EARTHQUAKE GEOLOGY OF THE CENTRAL TIEN SHAN, KYRGYZSTAN, CENTRAL ASIA

The central Kyrgyz Tien Shan offers excellent opportunities to study neotectonic and earthquake processes in a contractional setting. Four M³7.5 and seven additional M³7 earthquakes have occurred in the area in the past 120 years. Extensively preserved Quaternary geomorphic surfaces and well exposed Tertiary strata record the kinematics of faults and fault-related folds that have produced the several ranges and basins. The styles and kinematics of upper crustal deformation observable in the Tien Shan may offer useful analogies to understanding earthquake hazards posed by reverse faults in populated regions of western North America, such as the Los Angeles Basin and Puget Lowland.

We continue to explore four main aspects of earthquake geology in the central Tien Shan. First, rates of late Quaternary slip on the most active faults and fault-related folds match the GPS-measured ~20 mm/yr north-south rate of shortening. This indicates that the readily observable, Holocene faults accommodate the vast majority of shortening, and that off-fault, aseismic processes are not significant contributors to upper crustal deformation. Second, the timing and location of late 19th and 20th century earthquakes, when compared to regional paleoseismic data and fault slip rates, strongly suggest temporal and spatial clustering of earthquake strain release. Third, lengths of Holocene faults tend to be 20-60 km long, with 1- to 5-km-wide stepovers to adjacent active faults along strike and between basins. That the 1911 M~7.8 Chon-Kemin earthquake ruptured across a 10-km-wide stepover indicates that large, multi-segment or multi-fault ruptures are possible in other parts of the central Tien Shan. And fourth, geometries of fault-related folding suggest that many faults separated at the surface merge at relatively shallow depths. For example, the Issyk-Ata fault changes along strike from a gently south-dipping splay that penetrates the foreland Chu basin to a series of basement-involved, range-verging wedge thrusts, at a location coincident with thinning Tertiary basin strata. The multiple surface traces in this and other fault systems likely share common, continuous faults at seismogenic depths, and thus provide a mechanism for earthquake ruptures that include multiple faults or fault segments.