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Prehistoric earthquakes and fault scarps in the Barguzin fault zone (Baikal Rift system)
Russian Geology and Geophysics 48 (2007) 581–592 www.elsevier.com/locate/rgg
Prehistoric earthquakes and fault scarps in the Barguzin fault zone (Baikal Rift system) A.V. Chipizubov *, S.G. Arzhannikov, R.M. Semenov, O.P. Smekalin Institute of the Earth’s Crust, Siberian Branch of the RAS, 128 ul. Lermontova, Irkutsk, 664033, Russia Received 20 January 2006
Abstract We studied fault scarps produced by prehistoric earthquakes in the Barguzin fault zone and estimated the ages and the magnitudes of the scarp-forming events in each scarp segment from their structure, morphology, and scarp parameters. Morphological and structural data reveal two to four surface-rupturing events with displacement up to 5–9.5 m, and two events showed 14C ages of 4.5 and 9 kyr. The area of the Barguzin Fault may have experienced six 7.5 ≤ M < 8.0 earthquakes and two M ≥ 8.0 (8.0–8.2) events for the past 10–12 kyr. © 2007, IGM, Siberian Branch of the RAS. Published by Elsevier B.V. All rights reserved. Keywords: Paleoseismology; multiple fault scarps; dating fault scarps; magnitudes of prehistoric earthquakes; Barguzin fault
Introduction The ~200 km long Barguzin fault delineates the Barguzin rift basin in the northwest and consists of three segments: Ulyun in the southwest, Sarankhur in the center, and Shamanka in the northeast (Fig. 1). Over thirty years ago seismogeological studies revealed several fault scarps produced by prehistoric earthquakes in the zone of the Barguzin fault: Nesterikha, Sarankhur, Shamanka, Alla, Sukhitke, and Dyren (Solonenko, 1968, 1981). There is much controversy about the magnitudes of the scarp-forming events (Khromovskikh et al., 1993; Khromovskikh, 1995; Solonenko, 1981), and the catalog by Kondorskaya and Shebalin (1977) misses those events at all. The parameters of the fault scarps, namely an offset up to 10–20 m and a rupture length of 1.2–12 km (Solonenko, 1981), indicate that these are most likely the fragments of one or several (no more than three according to the number of fault segments) multiple-event fault scarps (Chipizubov, 1998; Strom, 1993, 1998). Or, they might have been produced by a single great earthquake. The large uncertainty required paleoseismology with advanced methods. We trenched the fault scarps in 1998 and 2000 and published the results of 1998 for the Ulyun and partly Sarankhur fault segments (Chipizubov et al., 2000) where we inferred two-event scarps with heights up to 12–18 m. Similar
* Corresponding author. E-mail address: [email protected] (A.V. Chipizubov)
fault scarps were examined in 2000 within the part of the Sarankhur segment that remained unexplored in 1998 and in a great part of the Shamanka segment. In this paper we report more details of fault scarp studies in the Barguzin fault which aimed at distinguishing segments in the scarp profiles and estimating the magnitudes of the rupture events. The main problem is to distinguish single- and multiple-event fault scarps, to estimate the number of scarp-forming events in the latter case, and to identify coeval scarp segments.
Single- and multiple-event segments of fault scarps Distinguishing single- and multiple-event segments of fault scarps produced by past earthquakes is a difficult but feasible task (Chipizubov, 1998). The problem can be solved through high-density trenching and sampling for radiometric dating, which is, however, quite costly. A cheaper way is to use common structural and morphological methods. First, detailed mapping reveals steps with different slope angles in multiple fault scarps. Beveled scarp profiles can be produced by several surface-rupturing events of different ages which may depart from the main rupture line (Chipizubov et al., 2000). Second, multiple fault scarps can be more easily distinguished in places where the rupture truncates river and creek terraces. There one can expect that the number and heights of terraces controlled by earthquake-induced erosion-base changes record the scarp-forming events of different ages. This approach has been applied lately in many regions worldwide.
. 1068-7971/$ - see front matter D 2007, IGM, Siberian Branch of the RAS. Published by Elsevier B.V. All rights reserved doi:10.1016/j.rgg.2007.06.004
Fig. 1. Fault scarps produced by prehistoric earthquakes in Barguzin fault (southern flank of Shamanka segment). 1 — bedrock; 2 — alluvium and colluvium in alluvial fans and glacial talus; 3, 4 — glacial deposits of terminal moraines of Last Glacial (3) and older glaciations (4); 5 — Barguzin fault, preserved fragments (a) of earthquake rupture (h is scarp height) and their poorly expressed segments (b) in insets; 6 — terrace benches; 7 — topography contour lines; 8 — divides; 9 — highway; 10 — insets and their numbers.
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For instance, the morphology of terraces gives evidence for four to six surface-rupturing events on the Furnace Creek fault and three or more events on the Death Valley fault in California (Brogan et al., 1991). The number of terraces on the upthrown wall of the Helan fault in China (epicentral area of the M = 8 earthquake of 1739) records three prehistoric earthquakes (Deng and Liao, 1996). Third, events of different ages can be inferred from scarp profiles (knickpoints, breaks in slope, etc.). The older, most often upper parts of scarps are flatter and the steeper segments at the scarp toe or free face commonly correspond to the youngest event. Methods for estimating the number of scarpforming events from scarp profiles were suggested over two decades ago (e.g., Vittori, 1991; Wallace, 1977) and analysis of scarp morphology has become of broad use in practical applications (Deng and Liao, 1996; Sanders and Slemmons, 1996). The relative ages of fault scarps not constrained by radiometric dating can be inferred from scarp parameters, especially, slope angles (Hanks and Schwortz, 1987; Lastochkin, 1981; Wallace, 1977), using a linear diffusion equation with constant coefficients (Andrews and Hanks, 1985; Bucknam and Andersen, 1979). Relative ages are often sufficient to reconstruct the scarp history. Scarps with equal heights and slope angles in rocks of proximal lithologies are presumably of the same age. Using detailed morphological profiling, together with 14C age ties, one can divide fault scarps into geomorphically expressed segments of different ages or, on the contrary, interpret several separate steps as a result of a single event (Chipizubov, 1998). In the latter case, the interpretation should be checked against rupture length/displacement ratios in known historic events to avoid confusion with multiple scarps produced by several past earthquakes that closely followed one another. This problem is insoluble even with high-density sampling for 14C: The ±25−100 yr uncertainty range of the radiometric age method can span several events within 50–200 years that extend the rupture length, as it was often observed through the last century.
