properties with depth (Global Vs30 Map Server; Roser and Gosar, 2010). Moreover, according to the Tajikistan Seismic Hazard Distribution Map (Giardini et al., 1999; WHO, 2010), the B-type soils in Tajikistan correspond to an area characterized by a very high PGA (Peak Ground Acceleration, (e.g., Castanos and Lomnitz, 2002)) value >4 m/s2 (with 10% chance of exceedance in 50 years (Giardini et al., 1999). This may suggest that the study area is strongly influenced by ground motion due to the very high considered surface acceleration, which can cause damage to soils and trees as well.
3. Material and methods The samples of juniper trees were collected from the upper part of the Archa Maydan River Valley, in the western part of the PamirAlay Mountains, at the elevation of 2150–2250 m a.s.l. The most important plant community is formed here by Juniperus semiglo-
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Table 1 Historical records of major seismic events M P 5 in the north-western Pamir region from 1900 to 2014 in maximum distance ca. 200 km from the tree-ring sampling area. Source: ANSS ComCat. No
Date
Magnitude
Epicentre location and depth
Distance from the sampling area
1 2 3 4 5 6 7 8 9 10 11 12 13 14 15 16 17 18 18 19 20 21
21-10-1907 28-12-1923 16-09-1924 05-07-1935 08-10-1935 11-01-1943 10-07-1949 19-07-1955 22-09-1956 23-08-1961 15-05-1982 15-02-1984 23-02-1984 22-01-1989 08-11-1992 02-10-1993 03-09-1998 20-01-1999 27-03-2004 10-05-2009 03-08-2010 10-11-2013
7.4 6.5 6.2 6.2 6.3 6.3 7.5 5.7 5.5 5.6 5.0 5.2 5.0 5.3 5.3 5.0 5.0 5.0 5.0 5.0 5.2 5.2
39.179°N 39.664°N 39.061°N 38.097°N 39.132°N 38.196°N 39.174°N 39.824°N 38.405°N 38.583°N 39.197°N 39.932°N 39.971°N 38.465°N 38.778°N 39.066°N 38.447°N 37.061°N 39.959°N 38.234°N 38.452°N 38.410°N
210 85 195 145 225 110 230 60 125 65 22 85 80 105 155 150 155 230 115 135 160 115
70.585°E 69.093°E 70.538°E 67.372°E 70.784°E 67.991°E 70.891°E 67.949°E 69.221°E 68.511°E 68.402°E 67.627°E 67.677°E 68.694°E 69.864°E 69.966°E 69.473°E 68.403°E 69.231°E 67.687°E 69.637°E 68.890°E
depth = 20.0 km depth = 20.0 km depth = 20.0 km depth = 15.0 km depth = 20.0 km depth = 35.0 km depth = 20.0 km depth = 15.0 km depth = 15.0 km depth = 25.0 km depth = 33.0 km depth = 22.3 km depth = 33.0 km depth = 33.0 km depth = 63.6 km depth = 14.2 km depth = 33.0 km dept = 33.0 km depth = 20.9 km depth = 19.4 km depth = 23.7 km depth = 23.0 km
Fig. 4. Taking cores from juniper trees; note the position of trees growing on steep scree slopes covered by a thick layer of loose sediment.
bosa and Juniperus seravschanica, with a significant participation of shrubs and dwarf shrubs of the genera Rosa, Berberis, Ephedra, Sorbus, and Cerasus. The juniper trees reach heights of up to 10 m and form an open forest (Fig. 4). Within the juniper forests there are also single specimens of Juniperus turkestanica, which are not relevant to the forest formation. The investigated trees are growing on lithomorphic soils (rankers) developed from Upper Palaeozoic schists, dolomites, limestones, and conglomerates. Soil cover is heterogeneous in terms of both its mechanical composition and development level. On steep slopes, depending on exposure and vegetation, soils range from fine-grained to coarse-skeletal underdeveloped types. The commonly observed soil profile contains debris of different-sized rock pieces (Rahmonov et al., 2016). For the present study, we collected increment core samples from 45 juniper trees (Juniperus semiglobosa and Juniperus zeravshanica) growing on the steep scree slopes (Fig. 4). Two cores per tree (one upslope and one downslope) were extracted parallel to slope direction. The coring procedure with a Pressler borer, sample preparation, and cross-dating were followed in accordance with standard procedures (e.g., Stokes and Smiley, 1968; Speer,
2010). Measurements of tree-ring width were performed using WinDENDRO (2015). The correctness of cross-dating ring width sequences was checked by using the program COFECHA (Holmes, 1983). Interestingly, these samples were poorly correlated with climate data, as shown in Fig. 5 presenting response function analysis between tree-ring indexed chronology and monthly mean temperature and monthly precipitation totals. This is in contrast to the samples taken from more stable areas of the western Pamir-Alay Mountains, where Opała et al. (2017) showed a distinct climatic signal and a more regular course of juniper curves without abrupt growth reductions and releases in sites not affected by geomorphic processes. The wood material in this study, collected from a less stable area covered by a thick layer of loose sediment, was analyzed for the occurrence of growth reductions connected with non-climatic signals. The calculation of growth reduction was done using the method described by Schweingruber (1990). The reduction values are calculated as the total TRW in the reduction period in relation to the same number of rings from the period preceding the reduction. The abrupt growth reduction is noted for sequences of more than three years. Growth changes were analyzed in a three
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Fig. 5. Response coefficient between tree-ring indexed chronology (the Archa Maydan site) and climate factors (monthly mean temperature and monthly precipitation totals). No statistically significant values have been found.
stage scale: 30–50% of change – moderate reductions, 51–70% – strong reductions, and >70% – very strong reductions. The age structure of the analyzed trees ranges from 150 years up to 500 years; however, the period from 1900 was analyzed in order to determine the influence of earthquakes of a known date of occurrence on growth-ring disturbances. The seismicity of the Pamir region was obtained for the period of 1900–2014 from available on-line seismic catalogues (USGS Earthquake Hazards Program, Storchak et al., 2013). The earthquakes taken into account for this research have occurred within a radius of c. 200 km from the sampling location (Fig. 1A, Table 1). 4. Results 4.1. Earthquake evidence based on tree-ring responses All samples were measured, cross-dated, and examined for growth-ring changes that might indicate disturbances as a result of earthquakes in the common period of 1900–2014. The analyzed trees, due to their position on the steep slope, have low moisture
availability and poorly developed soils. They are characterized by high level of tree-ring width variability (Figs. 6–8). The negative influence of unfavorable climatic conditions is less pronounced in these series in comparison with the more stable areas (Opała et al., 2017), but two periods are strongly influenced by extreme weather conditions (Fig. 6). Intensive growth reductions and locally absent rings are observed in 1917–1918 (in some trees also in 1919), which are a result of the exceptional drought occurring in 1917 (with April–September precipitation of 2.8 mm, while the average for this period is 125.7 mm). Growth reductions connected with extremely cold winters can be observed in the period 1970– 1978. The average winter temperature for this period was 1.2 °C below the long-term mean (1930–2007) for the Iskanderkul Meteorological Station. Seismic shaking influences the growth anomalies, especially in trees growing on steep slopes that are sensitive to mass movements. These perturbations occurred because the roots and stems of the trees growing in the Pamir-Alay tectonic unstable area were disturbed by sliding or creeping during or shortly after the earthquake event. Most of the 45 trees analyzed showed periods of non-climatic reduction of growth and recovery within an individual tree-ring series during the 1900–2014 period. However, the duration and intensity of these signals vary between individuals. These growth reductions can be recorded immediately after the earthquake or in the next year depending on the time of the event occurrence (time before, during, or after the vegetation season). Altogether, 80% of trees showed evidence of growth-ring disturbances occurring during or soon after several earthquakes between 1900 and 2014. These earthquakes resulted in growth suppression for 2–8 years in most of the sampled trees, but the duration of some growth suppression signals is longer due to successive earthquakes (e.g., 1923–1941, 1944–1964) (Fig. 6). After the suppression period, represented by narrow growth rings, recovery and accelerated growth can be observed. But these phases are not too long and are limited due to the strong seismicity of analyzed area and the influence of co-seismic and post-seismic geomorphic phenomenon.
