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Deformation and seismic effects in the ice cover of Lake Baikal
Russian Geology and Geophysics 50 (2009) 214–221 www.elsevier.com/locate/rgg
Deformation and seismic effects in the ice cover of Lake Baikal V.V. Ruzhich a, *, S.G. Psakhie b, E.N. Chernykh a, S.A. Bornyakov a, N.G. Granin c a
b
Institute of the Earth’s Crust, Siberian Branch of the RAS, 128 ul. Lermontova, Irkutsk, 664033, Russia Institute of Strength Physics and Materials Science, Siberian Branch of the RAS, 2/1 prosp. Akademichesky, Tomsk, 634021, Russia c Limnological Institute, Siberian Branch of the RAS, 3 ul. Ulan-Batorskaya, Irkutsk, 664033, Russia Received 25 March 2008; accepted 27 August 2008
Abstract The mechanics of the ice cover of Lake Baikal has been studied through monitoring of its deformation and seismic effects and full-size uniaxial compression and shear tests in 2005–2007. We measured the shear strength of ice specimens and large in situ blocks (σ = 0.2−1.9 MPa) and investigated it as a function of air temperature and ice structure. Deformation was analyzed in terms of various natural controls, such as air temperature and pressure, wind, sub-ice currents, and local earthquakes. Precise strain measurements along ice cracks were used to explore the strain behavior of ice, including the cases of dynamic failure (ice shocks). Measurements by seismic station Baikal-12 were used to monitor 4 5 diurnal background microseismicity variations and to record an ice quake with its magnitude (M = 0.3–0.8; E = 10 –10 J) comparable to a medium-size rock burst or a small earthquake. Ice quakes were studied in terms of their nucleation, dynamics, and aftereffects, as well as the strain and seismic responses of the ice, using sub-ice explosions in the latter case. The natural conditions of deformation in the elastoviscoplastic Baikal ice are similar to lithospheric processes and thus can be employed in tectonophysical modeling with scientific and practical implications for hazard mitigation. © 2009, IGM, Siberian Branch of the RAS. Published by Elsevier B.V. All rights reserved. Keywords: ice cover; major crack; shear strength; deformation; ice shock; dynamic loading; seismicity; physical modeling
Introduction Since long ago processes in the Baikal ice, which is mechanically similar to faulted crust, have attracted interest of geologists (Dobretsov et al., 2007; Gakkel’, 1959; Psakhie et al., 2005a, b; Sokol’nikov, 1960; Zhdanov et al., 2001, 2002). We studied winter ice mechanics in Lake Baikal mostly at a test site near the Angara source in the vicinity of Listvyanka Village (Fig. 1) as part of an SB RAS integration project run through 2005–2007 jointly by people from six academic institutions of Irkutsk, Tomsk, Novosibirsk, and Ulan-Ude. In this paper we report some project results with the main focus on natural dynamic failure of ice which can generate seismic motion in a way similar in many points to tectonic earthquakes. Thus we investigated ice shocks (ice quakes) as a model of seismotectonic and seismic processes, as well as strain patterns and physics of freshwater ice. Of special importance were the results obtained in full-size explosion tests in ice. At all stages of the project, we used advanced technical facilities. The reported data appear to be
* Corresponding author. E-mail address: [email protected] (V.V. Ruzhich)
novel and stimulating, especially because there is almost no experience in studying details of fracture and seismic effects in freshwater ice of Lake Baikal.
Mechanic properties of Baikal ice No knowledge of the Baikal ice rheology being available, we designed portable instruments allowing stress measurements under uniform uniaxial compression at a rate of the order of 10 µm/s. Measurements were run in wintertime on ice specimens and monolith blocks subject to compression and shear. Loading was applied to 5 × 5 × 5 cm ice cubes in situ with a special jack at air temperatures from +1 °C to –11 °C and yielded a compression strength of σs = 0.2− 1.7 MPa. The measured strength of ice samples depended on air temperature and on ice structure: It was σs = 0.2 MPa at 0 °C and 1.7 MPa at –11 °C and decreased in the presence of cracks, air and particle inclusions, or layering in the ice. Shear strength of ice cylinders 5 cm in height and 5 cm in diameter was σs = 0.2−0.4 MPa (at T = –6...–8 °C). Other experiments implied loading, with powerful jacks, applied to in situ ice blocks cut on three sides to 22 cm high, 64 cm
1068-7971/$ - see front matter D 2009, IGM, Siberian Branch of the RAS. Published by Elsevier B.V. All rights reserved. doi:10.1016/j.rgg.2008.08.005
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Fig. 2. Strain diagram for monolith ice prior to shear failure. Loading was by 50-ton jack set up in a niche cut in ice. Shear strength was 0.89 MPa.
Fig. 1. Location map of Listvyanka major ice crack near Angara source. 1. Observation sites. 2. Sites of data collection. 3. Major ice crack.
wide, and 70 cm long; the fourth side and the base of the blocks were left intact from below and from the face. The strength obtained in these experiments was σs = 0.6−0.89 MPa. See Figure 2 for the strain diagram of an ice block deforming to final failure as the ice reached the limit of its shear strength. We discovered that the ice was brittle and vulnerable to shocks like some rocks prone to brittle failure under dynamic loading. Compressive loading of some monolith samples free from cracks, air bubbles, or other structural defects, at temperatures from –5 °C to –11 °C, led to spontaneous violent fracture with shattering to flour dust. In natural conditions this mechanic property of the Baikal ice shows up as failure events we called ice quakes (ice shocks). Another essential property we discovered in tests of crushing cylindrical ice samples in plastic coating (i.e., constrained failure) was viscoplastic flow at T = –3 °C. The samples kept integrity under strain of no less than 47%, and the ice samples increased their shear strength in constrained conditions to as high as 1.9 MPa, though their structure was disturbed by air bubbles and cracks. We recorded acoustic signals generated by formation of numerous small cracks in the process of deformation. Note that crack growth in constrained samples becomes blocked as the cracks are compressed and healed by recrystallization to prevent the samples from failure even under high strain. According to Mellor (1979), ice possesses an unusual property of decreasing its shear strength to zero at growing hydrostatic pressure at normal temperatures, and increasing at growing strain rate. These physical and mechanic properties of constrained behavior of ice are worth special investigation as they can cause a notable effect on its rheology at different loads, strain rates, and temperatures.