Detailed structure, morphology and parameters of fault scarps As we already noted, 12–18 m high two-event fault scarps are traceable with short gaps in all three segments of the Barguzin fault. The fault scarps are 49 km long in the Ulyun segment, 69 km in the Sarankhur segment, with 4–7 km long gaps where they cannot be detected, and 53 km in the Shamanka segment. The fault scarps are the most prominent against broad flat surfaces of river and creek terraces in alluvial fans and in trains of glacial and slope wash talus at the range base. The lower the slope angles and the greater the lengths of these gently sloping surfaces, the more prominent the fault scarps. The scarp heights are only slightly greater than the fault offset. This morphology was observed in the Nesterikha, Sarankhur,
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Alla, and Sukhitke fault scarps, but no scarps were found in the fanheads of the Ulyun, Ulyunchikan (Ulyukchikan), Kurumkan, Sakhuli, and many other rivers in the course of earlier studies. In the cases when earthquake rupture follows the toes of large neotectonic escarpments, it is detectable as break in slope which can be identified in aerial photographs and by morphological analysis or is sometimes visible on eye examination. Geomorphically expressed fault scarps are traceable almost continuously along the Ulyun fault segment where only 1.2 km long Nesterikha scarp was known before (Fig. 2, inset 1). They are deciphered in aerial photographs and are clearly seen from the nearby highway. The ubiquitous 12–18 m high scarps (7–12 m of vertical offset) were formed by two surface-rupturing events. Two faulting events are inferred in the Nesterikha scarp from displaced terraces and from break in slope from 23–27° to 32–35° (Fig. 2, inset la; profile 1). An apparently single-event 15–16 m high fault scarp on the smooth surface of a talus train between Nesterikha and Ulyun villages turned out to be produced by two events (Fig. 2, inset 1b). The steeper (35°) young scarps (dark lines in inset 1b) are also traceable above and below the shallower (25°) older scarps (light lines). This morphology and elevation differences of the multiple scarps are inconsistent with their origin by erosion. Two surface-rupturing events are detectable from terrace-like surfaces preserved along the sides of narrow and deep ravines that cut the upthrown wall of the ruptured fault. Fault scarps in the Ulyun segment are 2–3 m high only on its ends and locally over its main extent, whereas the offset in each of the two events was most often about 4–5 m. Scarps south of the Nesterikha River are much lower. They are short and poorly pronounced 2–3 m high scarplets standing at 20° at the outlets of the Bannyi and Gremyachiy creeks, not necessarily produced by earthquakes. Earlier paleoseismology in the Sarankhur fault segment revealed only the 1.5 km long Sarankhur fault scarp (Fig. 2, inset 2b), and we revealed 4–8.5 to 18–21 m high and higher scarps in the fanheads of the Ukhshikhan, Epishka, Khonkhino, Upper Kurumkan (river and creek), Tekuna, Tokino, Sukhoi, and Sakhuli rivers and other nameless rivers and creeks, as well as in talus trains between the fans. The scarps stretch over a distance of 69 km with 4–7 km gaps (Fig. 1, inset 2; Fig. 3). Two-event scarps were found in the Epishka, Upper Sarankhur, Khonkhino, Sukhoi, and Sakhuli river valleys and in the valleys of some nameless creeks. Inset 2a in Fig. 2 sketches the normal and terrace scarps, with even four rupturing events recorded in terraces. The terraces were produced by an ephemeral stream incised into the upthrown fault wall. Two younger and two older events occurred 20 m apart. The slope angle of the scarp produced by the youngest event (33–39°, Profile 1 in inset 2) is comparable to that associated with the two younger events together (Profile 2 in inset 2); the scarp formed by the antepenultimate earthquake slopes 24–28° (Profile 3 in inset 2) and the scarp of the oldest event merged with that of the three younger earthquakes. All of them grade into the major scarp that stands at 30°. The
Fig. 2. Fault scarps produced by prehistoric earthquakes in Ulyun and southern Sarankhur segments of Barguzin fault. 1 — alluvium and colluvium in alluvial fans and river and creek valleys; 2 — glacial talus; 3 — swampy areas. Numerals denote offset (first number), scarp height (second number in parentheses), and slope angle in degrees. Other symbols as in Fig. 1.
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Fig. 3. Fault scarps produced by prehistoric earthquakes in northern Sarankhur segment of Barguzin fault. Symbols as in Fig. 1.
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scarp heights and offsets are shown on profiles in inset 2a of Fig. 2. Two events in the fanhead of the Upper Sarankhur River are detectable from terrace profiles and appear as a slope break in the left valley side (Fig. 2, inset 2b). The lower portion of the lower-angle scarp (24–27°) is traceable 20–25 m away from the upper portion of the steeper (30–35°) scarp produced by the youngest event (Profiles 1 and 2 in inset 2b). Two merged scarps of different ages make up a composite slope which stands at 36–38° (Profile 1 in inset 2b), though the scarp might be cut by lateral erosion which does not change the slope angle, but rocks accumulated at the toe decrease the apparent scarp height. A terrace and a break in slope of a 14 m high scarp that truncates the valley of the Khonkhino creek give evidence for two surface-rupturing events, each with an offset of 5 m; the slope angles of the younger and older scarps are 32 to 37° and 20 to 26°, respectively (Fig. 3.3; Profiles 9–11). Two events that produced a 17 m high fault scarp sloping 33–35° (traceable in contour lines in the 1:25,000 topographic map) which truncates diagonally the lower broad part of the Sakhuli valley are inferred only from displaced terraces. River incision associated with the older event cuts a terrace which was displaced by the younger event responsible for the modern incision. The amount of offset is 5 m on each of the two faults (Fig. 3, Profiles 5, 6). The segment of the Barguzin fault between the Tekuna and Sukhoi creeks 25– 30 m downstream of the two-event 36–39° steep scarps (Profile 8) includes 20–26° slopes with an offset of 3–4 to 5 m. Short (a few hundreds of meters) and relatively shallow scarps are traceable in the northeastern Sarankhur segment near Shamanka Village (Fig. 3, inset 4c). They are as high as 6 m and stand at 26–28° at the foot of a slope built of glacial deposits but are less than 2 m and only 18–20° in deposits of creek valleys (see 3 and 4 in Fig. 3). The site was trenched (K-5) and sampled for 14C analysis. Widespread older and younger fault scarps standing at 33–35° (age group 1) and 23–26° (age group 3) show comparable offsets of 3–6.5 and 4–6.5 m, respectively, in places where measurements were possible. Few offset measurements in scarps of age groups 2 and 4 gave 5.5 and 9.5 m, respectively (profiles in inset 2a of Fig. 2). The maximum displacement was measured in the Tun valley where the left lateral bar of a Last Glacial terminal moraine was uplifted for 40–50 m during at least two events with 20–25 m offset in each. The valley cut into the moraine was displaced 7.5 m by two younger events. We found no strike-slip motion reported in (Solonenko, 1968, 1981), neither in the Sarankhur fault scarp nor elsewhere. In our view, displacement of minor landforms on the upthrown fault wall relative to those in the downthrown wall (Solonenko, 1968) spaced at 15 m horizontally and 8.5 m vertically (Profile 1 in inset 2 of Fig. 1) cannot be reliable evidence for a 2–3 m left-lateral strike slip. The 2–3 m horizontal displacement can be accounted for by mere bending of ravines and ridges, observed in many small landforms of
the locality. In these conditions, only a horizontal displacement of 10–15 m or more can be reliably identified. Earlier studies within the Shamanka fault segment revealed the Shamanka, Alla, Sukhitke and Dyren fault scarps. Solonenko (1968) hypothesized that the Alla and Sukhitke scarps were produced by a single prehistoric earthquake. The most prominent multiple scarp up to 18–25 m high standing 35–38° is traceable for a length of 4 km in the talus train on both sides of the Alla valley. Northeast of the Alla valley, the scarp toe is delineated by a 30 m wide and 6 m deep graben. The offset of the two-event scarp is mainly 10–12 m. The amount of offset in each event reaches 5–6 m in the place where the scarp breaks in two steps of different ages. Three events can be identified on the left side of the Sukhotka (Sukhitka) River which are recorded in different heights of fault scarps at terraces of different ages and in a notable slope break of a two-event scarp profile. The first event (offset of 3 m) produced the oldest terrace, the second event displaced the latter and formed another terrace, which was 8 m displaced by the youngest event. The last earthquake was followed by gradual deep (50–60 m) incision. The vertical offset in each faulting event is shown on profiles 10–12 in Fig. 4 obtained using data courtesy of V.V. Nikolaev and M.G. Dem’yanovich. The fault scarp produced by the ultimate event currently stands at 28–30°, that formed by the penultimate event has a slope angle of 20–25°, and the scarp associated with the oldest event slopes 16°. Well-defined but shorter fault scarps occur at creek outlets that enter the Barguzin basin between the Endykhyk and Sheberso rivers (Fig. 4, inset b) and between the Alla and Dolsy Rivers (Fig. 1, inset 6b). No such prominent scarps are found southeast of the Alla River, where the basin junction with the flanking mountains is smoother, though seismic scarps are deciphered in aerial photographs (Fig. 1, inset 6). The Shamanka fault scarp described in (Solonenko, 1968) is located in the southwestern end of the fault segment. The scarp-forming events threw down the terminal moraine deposits of the early Last Glacial or of the older glaciation in the valleys of the Talinga and Iokhan rivers and several nameless creeks. The amount of horizontal displacement reaches 50 m in the terminal moraine of the Talinga glacier and 300 m in that of the Alla glacier. The latter was produced by an older (maximum) glaciation and is buried under alluvial deposits emerging only in the lateral moraine bars. No prominent fault scarps are found northeast of the Olos River. Thus, the fault scarps detected in the Barguzin fault zone are of at least four age groups. The youngest scarps (age group 1) and those of age group 3 with slope angles of 35° (32–37°) and 25° (24–26°), respectively, are traceable in all three fault segments with the longer scarps of age group 3 are found in their least active flanks. The scarps of age groups 2 and 4 occur discontinuously within the Sarankhur and Shamanka fault segments. These in the Sarankhur segment (Epishka and Lower Sarankhur interfluve) are two-event scarps of age groups 1–2 and 3–4. The scarps of groups 2 and 4 slope 30 and 20°, respectively. Scarps with these slope angles were encountered near Kurumkan Village (Tekun and
Fig. 4. Fault scarps produced by prehistoric earthquakes in northern Shamanka segment of Barguzin fault. Symbols as in Fig. 1.