Fig. 6. Percentage of samples with abrupt growth reduction in a three stage scale: 30–50% of change – moderate reductions, 51–70% – strong reductions, and >70% – very strong reductions. Main earthquake events and selected meteorological signals were also marked.
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The analysis of individual tree-ring series showed two different growth patterns as a result of earthquake events. The analysis of tree-ring width changes allowed for the identification of several events first beginning in 1900. The first type of pattern is a rapid decrease of tree-ring widths (e.g., after the earthquakes of 1923– 1924, 1998–1999) that continued for 2–4 years, after which the trees accelerated their growth. These disturbances are likely a direct effect of the partial destruction of the root system and the rapid reduction of nutrient supply to the trees. The second pattern of growth suppression represents slowly decreasing tree-ring widths (e.g., after the 1907 earthquake). This type of growth reduction could be connected with the gradual tilting of trees as well as the destruction of root hairs resulting from shaking and mass movements. The duration of these growth reduction periods is extended for more than 4 years. This type of growth reduction is less common in the analyzed trees. Figs. 7 and 8 show examples of the fluctuation of ring widths for four individual trees, representing both types of reduction patterns. The earthquake of 21 September 1907 triggered a gradual decrease in tree-ring widths noted from 1908 (Fig. 7A and B). Although the epicenter of this 7.4 M earthquake was located 210 km from the study area, it led to 60% growth reduction in trees five years after this event. It should be noted that very strong
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reductions of tree-ring widths were noted in only 6% of trees (see Fig. 6). The growth suppression could be a response to the tilting of trees. The first signs of recovery were noted between six (Tree: TJIS_45) (Fig. 7A) and nine (Tree: TJBAC_28) (Fig. 8B) years after this event. In rare cases the recovery occurred after two years (Tree:tj_cz19) (Fig. 7B). In most of the sequences, further growth recovery was disturbed by an extraordinary drought in 1917. A relatively different growth-ring width pattern was noted after earthquakes in 1923 and 1924, of 6.5 M and 6.2 M, respectively. These events influenced the growth reduction of trees upwards of 80%, and most of them showed strong and very strong reductions (Fig. 6). Sharp growth reduction was noted in 1924/1925 and lasted 2–4 years (Figs. 7A, B and 8A, B). The 11 September 1943 earthquake of 6.3 M was recorded in 50% of trees. In 1944 growth was reduced both sharply (TJIS_45, tj_cz19) (Fig. 7A and B) and slowly (TJBAC_28, TJ_CZ17) (Fig. 8A and B). The duration of these suppressions was prolonged due to the 10 July 1949 earthquake of 7.5 M, or even by the earthquake in 1955 (Figs. 7A and 8B). The earthquake of 19 July 1955, whose epicenter was located about 60 km from the research site, influenced the moderate and strong reduction observed in 37 trees over the next 2–3 years. This signal was accelerated by the 1956 earthquake. However, the one-year, deep reduction in 1957 could be partially connected with the cold
Fig. 7. Graph showing selected annual growth-ring plots: (A) core TJIS_45 – in close-up section including the growth suppression connected with the 1943 earthquake, the signal was strengthened by the 10 July 1949 earthquake of 7.5 M and by the earthquake in 1955; (B) core tj_cz19 – growth suppression connected with 1907, 1923/1924, 1935, 1943 and 1949 earthquakes is presented in close-up section.
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winter (Fig. 6). The following earthquakes, which occurred in the 1990s, did not produce clearly visible short- or long-term growth ring reductions in the analyzed trees (Figs. 6–8). A reduction in 1982 coincides with the earthquake of 15 May 1982, whose epicenter was only 22 km away from the sampling site. Most of the analyzed trees show a noticeable decline after the 1998 and 1999 earthquakes. As a result of these events, 78% of trees underwent an abrupt suppression in 2000 and 2001, the majority of which showed strong and very strong reductions (Figs. 6–8). The recovery was noted very fast, after 2 (Fig. 8A and B) to 4 (Fig. 7A) years. In the 21st century, the cores clearly recorded the earthquake of 8 January 2007 and the earthquake of 10 November 2013. The last earthquake event has been expressed as the abrupt growth suppression of 22 trees in 2014. A strong reduction was noted for most of these trees. Detrended tree-ring chronology from the western Pamir-Alay shows distinct fluctuations of tree-ring indices, which are an effect of seismic events and climatic variability. There are a few growth ring patterns clearly reflecting earthquake events (Fig. 9). The substantial 1907 and 1923 earthquakes took place after the vegetation period, therefore the growth suppression appeared in the following years. The 1943, 1955, 1982, and 1998 events occurred just before or during the vegetation period and therefore affected the immedi-
ate reduction of the tree-ring index in the year of earthquake. It should be noted, that the response to earthquakes of 5 M in 1982 and 1998 is different (Fig. 9) and it is not so clear like response to large ones. It can be explained by the occurrence of earthquakes in the different parts of the growing season and by the influence of other factors (local/climate). The earthquake in 1982 took place in the first part of vegetation period in wet season, when nutrients were introduced to the tree. Thus, during the second part of the vegetation season fast regeneration of root system took place. The earthquake in 1998 was recorded at the end of the summer, during the dry months. Lack of nutrient input did not allow to regenerate root system in the next vegetation season in 1999. The relatively deep reduction in 2000 can be a result of additional effect of low temperature in the winter (see Fig. 6). 4.2. Relation between co-seismic geomorphic processes and tree growth Earthquake-induced mass movements have been documented on the basis of the tree-ring variability of juniper trees. Rapidly and slowly decreasing growth rings correlate with seismic events whose epicenters were located within a maximum distance of 230 km from the study area. There were no injuries and other
Fig. 8. Graph showing selected annual growth-ring plots: (A) core TJBAC_28 – growth suppression connected with 1943, 1949 and 1999 is presented in close-up section; (B) core TJCZ_17 – distinct visible growth suppression connected with 1923/1924, 1935, 1943, 1949, 1955/1956 and 1998/1999 is presented in close-up section.
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Fig. 9. Undetrended tree-ring chronology based on 45 samples of juniper tree. Vertical lines indicate seismic events clearly recorded in tree-ring data; blue lines mark earthquakes occurring after vegetation period, red ones - just before or during the vegetation period.
