Adyr Faults: Generators of Strong Earthquakes in the Lake Issyk-Kul Depression (a Case Study of the Kultor Fault Zone)

Abstract

We have studied the eastern end of the Kultor adyr (foothill) fault in northern Lake Issyk-Kul region. Morphologically, the fault zone is a staircase of seismic fault scarps formed by movements along few adjacent ruptures gently tilted under the adyrs (northward). Numerous Saka-Wusun, Turkic and Karakhanid archeological monuments ended up to the slope parts of the fault scarps and were damaged or destroyed. Logging of paleoseismological trenches excavated across southernmost and youngest fault scarps has made it possible to determine the minimum age, recurrence period, and energy of strong historical earthquakes. Strong earthquakes along the studied part of the adyr fault occurred during the following periods: 60–170 AD; 530–620 AD; 1160–1250 AD; 1410–1460 AD. The time intervals among dates (strong earthquake reccurrence along the fault) is 200–600 years. The obtained parameters of the fault scarps were used to calculate the magnitudes of historical seismic events: M = 7.2 and M = 6.9 according to the heights of the studied fault scarps. These magnitudes can produce seismic oscillations in the Earth’s surface with I0 ≥ IX according to the MSK-64 Scale. We have confirmed our earlier supposition that the southern parts of the Issyk-Kul Depression were involved in uplift. A lateral longitudinal increase in the area of the adyrs occurs during strong earthquakes. These conclusions are important for a new seismic hazard assessment of the northern Lake Issyk-Kul region, the most populated part of Issyk-Kul Oblast, Kyrgyz Republic, as well as for other regions of the earth evolving in a postplatform orogenic regime.