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Sukhoi interfluve). The Sukhitke fault scarp and scarps stretching as far as the Olos River may be likewise of groups 2 and 4. Some fragments of the youngest fault scarp, which are hard to discriminate in two-event scarps, are actually of age group 2 or have slightly different ages than the other fragments of age group 1.
Dating fault scarps Samples for 14C dating were collected in five trenches excavated in the Barguzin fault, and only one trench bore no evident signature of failure and buried soils. Fault scarps in the Ulyun fault segment were trenched at two sites: one (K-3) in the lower one third of a 15 m high scarp produced by two events with 5–6 m vertical offset in each, and the other (K-2) in a low poorly expressed single-event scarp (Fig. 2, inset 1b). The log of trench K-3 (Fig. 5) shows a prominent classical colluvial wedge consisting of talus and blocks of soil with a laboratory 14C age of 4430±25 years BP (sample SOAN-3999) underlain by greenish-gray clay. The colluvium is overlain by a 3525±25 yr soil (sample SOAN-4000). These almost black fat soils apparently formed in limnic-bog environments. The younger soil is overlain by colluvium which, in turn, overlies a soil with a laboratory age about 3140±25 years (SORAN4001). The other thin colluvial wedge not necessarily was produced by an event in that very place but may result from gravity sliding of a weakly matted scarp triggered by a relatively distant earthquake. The calendar age of the youngest authentic event is bracketed between 4191 and 4626 years (Table 1). That event is apparently recorded in the 35° scarp step. The evidence of the older event responsible for the scarp that currently stands 24° is likely below the depth we excavated. The youngest event, possibly an earthquake-induced ground failure, occurred between 3601 and 3702 years BP (Table 1). Sampling in trench 2 (Fig. 5) dug in the decaying scarp end revealed a prehistoric event with the age between 4423 and 4652 years. This age is comparable to the calendar age obtained for the ultimate surface-rupturing event in trench 3 (4191–4626 years). Two trenches were logged within the Sarankhur segment of the Barguzin fault. One (K-1) exposed the toe of a scarp in glacial deposits in the Upper Sarankhur and Khara-Gorosun interfluve (Fig. 2, inset 2). Buried soil with a laboratory age of 3530±40 years (sample SORAN-3996) bears evidence of the youngest event about 3585 years ago (Table 1). This date is compatible with the calendar age of the ground failure event in trench K-3 (3601–3702 years), though it might rather represent the 4.5 kyr event. Another trench (K-5) was excavated in one relatively short and poorly sculptured scarp (a 2 m high scarp sloping 20°) in deposits of an ephemeral stream at the site where the Sarankhur fault segment pinches out near Shamanka Village (Fig. 3, inset b). The log shows a colluvial wedge that includes lumps of a buried soil with laboratory ages bracketed between 8220±90 (sample GIN11324) and 8950±120 (sample GIN-11325), and a block of
intercalated pebble, sand, and loam of nearly vertical bedding. The surface of the wedge traces the profile of the scarp as it was at its origin (Fig. 5, profile along K-5). The wedge is overlain by loam and loamy sand layers with different humus contents. The oldest layer with the greatest humus content has a laboratory age of 8180±60 years BP (sample GIN-11323). The calendar age of the ultimate prehistoric earthquake, and the only one revealed at that site, is bounded by 9101 and 9383 or 9250 and 9295 years (Table 1). The scarps produced by that event currently slope at angles from 20 to 28°. The steepest scarps are those up to 6 m high in glacial talus where the natural slope is much greater than in alluvial pebbles. In the common conditions at the site of trench K-3 (cobbles and boulders deposited by ephemeral torrents), scarps up to 6 m high with the age of 9 kyr can slope 24–26°. The scarps with slope angles of 25° (24–26°) and 35° (32–37°), the most frequently encountered in the Barguzin fault, formed 4.5 and 9 kyr ago, respectively. Neither trench exposed the slip planes of seismogenic faults, and the slip geometry remains unclear. It could be normal slip common of the area or reverse motion, as reverse planes were encountered in the neighbor North Baikal fault (Chipizubov et al., 2003).
Prehistoric earthquakes and estimates of their magnitudes Paleoseismic studies in the zone of the Barguzin fault show that the area was repeatedly shocked by large earthquakes for the past 10 kyr. The ultimate event, smaller than the others (M = 7) occurred 3600 years ago in the Lower Sarankhur– Khara-Gorosun interfluve. Signature of failure at 4.5 and 9 kyr is found in all three segments (Ulyun, Sarankhur, Shamanka) of the fault. The older fault scarps are traceable over long distances and involve even the least active flanks of the fault segments. Earthquake magnitudes (M) can be estimated using their ratios to the parameters of normal scarps (Chipizubov, 1998) M = 0.96 lg D + 6.7; S = 0.42, n = 22, r = 0.78,
(1)
M = 1.86 lg L + 4.16; S = 0.40, n = 20, r = 0.84,
(2)
lg L = 0.39M – 1.24; S = 0.18, n = 20, r = 0.84,
(3)
or equations for normal faults under extension (Lunina, 2001) lg L = 0.37 lg D + 1.38; S = 0.26, n = 14, r = 0.78
(4)
and equations for slip of any geometry with Dmax > 3 suggested in (Strom, 1998): M = 0.689 ln D + 6.35; r2 = 0.489,
(5)
where L is rupture length in km, D is cumulative displacement in m, S is standard error, n is the number of correlated pairs, and r is the correlation coefficient. Cumulative displacement (D) is an especially reliable parameter to estimate paleoearthquake magnitudes (Chipizubov, 1998). In our case, the length of coeval fault scarps
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Fig. 5. Trench logs. 1 — modern topsoil; 2 — buried soils with different humus contents (darker color corresponds to higher humus content); 3 — talus (large symbol) and fine debris (small symbol) with loamy fill; 4 — sand (large symbol) and loamy sand (small symbol); 5 — pebbles; 6 — greenish-brown loam; 7 — light-gray fine debris; 8 — sampling sites for radiocarbon, with laboratory sample numbers and 14C ages; 9 — large bedrock blocks.
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Table 1 Radiocarbon ages of buried soils from Barguzin Fault Trench
Sample
Laboratory age, years
Type
Thickness, cm
MRC 14C, years BP
Corrected age
Calendar age upper bound
mean
lower bound
3585
3733
3894
K-1
SOAN-3996
3530 ± 40
S
5
110
3420 ± 40
K-2
SOAN-3997
3970 ± 35
S
10
220
4190 ± 35
4423
4702
4971
K-2
SOAN-3998
3725 ± 45
S
10
220
3945 ± 45
4052
4411
4652
K-3
SOAN-3999
4430 ± 25
S
5
110
4320 ± 25
4626
4866
5115
K-3
SOAN-4000
3525 ± 25
S
5
110
3635 ± 25
3955
4064
4191
K-3
SOAN-4000
3525 ± 25
S
5
110
3415 ± 25
3601
3725
3864
K-3
SOAN-4001
3140 ± 75
S
5
110
3250 ± 75
3256
3471
3702
K-5
GIN-11325
8950 ± 120
S
20
440
8510 ± 120
9285
9502
9657
K-5
GIN-11324
8220 ± 90
S
5
110
8110 ± 90
8825
9045
9225
K-5
GIN-11324
8220 ± 90
S
5
110
8330 ± 90
9101
9295
9444
K-5
GIN-11323
8180 ± 60
S
5
110
8290 ± 60
9088
9250
9383
Note. Samples were analyzed at Institute of Geology, Novosibirsk (SOAN), Geological Institute, Moscow (GIN). S stands for organic soil, loamy sand, loam, etc.; MRC is the mean residence correction time, calculated as half of CAS, where CAS is the carbon age span of sample or time span of soil formation, calculated as the sample thickness times the carbon age trend, assumed to be 4.4 mm/yr (Chipizubov, 2000). Calendar age is calculated using the CALIB v. 3.03 program (Stuvier and Reimer, 1993), using the bidecadal atmospheric calibration data set, CAS = 300 yrs, and lab error multiplier = 2. Ages in bold are bound dates of earthquakes. * Thickness of organic soil in samples. ** Given is calendar-corrected age for upper soil contacts, which equals laboratory age minus MRC and provides an estimate of the radiocarbon age of the soil surface when buried. For lower soil contacts, it equals laboratory age plus MRC and estimates the inception of soil formation.