traces of the direct impact of rock particles on the analyzed tree stems. Therefore, it is suggested that the trees were not affected by large and rapid slope processes like rock falls, rock avalanches, massive landslides, or debris flows. The geomorphic features associated with these large-scale processes are observed in the sampling sites, but they are older than the currently living trees (see Fig. 2B). Thus, one can conclude that the observed changes in growth ring widths are related to a slow-medium velocity mass wasting process such as creeping. The slow downslope migration of soil and rock particles is accelerated during earthquake events. The slopes of the Pamir-Alay Mountains in the study area are covered by a thick layer of loose, unsaturated colluvial and glacial deposits (Figs. 2 and 10A). The seismic shaking of these materials influences the increase of creeping and disrupted shallow sliding activity (Fig. 10B). In general, the low-growing (max. 10 m) juniper trees with extensive root systems are resistant to tilting. Low velocity and shallow slope processes can mainly damage root hairs and smallish lateral and vertical roots, which are generally not deep. Limitation in water and mineral nutrient supply leads to the decrease of growth ring width (Fig. 10C). Recovery takes place a few years after the event, resulting from the rebuild of the fine root system in the upper soil (Fig. 10D). Its duration depends on several factors like type of regolith, slope inclination, and density of vegetation cover. A longer period of recovery was found in trees growing on steep unstable slopes, where post-seismic creep is more significant compared with relatively stable areas. Our studies allow for the determination of a maximum distance limit for regolith slide, taking into account the different intensity of earthquakes. A distance of c. 230 km for M 5.0–5.3 earthquakes indicates the large sensitivity of juniper trees for ground shaking (Fig. 11). 5. Discussion Many recent papers have shown that trees can be used as a unique tool for dating past earthquake events (Jacoby, 1997, 2010). Trees are usually affected by ground accelerations, seismic-induced landslides, rock falls, and hydrologic changes. This
phenomenon can help locate zones of disturbance and characterize the size and strength of an event. The response of trees to earthquake events must be analyzed in terms of climatic variations. Short-term growth suppression can be connected to extreme climate conditions (Schweingruber, 1996). The abrupt growth reduction observed in 1917–1918 was connected with an extreme drought, which was recorded in most dendrochronological records in Central Asia (Chen et al., 2013, 2016; Opała et al., 2017). The second period of growth suppression (1970–1978) observed in the analyzed trees was a result of extremely cold winters. The growth suppression resulting from historic earthquake events have rarely been analyzed in terms of growing seasons, but there are a few papers concerning timing of tree-ring response to earthquakeinduced landslide (Carrara and O’Neill, 2003) or normal faulting (Bekker, 2004). This finding allows for an explanation for why growth reduction does not appear in the year of the earthquake, but sometimes in the following year. On the basis of tree-ring chronology from the western Pamir-Alay we can observe differences between growth ring patterns connected with earthquakes and extreme weather conditions. The growth releases after the large earthquakes show irregular fluctuations (e.g., after the 1907, 1923 and 1943 earthquakes (see Fig. 9)). The extreme climatic events are marked as sharp, deep, and short reductions (e.g., after the 1917 extreme drought (see Fig. 9)). The effect of drought is restricted to one or two rings in analyzed trees, whereas the large earthquake-induced growth is recorded over several years. These findings are consistent with observations in other seismic areas (Jacoby, 1997). Juniper trees from the Pamir-Alay Mountains record ground shaking and movement triggered by earthquakes but the local hydrogeomorphological changes and dynamic stress caused a variety of effects in individual juniper trees. Differences in moisture availability and poorly developed soil on morphologically differentiated steep slope correspond with the variability of tree-ring variations in the individual series. However, dendrochronological data from the all investigated trees show clear connection between growth rings and earthquake-induced slope processes. Our results indicate that trees can record moderate seismic events whose epi-
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Fig. 10. Graph showing the effect of potentially damage to the root system on growth-ring pattern as a result of ground displacement: (A) juniper tree growing on slope covered by thick layer of loose deposits; (B) earthquake events and ground shaking; (C) damage root hairs and smallish roots leads to the decrease of growth ring width; (D) growth recovery as a result of root system rebuilding.
centers are located at a relatively large distance. Most commonly, the effect of earthquakes on the trees was studied in close distance to the epicenters (Sheppard and Jacoby, 1989; Yadav and Kulieshius, 1992; Kozaci, 2012). The direct impact of seismic events influences the external damage in trees, or strong and lengthy growth suppression. Many investigations have placed a greater emphasis on earthquake-induced large landslides than small slides or creep (Keefer, 1984, 2002; Rodrı´guez et al., 1999; Havenith et al., 2003; Evans et al., 2009; Havenith and Bourdeau, 2010; Šilhán et al., 2012; Jibson and Harp, 2016). Studies of previ-
ous earthquakes indicated a maximum epicentral distance limit for landslides of 20–30 km for an M 5.5 earthquake (Keefer, 1984), or even in extreme case of 245 km for an M 5.8 earthquake (Jibson and Harp, 2012). Our study shows the impact of M 5.0–5.5 earthquakes on growth ring variability within an epicenter distance of 230 km, which significantly exceeds the maximum distance of slides triggered by the seismic events of this magnitude noted by Keefer (1984) and Rodrı´guez et al. (1999) (Fig. 11). This exceedance is connected not only with the sensitivity of the high-mountain trees growing near upper timberline, which react even to slight
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Fig. 11. Distance from epicenter to ground displacement for earthquakes of different magnitudes. The results from the Pamir-Alay are compared with Keefer (1984) and Rodrı´guez et al. (1999) studies. Solid lines are approximate upper bounds of earthquake impact enclosing each group of data.
ground vibrations, but also with the characteristics of Pamir-Alay Mountains. One of the most important features of this area is the thick cover of loose sediments of different origins. Such sediment may have a tendency to develop site effects (Nakamura, 1989) in the study area. Usually, the amplitudes of seismic ground accelerations attenuate faster in less dense earth materials, fulfilled by fluids (water, gas, and oil) as well as in fractured or porous layers (Nakamura, 1989). The site effects such as amplification of vibration amplitude and occurrence of resonance frequency in the ground may significantly affect the damage of the tree roots. Rigid rocks characterized by higher velocity values weakly damp the seismic shaking, a consequence of which allows for the propagation of seismic waves over long distances and affects surface objects (Sokolov et al., 2012). If rigid rocks appear near the surface, the Vs30 values are much higher than the Vs30 observed in shallow, loose materials. Therefore, the Vs30 maps that show the distribution of averaged S-wave velocity calculated for shallow 30 m beneath the surface can be used to indicate areas where ground accelerations did not attenuate very quickly (Boore et al., 2011; Roser and Gosar, 2010). The Vs30 distribution in the Pamir and Pamir-Alay suggests that the shaking of the seismic event can propagate far away from the source due to the fact that wave attenuation can be weak. Local condition, such as mountain slopes and valley filled with thick loose material can amplify the seismic amplitudes. The effect of ‘‘locality” is crucial in this case. Guerrieri et al. (2007), in the paper concerning the earthquake environmental effects (EEE), showed the effects of the Verny earthquake from the pre-instrumental era. The study describes analysis of the secondary EEEs, which were used to evaluate epicentral and local intensities. The May 28 (June 8, Gregorian calendar) 1887 Verny earthquake was a large seismic event in Central Asia, which completely destroyed the regional center of Verny (Alma-Ata, Kazakhstan) causing hundreds of casualties (Guerrieri et al., 2007). The paper presents the description of environmental effects that were arranged ‘‘valley by valley”, which corresponded to local level
of EEEs. Guerrieri et al. (2007) reported that the M 7.3–7.5 Verny event influences the ground shaking even 560 km from the epicenter. The impact of large earthquakes M > 7.0 on ground stability and growth ring variability was noted in distances of more than 200 km, which is concurrent with other studies (e.g., Kitzberger et al., 1995). Gutenberg and Richter (1956), in their classical study, estimated the radius of perceptibility for several quakes that had occurred in California. Their research results showed that an M 6.0 earthquake or larger event indicates a radius length of about 200–350 km, and that an M 7.6 event has an affected area of a 450 km radius. The application of tree-ring data can confirm the distant impact of earthquake events, even on small-scale ground shaking. The growth suppression of analyzed juniper trees mainly results from seismic events and ground disturbances. In poorly populated high-mountain regions of Central Asia there is a lack of accurate observations about the impact of earthquakes on activity of geomorphic processes. The presented study are the first approach to define the possibility of reconstruction of seismic events and the resulting low-energy mass movements based on the tree-ring research in the Pamirs region. We realized that attempt of dendrochronological reconstruction of specific seismic events, especially moderate and light, is difficult due to influences of other factors, e.g. climatic. Assuming that the seismic events are the most important factor influencing tree growth on unstable areas and possible impact of climate can be faded out, one must take into consideration these uncertainties, when performing dendroclimatic reconstruction. 6. Conclusions The study of juniper wood samples from an unstable area of the western Pamir-Alay shows links between growth suppression and tectonic disturbances. Active tectonics and steep rock
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slopes partly covered by a thick layer of loose materials influence the occurrence of favorable conditions for frequent geomorphic processes. The earthquakes trigger not only large landslides and rock falls, but also slow-medium velocity mass wasting processes like soil creep. On the basis of the growth suppression and gradual recovery present in the dendrochronological data, which are not connected with extreme climate conditions, reliable evidence of earthquake events in the Pamir region and its surroundings can be distinguished. The research confirms that the dendrochronological method is a powerful tool for the analysis of not only climatic variability, but also for geophysical hazard detection. The research results show the impact of moderate earthquakes on growth ring variability occurring over a large distance from the seismic event epicenters. The exceedance in comparison with other studies is connected not only with the sensitivity of the high-mountain trees growing near the upper timberline, which even react to slight ground vibrations, but also with the characteristics of the study site (e.g., steep slopes, large amount of loose sediments). The identification of characteristic growth-ring patterns, which are a response to large earthquakes, will enable the reconstruction of these events from the past. The growth reductions after the large earthquakes show irregular fluctuations and gradual recovery while the extreme climatic events are marked as sharp, deep, and short reductions. However, the reconstruction of moderate and light earthquakes are more difficult to identify and sometimes may be erroneous. The existence of the oldgrowth juniper forest in the western Pamir-Alay will provide an opportunity to recognize tectonic disturbances occurring over the last hundred years, beyond historical observations. The obtained results can help to improve dendroclimatic reconstructions through the precise indication of one of the nonclimatic signals that are co-seismic geomorphic processes present in the dendrochronological series from the tectonically active regions.
