TECTONIC GEOMORPHOLOGY AND EARTHQUAKE HAZARD OF LARGE NORMAL FAULTS: AKSEHIR FAULT, SW TURKEY

The 100 km long Aksehir fault is an active normal fault with 7 geomorphic segments along the mountain front. Segment length varies from about 9 to 14 km. Total relief of the horst block is about 2 km.The fault and mountain block has a prominent bend to the west at the NW end and a second bend to the east at the SE end of the structure. The entire fault zone has experienced frequent moderate to strong earthquakes since the 16^th century, most recently in 2000 (M6) and 2002 (M6.5). Based on past historic seismicity the structure is active along its entire length.

Tectonic geomorphology of 32 drainage basins and mountain front facets suggests: 1) Mountain front facets (~100-900 m high) are triangular (34%) with slopes of 6-30°; trapezoidal (25%) with slopes of 31-38°; and combination (lower trapezoidal and upper triangular (41%) with slopes of 15-30°. Facets at the NW and SE segments ends are larger than the intervening facets. Steepest facets are along the central segments; 2) Mountain front sinuosity varies from about 1.1 on the NW segment to 2.4 on SE segment suggesting that the 6 northern segments are more active than the southernmost segment. Ratio of valley floor width to valley height varies from 0.2 to 0.6 consistent with tectonically active mountain fronts; and 3) Alluvial fans along the 4 northernmost segments are back-tilted to the south.

Large normal faults with a similar slip rate of 0.2-0.3 m/ka typically have strong earthquakes every few thousand years. Therefore, the many moderate to strong earthquakes on the Aksehir fault in the past few hundred years may be misleading. A paleoseismic evaluation could answer questions concerning the area’s earthquake hazard. Tectonic geomorphology suggests that the Aksehir fault is active, and larger earthquakes than those of the historic period are a potential threat.