with an offset of 6.5 m can reach 33 or 155 km, which according to (2) corresponds to magnitudes between 7.0 and 8.2. The magnitudes estimated from scarp offset (5 m on average, maximum of 6.5 m) using equations (1) and (5) were 7.48±0.42 and 7.7, respectively. Taking into account that surface rupture in the Baikal region appears at earthquake magnitudes M ≥ 7 (Chipizubov, 1998), the most probable magnitude can be assumed to range between 7.5 and 7.9. Or, it most likely, reached 8.0, as can be predicted proceeding from the 3 m offset in the M = 7.6 Muya earthquake of 1957 which would correspond to magnitudes 7.2 and 7.1 according to (1) and (5), respectively. The fault rupture length in an M = 8.0 event is up to 115 km, estimated by (3). Therefore, the earthquakes that occurred 4.5 and 9 kyr ago and left signature in all three segments of the Barguzin fault can have ruptured simultaneously the Ulyun and Sarankhur (114–124 km) or Sarankhur and Shamanka (100 km) segments. If each of the three segments (Ulyun, Sarankhur, and Shamanka) of the Barguzin fault generated two surface-rupturing events at 4.5 and 9.0 kyr, each of the six events could have a magnitude of M = 7.7 (7.5–7.9). A fault scarp with an offset of 6.5 m and a faulting length of 155 km (Ulyun, Sarankhur and partly Shamanka fault segments) can be produced by an M = 8.2 event. On the other hand, an offset of 6.5 m corresponds to a faulting length at least 26 km according to (4). A rupture length of the same order (L up to 20 km) at Dmax = 6.5 was obtained along the envelope in Fig. 5, a from (Strom and Nikonov, 1997). Then, there is a probability that two M = 7.7 earthquakes occurred on the
Ulyun fault segment and three similar events on the Sarankhur segment. It is most likely that the area experienced three M = 7.7 earthquakes about 4.5 kyr, with a recurrence time of hundreds of years; about 9.0 kyr there was one M = 8 event on the Ulyun and Sarankhur fault segments and one M = 7.7 event on the Shamanka segment. The parameters of the fault scarps of different ages within the three segments of the Barguzin fault and the magnitudes of the scarp-forming earthquakes are listed in Table 2. The oldest fault scarps with an offset from 3 to 9.5 m expressed on the surface as relatively short escarpments with slope angles of 20° (age group 4) are found locally in all three fault segments. They can have been produced either by one great event, as indicated by the large offset, or by several events. Altogether the zone of the Barguzin fault can have generated six 7.5 ≤ M < 8.0 and two M ≥ 8 (8.0–8.2) events for the past 10–12 kyr. There were much more 7.0 ≤ M < 7.5 earthquakes but they not always caused surface rupture or, if there was any, the scarps were not large enough to be detected today.
Conclusions The reported paleoseismic trenching of fault scarps in the Barguzin fault zone revealed several surface-rupturing earthquakes of magnitudes from 7 to 8.2 that occurred for the past 10–12 kyr. The M = 7.5–8.0 earthquakes had a mean recurrence interval of 2000 years and fell within a 7500 years long time span between 4.5 and 12 kyr. The largest events
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A.V. Chipizubov et al. / Russian Geology and Geophysics 48 (2007) 581–592 Table 2 Parameters of fault scarps of different ages and estimated earthquake magnitudes Nos
Age, kyr BP (slope)
Fault segment (I–III) and its part (river-river or river-village)
Measured fault scarp
Calculated scarp parameters and earthquake magnitudes
D, m
M1
L, km
M2
M5
M assumed
L3 (L4)
1
3.6
II, Khara-Gorosun
?
?
—
—
—
—
7.0
2
4.5
I, Nesterikha–Ulyun
6.5
45
7.5
7.2
7.6
(48–87)
7.7
3
4.5
II, Tun–Sakhuli
6.5
58
7.5
7.4
7.6
(48–87)
7.8
4
4.5
III, Shamanka R.–Alla
6.5
33
7.5
7.0
7.6
(48–87)
7.6
5
4.5
I–II, Nesterikha–Sakhuli
6.5
114
7.5
8.0
7.6
(48–87)
8.0
6
6–7 (30°)
II, Epishka
5.5
?
7.4
—
7.5
(45–82)
7.5
7
6–7 (30°)
III, Sukhotka–Olos
7
20
7.5
6.6
7.7
(49–89)
7.6
8
9
I, Nesterikha–Ulyunchik
6.5
49
7.5
7.3
7.6
(48–87)
7.8
9
9
II, Tun–Shamanka Village
6.5
69
7.5
7.6
7.6
(48–87)
7.9
10
9
III, Shamanka R.–Alla
6.5
33
7.5
7.0
7.6
(48–87)
7.6
11
9
I–II, Nesterikha–Shamanka
6.5
124
7.5
8.1
7.6
83–126
8.0
12
9
I–III, Nesterikha–Alla
6.5
155
7.5
8.2
7.6
89–130
8.1
13
12 (20°)
I, Gremyachi–Banny
3
5
7.2
5.5
7.1
(37–68)
7.2
14
12 (20°)
II, Epishka
9.5?
?
7.6
—
7.9
(55–100)
—
15
12 (20°)
II, Tekuna–Sukhoi
5
3
7.4
5.0
7.5
(43–79)
7.4
16
12 (20°)
III, Sukhotka–Olos
5
20
7.4
6.6
7.5
(43–79)
7.4
17
12 (20°)
I–III, Gremyachi–Olos
9.5
180
7.6
8.4
7.9
100–155
8.2
Note. Roman numerals denote segments of Barguzin fault: I — Ulyun, II — Sarankhur, III — Shamanka. The subscripts in calculated parameters M and L correspond to numbers of equations; L3 are given as mean values and (L4) as maximum values. Numbers in bold correspond to most probable events.
occurred in the earliest Holocene. One great event about 12 kyr ago can have ruptured more than 180 km of the Barguzin fault. Further paleoseismic studies in the Barguzin fault can provide more reliable evidence of the number, ages, and magnitudes of prehistoric earthquakes. Nevertheless, the refined parameters of fault scarps in the area already indicate that it repeatedly experienced M = 7.6–8.0 shocks. This fact call for a revision to the existing seismic risk zoning under the assumption that the seismic process is constant in time. The study was supported by grants 00-05-64928 and 04-05-64460 from the Russian Foundation for Basic Research.
References Andrews, D.J., Hanks, T.C., 1985. Scarp degraded by linear diffusion: Inverse solution for age. J. Geophys. Res. 90, 10,193–10,208. Brogan, G.E., Kellog, K.S., Slemmons, V., Terhune, C.L., 1991. Late Quaternary faulting along the Death Valley — Furnace Creek fault system, California and Nevada. US Geol. Surv. Bull. Bucknam, R.G., Andersen R.E., 1979. Estimation of fault-scarp ages from a scarp-height — slope-angle relationship. Geology 7, 11–14. Chipizubov, A.V., 1998. Recognition of one-act and coeval paleoseismodislocations and determination of paleoearthquake magnitudes by their scales, Geologiya i Geofizika (Russian Geology and Geophysics), 39 (3), 386–398 (398–410). Chipizubov, A.V., Melnikov, A.I., Stolpovsky, A.V., Baskakov, V.S., 2003. Fault scarps in zones of the North Baikal (southern flank) and Primorsky faults, in: Lithospheric Stress and Strain and Seismicity [in Russian]. Izd. SO RAN, Novosibirsk, 471–474.