Acknowledgements The authors wish to thank local people from Ayni District and Rahmonov family for their hospitality and assistance in the field. This project is supported by the Polish National Science Centre (NCN): grant number 2013/09/B/ST10/00634 ‘‘Comparison of thermal variability during the last millennium in the mountains of Central Asia and Central Europe”. P. Owczarek was financed by statutory founds of the University of Wroclaw. We gratefully acknowledge two anonymous reviewers for their constructive comments. References Alestalo, J., 1971. Dendrochronological interpretation of geomorphic processes. Fennia 105, 1–140. Allen, R.B., Bellingham, P.J., Wiser, S.K., 1999. Immediate damage by an earthquake to a temperate montane forest. Ecology 80, 708–714. http://dx.doi.org/10.1890/ 0012-9658(1999) 080[0708:IDBAET]2.0.CO;2. ANSS Comprehensive Earthquake Catalogue (ANSS ComCat) - USGS Earthquake Hazards Program.
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100 Years of earthquakes in the Pamir region as recorded in juniper wood: A case study of Tajikistan Piotr Owczarek a,⇑, Magdalena Opała-Owczarek b, Oimahmad Rahmonov c, Maciej Mendecki d a
Institute of Geography and Regional Development, University of Wroclaw, Uniwersytecki Sq. 1, 50-137 Wroclaw, Poland Department of Climatology, Faculty of Earth Sciences, University of Silesia in Katowice, ul. Bedzinska 60, 41-200 Sosnowiec, Poland c Department of Physical Geography, Faculty of Earth Sciences, University of Silesia in Katowice, ul. Bedzinska 60, 41-200 Sosnowiec, Poland d Department of Applied Geology, Faculty of Earth Sciences, University of Silesia in Katowice, ul. Bedzinska 60, 41-200 Sosnowiec, Poland b
a r t i c l e
i n f o
Article history: Received 31 October 2016 Received in revised form 21 January 2017 Accepted 5 February 2017 Available online 9 February 2017 Keywords: Central Asia Pamir region Earthquake Tree-ring Paleoseismology Juniperus
a b s t r a c t Active tectonics reflect high seismicity rates in the Pamir and surrounding areas. Long-lived trees growing in the western Pamir-Alay mountains are affected by ground accelerations and seismic-induced geomorphic processes. At a high-mountain site in the Pamir-Alay, 45 juniper trees were sampled in order to analyze the potential role of these trees in the reconstruction of disturbance events. Although the trees have a maximum age c. 500 years, only the period of the last 100 years was analyzed, aiming for the dendrochronological identification of earthquake events known by historic documents and measurements. Analysis mainly showed non-climatic growth suppression and releases. These signals were identified as the results of tectonic disturbances. Tree-ring data recorded many earthquakes from which the most distinct were: 1907, 1923, 1943, 1955, 1982, and 1998. The 1907 and 1923 large earthquakes took place after the vegetation period; therefore, growth reduction appeared in the following year. The 1943, 1955, 1982, and 1998 events occurred just before or during the vegetation period and therefore induced the immediate reduction of tree rings in the earthquake year. The identification of the characteristic growth ring patterns, which are a response to large earthquakes, will enable the reconstruction of these events in the past on the basis of centuries-long dendrochronological records. Ó 2017 Elsevier Ltd. All rights reserved.
1. Introduction High-mountain ecosystems belong among the most sensitive ecological systems and record various climatic and non-climatic factors (Schweingruber, 1996). Local site conditions, including, e.g., relief, tectonic and geomorphic deformations, soil and rock characteristics, exposition, and topoclimate, can influence the growth, vitality, and death of trees. In the trees growing in the mountains of Central Asia, due to differentiation of habitat conditions, we can find many signals connected with ecological, geomorphological, meteorological, and geophysical threats (e.g. Esper et al., 2002; Solomina, 2002; Opała et al., 2017). The westernmost part of the India-Eurasia plate collision zone is characterized by ongoing convergence that reaches 29 ± 1 mm/yr (Mohadjer et al., 2010; Ischuk et al., 2013). Active tectonics reflect high seismicity rates in the Pamir-Alay Mountains (Lukk et al.,
⇑ Corresponding author. E-mail addresses: [email protected] (P. Owczarek), [email protected] (M. Opała-Owczarek), [email protected] (O. Rahmonov), [email protected] (M. Mendecki). http://dx.doi.org/10.1016/j.jseaes.2017.02.011 1367-9120/Ó 2017 Elsevier Ltd. All rights reserved.
1995; Rautian and Leith, 2002; Schurr et al., 2014). This area, one of the most seismically active in Central Asia, is characterized by the occurrence of a large number of deep earthquakes that are disastrous to life and property (Evans et al., 2009). In the Pamir region, earthquakes larger than a magnitude of 5 were first been reported beginning in the 1860s (Musketov and Orlov, 1893; Shebalin and Kondorskaya, 1982). Older events can be reconstructed on the basis of historical sources, sedimentary sequences, and geomorphological analysis. One of the reliable proxy sources on earthquakes spanning from ancient to modern times are dendrochronological records (Jacoby, 1997, 2010). Strong earthquakes affect trees directly by not only causing growth-ring disturbances, but also tree mortality (Allen et al., 1999; Jacoby, 1997). Wood and Neumann (1931) presented criteria for the modified earthquake Mercalli intensity scale based on the effects of the shaking of trees, whereby broken stems indicate the VIII + quake intensity. Large-scale faulting can indirectly influence the growth disturbances as a result of mass movement or changes in the ground water level (Page, 1970; Jacoby et al., 1992). Displacements of the Earth’s crust combined with geomorphic processes on steep slopes affect the changes in the position of trees, and thus the
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changes in wood anatomy (Alestalo, 1971; Shroder, 1980; Stoffel and Bollschweiler, 2008; Stoffel et al., 2010; Stoffel and Corona, 2014). The earthquake magnitude, distance from the epicenter, and their frequency influence the degree of tree inclination, changes in crown shape, breakages, and uprooting. Allen et al. (1999) described earthquake-induced damage to mountain beech trees (Nothofagus solandri) (c. 24% tree mortality in the research plot) growing 10 km from the 1994 event epicenter in the Southern Alps, New Zealand. Kozaci (2012) also reported the decapitation of the crown of Scots pines (Pinus silvestris) growing on the North Anatolian faults as a result of a 1943 earthquake. The drastic growth reduction of conifer tree rings, observed from 1813, was noted in trees growing on the San Andreas Fault (Sheppard and Jacoby, 1989). These growth reductions are the results of 1812 ‘‘San Juan Capistrano” earthquake. With increasing distance from the earthquake epicenter the dendrochronological signal becomes weaker, but is still visible. Growth suppression as a result of ground shaking or growth releases after the death of neighboring
trees in the years following a disturbance is observed hundreds of km from the epicenter (Sheppard and Jacoby, 1989; Kitzberger et al., 1995; Jacoby, 1997). However, even though many studies have reported the use of tree rings as a proxy method for the reconstruction of past earthquake events in different seismic areas, there has been very little research conducted in the high mountains of Central Asia. The previous research has only concentrated on the northern Tian-Shan in Kazakhstan (Yadav and Kulieshius, 1992) and the south-western Karakoram in Pakistan (Bokhari et al., 2010, 2013). The presented study is the first-of-its-kind investigation into the Pamir-Alay Mountains, western Tajikistan, which deals with dendroseismology. In this paper, we explore the juniper tree as a source of information about seismic activity. The purpose of this investigation is: (1) to identify growth-ring disturbances connected with earthquake events from the beginning of 20th century; (2) to analyze the potential of the juniper tree in the reconstruction of disturbance events which took place before instrumental observations.