Our observations show that the ice rheology changes with depth, from top to bottom where, at the ice-water interface, it has a temperature about +0.3 °C (Duchkov et al., 2007) and shows viscoplastic behavior. Thus, taken in a first approximation, the Baikal ice has a two-layer structure, with the brittle upper layer about the air temperature at T = –20...–35 °C. In general, ice mechanics can vary in a broad range at different loading conditions from elasticity to nonlinear viscosity being subject to creep or brittle failure. Nevertheless, with all this knowledge, ice and, especially, the ice of Lake Baikal, remains poorly understood in terms of physics and mechanics. Causes of ice deformation in the Baikal basin The causes of ice deformation and fracture have important dynamic implications. The Baikal freshwater ice forms in special weather and structural conditions, and has its physics and mechanics slightly different from those in marine ice (Galazii, 1993; Makshtas et al., 2007; Pomytkin, 1960; Psakhie et al., 2005a; Sokol’nikov, 1960, 1970; Zhdanov et al., 2001, 2002). The ice cover of Baikal is unique due to an extremely large size of the lake, which is 636 km long and 25–79 km wide, with the 31,500 km2 total area of the natural ice laboratory. Ice is subject to weather changes (air temperature, wind, solar radiation, etc.) controlled by westerly cyclones and by the Siberian anticyclone. From the beginning of our project, we understood that the exceptionally transparent and clear Baikal ice differed physically and mechanically from marine ice (Dortman, 1976; Mel’nikov, 1981). Ice is known to have high thermal expansion (9.1×10−5 at 0 °C), i.e., with an average lake width of 50 km, ice can expand for up to 45.5 m in daytime in a single event and contract correspondingly in the night. Thus, the effect of diurnal air temperature variations on the ice constrained by the shore, where it is in many places tightly welded with bottom sediments and bedrock, is to produce destructive periodic compression-to-extension change. As the surface contracts in the nighttime, ice cracks open, and let water penetrate therein making patches of ice-free water in major crack zones. Rhythmic expansion of the ice surface for many
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days causes its hummocking, especially intense along thermal sutures or major cracks which are boundaries of ice plates. The ice cover expansion is the most effective at highest temperature contrasts between night cooling and day warming of air and ice. Weather changes in winter and spring (January–April) produce the respective long-term temperature trends, which, in turn, favor ice divergence and convergence. Cumulative seasonal compressive strain in major cracks of different generations can reach 6–10 m, as estimated in onshore Listvyanka Village from the height of ice ridges and amount of underthrust of ice plates. The total winter expansion across the lake inferred as the first approximation from ice crack patterns in aerial images is 50–100 m. This estimate is consistent with data by Sokol’nikov (1960) that an ice plate can underthrust to 20 m the greatest, and there may be several such places. Expansion along the lake is obviously still greater, as it is clearly seen in images from the area of Ol’khon Island and Svyatoi Nos Peninsula with large fields of ice ridges and arc-shaped large cracks oriented across the lake. Dynamic failure of ice, with prominent seismic effects, is the most active after February 10, and especially in March, which coincides with the season of strongest temperature change. This is the season when the Baikal ice is the most monolithic, stiff, and elastic. It can accumulate maximum elastic energy on sunny days at temperature contrasts of 5–10 °C or more by warming and related thermal expansion which sharply increases the ice surface area. Other deformation controls may come from sub-ice convective currents or ruggedness of the ice top and bottom because of hummocking, which increases the efficiency of wind action and deformation. The deformation activity increases in late March–early April as snow, which damps sharp temperature contrasts, becomes blown off. The snow-free places exposed to insolation and heating are subject to geostrophic flow (Zhdanov et al., 2001, 2002). Sub-ice currents persist throughout the water column during the ice period (Sokol’nikov, 1970, Ravens et al., 2000, Zhdanov et al., 2001, 2002), though their rates decrease to no more than 10 cm/s after freeze up, as ice screens the wind effect. They keep their pattern, with the general cyclonic circulation and separate circulation cells in the lake subbasins. Outflow into the Angara gives rise to an anticyclone circulation cell in the Listvenichnyi Bay, both in the ice and ice-free seasons. At the boundary of this cell there is a zone of convergence located near the Listvenichnyi Cape just where a major crack commonly appears running from Berezovyi Cape to Tankhoi Village. The system of sub-ice currents might cause some influence on formation of major cracks but a more significant reason is that the capes are stress concentrators, and almost all transverse cracks begin at capes. The coefficients of friction of wind and sub-ice currents against ice are 0.0012 and 0.0055, respectively (Makshtas et al., 2007). Baikal wind in winter storm weather can reach a speed of 20–30 m/s or more while sub-ice currents are never faster than 10 cm/s. Thus, the action of wind is naturally about two orders of magnitude greater than that of sub-ice currents.
Another agent is winter water level fall of tens of centimeters, which, by outflow through the Angara, is smooth and evident in ice subsidence near the shore. Air pressure changes may likewise contribute to vertical deformation of ice and cause transversal lake level variations of 10 cm or more (Pomytkin, 1960). In addition to those factors, the Baikal ice and its seismic background can experience occasional, sometimes very strong, impact from tectonic earthquakes often generated in the basement of the Baikal basin, most frequently at crustal depths between 5 and 20 km. The M = 6.0 South Baikal earthquake of 25 February 1999 located 10 km south of Listvyanka Village (Ruzhich et al., 2002) was one such event. The ice rupture it caused was the strongest in the vicinity of Tankhoi Village where cracks reached 1 m wide and ran for kilometers along the shore. There is also historical evidence of ice displacement and fracture during the Tsagan earthquake of 11 December 1862 when seismic shaking intensity reached X on the MSK-64 scale. Strain measurements in seismogenic ice cracks Like processes in crustal material, deformation of ice caused by various factors (see above) is associated with energy release in waves of different frequencies, including the seismoacoustic band relevant to our study. To investigate details of ice deformation, we measured motions in cracks of different scales using a specially designed precise instrument Sdvig-3M (Dimaki et al., 2006). Below we report some measurement results from large ice cracks. These cracks, called fissures in the case of Baikal, opened to 10–20 cm on night cooling and closed to the same or slightly smaller magnitude by warming-driven compression in daytime, the respective diurnal rate being 1–2 cm/h. Dynamic failure, such as tectonic earthquakes or ice shocks, are associated with slip rates of a much greater order of magnitude. An ice quake, with violent fracture of ice accompanied by rustle, boom, and shaking, resembles a small tectonic earthquake for any eye-witness, especially for one standing close to a large crack. See Figure 3 for the location of strain meters (a side and a central ones) at the site of the Listvyanka large crack during an ice quake. On witnessing the shock, we estimated the amount of horizontal displacement of the ice plate reaching 1.5–2.0 m in 3–5 s, i.e., the motion was quite fast. Note for comparison that fault slip rates in large earthquakes are as high as 5–10 m/s. Large ice shocks at a site occur every 4–15 days, thus making no more than ten most strongly felt events, which can amount to hundreds of events over the whole lake recorded by onshore seismic stations. See Fig. 4 for the strain curve of motion on the Listvyanka crack in the ice shock we witnessed. Rupture began from Listvenichnyi Cape, which confirms our hypothesis on the near-shore location of the concentrators of destructive stress in the ice. Figure 5 shows displacement in two smaller cracks (1, 2) located 60 and 90 m far from the large crack, during the same event. Compression grew rapidly prior to the shock and was followed by a pulse of opening and then by the return motion.
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Fig. 3. Ice shock and hummocking on 22 March 2007 at 12:44, local time. See strain meters of which one is sinking under broken ice.