Chipizubov, A.V., Semenov, P.M., Arzhannikov, S.G., Smekalin, O.P. 2000. New data on paleoseismic dislocations within the Barguzin fault zone, Baikal rift system, Dokl. Earth Sci. 372, 4, 751. Deng, Q., Liao, Y., 1996. Paleoseismology along the range-front fault of Helan Mountains, North Central China. J. Geophys. Res. 101, 5,873– 5,893. Hanks, T.C, Schwartz, D.P., 1987. Morphologic dating of the pre-1983 fault scarp on the Lost River fault at Doublespring Pass Road, Custer County, Idaho. Bull. Seismol. Soc. Amer. 77 (3), 837–846. Khromovskikh, V.S., 1995. Paleoseismology as applied to mapping earthquake source areas (example of the Baikal Rift system), in: Ulomov, V.I. (Ed.), Seismicity and Seismic Zoning of Northern Eurasia [in Russian], issues 2–3. OIFZ RAN, Moscow, 300–307. Khromovskikh, V.S., Chipizubov, A.V., Kurushin, R. A., Smekalin, O. P., Del’yansky, E.A., 1993. New data on seismic fault scarps in the Baikal rift, in: Ulomov, V.I. (Ed.), Seismicity and Seismic Zoning in Northern Eurasia [in Russian], issue 1. Nauka, Moscow, 256-264. Kondorskaya, N.V., N.V. Shebalin (Eds.), 1977. New Catalog of Large Earthquakes on the USSR Territory [in Russian], Nauka, Moscow. Lastochkin, S.V., 1981. Morphologic dating of fault scarps. Geomorfologiya 1, 91–94. Lunina, O.V., 2001. Lithospheric stress as a control over relations between fault parameters and earthquake magnitudes, Geologiya i Geofizika (Russian Geology and Geophysics) 42 (9), 1389–1398 (1322–1330). Sanders, C.O., Slemmons, D.B., 1996. Geomorphic evidence for Holocene earthquakes in the Olinghouse fault zone, Western Nevada. Bull. Seismol. Soc. Amer. 86 (6), 1784–1792. Solonenko, V.P. (Ed.), 1968. Seismotectonics and Seismicity of the Baikal Rift System [in Russian]. Nauka, Moscow. Solonenko, V.P. (Ed.), 1981. Seismogeology and Detailed Seismic Zoning of the Baikal Region [in Russian]. Nauka, Novosibirsk. Strom, A.L. 1998. Quantitative Parameters of Seismic Rupture and Their Use in Paleoseismology and Engineering Geology [in Russian]. Author’s abstract, PhD Thesis, OIFZ RAN, Moscow.
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Strom, A.L., 1993. Parameters of fault scarps produced by historic and prehistoric earthquakes: A comparative analysis. Izv. RAN, Ser. Fizika Zemli 9, 38–42. Strom, A.L., Nikonov, A.A., 1997. Relationship between parameters of fault scarps and magnitudes of the scarp-forming events. Izv. RAN, Ser. Fizika Zemli 12, 55–67.
Stuiver. M., Reimer, P.J., 1993. Extended 14C data base and revised CALIB 3.0 14C age calibration program. Radiocarbon 35 (2), 215–230. Vittori, E., Labini, S.S., Serva, L., 1991. Palaeoseismology: Review of the state-of-the-art. Tectonophysics, 9–32. Wallace, R.E., 1977. Profiles and ages of young fault scarps, north-central Nevada. Geol. Soc. Amer. Bull. 88 (9), 1267–1281.
Prehistoric earthquakes and fault scarps in the Barguzin fault zone (Baikal Rift system) A.V. Chipizubov *, S.G. Arzhannikov, R.M. Semenov, O.P. Smekalin Institute of the Earth’s Crust, Siberian Branch of the RAS, 128 ul. Lermontova, Irkutsk, 664033, Russia Received 20 January 2006
Abstract We studied fault scarps produced by prehistoric earthquakes in the Barguzin fault zone and estimated the ages and the magnitudes of the scarp-forming events in each scarp segment from their structure, morphology, and scarp parameters. Morphological and structural data reveal two to four surface-rupturing events with displacement up to 5–9.5 m, and two events showed 14C ages of 4.5 and 9 kyr. The area of the Barguzin Fault may have experienced six 7.5 ≤ M < 8.0 earthquakes and two M ≥ 8.0 (8.0–8.2) events for the past 10–12 kyr. © 2007, IGM, Siberian Branch of the RAS. Published by Elsevier B.V. All rights reserved. Keywords: Paleoseismology; multiple fault scarps; dating fault scarps; magnitudes of prehistoric earthquakes; Barguzin fault
Introduction The ~200 km long Barguzin fault delineates the Barguzin rift basin in the northwest and consists of three segments: Ulyun in the southwest, Sarankhur in the center, and Shamanka in the northeast (Fig. 1). Over thirty years ago seismogeological studies revealed several fault scarps produced by prehistoric earthquakes in the zone of the Barguzin fault: Nesterikha, Sarankhur, Shamanka, Alla, Sukhitke, and Dyren (Solonenko, 1968, 1981). There is much controversy about the magnitudes of the scarp-forming events (Khromovskikh et al., 1993; Khromovskikh, 1995; Solonenko, 1981), and the catalog by Kondorskaya and Shebalin (1977) misses those events at all. The parameters of the fault scarps, namely an offset up to 10–20 m and a rupture length of 1.2–12 km (Solonenko, 1981), indicate that these are most likely the fragments of one or several (no more than three according to the number of fault segments) multiple-event fault scarps (Chipizubov, 1998; Strom, 1993, 1998). Or, they might have been produced by a single great earthquake. The large uncertainty required paleoseismology with advanced methods. We trenched the fault scarps in 1998 and 2000 and published the results of 1998 for the Ulyun and partly Sarankhur fault segments (Chipizubov et al., 2000) where we inferred two-event scarps with heights up to 12–18 m. Similar
* Corresponding author. E-mail address: [email protected] (A.V. Chipizubov)
fault scarps were examined in 2000 within the part of the Sarankhur segment that remained unexplored in 1998 and in a great part of the Shamanka segment. In this paper we report more details of fault scarp studies in the Barguzin fault which aimed at distinguishing segments in the scarp profiles and estimating the magnitudes of the rupture events. The main problem is to distinguish single- and multiple-event fault scarps, to estimate the number of scarp-forming events in the latter case, and to identify coeval scarp segments.
Single- and multiple-event segments of fault scarps Distinguishing single- and multiple-event segments of fault scarps produced by past earthquakes is a difficult but feasible task (Chipizubov, 1998). The problem can be solved through high-density trenching and sampling for radiometric dating, which is, however, quite costly. A cheaper way is to use common structural and morphological methods. First, detailed mapping reveals steps with different slope angles in multiple fault scarps. Beveled scarp profiles can be produced by several surface-rupturing events of different ages which may depart from the main rupture line (Chipizubov et al., 2000). Second, multiple fault scarps can be more easily distinguished in places where the rupture truncates river and creek terraces. There one can expect that the number and heights of terraces controlled by earthquake-induced erosion-base changes record the scarp-forming events of different ages. This approach has been applied lately in many regions worldwide.
. 1068-7971/$ - see front matter D 2007, IGM, Siberian Branch of the RAS. Published by Elsevier B.V. All rights reserved doi:10.1016/j.rgg.2007.06.004
Fig. 1. Fault scarps produced by prehistoric earthquakes in Barguzin fault (southern flank of Shamanka segment). 1 — bedrock; 2 — alluvium and colluvium in alluvial fans and glacial talus; 3, 4 — glacial deposits of terminal moraines of Last Glacial (3) and older glaciations (4); 5 — Barguzin fault, preserved fragments (a) of earthquake rupture (h is scarp height) and their poorly expressed segments (b) in insets; 6 — terrace benches; 7 — topography contour lines; 8 — divides; 9 — highway; 10 — insets and their numbers.
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For instance, the morphology of terraces gives evidence for four to six surface-rupturing events on the Furnace Creek fault and three or more events on the Death Valley fault in California (Brogan et al., 1991). The number of terraces on the upthrown wall of the Helan fault in China (epicentral area of the M = 8 earthquake of 1739) records three prehistoric earthquakes (Deng and Liao, 1996). Third, events of different ages can be inferred from scarp profiles (knickpoints, breaks in slope, etc.). The older, most often upper parts of scarps are flatter and the steeper segments at the scarp toe or free face commonly correspond to the youngest event. Methods for estimating the number of scarpforming events from scarp profiles were suggested over two decades ago (e.g., Vittori, 1991; Wallace, 1977) and analysis of scarp morphology has become of broad use in practical applications (Deng and Liao, 1996; Sanders and Slemmons, 1996). The relative ages of fault scarps not constrained by radiometric dating can be inferred from scarp parameters, especially, slope angles (Hanks and Schwortz, 1987; Lastochkin, 1981; Wallace, 1977), using a linear diffusion equation with constant coefficients (Andrews and Hanks, 1985; Bucknam and Andersen, 1979). Relative ages are often sufficient to reconstruct the scarp history. Scarps with equal heights and slope angles in rocks of proximal lithologies are presumably of the same age. Using detailed morphological profiling, together with 14C age ties, one can divide fault scarps into geomorphically expressed segments of different ages or, on the contrary, interpret several separate steps as a result of a single event (Chipizubov, 1998). In the latter case, the interpretation should be checked against rupture length/displacement ratios in known historic events to avoid confusion with multiple scarps produced by several past earthquakes that closely followed one another. This problem is insoluble even with high-density sampling for 14C: The ±25−100 yr uncertainty range of the radiometric age method can span several events within 50–200 years that extend the rupture length, as it was often observed through the last century.