Fig. 1. (A) Major earthquake events between 1900 and 2014 in the north-western part of the Pamir region, black rectangle shows the study area; (B) detailed location of treering sampling site in the upper part of the Archa Maydan River Valley. Source: ANSS ComCat.
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2. Study area 2.1. General description The study area is located in the western part of the Pamir-Alay Mountains, which belongs to the extensive Pamir region (Fig. 1A). The Pamir-Alay Mountains, known as the Fann Mountains (Chimtarga peak 5489 m a.s.l.) in this area, consists of a typical highmountain relief characterized by steep slopes with numerous traces of mass movement activity. Small valley and cirque glaciers are situated in the highest part of the mountains. Large talus cones and landslides are typical geomorphic forms that are widespread throughout the mountains (Fig. 2A and B). Its deep valleys often have irregular longitudinal profiles with alternating wide and narrow zones, which are separated by Pleistocene frontal moraines (Fig. 2C and D). Trees were sampled in the upper part of the Archa Maydan River within the Zarafshan Range at the elevation 2150– 2250 m (Fig. 1B). This part of the Pamir-Alay Mountains, surrounded by desert plains, has a typical arid continental climate. The mean annual total precipitation is 271.3 mm with maximum in spring (March–May). The mean annual air temperature 6.6 °C (Iskanderkul Meteorological Station).
2.2. Geological background The Pamir region is composed of continental crust of about 70 km in thickness (Chen and Molnar, 1981; Holt and Wallace, 1990) and three major tectonostratigraphic units can be distinguished there. The region is divided into the North, Central, and
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South Pamir (Schurr et al., 2014). Furthermore, the Pamir syntax (a convergence of the Pamir ranges towards a single point) is surrounded by sedimentation basins from the west (the Tadjik-Afghan Basin), the east (the Tarim Basin), and the south (Indus Valley). The Pamir is bordered from the north by the Tian-Shan unit (Burtman and Molnar, 1993). The sedimentation and rock origins of the Pamir region stem from Carboniferous until Neogene and are related to various geological processes such as basin deposition (terrigenous and marine), volcanic activity, and the accretion of subduction complexes. The collision between the Indian and Eurasian plates in the Cainozoic Era caused deformations in the Pamir region and formed many systems of faults and folds (Chen and Molnar, 1981; Holt and Wallace, 1990; Burtman and Molnar, 1993; Ducea et al., 2003; Schurr et al., 2014). The External Pamir complex comprises the Pamir-Alay region and the eastern part of the Tadjik Depression (Tadjik-Afghan Basin), and it changes gradually. The geological structures mainly include Upper Palaeozoic and, to a lesser extent, Carboniferous, Cretaceous, and Neogene rocks. It consists mainly of thick terrigenous sandstone but also marine deposits as schists, limestone, marls, and clay. During the Neocene, when the collision between India and Eurasia had started, the massif was isolated and a very thick sequence of non-marine sediment was accumulated within the Tadjik Basin (Burtman and Molnar, 1993; Nicolaev, 2002; Ducea et al., 2003). 2.3. Seismicity Seismic and neo-tectonics studies pointed out that the Pamir region moves northward en bloc and collides with the Tian-Shan,
Fig. 2. The main relief features of the western Pamir-Alay: (A) different generations of talus cones; (B) large inactive landslide (dashed line) that developed on the right slope of the U-shaped valley; (C) large frontal moraine dissected by outflow from glacier lakes, upper part of the Fann Mountains; (D) U-shape valley filled by glacial, glacifluvial and slope deposits, the arrow indicate location of the frontal moraine presented in (C).
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Fig. 3. Shaded map of Central Asia region with marked main fault and thrust systems, compiled with the Vs30 hazard map. Abbreviations are as follows: MAT – Main Alay Thrust, PTS – Pamir Thrust System, SKF - Sarez-Karakul Fault, DKF - Daravaz-Karakul Fault, CBF - Central Badakhshan Fault, SMTS - Sarez-Murghab Thrust System, HF – Herat Fault, MKT – Main Karakoram Thrust, FBFS – Fergana Basin Fault System, TSFS – Tian-Shan Fault System and KF – Karakoram Fault (after Burtman and Molnar, 1993; Nicolaev, 2002; Ducea et al., 2003; Mohadjer et al., 2010, 2016, simplified). Source: Global Vs30 Map Server.
which is reflected by high seismicity along this frontal Pamir thrust system (PTS) and the Main Alay thrust system (MAT) (Schurr et al., 2014) (Fig. 3). The western Pamir shows higher seismic deformation rates, explained by the ongoing collapse of the western margin of the Pamir Plateau and the westward extrusion of Pamir rocks into the Tadjik Depression. Further to the south the PTS connects to the system of faults in the Hindu Kush where the meridian Daravaz-Karakul (DKF) and Central Badakhshan (CBF) faults can be distinguished (Fig. 3). The CBF extends in latitudinal directions as the Herat Fault (HR), while the southern and eastern Pamir is separated from the Kunlun Shan by the Karakoram faults and thrust system (e.g., the Main Karakoram Thrust (MKT) and the Karakoram Fault (KF)) (Fig. 3). Within the Pamir, the SarezKarakul Fault (SKF) system separates the mostly undeformed high plateau of the eastern Pamir from the seismically active, highrelief, and deeply incised western Pamir. The SKF connects with a large fault complex called the Sarez-Murghab Thrust System (SMTS). What’s more, on the border with the Tarim Basin the seismicity also shows active deformation at the eastern end of the PTS (Fig. 3) (Mohadjer et al., 2010). The seismic hazard for Central Asia as well as for Tajikistan is caused by the tectonic stresses associated with the Indian subcontinent continuing to move northward against Eurasia. This moving process manifests as a large number of earthquakes (King et al., 1999). The seismic events are mainly focused in the vicinity of the Pamir thrust system and Central Badakhshan fault (Fig. 3). However, the seismic event locations appear around the Pamir region, and one event occurred very close to the study area (M = 5, 1982-05-14) (Fig. 1, Table 1). Part of the earthquake events exceed P 6.5 magnitude. A seismic hazard map based on the Vs30
estimation for the Pamir region is shown in Fig. 3. The Vs30 parameter is the most important parameter in the classification of the soil in response to the average shear-wave velocity in the topmost 30 m of sediments or rock. This quantity is also used in the building code Eurocode 8 (EC8) to classify sites according to soil type into five major categories (A, B, C, D and E), and two specific geoengineering categories that correspond to very loose or liquefiable material (Roser and Gosar, 2010). The study area, according to United States Geological Survey (Global Vs30 Map Server), is characterized as a B-type soil which then can be described as deposits of very dense sand, gravel, or very stiff clay (eroded rocks), with Vs P 760 m/s and < 6 m of soil above rock, characterized by a gradual increase in the mechanical properties with depth (Global Vs30 Map Server; Roser and Gosar, 2010). Moreover, according to the Tajikistan Seismic Hazard Distribution Map (Giardini et al., 1999; WHO, 2010), the B-type soils in Tajikistan correspond to an area characterized by a very high PGA (Peak Ground Acceleration, (e.g., Castanos and Lomnitz, 2002)) value >4 m/s2 (with 10% chance of exceedance in 50 years (Giardini et al., 1999). This may suggest that the study area is strongly influenced by ground motion due to the very high considered surface acceleration, which can cause damage to soils and trees as well.