The recovery to the initial opening, which was incomplete for crack 2, occurred slowly for several hours. A physical interpretation of this pattern may be that the response of ice fractured over a large area has two phases of rapid elastic and slow viscoplastic motion. Note that we observed the same response to dynamic loading in other similar ice events, as well as in crustal surface rupture caused by explosions or earthquakes (Psakhie et al., 2005b; Ruzhich et al., 1999). The above ice quake synopsis highlights essential details of failure in kilometers-long cracks under compression, which is very similar to seismic slip in large earthquakes. Seismic effects in the ice cover of Lake Baikal So far ice seismicity in the Baikal rift has been very poorly explored. In our project, seismic signals were recorded by a
Fig. 4. Strain dynamics in Listvyanka zone of major crack during ice quake of 22 March 2007. See that the sides of a subsidiary crack set in motion before the central part of the crack zone. Amount of slip in central crack subject to major rupture was much greater than in the subsidiary crack.
digital seismic station Baikal-12 with SK-1P and OSP-2M channels, for velocity and acceleration data, respectively. The sensors were placed in an ice trench and oriented in the vertical (Z) and horizontal (N–S, E–W) directions. Background microseismicity. We monitored daily background microseismisity in the Baikal ice as a record of its brittle failure and obtained the following results. The seismograms of Fig. 6 correspond to microseismic background variations in ice and bedrock at the site of the Listvyanka onshore seismic station correlated with variations of air temperature between 14 and 23 March 2007, with daily mean slip velocities sampled at every 2 minutes. The mean slip velocities were about 10 µm/s for ice and 0.01 µm/s for ground, with daily variations of 5–10 µm/s and 0.025–0.04 µm/s, respectively. Microseismic and temperature variations correlate at 0.78, with a lag of about 4 hours because ice warms up slowly.
Fig. 5. Slip in two NW ice cracks during ice quake of 22 March 2007. See text for explanation.
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Fig. 6. Background microseismicity (B(t)) in ice (a) and bedrock (b) as a response to diurnal air temperature variations (c).
The background microseismicity has a relatively regular pattern with peaks about midnight and troughs in the daytime between 10 a.m. and noon. However, the highest peaks of 50–100 µm/s occurred about the noon, as well as most ice shocks. The 3–4 h lag of ice failure response to temperature changes corresponds to the lag of microseismicity with respect to the temperature peaks and troughs. According to data from the Listvyanka onshore seismic station, the maximum of background seismicity is associated with thermal expansion of rocks in the daytime and the minimum is at the nighttime cooling. These measurement data can be explained as follows. The nighttime air cooling, with regard to the lag of the ice temperature response, causes ice shortening and failure by inception or growth of numerous small cracks accompanied by a moderate increase in microseismicity. The daytime warming pattern is different. Numerous small pulses give way in the morning to scarce but higher-energy ones corresponding to compressive ice fracture as a consequence of gradual warming and thermal expansion. Thereby the extensional cracks that appeared before close and freeze together but the shears become more active, and large cracks grow as reverse and over- and under-thrust faults. In the noon time (between 12–16 h), when air temperature rises to warm the ice up gradually, its compressive destruction is the strongest being attendant with large ice shocks and seismicity peaks much above the nighttime maximum of decompressive failure. The daytime low of background seismicity may result from transition to a different deformation regime, namely, to compression which shows up in a different way, especially prominent before and after ice quakes.
Note that the ice quake of 22 March 2007 nucleated during air temperature rise on the night of 21 March. The shock was preceded by about 9 hours of seismic quiescence (Fig. 6, a). Therefore, air temperature change is the main cause of deformation in the Baikal ice. The seismic effects associated with this deformation are similar to nucleation of earthquakes or rock bursts. Ice quake energy. The energy of the ice shock of 22 March 2007 was estimated using seismic records of attenuation patterns in three small sub-ice test explosions. For this, we applied the linear theory of seismic attenuation to calculate the change of maximum slip velocity with distance. According to theoretical calculations and a large amount of experimental data, this change follows the law R U = U0 R 0
−n
at a distance of 8–10 radiuses R0 from the source, where n = 1.4–1.8. This relationship is commonly used for describing measured distance-dependent changes of amplitudes and periods of seismic waves. The maximum slip velocity of 5.3 mm/s was in the vertical component during the peak phase. See Table 1 for data on small explosions in which the maximum values were recorded in the OSP-2M channel. We performed 45– 105 g test explosions with a metal cartridge in order to decipher the seismic records collected during ice deformation and correlate them with those of crustal failure.
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Table 1 Measured slip velocity and acceleration in ice during test explosions on 21 March 2007 Explosion no.
Time, h
Q, g
R, m
1
14–15
45
115
2
3
14–25
105
17–45
45
155
140
Kinematic parameter
N–S
E–W
Z
Shear, mm
0.2
0.4
0.16
0.5
Velocity, mm/s
1.3
8.3
3.1
9.0
Acceleration, mm/s2
114
587
289
664.1
Shear, mm
0.2
0.6
0.45
0.8
Velocity, mm/s
1.9
5.6
3.7
7.0
Acceleration, mm/s2
136
432
349
571.8
Shear, mm
0.29
0.32
0.26
0.5
Velocity, mm/s
2.1
3.6
5.3
6.7
239
191
386
492.5
Acceleration, mm/s
The observed distance-dependent attenuation of seismic signals is commonly analyzed through slip velocities. For ice this relationship is (Sadovskii, 1999) R U = 5382 1/3 Q
Absolute value
−1.5
,
where U is the slip velocity in cm/s, R is the distance in m, and Q is the explosive weight in kg. From the ice shock record we found out that the maximum slip acceleration was 2340 mm/s2 in the negative phase along the N–S component, with the respective slip velocity of 87.5 mm/s and a displacement of 4.6 mm. The inferred estimate of shaking intensity at the ice rupture site corresponds to IV–V on the MSK-64 scale. Proceeding from the shortest distance of seismographs from the crack (40 m), the ice shock energy presented as equivalent distance-dependent energy attenuation for the known amount of explosive is 104–105 J or 0.3–0.8 in magnitude (Aptikaev, 1969). This is the first energy estimate for an ice quake on Lake Baikal and is rather approximate because the calculation results in this case largely depend on the accuracy of measured distance. At long distances of the order of 500 m the energy is 107 J. The obtained estimates show that ice quakes are comparable in energy with small tectonic earthquakes or medium-size rock bursts. Their energy can depend on ice thickness and rheology,
2
and on the size and structure of specific large compression cracks. Ice quake as a model of an earthquake. Seismic records were used to estimate the rate of ice quake-induced rupture which may cover many kilometers. Seismic arrivals from the ice quake recorded by spaced seismographs showed offshore propagation of motion along the Listvyanka crack at 0.1– 0.2 km/s. Note for comparison that motion in large tectonic earthquakes can propagate at 1–3 km/s. A digital seismic station Baikal-12 located 40 m far from an active large crack recorded an ice shock on 22 March 2007, at 12:44. That was the first event on Baikal recorded at a short distance from the crack, which allowed some important calculations of shaking intensity with standard methods of engineering seismology. See Figure 7 for slip acceleration patterns recorded by three OSP-2M channels. Structural measurements showed that the zone of the Listvyanka crack consisted of several large en-echelon rightlateral cracks, with almost zero rupture rates on their ends and fastest rupture, apparently, in their middle. Therefore, ice quake-induced rupture of large cracks is discontinuous and spanning some time interval. We examined a 50–60 m wide neck between two cracks in the Listvyanka zone after the quake and saw that it became cut into fragments by long 0.3–0.4 m wide fissures striking in the N–S and W–E
Fig. 7. Piece of ice seismic record during ice quake of 22 March 2007 at 12:44.
