An insight into earthquake-triggered tsunamis in the largest Lake Sevan in the Caucasus

Studies of geological hazards and their interactions are of fundamental importance for Armenia, which has a relatively small area with a high density of active geological structures. The study area is located at a complex geodynamic intersection, where numerous active tectonic and volcanic influences, gravity-induced forcing, and geological processes of other origin have occurred (Figure 1).

Figure 1

Active tectonics north of the Arabian plate. (1) Major strike-slip faults, (2) Major thrust faults, (3) Focal mechanisms of Mw > 4.8 (CMT Harvard) earthquake, (4) Mechanism of 3

It is of great importance to study and assess the risks of active geological processes and their possible development in the Lake Sevan basin, taking into account its mountainous terrain, active faults with high seismic potential and the surrounding population of about 300,000 people. Lake Sevan is one of the largest high-altitude (alpine) freshwater lakes in Eurasia. It is located at an altitude of 1,900 meters above sea level.

The water level area of ​​the lake is 1,241 km2, which constitutes 4.2% of the total area of ​​the Republic of Armenia. Lake Sevan dates back to the Neogene-Quaternary era and tectonic-volcanic origin. It consists of two parts: the small northern Sevan (the deepest part) and the large southern Sevan. Large Southern Sevan is twice the size of Small Sevan1. The entire lake is about 75 km long, and has a maximum depth of ≈ 80 m.

Strong earthquakes are among the most important causes of large collapses and even lake tsunamis3․ Although most lake tsunamis are caused by landslides, it is also important to consider the contribution of seismic effects and related surface fractures to tsunami potential. No confirmed cases of ancient tsunamis have been identified in Lake Sevan. However, lightning waves triggered by earthquakes 1 and lake tsunamis 2,4 have been proposed.

It is important to comment that seismic activity alone may not be the only factor responsible for generating the lake tsunami. Other factors, such as earthquake magnitude and location, surface faulting, and kinematics, along with the geometric and hydrodynamic properties of the lake basin, can also influence the generation and propagation of tsunami waves5,6.

Tsunamis in lakes can be generated by fault displacement beneath or around lake systems. A fault displaces the ground in a vertical motion through reverse, normal, or oblique faulting processes, displacing the water column above and causing a tsunami. In a closed basin such as a lake, the tsunami is referred to as the primary wave generated by the co-seismic displacement generated by the earthquake, and the tsunami is referred to as the harmonic resonance within the lake.

According to the Indian Tsunami Early Warning Center (ITEWC)7, at the time of any earthquake, only the subepicentral parameters and its magnitude are available in near-real time. Because fault geometry information becomes available later, it cannot be used for real-time tsunami prediction for areas near the source. Furthermore, tsunami modeling, especially coastal inundation, cannot be run in real time to generate operational tsunami warnings, as running the model takes significant computational time according to ITEWC7. For operational quantitative tsunami forecasting, there must be a way to quickly estimate travel times and the preceding period based on rapidly available earthquake parameters (based on earthquake magnitude and epicenter only).

For the above reasons, the best way to understand and evaluate potential tsunami risk is to create historical scenarios or hypothetical earthquake scenarios, such as ITEWC7.

Several hypothetical Lake Tahoe intermountain seismic scenarios were also discussed and calculated by Ichinose et al.3 to estimate the distribution of coastal wave heights in greater detail than the previous study by Dewey and Dise8.

The same approach was applied to the revised tsunami source model for the 1707 Hue earthquake and a tsunami inundation simulation at Lake Ryujin in Kyushu, Japan.

Therefore, any comprehensive lake tsunami assessment requires the use of innovative methods to determine interactions between various geohazards and their associated hazards, especially in areas adjacent to populated areas, railways and national highways.

Active fault setting

The Lake Sevan basin is controlled by the active parts of the Pampak-Sevan-Syunik Fault (PSSF) and the Noratus-Kanagi Fault (NQF), which not only border the lake to the northeast and southwest, but also penetrate its central part. (Figure 2)4. The activity of these faults has been extensively studied, and furthermore, the main PSSF can generate M>>7.0 earthquakes1,2,10,11,12,13. The 490 km long PSSF is located in the Lesser Caucasus Mountain Belt and consists of five large parts, separated by transgressive zones10,13,14.

Figure 2

Active rifting (from 4 and earthquakes (from 2023 Armenian Earthquake Catalog) of the Lake Sevan Basin

In an extensional EW setting, the right-sided PSS fault (Figure 2) was inherited from major thrust faults that had been deforming the belt since the continental collision of the South Armenian Block with the Eurasian margin during the Paleocene and into the early Eocene10,15. Furthermore, the fault is aligned along the Sevan-Hakari suture zone10,15,16 (Figures 2, 3). In the lake depression, the PSSF is divided into several branches that control the almond-shaped basin, including the two main submarine sections: the Vanadzor-Artanesh with dominant reverse fault kinematics and the Dzknagit-Khunarhasar fault with dominant normal faulting. -Error component 4, which this study mainly focuses on (Figure 2).

Figure 3

Geological cross section AA' of the Lesser Caucasus region of Armenia, shown in Figure 2 (modified from 10,15)

The base of the Lake Sevan depression is composed of Middle Jurassic to Lower Cretaceous ophiolites 15, Upper Cretaceous to Eocene sedimentary rocks (limestone, tuff, tuffobreccia, and sandstone) and by post-Oligocene plutons 17.