3. Material and methods The samples of juniper trees were collected from the upper part of the Archa Maydan River Valley, in the western part of the PamirAlay Mountains, at the elevation of 2150–2250 m a.s.l. The most important plant community is formed here by Juniperus semiglo-
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Table 1 Historical records of major seismic events M P 5 in the north-western Pamir region from 1900 to 2014 in maximum distance ca. 200 km from the tree-ring sampling area. Source: ANSS ComCat. No
Date
Magnitude
Epicentre location and depth
Distance from the sampling area
1 2 3 4 5 6 7 8 9 10 11 12 13 14 15 16 17 18 18 19 20 21
21-10-1907 28-12-1923 16-09-1924 05-07-1935 08-10-1935 11-01-1943 10-07-1949 19-07-1955 22-09-1956 23-08-1961 15-05-1982 15-02-1984 23-02-1984 22-01-1989 08-11-1992 02-10-1993 03-09-1998 20-01-1999 27-03-2004 10-05-2009 03-08-2010 10-11-2013
7.4 6.5 6.2 6.2 6.3 6.3 7.5 5.7 5.5 5.6 5.0 5.2 5.0 5.3 5.3 5.0 5.0 5.0 5.0 5.0 5.2 5.2
39.179°N 39.664°N 39.061°N 38.097°N 39.132°N 38.196°N 39.174°N 39.824°N 38.405°N 38.583°N 39.197°N 39.932°N 39.971°N 38.465°N 38.778°N 39.066°N 38.447°N 37.061°N 39.959°N 38.234°N 38.452°N 38.410°N
210 85 195 145 225 110 230 60 125 65 22 85 80 105 155 150 155 230 115 135 160 115
70.585°E 69.093°E 70.538°E 67.372°E 70.784°E 67.991°E 70.891°E 67.949°E 69.221°E 68.511°E 68.402°E 67.627°E 67.677°E 68.694°E 69.864°E 69.966°E 69.473°E 68.403°E 69.231°E 67.687°E 69.637°E 68.890°E
depth = 20.0 km depth = 20.0 km depth = 20.0 km depth = 15.0 km depth = 20.0 km depth = 35.0 km depth = 20.0 km depth = 15.0 km depth = 15.0 km depth = 25.0 km depth = 33.0 km depth = 22.3 km depth = 33.0 km depth = 33.0 km depth = 63.6 km depth = 14.2 km depth = 33.0 km dept = 33.0 km depth = 20.9 km depth = 19.4 km depth = 23.7 km depth = 23.0 km
Fig. 4. Taking cores from juniper trees; note the position of trees growing on steep scree slopes covered by a thick layer of loose sediment.
bosa and Juniperus seravschanica, with a significant participation of shrubs and dwarf shrubs of the genera Rosa, Berberis, Ephedra, Sorbus, and Cerasus. The juniper trees reach heights of up to 10 m and form an open forest (Fig. 4). Within the juniper forests there are also single specimens of Juniperus turkestanica, which are not relevant to the forest formation. The investigated trees are growing on lithomorphic soils (rankers) developed from Upper Palaeozoic schists, dolomites, limestones, and conglomerates. Soil cover is heterogeneous in terms of both its mechanical composition and development level. On steep slopes, depending on exposure and vegetation, soils range from fine-grained to coarse-skeletal underdeveloped types. The commonly observed soil profile contains debris of different-sized rock pieces (Rahmonov et al., 2016). For the present study, we collected increment core samples from 45 juniper trees (Juniperus semiglobosa and Juniperus zeravshanica) growing on the steep scree slopes (Fig. 4). Two cores per tree (one upslope and one downslope) were extracted parallel to slope direction. The coring procedure with a Pressler borer, sample preparation, and cross-dating were followed in accordance with standard procedures (e.g., Stokes and Smiley, 1968; Speer,
2010). Measurements of tree-ring width were performed using WinDENDRO (2015). The correctness of cross-dating ring width sequences was checked by using the program COFECHA (Holmes, 1983). Interestingly, these samples were poorly correlated with climate data, as shown in Fig. 5 presenting response function analysis between tree-ring indexed chronology and monthly mean temperature and monthly precipitation totals. This is in contrast to the samples taken from more stable areas of the western Pamir-Alay Mountains, where Opała et al. (2017) showed a distinct climatic signal and a more regular course of juniper curves without abrupt growth reductions and releases in sites not affected by geomorphic processes. The wood material in this study, collected from a less stable area covered by a thick layer of loose sediment, was analyzed for the occurrence of growth reductions connected with non-climatic signals. The calculation of growth reduction was done using the method described by Schweingruber (1990). The reduction values are calculated as the total TRW in the reduction period in relation to the same number of rings from the period preceding the reduction. The abrupt growth reduction is noted for sequences of more than three years. Growth changes were analyzed in a three
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Fig. 5. Response coefficient between tree-ring indexed chronology (the Archa Maydan site) and climate factors (monthly mean temperature and monthly precipitation totals). No statistically significant values have been found.
stage scale: 30–50% of change – moderate reductions, 51–70% – strong reductions, and >70% – very strong reductions. The age structure of the analyzed trees ranges from 150 years up to 500 years; however, the period from 1900 was analyzed in order to determine the influence of earthquakes of a known date of occurrence on growth-ring disturbances. The seismicity of the Pamir region was obtained for the period of 1900–2014 from available on-line seismic catalogues (USGS Earthquake Hazards Program, Storchak et al., 2013). The earthquakes taken into account for this research have occurred within a radius of c. 200 km from the sampling location (Fig. 1A, Table 1). 4. Results 4.1. Earthquake evidence based on tree-ring responses All samples were measured, cross-dated, and examined for growth-ring changes that might indicate disturbances as a result of earthquakes in the common period of 1900–2014. The analyzed trees, due to their position on the steep slope, have low moisture
availability and poorly developed soils. They are characterized by high level of tree-ring width variability (Figs. 6–8). The negative influence of unfavorable climatic conditions is less pronounced in these series in comparison with the more stable areas (Opała et al., 2017), but two periods are strongly influenced by extreme weather conditions (Fig. 6). Intensive growth reductions and locally absent rings are observed in 1917–1918 (in some trees also in 1919), which are a result of the exceptional drought occurring in 1917 (with April–September precipitation of 2.8 mm, while the average for this period is 125.7 mm). Growth reductions connected with extremely cold winters can be observed in the period 1970– 1978. The average winter temperature for this period was 1.2 °C below the long-term mean (1930–2007) for the Iskanderkul Meteorological Station. Seismic shaking influences the growth anomalies, especially in trees growing on steep slopes that are sensitive to mass movements. These perturbations occurred because the roots and stems of the trees growing in the Pamir-Alay tectonic unstable area were disturbed by sliding or creeping during or shortly after the earthquake event. Most of the 45 trees analyzed showed periods of non-climatic reduction of growth and recovery within an individual tree-ring series during the 1900–2014 period. However, the duration and intensity of these signals vary between individuals. These growth reductions can be recorded immediately after the earthquake or in the next year depending on the time of the event occurrence (time before, during, or after the vegetation season). Altogether, 80% of trees showed evidence of growth-ring disturbances occurring during or soon after several earthquakes between 1900 and 2014. These earthquakes resulted in growth suppression for 2–8 years in most of the sampled trees, but the duration of some growth suppression signals is longer due to successive earthquakes (e.g., 1923–1941, 1944–1964) (Fig. 6). After the suppression period, represented by narrow growth rings, recovery and accelerated growth can be observed. But these phases are not too long and are limited due to the strong seismicity of analyzed area and the influence of co-seismic and post-seismic geomorphic phenomenon.
Fig. 6. Percentage of samples with abrupt growth reduction in a three stage scale: 30–50% of change – moderate reductions, 51–70% – strong reductions, and >70% – very strong reductions. Main earthquake events and selected meteorological signals were also marked.