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Fig. 8. Background microseismicity change and three ice shocks. Piece of record during ice quake of 22 March 2007. Figures at peaks mark local time of events.
directions. Note that the next crack was much less strongly reactivated judging by the amount of displacement. This pattern has bearing on rupture kinematics of large cracks in the Baikal ice useful as a model of rupture in a tectonic earthquake. Another piece of record (Fig. 8) shows background seismicity variations at the observation site during the quake of 22.03.2007. See that increase in background seismicity corresponding to nucleation of the culminating failure began about three hours before the first shock of 12:44. The main shock was followed by two similar events in more distant fragments of the crack zone, 96 and then 76 minutes later, which may be aftershocks or independent events in a long major crack. Then the seismic background decreased gradually to the minimum by 18 h. The whole event lasted about 8–9 hours and produced typical sets of thrusts and shear joints as in seismogenic reverse-oblique faulting. The reported seismogenic ice rupture resembles that in large and great events (Psakhie et al., 2007; Ruzhich, 2007).
Conclusions The knowledge gained through the reported strain and seismoacoustic measurements confirms that there are several agents responsible for ice deformation on Lake Baikal. They are, namely, air temperature variations, winds, sub-ice currents, and air pressure change, ordered according to their significance. This combination resembles slow deformation in the crust and thus makes mechanic processes in the ice a good model of seismotectonic processes. Ice quakes share many features of similarity with tectonic earthquakes in nucleation and failure dynamics. Seismic processes we observed in deformation of ice fields released 104–105 J (M = 0.3–0.8) of energy and had a shaking intensity of III–IV MSK-64 comparable with small earthquakes or medium-size rock bursts. The deformation regime and generation of elastic waves in initiation and growth of ice cracks turned out to be similar to those in faults or in areas of pending rock bursts. The experimentally observed similarity opens up new avenues in tectonic and seismotectonic modeling using of the exceptional natural ice laboratory of Lake Baikal, which has
important scientific and practical implications for prediction of earthquakes and rock bursts. We greatly appreciate the overall support of the Baikal ice project from Academician N.L. Dobretsov. The study was carried out as part of Integration Projects 26 and 27 and Multidisciplinary Project 6.18 of the Siberian Branch of the Russian Academy of Sciences.
References Aptikaev, F.F., 1969. Seismic Motion in Explosions and Earthquakes [in Russian]. Nauka, Moscow. Dimaki, A.V., Astafurov, S.V., Shilko, E.V., Ruzhich, V.V., Psakhie, S.G., 2006. Sdvig-3M digital stations for recording fault motion, in: Lithospheric Geodynamics and Stress. Proc. Intern. Conf. [in Russian]. Institute of Mining SB RAS, Novosibirsk, pp. 193–199. Dobretsov, N.L., Psakhie, S.G., Ruzhich, V.V., Popov, V.L., Shilko, E.V., Granin, N.G., Timofeev, V.Yu., Astafurov, S.V., Dimaki, A.V., Starchevich, Ya., 2007. Ice cover of Lake Baikal as a model for studying tectonic processes in the Earth’s crust. Doklady Earth Sci., 413 (2), 155. Dortman, N.B. (Ed.), 1976. Physical properties of Rocks and Mineral Deposits. A Geophysical Handbook [in Russian]. Nedra, Moscow. Duchkov, A.D., Istomin, V.E., Kazantsev, S.A., 2007. The temperature regime of ice in Lake Baikal and related stress in ice plates. Fizicheskaya Mezomekhanika 10, 87–92. Gakkel’, Ya.Ya., 1959. Natural “Modeling” Using Ice Crust [in Russian]. Volume 90, Izd. VGO, Moscow, 21–32. Galasii, G.I. (Ed.), 1993. Atlas of Lake Baikal [in Russian], FSGKR, Moscow. Makshtas, A.P., Marchenko, A.V., Shutilin, S.V., 2007. Modeling hummocking processes in marine ice, in: Modern Problems of Rapid and Catastrophic Processes [in Russian]. Nauka, Moscow, pp. 134–150. Mellor, M., 1979. Mechanical properties of polycrystalline ice, in: Tryde, P. (Ed.), Physics and Mechanics of Ice. IUTAM Symposium, Copenhagen, pp. 217–245. Mel’nikov, N.V. (Ed.), 1981. Distribution and Correlation of Physical Parameters of Rocks. A Handbook [in Russian]. Nedra, Moscow. Pomytkin, B.A., 1960. On lake level change in Baikal. Transactions, Baikal Limnological Station [in Russian]. Izd. AN SSSR, Moscow, pp. 242–263. Psakhie, S.G., Ruzhich, V.V., Granin, N.G., Chenskii, A.G., Chernyhk, E.N., Shilko, E.V., Dimaki, A.V., Astafurov, S.V., 2005a. Investigating the evolution of seismic processes in the crust using a model of the ice cover of Lake Baikal, in: Current Deformation and Natural Hazard in Central Asia [in Russian]. IZK, Irkutsk, Issue 3, pp. 225–229. Psakhie, S.G., Ruzhich, V.V., Shilko, E.V., Popov, V.L., Astafurov, S.V., 2007. A new way to manage displacements in zones of active faults. Tribol. Intern. 40 (6), 995–1003.
V.V. Ruzhich et al. / Russian Geology and Geophysics 50 (2009) 214–221 Psakhie, S.G., Ruzhich, V.V., Shilko, E.V., Popov, V.L., Dimaki, A.V., Astafurov, S.V., Lopatin, V.V., 2005b. Influence of the state of interfaces on the character of local displacements in fault-block and interfacial media. Technical Physics Letters 31 (8), 712–715. Ravens, T.M., Kocsis, O., Wuest, A., Granin, N.G., 2000. Small-scale turbulence and vertical mixing in Lake Baikal. Limnol. Oceanogr. 45 (1), 159–173. Ruzhich, V.V., 2007. Experimental methods in seismogeology, in: Problems of Modern Seismogeology and Geodynamics of Eastern and Central Asia. Proc. All-Russian Workshop [in Russian]. Book 2, IZK, Irkutsk, pp. 118– 124. Ruzhich, V.V., Semenov, P.M., Alakshin, A.M., Melnikova, V.I., Chipizubov, A.V., Arzhannikov S.G., Smekalin, O.P., Emel’yanova, I.A., Dem’yanovich, M.G., Radziminovich, N.A., 2002. The 25 February 1999 South Baikal earthquake: seismotectonics and regional geodynamics. Geologiya i Geofizika (Russian Geology and Geophysics) 43 (5), 470–492 (448–458).
221
Ruzhich, V.V., Truskov, V.A., Chernykh, E.N., Smekalin, O.P., 1999. Neotectonic movements in fault zones of the Baikal region and their origin mechanisms. Geologiya i Geofizika (Russian Geology and Geophysics) 40 (3), 360–372 (356–368). Sadovskii, M.A., 1999. Geophysics and Physics of Explosion. Selected Works [in Russian]. Nauka, Moscow. Sokol’nikov, V.M., 1960. Vertical and horizontal displacement and strain in the ice cover of Lake Baikal. Transactions, Baikal Limnological Station [in Russian]. Izd. Akad. Nauk SSSR, Moscow, Vol. 1, pp. 291–350. Sokol’nikov, V.M., 1970. Deep currents and water exchange in Lake Baikal. in: Currents and Water Diffusion in Lake Baikal. Heat and Water Loss before Freezing-up [in Russian]. Nauka, Leningrad, pp. 67–74. Zhdanov, A.A., Granin, N.G., Shimaraev, M.N., 2001. The generation mechanism of under-ice currents in Lake Baikal. Doklady Earth Sci. 377A (3), 329. Zhdanov, A.A., Granin, N.G., Shimaraev, M.N., 2002. Sub-ice currents in Lake Baikal, from new experimental data. Geografiya i Prirodnye Resursy, No. 1, 79–83.
