Paleoseismic record of three holocene earthquakes rupturing the Issyk-Ata fault near Bishkek, north Kyrgyzstan

The northern edge of the western central Tien Shan range is bounded by the Issyk-Ata fault situated south of Bishkek, the capital of Kyrgyzstan. Contraction in this thick-skinned orogen occurs with low-strain accumulation and long earthquake recurrence intervals. In the nineteenth to twentieth centuries, a sequence of large earthquakes with magnitudes between 6.9 and 8 affected the northern Tien Shan but left nearly the entire extent of the Issyk-Ata fault unruptured. Here, the only known historic earthquake ruptured in A.D. 1885 (M6.9) along the western end of the Issyk-Ata fault. Because earthquakes in low-strain regions often tend to cluster in time and may promote failure along nearby structures, the earthquake history of the northern Tien Shan represents an exceptional structural setting for studying fault behavior affected by an intraplate earthquake sequence. We present a paleoseismological study from one site (Belek) along the Issyk-Ata fault located east of the A.D. 1885 epicentral area. Our analysis combines a range of tools, including photogrammetry, differential Global Positioning System, 3D visualization, and age modeling with different dating methods (infrared stimulated luminescence, radiocarbon, U-series) to improve the reliability of an event chronology for the trench stratigraphy and fault geometry. We were able to distinguish three different surfacerupturing paleoearthquakes, these affected the area before 10:5 ± 1:1 cal ka B.P., at ∼5:6 ± 1:0 cal ka B.P., and at ∼630 ± 100 cal B.P., respectively. Associated paleomagnitudes for the last two earthquakes range betweenM6.7 and 7.4, with a cumulative slip rate of 0:7 ± 0:2 mm/yr. We did not find evidence for the A.D. 1885 event at Belek. Our study yielded two main overall results: first, it extends the regional historic and paleoseismic record, second, the documented rupture events along the Issyk-Ata fault suggest that this fault was not affected in its entirety, instead, these events indicate segmented rupture behavior.