Strike-slip faults in the Lesser Caucasus show variable vertical kinematics, not only at a local scale, but also at a multi-kilometre scale10,15.

The kinetics changes along the PSSF region. Along the southern parts, the PSSF generally shows oblique slip with a normal component, while along the northern parts it shows a reverse component, such as the Vanadzor-Artanish section10,18.

This difference is explained by the gradual change in fault direction from N105° (in the north) to N155° (in the south) with respect to the stress field. In this context, morphological features such as pressure highs, deflective anomalies, displacement currents, triangular flanks, and overhanging pools also provide evidence of recent tectonic activity.

According to Karakhanyan1, recent seismic activity in the Lake Sevan basin is represented by several earthquakes that occurred in 1933, 1936, 1945, 1947, 1992, 1993 and 1996, with a magnitude ranging from 4.3 to 5.0.

Vanadzor-Artanesh sector

The Vanadzor-Artanich sector, with an elevation of about 120°N and an inclination angle of about 89°N, is characterized by a rupture zone 82 km long and 39 km wide (at the fault plane). For the purposes of tsunami simulation in this study, we focus on the southern part of the section with a length of about 35 km and a width of 22 km (Table 1) according to the vertical displacement of 2 m (determined by the old seismic survey). The study near the village of Tsufagio (Avagyan et al. 19 is in preparation )). This part of the section lies entirely within the Lake Sevan basin, and is bounded by the deepest northeastern part of the lake, with a maximum depth of 80 metres. The reverse fault component of the Vanadzor-Artanish portion of the PSS fault is well expressed in the terrain (Figs. 2, 4a, c) and is represented by a series of stresses as observed at the Violetovo and Semenovka sites 4,13,14. At approximately the site of Violetovo, the PSS fault splits into two branches (Figure 2) that border the Dzknagit Depression.

Table 1. Earthquake-tsunami scenariosFigure. 4

Bathymetric maps of Lake Sevan overlaid on a digital elevation model (DEM) of the basin, the analysis of which shows fault segments running through the lake floor. (a) Arrows indicate the Vanadzor-Artanish segment with the dominant inverse component manifesting onshore (a, c). (b) Same for Dezknagit-Khunarhasar strike (revised from 1.33)

Paleoseismic studies conducted in the Violetovo and Semenovka areas identified three seismic events by 14C analysis (Figure 2), including one seismic event that occurred in 7539-5638 BC and was followed by two more seismic events in 3078-200 BC and in 2333-403 BC. Birth 4,13. The instantaneous magnitude of the last two seismic events was previously calculated according to Wells and Coppersmith20 at approximately 7.3 Mw and 7.213,14,21 Mw.

The foci of the three historical events that occurred in 995 AD (M = 6.2), 1187 AD (M = 6.2) and 1853 AD (M = 6.0) have also been identified on the northeastern shore of Small Sevan along Vanadzor-Artanish and Dzknaget. -Khanharhissar clips 1.

A reverse fault surface rupture with a strike-slip component was recently identified in the Tsovagyugh area (Fig. 2) suggesting a M ~ 7.0 paleoearthquake (Avagyan et al. 19 in preparation). Reverse fault kinematics with a strike-slip component are observed in the northeastern part of the lake (Fig. 4c). These observations allow modeling a scenario for the Vanadzor-Artanish section with mainly reverse fault movement. In addition to vertical displacement, there is also horizontal displacement; However, this horizontal movement is not large. Therefore, given that the vertical component plays a more important role in tsunami generation, we chose to focus primarily on the vertical component in this simplified version of our study.

Dzknagit-Khunarhasar sector

As for Dzknaget-Khonarhasar4, this part has mostly right-sided strike-slip kinematics, cutting the entire central part of the lake (Fig. 2, 4b). However, a small difference in dip near the vertical plane of the fault may affect the kinematics of the vertical component. According to existing morphological and geological data1,13,14,21,22,23 the vertical component is primarily a natural component. With a strike of ~310°N, a length of ~146 km and a width of ~26 km (in the fault plane), the Dzknaget-Khonarhasar sector is generally straight, implying a nearly vertical fault plane dipping 88°N (Figure 1a). 2). According to the data of large-scale paleoseismic studies1,4,13,14,22, the following seismic events were identified according to 14C analysis at a distance of 10-12 km from the southeastern shore of Great Sevan: 5655-4992 BC, 3498-1262 BC. And the twelfth and ninth centuries BC. (Figure 2). These three powerful earthquakes had a magnitude of Mw 7.0–7.5 according to Wells and Coopersmith20 and were accompanied by surface ruptures1,4,13,22.

After seismic stations were installed in the area, several well-correlated minor seismic events with the Dzknagit-Khunarhasar section on the lake bed were recorded using the instruments1 (Figure 2). The above data provide evidence that the faults at the bottom of Lake Sevan are active and capable of generating strong earthquakes. The presence of long and capable faults in both Little Sevan and Greater Sevan should be taken into account by any seismic models and in seismic hazard assessments.

Taking into account all the important findings of previous studies, this study applies the advanced empirical seismic source scaling laws of Tingbaijam et al.24 and the moment magnitude scaling of Aki25 to produce updated Mwmax values ​​for the seismic scenarios developed for the Vanadzor-Artanish and Dzknaget-Khonarhasar sectors. To conduct a tsunami simulation in the lake.

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