Editorial responsibility: V.S. Seleznev
Deformation and seismic effects in the ice cover of Lake Baikal V.V. Ruzhich a, *, S.G. Psakhie b, E.N. Chernykh a, S.A. Bornyakov a, N.G. Granin c a
b
Institute of the Earth’s Crust, Siberian Branch of the RAS, 128 ul. Lermontova, Irkutsk, 664033, Russia Institute of Strength Physics and Materials Science, Siberian Branch of the RAS, 2/1 prosp. Akademichesky, Tomsk, 634021, Russia c Limnological Institute, Siberian Branch of the RAS, 3 ul. Ulan-Batorskaya, Irkutsk, 664033, Russia Received 25 March 2008; accepted 27 August 2008
Abstract The mechanics of the ice cover of Lake Baikal has been studied through monitoring of its deformation and seismic effects and full-size uniaxial compression and shear tests in 2005–2007. We measured the shear strength of ice specimens and large in situ blocks (σ = 0.2−1.9 MPa) and investigated it as a function of air temperature and ice structure. Deformation was analyzed in terms of various natural controls, such as air temperature and pressure, wind, sub-ice currents, and local earthquakes. Precise strain measurements along ice cracks were used to explore the strain behavior of ice, including the cases of dynamic failure (ice shocks). Measurements by seismic station Baikal-12 were used to monitor 4 5 diurnal background microseismicity variations and to record an ice quake with its magnitude (M = 0.3–0.8; E = 10 –10 J) comparable to a medium-size rock burst or a small earthquake. Ice quakes were studied in terms of their nucleation, dynamics, and aftereffects, as well as the strain and seismic responses of the ice, using sub-ice explosions in the latter case. The natural conditions of deformation in the elastoviscoplastic Baikal ice are similar to lithospheric processes and thus can be employed in tectonophysical modeling with scientific and practical implications for hazard mitigation. © 2009, IGM, Siberian Branch of the RAS. Published by Elsevier B.V. All rights reserved. Keywords: ice cover; major crack; shear strength; deformation; ice shock; dynamic loading; seismicity; physical modeling
Introduction Since long ago processes in the Baikal ice, which is mechanically similar to faulted crust, have attracted interest of geologists (Dobretsov et al., 2007; Gakkel’, 1959; Psakhie et al., 2005a, b; Sokol’nikov, 1960; Zhdanov et al., 2001, 2002). We studied winter ice mechanics in Lake Baikal mostly at a test site near the Angara source in the vicinity of Listvyanka Village (Fig. 1) as part of an SB RAS integration project run through 2005–2007 jointly by people from six academic institutions of Irkutsk, Tomsk, Novosibirsk, and Ulan-Ude. In this paper we report some project results with the main focus on natural dynamic failure of ice which can generate seismic motion in a way similar in many points to tectonic earthquakes. Thus we investigated ice shocks (ice quakes) as a model of seismotectonic and seismic processes, as well as strain patterns and physics of freshwater ice. Of special importance were the results obtained in full-size explosion tests in ice. At all stages of the project, we used advanced technical facilities. The reported data appear to be
* Corresponding author. E-mail address: [email protected] (V.V. Ruzhich)
novel and stimulating, especially because there is almost no experience in studying details of fracture and seismic effects in freshwater ice of Lake Baikal.
Mechanic properties of Baikal ice No knowledge of the Baikal ice rheology being available, we designed portable instruments allowing stress measurements under uniform uniaxial compression at a rate of the order of 10 µm/s. Measurements were run in wintertime on ice specimens and monolith blocks subject to compression and shear. Loading was applied to 5 × 5 × 5 cm ice cubes in situ with a special jack at air temperatures from +1 °C to –11 °C and yielded a compression strength of σs = 0.2− 1.7 MPa. The measured strength of ice samples depended on air temperature and on ice structure: It was σs = 0.2 MPa at 0 °C and 1.7 MPa at –11 °C and decreased in the presence of cracks, air and particle inclusions, or layering in the ice. Shear strength of ice cylinders 5 cm in height and 5 cm in diameter was σs = 0.2−0.4 MPa (at T = –6...–8 °C). Other experiments implied loading, with powerful jacks, applied to in situ ice blocks cut on three sides to 22 cm high, 64 cm
1068-7971/$ - see front matter D 2009, IGM, Siberian Branch of the RAS. Published by Elsevier B.V. All rights reserved. doi:10.1016/j.rgg.2008.08.005
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Fig. 2. Strain diagram for monolith ice prior to shear failure. Loading was by 50-ton jack set up in a niche cut in ice. Shear strength was 0.89 MPa.
Fig. 1. Location map of Listvyanka major ice crack near Angara source. 1. Observation sites. 2. Sites of data collection. 3. Major ice crack.
wide, and 70 cm long; the fourth side and the base of the blocks were left intact from below and from the face. The strength obtained in these experiments was σs = 0.6−0.89 MPa. See Figure 2 for the strain diagram of an ice block deforming to final failure as the ice reached the limit of its shear strength. We discovered that the ice was brittle and vulnerable to shocks like some rocks prone to brittle failure under dynamic loading. Compressive loading of some monolith samples free from cracks, air bubbles, or other structural defects, at temperatures from –5 °C to –11 °C, led to spontaneous violent fracture with shattering to flour dust. In natural conditions this mechanic property of the Baikal ice shows up as failure events we called ice quakes (ice shocks). Another essential property we discovered in tests of crushing cylindrical ice samples in plastic coating (i.e., constrained failure) was viscoplastic flow at T = –3 °C. The samples kept integrity under strain of no less than 47%, and the ice samples increased their shear strength in constrained conditions to as high as 1.9 MPa, though their structure was disturbed by air bubbles and cracks. We recorded acoustic signals generated by formation of numerous small cracks in the process of deformation. Note that crack growth in constrained samples becomes blocked as the cracks are compressed and healed by recrystallization to prevent the samples from failure even under high strain. According to Mellor (1979), ice possesses an unusual property of decreasing its shear strength to zero at growing hydrostatic pressure at normal temperatures, and increasing at growing strain rate. These physical and mechanic properties of constrained behavior of ice are worth special investigation as they can cause a notable effect on its rheology at different loads, strain rates, and temperatures.
