Transpression along the Altyn Tagh fault and terminations of large ruptures at the Aksay restraining bend: results from numerical modeling and the observed earthquake record

Field evidence and numerical modeling suggest that the Aksay restraining double-bend of the Altyn Tagh fault (ATF) may serve as a barrier to great earthquakes by terminating major strike-slip ruptures. The Altyn Tagh fault generally defines the northwestern margin of the Tibet-Qinghai plateau, accommodating transpressive northeastward motion of the plateau with respect to the Tarim block and stable Eurasia to its north and west. Transpression is enhanced at major restraining double-bends along the ATF (including the Sulamu Tagh, Akato Tagh, and Aksay bends). Contemporary activity of two parallel fault strands within the Aksay bend and stepover (see figure) requires separate rupture of each fault, implying that the bend stops incoming ruptures on either fault. Geomorphic observations indicate that slip is transferred between the northern and southern strands of the ATF at the Aksay bend, and multi-cycle numerical rupture models of the system suggest that this tradeoff in slip is manifested by ruptures tapering as they enter the zone of high fault-normal stresses within the most transpressive portion of the bend. We present new field data that require a renewed interpretation of controversial slip rate sites near Old Aksay town (e.g., Meriaux et al., 2005), report a paleoseismologic record for the tail end of the northern strand, and introduce a new slip rate site on the southern strand that complements work by Wang et al. (2004). Slip rate data indicate similar slip rates (~6-9 mm/yr) on both strands where they enter opposite sides of the bend, and paleoseismologic data from the relatively poorly expressed western end of the northern strand show multiple Quaternary events. These field data corroborate predictions from dynamic rupture models, which indicate fewer, larger events on incoming strands and smaller, more frequent events on each strand beyond the bend. Our combined modeling and field results together suggest that major fault bends like that at Aksay serve as important impediments to throughgoing strike-slip rupture. References: Meriaux, A.-S. et al. (2005), J. Geophys. Res. 110, doi:10.1029/2004JB003210. Wang, F. et al. (2004), Study on Holocene strike-slip rates of the middle Altyn Tagh fault by terraces offset measurement, Seismology and Geology 26(1), p. 61-70 (in Chinese). The Aksay bend of the Altyn Tagh fault. Slip transfers between southern (SATF) and northern (NATF) strands across the bend. Great strike-slip ruptures entering the bend on either fault will be arrested by high fault-normal stresses, leaving heterogeneous stresses that promote smaller local events on the tail end of each strand.