Detailed structure, morphology and parameters of fault scarps As we already noted, 12–18 m high two-event fault scarps are traceable with short gaps in all three segments of the Barguzin fault. The fault scarps are 49 km long in the Ulyun segment, 69 km in the Sarankhur segment, with 4–7 km long gaps where they cannot be detected, and 53 km in the Shamanka segment. The fault scarps are the most prominent against broad flat surfaces of river and creek terraces in alluvial fans and in trains of glacial and slope wash talus at the range base. The lower the slope angles and the greater the lengths of these gently sloping surfaces, the more prominent the fault scarps. The scarp heights are only slightly greater than the fault offset. This morphology was observed in the Nesterikha, Sarankhur,
583
Alla, and Sukhitke fault scarps, but no scarps were found in the fanheads of the Ulyun, Ulyunchikan (Ulyukchikan), Kurumkan, Sakhuli, and many other rivers in the course of earlier studies. In the cases when earthquake rupture follows the toes of large neotectonic escarpments, it is detectable as break in slope which can be identified in aerial photographs and by morphological analysis or is sometimes visible on eye examination. Geomorphically expressed fault scarps are traceable almost continuously along the Ulyun fault segment where only 1.2 km long Nesterikha scarp was known before (Fig. 2, inset 1). They are deciphered in aerial photographs and are clearly seen from the nearby highway. The ubiquitous 12–18 m high scarps (7–12 m of vertical offset) were formed by two surface-rupturing events. Two faulting events are inferred in the Nesterikha scarp from displaced terraces and from break in slope from 23–27° to 32–35° (Fig. 2, inset la; profile 1). An apparently single-event 15–16 m high fault scarp on the smooth surface of a talus train between Nesterikha and Ulyun villages turned out to be produced by two events (Fig. 2, inset 1b). The steeper (35°) young scarps (dark lines in inset 1b) are also traceable above and below the shallower (25°) older scarps (light lines). This morphology and elevation differences of the multiple scarps are inconsistent with their origin by erosion. Two surface-rupturing events are detectable from terrace-like surfaces preserved along the sides of narrow and deep ravines that cut the upthrown wall of the ruptured fault. Fault scarps in the Ulyun segment are 2–3 m high only on its ends and locally over its main extent, whereas the offset in each of the two events was most often about 4–5 m. Scarps south of the Nesterikha River are much lower. They are short and poorly pronounced 2–3 m high scarplets standing at 20° at the outlets of the Bannyi and Gremyachiy creeks, not necessarily produced by earthquakes. Earlier paleoseismology in the Sarankhur fault segment revealed only the 1.5 km long Sarankhur fault scarp (Fig. 2, inset 2b), and we revealed 4–8.5 to 18–21 m high and higher scarps in the fanheads of the Ukhshikhan, Epishka, Khonkhino, Upper Kurumkan (river and creek), Tekuna, Tokino, Sukhoi, and Sakhuli rivers and other nameless rivers and creeks, as well as in talus trains between the fans. The scarps stretch over a distance of 69 km with 4–7 km gaps (Fig. 1, inset 2; Fig. 3). Two-event scarps were found in the Epishka, Upper Sarankhur, Khonkhino, Sukhoi, and Sakhuli river valleys and in the valleys of some nameless creeks. Inset 2a in Fig. 2 sketches the normal and terrace scarps, with even four rupturing events recorded in terraces. The terraces were produced by an ephemeral stream incised into the upthrown fault wall. Two younger and two older events occurred 20 m apart. The slope angle of the scarp produced by the youngest event (33–39°, Profile 1 in inset 2) is comparable to that associated with the two younger events together (Profile 2 in inset 2); the scarp formed by the antepenultimate earthquake slopes 24–28° (Profile 3 in inset 2) and the scarp of the oldest event merged with that of the three younger earthquakes. All of them grade into the major scarp that stands at 30°. The
Fig. 2. Fault scarps produced by prehistoric earthquakes in Ulyun and southern Sarankhur segments of Barguzin fault. 1 — alluvium and colluvium in alluvial fans and river and creek valleys; 2 — glacial talus; 3 — swampy areas. Numerals denote offset (first number), scarp height (second number in parentheses), and slope angle in degrees. Other symbols as in Fig. 1.
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Fig. 3. Fault scarps produced by prehistoric earthquakes in northern Sarankhur segment of Barguzin fault. Symbols as in Fig. 1.
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scarp heights and offsets are shown on profiles in inset 2a of Fig. 2. Two events in the fanhead of the Upper Sarankhur River are detectable from terrace profiles and appear as a slope break in the left valley side (Fig. 2, inset 2b). The lower portion of the lower-angle scarp (24–27°) is traceable 20–25 m away from the upper portion of the steeper (30–35°) scarp produced by the youngest event (Profiles 1 and 2 in inset 2b). Two merged scarps of different ages make up a composite slope which stands at 36–38° (Profile 1 in inset 2b), though the scarp might be cut by lateral erosion which does not change the slope angle, but rocks accumulated at the toe decrease the apparent scarp height. A terrace and a break in slope of a 14 m high scarp that truncates the valley of the Khonkhino creek give evidence for two surface-rupturing events, each with an offset of 5 m; the slope angles of the younger and older scarps are 32 to 37° and 20 to 26°, respectively (Fig. 3.3; Profiles 9–11). Two events that produced a 17 m high fault scarp sloping 33–35° (traceable in contour lines in the 1:25,000 topographic map) which truncates diagonally the lower broad part of the Sakhuli valley are inferred only from displaced terraces. River incision associated with the older event cuts a terrace which was displaced by the younger event responsible for the modern incision. The amount of offset is 5 m on each of the two faults (Fig. 3, Profiles 5, 6). The segment of the Barguzin fault between the Tekuna and Sukhoi creeks 25– 30 m downstream of the two-event 36–39° steep scarps (Profile 8) includes 20–26° slopes with an offset of 3–4 to 5 m. Short (a few hundreds of meters) and relatively shallow scarps are traceable in the northeastern Sarankhur segment near Shamanka Village (Fig. 3, inset 4c). They are as high as 6 m and stand at 26–28° at the foot of a slope built of glacial deposits but are less than 2 m and only 18–20° in deposits of creek valleys (see 3 and 4 in Fig. 3). The site was trenched (K-5) and sampled for 14C analysis. Widespread older and younger fault scarps standing at 33–35° (age group 1) and 23–26° (age group 3) show comparable offsets of 3–6.5 and 4–6.5 m, respectively, in places where measurements were possible. Few offset measurements in scarps of age groups 2 and 4 gave 5.5 and 9.5 m, respectively (profiles in inset 2a of Fig. 2). The maximum displacement was measured in the Tun valley where the left lateral bar of a Last Glacial terminal moraine was uplifted for 40–50 m during at least two events with 20–25 m offset in each. The valley cut into the moraine was displaced 7.5 m by two younger events. We found no strike-slip motion reported in (Solonenko, 1968, 1981), neither in the Sarankhur fault scarp nor elsewhere. In our view, displacement of minor landforms on the upthrown fault wall relative to those in the downthrown wall (Solonenko, 1968) spaced at 15 m horizontally and 8.5 m vertically (Profile 1 in inset 2 of Fig. 1) cannot be reliable evidence for a 2–3 m left-lateral strike slip. The 2–3 m horizontal displacement can be accounted for by mere bending of ravines and ridges, observed in many small landforms of
the locality. In these conditions, only a horizontal displacement of 10–15 m or more can be reliably identified. Earlier studies within the Shamanka fault segment revealed the Shamanka, Alla, Sukhitke and Dyren fault scarps. Solonenko (1968) hypothesized that the Alla and Sukhitke scarps were produced by a single prehistoric earthquake. The most prominent multiple scarp up to 18–25 m high standing 35–38° is traceable for a length of 4 km in the talus train on both sides of the Alla valley. Northeast of the Alla valley, the scarp toe is delineated by a 30 m wide and 6 m deep graben. The offset of the two-event scarp is mainly 10–12 m. The amount of offset in each event reaches 5–6 m in the place where the scarp breaks in two steps of different ages. Three events can be identified on the left side of the Sukhotka (Sukhitka) River which are recorded in different heights of fault scarps at terraces of different ages and in a notable slope break of a two-event scarp profile. The first event (offset of 3 m) produced the oldest terrace, the second event displaced the latter and formed another terrace, which was 8 m displaced by the youngest event. The last earthquake was followed by gradual deep (50–60 m) incision. The vertical offset in each faulting event is shown on profiles 10–12 in Fig. 4 obtained using data courtesy of V.V. Nikolaev and M.G. Dem’yanovich. The fault scarp produced by the ultimate event currently stands at 28–30°, that formed by the penultimate event has a slope angle of 20–25°, and the scarp associated with the oldest event slopes 16°. Well-defined but shorter fault scarps occur at creek outlets that enter the Barguzin basin between the Endykhyk and Sheberso rivers (Fig. 4, inset b) and between the Alla and Dolsy Rivers (Fig. 1, inset 6b). No such prominent scarps are found southeast of the Alla River, where the basin junction with the flanking mountains is smoother, though seismic scarps are deciphered in aerial photographs (Fig. 1, inset 6). The Shamanka fault scarp described in (Solonenko, 1968) is located in the southwestern end of the fault segment. The scarp-forming events threw down the terminal moraine deposits of the early Last Glacial or of the older glaciation in the valleys of the Talinga and Iokhan rivers and several nameless creeks. The amount of horizontal displacement reaches 50 m in the terminal moraine of the Talinga glacier and 300 m in that of the Alla glacier. The latter was produced by an older (maximum) glaciation and is buried under alluvial deposits emerging only in the lateral moraine bars. No prominent fault scarps are found northeast of the Olos River. Thus, the fault scarps detected in the Barguzin fault zone are of at least four age groups. The youngest scarps (age group 1) and those of age group 3 with slope angles of 35° (32–37°) and 25° (24–26°), respectively, are traceable in all three fault segments with the longer scarps of age group 3 are found in their least active flanks. The scarps of age groups 2 and 4 occur discontinuously within the Sarankhur and Shamanka fault segments. These in the Sarankhur segment (Epishka and Lower Sarankhur interfluve) are two-event scarps of age groups 1–2 and 3–4. The scarps of groups 2 and 4 slope 30 and 20°, respectively. Scarps with these slope angles were encountered near Kurumkan Village (Tekun and
Fig. 4. Fault scarps produced by prehistoric earthquakes in northern Shamanka segment of Barguzin fault. Symbols as in Fig. 1.