Our observations show that the ice rheology changes with depth, from top to bottom where, at the ice-water interface, it has a temperature about +0.3 °C (Duchkov et al., 2007) and shows viscoplastic behavior. Thus, taken in a first approximation, the Baikal ice has a two-layer structure, with the brittle upper layer about the air temperature at T = –20...–35 °C. In general, ice mechanics can vary in a broad range at different loading conditions from elasticity to nonlinear viscosity being subject to creep or brittle failure. Nevertheless, with all this knowledge, ice and, especially, the ice of Lake Baikal, remains poorly understood in terms of physics and mechanics. Causes of ice deformation in the Baikal basin The causes of ice deformation and fracture have important dynamic implications. The Baikal freshwater ice forms in special weather and structural conditions, and has its physics and mechanics slightly different from those in marine ice (Galazii, 1993; Makshtas et al., 2007; Pomytkin, 1960; Psakhie et al., 2005a; Sokol’nikov, 1960, 1970; Zhdanov et al., 2001, 2002). The ice cover of Baikal is unique due to an extremely large size of the lake, which is 636 km long and 25–79 km wide, with the 31,500 km2 total area of the natural ice laboratory. Ice is subject to weather changes (air temperature, wind, solar radiation, etc.) controlled by westerly cyclones and by the Siberian anticyclone. From the beginning of our project, we understood that the exceptionally transparent and clear Baikal ice differed physically and mechanically from marine ice (Dortman, 1976; Mel’nikov, 1981). Ice is known to have high thermal expansion (9.1×10−5 at 0 °C), i.e., with an average lake width of 50 km, ice can expand for up to 45.5 m in daytime in a single event and contract correspondingly in the night. Thus, the effect of diurnal air temperature variations on the ice constrained by the shore, where it is in many places tightly welded with bottom sediments and bedrock, is to produce destructive periodic compression-to-extension change. As the surface contracts in the nighttime, ice cracks open, and let water penetrate therein making patches of ice-free water in major crack zones. Rhythmic expansion of the ice surface for many
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days causes its hummocking, especially intense along thermal sutures or major cracks which are boundaries of ice plates. The ice cover expansion is the most effective at highest temperature contrasts between night cooling and day warming of air and ice. Weather changes in winter and spring (January–April) produce the respective long-term temperature trends, which, in turn, favor ice divergence and convergence. Cumulative seasonal compressive strain in major cracks of different generations can reach 6–10 m, as estimated in onshore Listvyanka Village from the height of ice ridges and amount of underthrust of ice plates. The total winter expansion across the lake inferred as the first approximation from ice crack patterns in aerial images is 50–100 m. This estimate is consistent with data by Sokol’nikov (1960) that an ice plate can underthrust to 20 m the greatest, and there may be several such places. Expansion along the lake is obviously still greater, as it is clearly seen in images from the area of Ol’khon Island and Svyatoi Nos Peninsula with large fields of ice ridges and arc-shaped large cracks oriented across the lake. Dynamic failure of ice, with prominent seismic effects, is the most active after February 10, and especially in March, which coincides with the season of strongest temperature change. This is the season when the Baikal ice is the most monolithic, stiff, and elastic. It can accumulate maximum elastic energy on sunny days at temperature contrasts of 5–10 °C or more by warming and related thermal expansion which sharply increases the ice surface area. Other deformation controls may come from sub-ice convective currents or ruggedness of the ice top and bottom because of hummocking, which increases the efficiency of wind action and deformation. The deformation activity increases in late March–early April as snow, which damps sharp temperature contrasts, becomes blown off. The snow-free places exposed to insolation and heating are subject to geostrophic flow (Zhdanov et al., 2001, 2002). Sub-ice currents persist throughout the water column during the ice period (Sokol’nikov, 1970, Ravens et al., 2000, Zhdanov et al., 2001, 2002), though their rates decrease to no more than 10 cm/s after freeze up, as ice screens the wind effect. They keep their pattern, with the general cyclonic circulation and separate circulation cells in the lake subbasins. Outflow into the Angara gives rise to an anticyclone circulation cell in the Listvenichnyi Bay, both in the ice and ice-free seasons. At the boundary of this cell there is a zone of convergence located near the Listvenichnyi Cape just where a major crack commonly appears running from Berezovyi Cape to Tankhoi Village. The system of sub-ice currents might cause some influence on formation of major cracks but a more significant reason is that the capes are stress concentrators, and almost all transverse cracks begin at capes. The coefficients of friction of wind and sub-ice currents against ice are 0.0012 and 0.0055, respectively (Makshtas et al., 2007). Baikal wind in winter storm weather can reach a speed of 20–30 m/s or more while sub-ice currents are never faster than 10 cm/s. Thus, the action of wind is naturally about two orders of magnitude greater than that of sub-ice currents.
Another agent is winter water level fall of tens of centimeters, which, by outflow through the Angara, is smooth and evident in ice subsidence near the shore. Air pressure changes may likewise contribute to vertical deformation of ice and cause transversal lake level variations of 10 cm or more (Pomytkin, 1960). In addition to those factors, the Baikal ice and its seismic background can experience occasional, sometimes very strong, impact from tectonic earthquakes often generated in the basement of the Baikal basin, most frequently at crustal depths between 5 and 20 km. The M = 6.0 South Baikal earthquake of 25 February 1999 located 10 km south of Listvyanka Village (Ruzhich et al., 2002) was one such event. The ice rupture it caused was the strongest in the vicinity of Tankhoi Village where cracks reached 1 m wide and ran for kilometers along the shore. There is also historical evidence of ice displacement and fracture during the Tsagan earthquake of 11 December 1862 when seismic shaking intensity reached X on the MSK-64 scale. Strain measurements in seismogenic ice cracks Like processes in crustal material, deformation of ice caused by various factors (see above) is associated with energy release in waves of different frequencies, including the seismoacoustic band relevant to our study. To investigate details of ice deformation, we measured motions in cracks of different scales using a specially designed precise instrument Sdvig-3M (Dimaki et al., 2006). Below we report some measurement results from large ice cracks. These cracks, called fissures in the case of Baikal, opened to 10–20 cm on night cooling and closed to the same or slightly smaller magnitude by warming-driven compression in daytime, the respective diurnal rate being 1–2 cm/h. Dynamic failure, such as tectonic earthquakes or ice shocks, are associated with slip rates of a much greater order of magnitude. An ice quake, with violent fracture of ice accompanied by rustle, boom, and shaking, resembles a small tectonic earthquake for any eye-witness, especially for one standing close to a large crack. See Figure 3 for the location of strain meters (a side and a central ones) at the site of the Listvyanka large crack during an ice quake. On witnessing the shock, we estimated the amount of horizontal displacement of the ice plate reaching 1.5–2.0 m in 3–5 s, i.e., the motion was quite fast. Note for comparison that fault slip rates in large earthquakes are as high as 5–10 m/s. Large ice shocks at a site occur every 4–15 days, thus making no more than ten most strongly felt events, which can amount to hundreds of events over the whole lake recorded by onshore seismic stations. See Fig. 4 for the strain curve of motion on the Listvyanka crack in the ice shock we witnessed. Rupture began from Listvenichnyi Cape, which confirms our hypothesis on the near-shore location of the concentrators of destructive stress in the ice. Figure 5 shows displacement in two smaller cracks (1, 2) located 60 and 90 m far from the large crack, during the same event. Compression grew rapidly prior to the shock and was followed by a pulse of opening and then by the return motion.
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Fig. 3. Ice shock and hummocking on 22 March 2007 at 12:44, local time. See strain meters of which one is sinking under broken ice.