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The analysis of individual tree-ring series showed two different growth patterns as a result of earthquake events. The analysis of tree-ring width changes allowed for the identification of several events first beginning in 1900. The first type of pattern is a rapid decrease of tree-ring widths (e.g., after the earthquakes of 1923– 1924, 1998–1999) that continued for 2–4 years, after which the trees accelerated their growth. These disturbances are likely a direct effect of the partial destruction of the root system and the rapid reduction of nutrient supply to the trees. The second pattern of growth suppression represents slowly decreasing tree-ring widths (e.g., after the 1907 earthquake). This type of growth reduction could be connected with the gradual tilting of trees as well as the destruction of root hairs resulting from shaking and mass movements. The duration of these growth reduction periods is extended for more than 4 years. This type of growth reduction is less common in the analyzed trees. Figs. 7 and 8 show examples of the fluctuation of ring widths for four individual trees, representing both types of reduction patterns. The earthquake of 21 September 1907 triggered a gradual decrease in tree-ring widths noted from 1908 (Fig. 7A and B). Although the epicenter of this 7.4 M earthquake was located 210 km from the study area, it led to 60% growth reduction in trees five years after this event. It should be noted that very strong
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reductions of tree-ring widths were noted in only 6% of trees (see Fig. 6). The growth suppression could be a response to the tilting of trees. The first signs of recovery were noted between six (Tree: TJIS_45) (Fig. 7A) and nine (Tree: TJBAC_28) (Fig. 8B) years after this event. In rare cases the recovery occurred after two years (Tree:tj_cz19) (Fig. 7B). In most of the sequences, further growth recovery was disturbed by an extraordinary drought in 1917. A relatively different growth-ring width pattern was noted after earthquakes in 1923 and 1924, of 6.5 M and 6.2 M, respectively. These events influenced the growth reduction of trees upwards of 80%, and most of them showed strong and very strong reductions (Fig. 6). Sharp growth reduction was noted in 1924/1925 and lasted 2–4 years (Figs. 7A, B and 8A, B). The 11 September 1943 earthquake of 6.3 M was recorded in 50% of trees. In 1944 growth was reduced both sharply (TJIS_45, tj_cz19) (Fig. 7A and B) and slowly (TJBAC_28, TJ_CZ17) (Fig. 8A and B). The duration of these suppressions was prolonged due to the 10 July 1949 earthquake of 7.5 M, or even by the earthquake in 1955 (Figs. 7A and 8B). The earthquake of 19 July 1955, whose epicenter was located about 60 km from the research site, influenced the moderate and strong reduction observed in 37 trees over the next 2–3 years. This signal was accelerated by the 1956 earthquake. However, the one-year, deep reduction in 1957 could be partially connected with the cold
Fig. 7. Graph showing selected annual growth-ring plots: (A) core TJIS_45 – in close-up section including the growth suppression connected with the 1943 earthquake, the signal was strengthened by the 10 July 1949 earthquake of 7.5 M and by the earthquake in 1955; (B) core tj_cz19 – growth suppression connected with 1907, 1923/1924, 1935, 1943 and 1949 earthquakes is presented in close-up section.
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winter (Fig. 6). The following earthquakes, which occurred in the 1990s, did not produce clearly visible short- or long-term growth ring reductions in the analyzed trees (Figs. 6–8). A reduction in 1982 coincides with the earthquake of 15 May 1982, whose epicenter was only 22 km away from the sampling site. Most of the analyzed trees show a noticeable decline after the 1998 and 1999 earthquakes. As a result of these events, 78% of trees underwent an abrupt suppression in 2000 and 2001, the majority of which showed strong and very strong reductions (Figs. 6–8). The recovery was noted very fast, after 2 (Fig. 8A and B) to 4 (Fig. 7A) years. In the 21st century, the cores clearly recorded the earthquake of 8 January 2007 and the earthquake of 10 November 2013. The last earthquake event has been expressed as the abrupt growth suppression of 22 trees in 2014. A strong reduction was noted for most of these trees. Detrended tree-ring chronology from the western Pamir-Alay shows distinct fluctuations of tree-ring indices, which are an effect of seismic events and climatic variability. There are a few growth ring patterns clearly reflecting earthquake events (Fig. 9). The substantial 1907 and 1923 earthquakes took place after the vegetation period, therefore the growth suppression appeared in the following years. The 1943, 1955, 1982, and 1998 events occurred just before or during the vegetation period and therefore affected the immedi-
ate reduction of the tree-ring index in the year of earthquake. It should be noted, that the response to earthquakes of 5 M in 1982 and 1998 is different (Fig. 9) and it is not so clear like response to large ones. It can be explained by the occurrence of earthquakes in the different parts of the growing season and by the influence of other factors (local/climate). The earthquake in 1982 took place in the first part of vegetation period in wet season, when nutrients were introduced to the tree. Thus, during the second part of the vegetation season fast regeneration of root system took place. The earthquake in 1998 was recorded at the end of the summer, during the dry months. Lack of nutrient input did not allow to regenerate root system in the next vegetation season in 1999. The relatively deep reduction in 2000 can be a result of additional effect of low temperature in the winter (see Fig. 6). 4.2. Relation between co-seismic geomorphic processes and tree growth Earthquake-induced mass movements have been documented on the basis of the tree-ring variability of juniper trees. Rapidly and slowly decreasing growth rings correlate with seismic events whose epicenters were located within a maximum distance of 230 km from the study area. There were no injuries and other
Fig. 8. Graph showing selected annual growth-ring plots: (A) core TJBAC_28 – growth suppression connected with 1943, 1949 and 1999 is presented in close-up section; (B) core TJCZ_17 – distinct visible growth suppression connected with 1923/1924, 1935, 1943, 1949, 1955/1956 and 1998/1999 is presented in close-up section.
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Fig. 9. Undetrended tree-ring chronology based on 45 samples of juniper tree. Vertical lines indicate seismic events clearly recorded in tree-ring data; blue lines mark earthquakes occurring after vegetation period, red ones - just before or during the vegetation period.
traces of the direct impact of rock particles on the analyzed tree stems. Therefore, it is suggested that the trees were not affected by large and rapid slope processes like rock falls, rock avalanches, massive landslides, or debris flows. The geomorphic features associated with these large-scale processes are observed in the sampling sites, but they are older than the currently living trees (see Fig. 2B). Thus, one can conclude that the observed changes in growth ring widths are related to a slow-medium velocity mass wasting process such as creeping. The slow downslope migration of soil and rock particles is accelerated during earthquake events. The slopes of the Pamir-Alay Mountains in the study area are covered by a thick layer of loose, unsaturated colluvial and glacial deposits (Figs. 2 and 10A). The seismic shaking of these materials influences the increase of creeping and disrupted shallow sliding activity (Fig. 10B). In general, the low-growing (max. 10 m) juniper trees with extensive root systems are resistant to tilting. Low velocity and shallow slope processes can mainly damage root hairs and smallish lateral and vertical roots, which are generally not deep. Limitation in water and mineral nutrient supply leads to the decrease of growth ring width (Fig. 10C). Recovery takes place a few years after the event, resulting from the rebuild of the fine root system in the upper soil (Fig. 10D). Its duration depends on several factors like type of regolith, slope inclination, and density of vegetation cover. A longer period of recovery was found in trees growing on steep unstable slopes, where post-seismic creep is more significant compared with relatively stable areas. Our studies allow for the determination of a maximum distance limit for regolith slide, taking into account the different intensity of earthquakes. A distance of c. 230 km for M 5.0–5.3 earthquakes indicates the large sensitivity of juniper trees for ground shaking (Fig. 11). 5. Discussion Many recent papers have shown that trees can be used as a unique tool for dating past earthquake events (Jacoby, 1997, 2010). Trees are usually affected by ground accelerations, seismic-induced landslides, rock falls, and hydrologic changes. This
phenomenon can help locate zones of disturbance and characterize the size and strength of an event. The response of trees to earthquake events must be analyzed in terms of climatic variations. Short-term growth suppression can be connected to extreme climate conditions (Schweingruber, 1996). The abrupt growth reduction observed in 1917–1918 was connected with an extreme drought, which was recorded in most dendrochronological records in Central Asia (Chen et al., 2013, 2016; Opała et al., 2017). The second period of growth suppression (1970–1978) observed in the analyzed trees was a result of extremely cold winters. The growth suppression resulting from historic earthquake events have rarely been analyzed in terms of growing seasons, but there are a few papers concerning timing of tree-ring response to earthquakeinduced landslide (Carrara and O’Neill, 2003) or normal faulting (Bekker, 2004). This finding allows for an explanation for why growth reduction does not appear in the year of the earthquake, but sometimes in the following year. On the basis of tree-ring chronology from the western Pamir-Alay we can observe differences between growth ring patterns connected with earthquakes and extreme weather conditions. The growth releases after the large earthquakes show irregular fluctuations (e.g., after the 1907, 1923 and 1943 earthquakes (see Fig. 9)). The extreme climatic events are marked as sharp, deep, and short reductions (e.g., after the 1917 extreme drought (see Fig. 9)). The effect of drought is restricted to one or two rings in analyzed trees, whereas the large earthquake-induced growth is recorded over several years. These findings are consistent with observations in other seismic areas (Jacoby, 1997). Juniper trees from the Pamir-Alay Mountains record ground shaking and movement triggered by earthquakes but the local hydrogeomorphological changes and dynamic stress caused a variety of effects in individual juniper trees. Differences in moisture availability and poorly developed soil on morphologically differentiated steep slope correspond with the variability of tree-ring variations in the individual series. However, dendrochronological data from the all investigated trees show clear connection between growth rings and earthquake-induced slope processes. Our results indicate that trees can record moderate seismic events whose epi-
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Fig. 10. Graph showing the effect of potentially damage to the root system on growth-ring pattern as a result of ground displacement: (A) juniper tree growing on slope covered by thick layer of loose deposits; (B) earthquake events and ground shaking; (C) damage root hairs and smallish roots leads to the decrease of growth ring width; (D) growth recovery as a result of root system rebuilding.