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Sukhoi interfluve). The Sukhitke fault scarp and scarps stretching as far as the Olos River may be likewise of groups 2 and 4. Some fragments of the youngest fault scarp, which are hard to discriminate in two-event scarps, are actually of age group 2 or have slightly different ages than the other fragments of age group 1.
Dating fault scarps Samples for 14C dating were collected in five trenches excavated in the Barguzin fault, and only one trench bore no evident signature of failure and buried soils. Fault scarps in the Ulyun fault segment were trenched at two sites: one (K-3) in the lower one third of a 15 m high scarp produced by two events with 5–6 m vertical offset in each, and the other (K-2) in a low poorly expressed single-event scarp (Fig. 2, inset 1b). The log of trench K-3 (Fig. 5) shows a prominent classical colluvial wedge consisting of talus and blocks of soil with a laboratory 14C age of 4430±25 years BP (sample SOAN-3999) underlain by greenish-gray clay. The colluvium is overlain by a 3525±25 yr soil (sample SOAN-4000). These almost black fat soils apparently formed in limnic-bog environments. The younger soil is overlain by colluvium which, in turn, overlies a soil with a laboratory age about 3140±25 years (SORAN4001). The other thin colluvial wedge not necessarily was produced by an event in that very place but may result from gravity sliding of a weakly matted scarp triggered by a relatively distant earthquake. The calendar age of the youngest authentic event is bracketed between 4191 and 4626 years (Table 1). That event is apparently recorded in the 35° scarp step. The evidence of the older event responsible for the scarp that currently stands 24° is likely below the depth we excavated. The youngest event, possibly an earthquake-induced ground failure, occurred between 3601 and 3702 years BP (Table 1). Sampling in trench 2 (Fig. 5) dug in the decaying scarp end revealed a prehistoric event with the age between 4423 and 4652 years. This age is comparable to the calendar age obtained for the ultimate surface-rupturing event in trench 3 (4191–4626 years). Two trenches were logged within the Sarankhur segment of the Barguzin fault. One (K-1) exposed the toe of a scarp in glacial deposits in the Upper Sarankhur and Khara-Gorosun interfluve (Fig. 2, inset 2). Buried soil with a laboratory age of 3530±40 years (sample SORAN-3996) bears evidence of the youngest event about 3585 years ago (Table 1). This date is compatible with the calendar age of the ground failure event in trench K-3 (3601–3702 years), though it might rather represent the 4.5 kyr event. Another trench (K-5) was excavated in one relatively short and poorly sculptured scarp (a 2 m high scarp sloping 20°) in deposits of an ephemeral stream at the site where the Sarankhur fault segment pinches out near Shamanka Village (Fig. 3, inset b). The log shows a colluvial wedge that includes lumps of a buried soil with laboratory ages bracketed between 8220±90 (sample GIN11324) and 8950±120 (sample GIN-11325), and a block of
intercalated pebble, sand, and loam of nearly vertical bedding. The surface of the wedge traces the profile of the scarp as it was at its origin (Fig. 5, profile along K-5). The wedge is overlain by loam and loamy sand layers with different humus contents. The oldest layer with the greatest humus content has a laboratory age of 8180±60 years BP (sample GIN-11323). The calendar age of the ultimate prehistoric earthquake, and the only one revealed at that site, is bounded by 9101 and 9383 or 9250 and 9295 years (Table 1). The scarps produced by that event currently slope at angles from 20 to 28°. The steepest scarps are those up to 6 m high in glacial talus where the natural slope is much greater than in alluvial pebbles. In the common conditions at the site of trench K-3 (cobbles and boulders deposited by ephemeral torrents), scarps up to 6 m high with the age of 9 kyr can slope 24–26°. The scarps with slope angles of 25° (24–26°) and 35° (32–37°), the most frequently encountered in the Barguzin fault, formed 4.5 and 9 kyr ago, respectively. Neither trench exposed the slip planes of seismogenic faults, and the slip geometry remains unclear. It could be normal slip common of the area or reverse motion, as reverse planes were encountered in the neighbor North Baikal fault (Chipizubov et al., 2003).
Prehistoric earthquakes and estimates of their magnitudes Paleoseismic studies in the zone of the Barguzin fault show that the area was repeatedly shocked by large earthquakes for the past 10 kyr. The ultimate event, smaller than the others (M = 7) occurred 3600 years ago in the Lower Sarankhur– Khara-Gorosun interfluve. Signature of failure at 4.5 and 9 kyr is found in all three segments (Ulyun, Sarankhur, Shamanka) of the fault. The older fault scarps are traceable over long distances and involve even the least active flanks of the fault segments. Earthquake magnitudes (M) can be estimated using their ratios to the parameters of normal scarps (Chipizubov, 1998) M = 0.96 lg D + 6.7; S = 0.42, n = 22, r = 0.78,
(1)
M = 1.86 lg L + 4.16; S = 0.40, n = 20, r = 0.84,
(2)
lg L = 0.39M – 1.24; S = 0.18, n = 20, r = 0.84,
(3)
or equations for normal faults under extension (Lunina, 2001) lg L = 0.37 lg D + 1.38; S = 0.26, n = 14, r = 0.78
(4)
and equations for slip of any geometry with Dmax > 3 suggested in (Strom, 1998): M = 0.689 ln D + 6.35; r2 = 0.489,
(5)
where L is rupture length in km, D is cumulative displacement in m, S is standard error, n is the number of correlated pairs, and r is the correlation coefficient. Cumulative displacement (D) is an especially reliable parameter to estimate paleoearthquake magnitudes (Chipizubov, 1998). In our case, the length of coeval fault scarps
A.V. Chipizubov et al. / Russian Geology and Geophysics 48 (2007) 581–592
589
Fig. 5. Trench logs. 1 — modern topsoil; 2 — buried soils with different humus contents (darker color corresponds to higher humus content); 3 — talus (large symbol) and fine debris (small symbol) with loamy fill; 4 — sand (large symbol) and loamy sand (small symbol); 5 — pebbles; 6 — greenish-brown loam; 7 — light-gray fine debris; 8 — sampling sites for radiocarbon, with laboratory sample numbers and 14C ages; 9 — large bedrock blocks.