The recovery to the initial opening, which was incomplete for crack 2, occurred slowly for several hours. A physical interpretation of this pattern may be that the response of ice fractured over a large area has two phases of rapid elastic and slow viscoplastic motion. Note that we observed the same response to dynamic loading in other similar ice events, as well as in crustal surface rupture caused by explosions or earthquakes (Psakhie et al., 2005b; Ruzhich et al., 1999). The above ice quake synopsis highlights essential details of failure in kilometers-long cracks under compression, which is very similar to seismic slip in large earthquakes. Seismic effects in the ice cover of Lake Baikal So far ice seismicity in the Baikal rift has been very poorly explored. In our project, seismic signals were recorded by a
Fig. 4. Strain dynamics in Listvyanka zone of major crack during ice quake of 22 March 2007. See that the sides of a subsidiary crack set in motion before the central part of the crack zone. Amount of slip in central crack subject to major rupture was much greater than in the subsidiary crack.
digital seismic station Baikal-12 with SK-1P and OSP-2M channels, for velocity and acceleration data, respectively. The sensors were placed in an ice trench and oriented in the vertical (Z) and horizontal (N–S, E–W) directions. Background microseismicity. We monitored daily background microseismisity in the Baikal ice as a record of its brittle failure and obtained the following results. The seismograms of Fig. 6 correspond to microseismic background variations in ice and bedrock at the site of the Listvyanka onshore seismic station correlated with variations of air temperature between 14 and 23 March 2007, with daily mean slip velocities sampled at every 2 minutes. The mean slip velocities were about 10 µm/s for ice and 0.01 µm/s for ground, with daily variations of 5–10 µm/s and 0.025–0.04 µm/s, respectively. Microseismic and temperature variations correlate at 0.78, with a lag of about 4 hours because ice warms up slowly.
Fig. 5. Slip in two NW ice cracks during ice quake of 22 March 2007. See text for explanation.
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Fig. 6. Background microseismicity (B(t)) in ice (a) and bedrock (b) as a response to diurnal air temperature variations (c).
The background microseismicity has a relatively regular pattern with peaks about midnight and troughs in the daytime between 10 a.m. and noon. However, the highest peaks of 50–100 µm/s occurred about the noon, as well as most ice shocks. The 3–4 h lag of ice failure response to temperature changes corresponds to the lag of microseismicity with respect to the temperature peaks and troughs. According to data from the Listvyanka onshore seismic station, the maximum of background seismicity is associated with thermal expansion of rocks in the daytime and the minimum is at the nighttime cooling. These measurement data can be explained as follows. The nighttime air cooling, with regard to the lag of the ice temperature response, causes ice shortening and failure by inception or growth of numerous small cracks accompanied by a moderate increase in microseismicity. The daytime warming pattern is different. Numerous small pulses give way in the morning to scarce but higher-energy ones corresponding to compressive ice fracture as a consequence of gradual warming and thermal expansion. Thereby the extensional cracks that appeared before close and freeze together but the shears become more active, and large cracks grow as reverse and over- and under-thrust faults. In the noon time (between 12–16 h), when air temperature rises to warm the ice up gradually, its compressive destruction is the strongest being attendant with large ice shocks and seismicity peaks much above the nighttime maximum of decompressive failure. The daytime low of background seismicity may result from transition to a different deformation regime, namely, to compression which shows up in a different way, especially prominent before and after ice quakes.
Note that the ice quake of 22 March 2007 nucleated during air temperature rise on the night of 21 March. The shock was preceded by about 9 hours of seismic quiescence (Fig. 6, a). Therefore, air temperature change is the main cause of deformation in the Baikal ice. The seismic effects associated with this deformation are similar to nucleation of earthquakes or rock bursts. Ice quake energy. The energy of the ice shock of 22 March 2007 was estimated using seismic records of attenuation patterns in three small sub-ice test explosions. For this, we applied the linear theory of seismic attenuation to calculate the change of maximum slip velocity with distance. According to theoretical calculations and a large amount of experimental data, this change follows the law R U = U0 R 0
−n
at a distance of 8–10 radiuses R0 from the source, where n = 1.4–1.8. This relationship is commonly used for describing measured distance-dependent changes of amplitudes and periods of seismic waves. The maximum slip velocity of 5.3 mm/s was in the vertical component during the peak phase. See Table 1 for data on small explosions in which the maximum values were recorded in the OSP-2M channel. We performed 45– 105 g test explosions with a metal cartridge in order to decipher the seismic records collected during ice deformation and correlate them with those of crustal failure.
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Table 1 Measured slip velocity and acceleration in ice during test explosions on 21 March 2007 Explosion no.
Time, h
Q, g
R, m
1
14–15
45
115
2
3
14–25
105
17–45
45
155
140
Kinematic parameter
N–S
E–W
Z
Shear, mm
0.2
0.4
0.16
0.5
Velocity, mm/s
1.3
8.3
3.1
9.0
Acceleration, mm/s2
114
587
289
664.1
Shear, mm
0.2
0.6
0.45
0.8
Velocity, mm/s
1.9
5.6
3.7
7.0
Acceleration, mm/s2
136
432
349
571.8
Shear, mm
0.29
0.32
0.26
0.5
Velocity, mm/s
2.1
3.6
5.3
6.7
239
191
386
492.5
Acceleration, mm/s
The observed distance-dependent attenuation of seismic signals is commonly analyzed through slip velocities. For ice this relationship is (Sadovskii, 1999) R U = 5382 1/3 Q
Absolute value
−1.5
,
where U is the slip velocity in cm/s, R is the distance in m, and Q is the explosive weight in kg. From the ice shock record we found out that the maximum slip acceleration was 2340 mm/s2 in the negative phase along the N–S component, with the respective slip velocity of 87.5 mm/s and a displacement of 4.6 mm. The inferred estimate of shaking intensity at the ice rupture site corresponds to IV–V on the MSK-64 scale. Proceeding from the shortest distance of seismographs from the crack (40 m), the ice shock energy presented as equivalent distance-dependent energy attenuation for the known amount of explosive is 104–105 J or 0.3–0.8 in magnitude (Aptikaev, 1969). This is the first energy estimate for an ice quake on Lake Baikal and is rather approximate because the calculation results in this case largely depend on the accuracy of measured distance. At long distances of the order of 500 m the energy is 107 J. The obtained estimates show that ice quakes are comparable in energy with small tectonic earthquakes or medium-size rock bursts. Their energy can depend on ice thickness and rheology,
2
and on the size and structure of specific large compression cracks. Ice quake as a model of an earthquake. Seismic records were used to estimate the rate of ice quake-induced rupture which may cover many kilometers. Seismic arrivals from the ice quake recorded by spaced seismographs showed offshore propagation of motion along the Listvyanka crack at 0.1– 0.2 km/s. Note for comparison that motion in large tectonic earthquakes can propagate at 1–3 km/s. A digital seismic station Baikal-12 located 40 m far from an active large crack recorded an ice shock on 22 March 2007, at 12:44. That was the first event on Baikal recorded at a short distance from the crack, which allowed some important calculations of shaking intensity with standard methods of engineering seismology. See Figure 7 for slip acceleration patterns recorded by three OSP-2M channels. Structural measurements showed that the zone of the Listvyanka crack consisted of several large en-echelon rightlateral cracks, with almost zero rupture rates on their ends and fastest rupture, apparently, in their middle. Therefore, ice quake-induced rupture of large cracks is discontinuous and spanning some time interval. We examined a 50–60 m wide neck between two cracks in the Listvyanka zone after the quake and saw that it became cut into fragments by long 0.3–0.4 m wide fissures striking in the N–S and W–E
Fig. 7. Piece of ice seismic record during ice quake of 22 March 2007 at 12:44.
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Fig. 8. Background microseismicity change and three ice shocks. Piece of record during ice quake of 22 March 2007. Figures at peaks mark local time of events.
directions. Note that the next crack was much less strongly reactivated judging by the amount of displacement. This pattern has bearing on rupture kinematics of large cracks in the Baikal ice useful as a model of rupture in a tectonic earthquake. Another piece of record (Fig. 8) shows background seismicity variations at the observation site during the quake of 22.03.2007. See that increase in background seismicity corresponding to nucleation of the culminating failure began about three hours before the first shock of 12:44. The main shock was followed by two similar events in more distant fragments of the crack zone, 96 and then 76 minutes later, which may be aftershocks or independent events in a long major crack. Then the seismic background decreased gradually to the minimum by 18 h. The whole event lasted about 8–9 hours and produced typical sets of thrusts and shear joints as in seismogenic reverse-oblique faulting. The reported seismogenic ice rupture resembles that in large and great events (Psakhie et al., 2007; Ruzhich, 2007).