centers are located at a relatively large distance. Most commonly, the effect of earthquakes on the trees was studied in close distance to the epicenters (Sheppard and Jacoby, 1989; Yadav and Kulieshius, 1992; Kozaci, 2012). The direct impact of seismic events influences the external damage in trees, or strong and lengthy growth suppression. Many investigations have placed a greater emphasis on earthquake-induced large landslides than small slides or creep (Keefer, 1984, 2002; Rodrı´guez et al., 1999; Havenith et al., 2003; Evans et al., 2009; Havenith and Bourdeau, 2010; Šilhán et al., 2012; Jibson and Harp, 2016). Studies of previ-
ous earthquakes indicated a maximum epicentral distance limit for landslides of 20–30 km for an M 5.5 earthquake (Keefer, 1984), or even in extreme case of 245 km for an M 5.8 earthquake (Jibson and Harp, 2012). Our study shows the impact of M 5.0–5.5 earthquakes on growth ring variability within an epicenter distance of 230 km, which significantly exceeds the maximum distance of slides triggered by the seismic events of this magnitude noted by Keefer (1984) and Rodrı´guez et al. (1999) (Fig. 11). This exceedance is connected not only with the sensitivity of the high-mountain trees growing near upper timberline, which react even to slight
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Fig. 11. Distance from epicenter to ground displacement for earthquakes of different magnitudes. The results from the Pamir-Alay are compared with Keefer (1984) and Rodrı´guez et al. (1999) studies. Solid lines are approximate upper bounds of earthquake impact enclosing each group of data.
ground vibrations, but also with the characteristics of Pamir-Alay Mountains. One of the most important features of this area is the thick cover of loose sediments of different origins. Such sediment may have a tendency to develop site effects (Nakamura, 1989) in the study area. Usually, the amplitudes of seismic ground accelerations attenuate faster in less dense earth materials, fulfilled by fluids (water, gas, and oil) as well as in fractured or porous layers (Nakamura, 1989). The site effects such as amplification of vibration amplitude and occurrence of resonance frequency in the ground may significantly affect the damage of the tree roots. Rigid rocks characterized by higher velocity values weakly damp the seismic shaking, a consequence of which allows for the propagation of seismic waves over long distances and affects surface objects (Sokolov et al., 2012). If rigid rocks appear near the surface, the Vs30 values are much higher than the Vs30 observed in shallow, loose materials. Therefore, the Vs30 maps that show the distribution of averaged S-wave velocity calculated for shallow 30 m beneath the surface can be used to indicate areas where ground accelerations did not attenuate very quickly (Boore et al., 2011; Roser and Gosar, 2010). The Vs30 distribution in the Pamir and Pamir-Alay suggests that the shaking of the seismic event can propagate far away from the source due to the fact that wave attenuation can be weak. Local condition, such as mountain slopes and valley filled with thick loose material can amplify the seismic amplitudes. The effect of ‘‘locality” is crucial in this case. Guerrieri et al. (2007), in the paper concerning the earthquake environmental effects (EEE), showed the effects of the Verny earthquake from the pre-instrumental era. The study describes analysis of the secondary EEEs, which were used to evaluate epicentral and local intensities. The May 28 (June 8, Gregorian calendar) 1887 Verny earthquake was a large seismic event in Central Asia, which completely destroyed the regional center of Verny (Alma-Ata, Kazakhstan) causing hundreds of casualties (Guerrieri et al., 2007). The paper presents the description of environmental effects that were arranged ‘‘valley by valley”, which corresponded to local level
of EEEs. Guerrieri et al. (2007) reported that the M 7.3–7.5 Verny event influences the ground shaking even 560 km from the epicenter. The impact of large earthquakes M > 7.0 on ground stability and growth ring variability was noted in distances of more than 200 km, which is concurrent with other studies (e.g., Kitzberger et al., 1995). Gutenberg and Richter (1956), in their classical study, estimated the radius of perceptibility for several quakes that had occurred in California. Their research results showed that an M 6.0 earthquake or larger event indicates a radius length of about 200–350 km, and that an M 7.6 event has an affected area of a 450 km radius. The application of tree-ring data can confirm the distant impact of earthquake events, even on small-scale ground shaking. The growth suppression of analyzed juniper trees mainly results from seismic events and ground disturbances. In poorly populated high-mountain regions of Central Asia there is a lack of accurate observations about the impact of earthquakes on activity of geomorphic processes. The presented study are the first approach to define the possibility of reconstruction of seismic events and the resulting low-energy mass movements based on the tree-ring research in the Pamirs region. We realized that attempt of dendrochronological reconstruction of specific seismic events, especially moderate and light, is difficult due to influences of other factors, e.g. climatic. Assuming that the seismic events are the most important factor influencing tree growth on unstable areas and possible impact of climate can be faded out, one must take into consideration these uncertainties, when performing dendroclimatic reconstruction. 6. Conclusions The study of juniper wood samples from an unstable area of the western Pamir-Alay shows links between growth suppression and tectonic disturbances. Active tectonics and steep rock
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slopes partly covered by a thick layer of loose materials influence the occurrence of favorable conditions for frequent geomorphic processes. The earthquakes trigger not only large landslides and rock falls, but also slow-medium velocity mass wasting processes like soil creep. On the basis of the growth suppression and gradual recovery present in the dendrochronological data, which are not connected with extreme climate conditions, reliable evidence of earthquake events in the Pamir region and its surroundings can be distinguished. The research confirms that the dendrochronological method is a powerful tool for the analysis of not only climatic variability, but also for geophysical hazard detection. The research results show the impact of moderate earthquakes on growth ring variability occurring over a large distance from the seismic event epicenters. The exceedance in comparison with other studies is connected not only with the sensitivity of the high-mountain trees growing near the upper timberline, which even react to slight ground vibrations, but also with the characteristics of the study site (e.g., steep slopes, large amount of loose sediments). The identification of characteristic growth-ring patterns, which are a response to large earthquakes, will enable the reconstruction of these events from the past. The growth reductions after the large earthquakes show irregular fluctuations and gradual recovery while the extreme climatic events are marked as sharp, deep, and short reductions. However, the reconstruction of moderate and light earthquakes are more difficult to identify and sometimes may be erroneous. The existence of the oldgrowth juniper forest in the western Pamir-Alay will provide an opportunity to recognize tectonic disturbances occurring over the last hundred years, beyond historical observations. The obtained results can help to improve dendroclimatic reconstructions through the precise indication of one of the nonclimatic signals that are co-seismic geomorphic processes present in the dendrochronological series from the tectonically active regions.
Acknowledgements The authors wish to thank local people from Ayni District and Rahmonov family for their hospitality and assistance in the field. This project is supported by the Polish National Science Centre (NCN): grant number 2013/09/B/ST10/00634 ‘‘Comparison of thermal variability during the last millennium in the mountains of Central Asia and Central Europe”. P. Owczarek was financed by statutory founds of the University of Wroclaw. We gratefully acknowledge two anonymous reviewers for their constructive comments. References Alestalo, J., 1971. Dendrochronological interpretation of geomorphic processes. Fennia 105, 1–140. Allen, R.B., Bellingham, P.J., Wiser, S.K., 1999. Immediate damage by an earthquake to a temperate montane forest. Ecology 80, 708–714. http://dx.doi.org/10.1890/ 0012-9658(1999) 080[0708:IDBAET]2.0.CO;2. ANSS Comprehensive Earthquake Catalogue (ANSS ComCat) - USGS Earthquake Hazards Program.
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