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Table 1 Radiocarbon ages of buried soils from Barguzin Fault Trench
Sample
Laboratory age, years
Type
Thickness, cm
MRC 14C, years BP
Corrected age
Calendar age upper bound
mean
lower bound
3585
3733
3894
K-1
SOAN-3996
3530 ± 40
S
5
110
3420 ± 40
K-2
SOAN-3997
3970 ± 35
S
10
220
4190 ± 35
4423
4702
4971
K-2
SOAN-3998
3725 ± 45
S
10
220
3945 ± 45
4052
4411
4652
K-3
SOAN-3999
4430 ± 25
S
5
110
4320 ± 25
4626
4866
5115
K-3
SOAN-4000
3525 ± 25
S
5
110
3635 ± 25
3955
4064
4191
K-3
SOAN-4000
3525 ± 25
S
5
110
3415 ± 25
3601
3725
3864
K-3
SOAN-4001
3140 ± 75
S
5
110
3250 ± 75
3256
3471
3702
K-5
GIN-11325
8950 ± 120
S
20
440
8510 ± 120
9285
9502
9657
K-5
GIN-11324
8220 ± 90
S
5
110
8110 ± 90
8825
9045
9225
K-5
GIN-11324
8220 ± 90
S
5
110
8330 ± 90
9101
9295
9444
K-5
GIN-11323
8180 ± 60
S
5
110
8290 ± 60
9088
9250
9383
Note. Samples were analyzed at Institute of Geology, Novosibirsk (SOAN), Geological Institute, Moscow (GIN). S stands for organic soil, loamy sand, loam, etc.; MRC is the mean residence correction time, calculated as half of CAS, where CAS is the carbon age span of sample or time span of soil formation, calculated as the sample thickness times the carbon age trend, assumed to be 4.4 mm/yr (Chipizubov, 2000). Calendar age is calculated using the CALIB v. 3.03 program (Stuvier and Reimer, 1993), using the bidecadal atmospheric calibration data set, CAS = 300 yrs, and lab error multiplier = 2. Ages in bold are bound dates of earthquakes. * Thickness of organic soil in samples. ** Given is calendar-corrected age for upper soil contacts, which equals laboratory age minus MRC and provides an estimate of the radiocarbon age of the soil surface when buried. For lower soil contacts, it equals laboratory age plus MRC and estimates the inception of soil formation.
with an offset of 6.5 m can reach 33 or 155 km, which according to (2) corresponds to magnitudes between 7.0 and 8.2. The magnitudes estimated from scarp offset (5 m on average, maximum of 6.5 m) using equations (1) and (5) were 7.48±0.42 and 7.7, respectively. Taking into account that surface rupture in the Baikal region appears at earthquake magnitudes M ≥ 7 (Chipizubov, 1998), the most probable magnitude can be assumed to range between 7.5 and 7.9. Or, it most likely, reached 8.0, as can be predicted proceeding from the 3 m offset in the M = 7.6 Muya earthquake of 1957 which would correspond to magnitudes 7.2 and 7.1 according to (1) and (5), respectively. The fault rupture length in an M = 8.0 event is up to 115 km, estimated by (3). Therefore, the earthquakes that occurred 4.5 and 9 kyr ago and left signature in all three segments of the Barguzin fault can have ruptured simultaneously the Ulyun and Sarankhur (114–124 km) or Sarankhur and Shamanka (100 km) segments. If each of the three segments (Ulyun, Sarankhur, and Shamanka) of the Barguzin fault generated two surface-rupturing events at 4.5 and 9.0 kyr, each of the six events could have a magnitude of M = 7.7 (7.5–7.9). A fault scarp with an offset of 6.5 m and a faulting length of 155 km (Ulyun, Sarankhur and partly Shamanka fault segments) can be produced by an M = 8.2 event. On the other hand, an offset of 6.5 m corresponds to a faulting length at least 26 km according to (4). A rupture length of the same order (L up to 20 km) at Dmax = 6.5 was obtained along the envelope in Fig. 5, a from (Strom and Nikonov, 1997). Then, there is a probability that two M = 7.7 earthquakes occurred on the
Ulyun fault segment and three similar events on the Sarankhur segment. It is most likely that the area experienced three M = 7.7 earthquakes about 4.5 kyr, with a recurrence time of hundreds of years; about 9.0 kyr there was one M = 8 event on the Ulyun and Sarankhur fault segments and one M = 7.7 event on the Shamanka segment. The parameters of the fault scarps of different ages within the three segments of the Barguzin fault and the magnitudes of the scarp-forming earthquakes are listed in Table 2. The oldest fault scarps with an offset from 3 to 9.5 m expressed on the surface as relatively short escarpments with slope angles of 20° (age group 4) are found locally in all three fault segments. They can have been produced either by one great event, as indicated by the large offset, or by several events. Altogether the zone of the Barguzin fault can have generated six 7.5 ≤ M < 8.0 and two M ≥ 8 (8.0–8.2) events for the past 10–12 kyr. There were much more 7.0 ≤ M < 7.5 earthquakes but they not always caused surface rupture or, if there was any, the scarps were not large enough to be detected today.
Conclusions The reported paleoseismic trenching of fault scarps in the Barguzin fault zone revealed several surface-rupturing earthquakes of magnitudes from 7 to 8.2 that occurred for the past 10–12 kyr. The M = 7.5–8.0 earthquakes had a mean recurrence interval of 2000 years and fell within a 7500 years long time span between 4.5 and 12 kyr. The largest events
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A.V. Chipizubov et al. / Russian Geology and Geophysics 48 (2007) 581–592 Table 2 Parameters of fault scarps of different ages and estimated earthquake magnitudes Nos
Age, kyr BP (slope)
Fault segment (I–III) and its part (river-river or river-village)
Measured fault scarp
Calculated scarp parameters and earthquake magnitudes
D, m
M1
L, km
M2
M5
M assumed
L3 (L4)
1
3.6
II, Khara-Gorosun
?
?
—
—
—
—
7.0
2
4.5
I, Nesterikha–Ulyun
6.5
45
7.5
7.2
7.6
(48–87)
7.7
3
4.5
II, Tun–Sakhuli
6.5
58
7.5
7.4
7.6
(48–87)
7.8
4
4.5
III, Shamanka R.–Alla
6.5
33
7.5
7.0
7.6
(48–87)
7.6
5
4.5
I–II, Nesterikha–Sakhuli
6.5
114
7.5
8.0
7.6
(48–87)
8.0
6
6–7 (30°)
II, Epishka
5.5
?
7.4
—
7.5
(45–82)
7.5
7
6–7 (30°)
III, Sukhotka–Olos
7
20
7.5
6.6
7.7
(49–89)
7.6
8
9
I, Nesterikha–Ulyunchik
6.5
49
7.5
7.3
7.6
(48–87)
7.8
9
9
II, Tun–Shamanka Village
6.5
69
7.5
7.6
7.6
(48–87)
7.9
10
9
III, Shamanka R.–Alla
6.5
33
7.5
7.0
7.6
(48–87)
7.6
11
9
I–II, Nesterikha–Shamanka
6.5
124
7.5
8.1
7.6
83–126
8.0
12
9
I–III, Nesterikha–Alla
6.5
155
7.5
8.2
7.6
89–130
8.1
13
12 (20°)
I, Gremyachi–Banny
3
5
7.2
5.5
7.1
(37–68)
7.2
14
12 (20°)
II, Epishka
9.5?
?
7.6
—
7.9
(55–100)
—
15
12 (20°)
II, Tekuna–Sukhoi
5
3
7.4
5.0
7.5
(43–79)
7.4
16
12 (20°)
III, Sukhotka–Olos
5
20
7.4
6.6
7.5
(43–79)
7.4
17
12 (20°)
I–III, Gremyachi–Olos
9.5
180
7.6
8.4
7.9
100–155
8.2
Note. Roman numerals denote segments of Barguzin fault: I — Ulyun, II — Sarankhur, III — Shamanka. The subscripts in calculated parameters M and L correspond to numbers of equations; L3 are given as mean values and (L4) as maximum values. Numbers in bold correspond to most probable events.
occurred in the earliest Holocene. One great event about 12 kyr ago can have ruptured more than 180 km of the Barguzin fault. Further paleoseismic studies in the Barguzin fault can provide more reliable evidence of the number, ages, and magnitudes of prehistoric earthquakes. Nevertheless, the refined parameters of fault scarps in the area already indicate that it repeatedly experienced M = 7.6–8.0 shocks. This fact call for a revision to the existing seismic risk zoning under the assumption that the seismic process is constant in time. The study was supported by grants 00-05-64928 and 04-05-64460 from the Russian Foundation for Basic Research.
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