Conclusions The knowledge gained through the reported strain and seismoacoustic measurements confirms that there are several agents responsible for ice deformation on Lake Baikal. They are, namely, air temperature variations, winds, sub-ice currents, and air pressure change, ordered according to their significance. This combination resembles slow deformation in the crust and thus makes mechanic processes in the ice a good model of seismotectonic processes. Ice quakes share many features of similarity with tectonic earthquakes in nucleation and failure dynamics. Seismic processes we observed in deformation of ice fields released 104–105 J (M = 0.3–0.8) of energy and had a shaking intensity of III–IV MSK-64 comparable with small earthquakes or medium-size rock bursts. The deformation regime and generation of elastic waves in initiation and growth of ice cracks turned out to be similar to those in faults or in areas of pending rock bursts. The experimentally observed similarity opens up new avenues in tectonic and seismotectonic modeling using of the exceptional natural ice laboratory of Lake Baikal, which has
important scientific and practical implications for prediction of earthquakes and rock bursts. We greatly appreciate the overall support of the Baikal ice project from Academician N.L. Dobretsov. The study was carried out as part of Integration Projects 26 and 27 and Multidisciplinary Project 6.18 of the Siberian Branch of the Russian Academy of Sciences.
References Aptikaev, F.F., 1969. Seismic Motion in Explosions and Earthquakes [in Russian]. Nauka, Moscow. Dimaki, A.V., Astafurov, S.V., Shilko, E.V., Ruzhich, V.V., Psakhie, S.G., 2006. Sdvig-3M digital stations for recording fault motion, in: Lithospheric Geodynamics and Stress. Proc. Intern. Conf. [in Russian]. Institute of Mining SB RAS, Novosibirsk, pp. 193–199. Dobretsov, N.L., Psakhie, S.G., Ruzhich, V.V., Popov, V.L., Shilko, E.V., Granin, N.G., Timofeev, V.Yu., Astafurov, S.V., Dimaki, A.V., Starchevich, Ya., 2007. Ice cover of Lake Baikal as a model for studying tectonic processes in the Earth’s crust. Doklady Earth Sci., 413 (2), 155. Dortman, N.B. (Ed.), 1976. Physical properties of Rocks and Mineral Deposits. A Geophysical Handbook [in Russian]. Nedra, Moscow. Duchkov, A.D., Istomin, V.E., Kazantsev, S.A., 2007. The temperature regime of ice in Lake Baikal and related stress in ice plates. Fizicheskaya Mezomekhanika 10, 87–92. Gakkel’, Ya.Ya., 1959. Natural “Modeling” Using Ice Crust [in Russian]. Volume 90, Izd. VGO, Moscow, 21–32. Galasii, G.I. (Ed.), 1993. Atlas of Lake Baikal [in Russian], FSGKR, Moscow. Makshtas, A.P., Marchenko, A.V., Shutilin, S.V., 2007. Modeling hummocking processes in marine ice, in: Modern Problems of Rapid and Catastrophic Processes [in Russian]. Nauka, Moscow, pp. 134–150. Mellor, M., 1979. Mechanical properties of polycrystalline ice, in: Tryde, P. (Ed.), Physics and Mechanics of Ice. IUTAM Symposium, Copenhagen, pp. 217–245. Mel’nikov, N.V. (Ed.), 1981. Distribution and Correlation of Physical Parameters of Rocks. A Handbook [in Russian]. Nedra, Moscow. Pomytkin, B.A., 1960. On lake level change in Baikal. Transactions, Baikal Limnological Station [in Russian]. Izd. AN SSSR, Moscow, pp. 242–263. Psakhie, S.G., Ruzhich, V.V., Granin, N.G., Chenskii, A.G., Chernyhk, E.N., Shilko, E.V., Dimaki, A.V., Astafurov, S.V., 2005a. Investigating the evolution of seismic processes in the crust using a model of the ice cover of Lake Baikal, in: Current Deformation and Natural Hazard in Central Asia [in Russian]. IZK, Irkutsk, Issue 3, pp. 225–229. Psakhie, S.G., Ruzhich, V.V., Shilko, E.V., Popov, V.L., Astafurov, S.V., 2007. A new way to manage displacements in zones of active faults. Tribol. Intern. 40 (6), 995–1003.
V.V. Ruzhich et al. / Russian Geology and Geophysics 50 (2009) 214–221 Psakhie, S.G., Ruzhich, V.V., Shilko, E.V., Popov, V.L., Dimaki, A.V., Astafurov, S.V., Lopatin, V.V., 2005b. Influence of the state of interfaces on the character of local displacements in fault-block and interfacial media. Technical Physics Letters 31 (8), 712–715. Ravens, T.M., Kocsis, O., Wuest, A., Granin, N.G., 2000. Small-scale turbulence and vertical mixing in Lake Baikal. Limnol. Oceanogr. 45 (1), 159–173. Ruzhich, V.V., 2007. Experimental methods in seismogeology, in: Problems of Modern Seismogeology and Geodynamics of Eastern and Central Asia. Proc. All-Russian Workshop [in Russian]. Book 2, IZK, Irkutsk, pp. 118– 124. Ruzhich, V.V., Semenov, P.M., Alakshin, A.M., Melnikova, V.I., Chipizubov, A.V., Arzhannikov S.G., Smekalin, O.P., Emel’yanova, I.A., Dem’yanovich, M.G., Radziminovich, N.A., 2002. The 25 February 1999 South Baikal earthquake: seismotectonics and regional geodynamics. Geologiya i Geofizika (Russian Geology and Geophysics) 43 (5), 470–492 (448–458).
221
Ruzhich, V.V., Truskov, V.A., Chernykh, E.N., Smekalin, O.P., 1999. Neotectonic movements in fault zones of the Baikal region and their origin mechanisms. Geologiya i Geofizika (Russian Geology and Geophysics) 40 (3), 360–372 (356–368). Sadovskii, M.A., 1999. Geophysics and Physics of Explosion. Selected Works [in Russian]. Nauka, Moscow. Sokol’nikov, V.M., 1960. Vertical and horizontal displacement and strain in the ice cover of Lake Baikal. Transactions, Baikal Limnological Station [in Russian]. Izd. Akad. Nauk SSSR, Moscow, Vol. 1, pp. 291–350. Sokol’nikov, V.M., 1970. Deep currents and water exchange in Lake Baikal. in: Currents and Water Diffusion in Lake Baikal. Heat and Water Loss before Freezing-up [in Russian]. Nauka, Leningrad, pp. 67–74. Zhdanov, A.A., Granin, N.G., Shimaraev, M.N., 2001. The generation mechanism of under-ice currents in Lake Baikal. Doklady Earth Sci. 377A (3), 329. Zhdanov, A.A., Granin, N.G., Shimaraev, M.N., 2002. Sub-ice currents in Lake Baikal, from new experimental data. Geografiya i Prirodnye Resursy, No. 1, 79–83.
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