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IUGG XXIV General Assembly July 2-13, 2007 Perugia, Italy
(S) - IASPEI - International Association of Seismology and Physics of the Earth's
Interior
JSS005 Oral Presentation 1909
The role of complex step-over geometry on the 1939 earthquake rupture
propagation in the Niksar Region, North Anatolian fault system
Dr. Omer Emre
Geology Dept General Directorate of Mineral Research and Explor IASPEI
The 1939 Erzincan earthquake (M: 7.9) is the largest event in a sequence of earthquake in 20th century
along the North Anatolian Fault System (NAFS) produced a 360 km-long surface rupture. The event
nucleated in the eastern end of the rupture and unilaterally propagated toward to west. The rupture
extended along the master strand of the NAFS between Erzincan and Niksar basins. However, a part of
65 km-long western portion of the rupture directed towards on the Ezinepazar splay. Three years later,
the 1942 earthquake (M: 6.9) occurred on Erbaa-Niksar fault on the master strand. It produced a 47
km-long surface rupture. We performed detail mapping on fault and step-over geometry to discuss fault
interaction in the both events. In the region, the master strand of NAFS consists of Resadiye segment
and Erbaa-Niksar fault. However, Karayaka reverse fault and strike-slip Esencay and Ezinepazar faults
are major splays. Niksar pull-apart basin located on the main strand which has an 11 km width and
separates the eastern and the central NAFS. The basin characterized by serial normal fault on the both
margins. The Erbaa-Niksar fault divides into two main geometric sections by a 12 km-long restraining
step-over which is characterized with reverse fault zone and push-up structure. The length of the
Esencay fault is 70 km. It is also divided into two sections by a restraining step-over which is 7.5 kmlong
and 1.5 km width. Eastern section of the fault includes a serial subsection in left stepping.
Ezinepazar fault that included in the 1939 rupture as the western tip segment runs into Anatolian
micro-plate. A local releasing step-over separates Reşadiye and Ezinepazar segment. Although
displacements were 3.5-4.0 m along Reşadiye segment east of Niksar basin, the amount of the
maximum displacement was about 2 m on the Ezinepazar segment of the 1939 rupture. Hence, the
amount of lateral slip abruptly decreased on the Ezinepazar segment. However, the data indicated that
a 26 km eastern section of the Erbaa-Niksar fault and normal faults on the both margin of Niksar
releasing step-over ruptured during the both 1939 and 1942 events. Surface faulting of the 1939 event
terminated at the restraining step-over on the Erbaa-Niksar fault. However, the surface faulting of 1942
event covered entire fault including the 26 km-long eastern section. On the other hand, there is no data
for surface faulting on an 18 km-long the eastern section of the Esencay fault. We concluded that
structural complexity in the region played a significant role on the deviating of the 1939 rupture from
the master strand to the Ezinepazar splay. This study reveals that not only Niksar releasing step-over
but also two large restraining step-over on the Niksar-Erbaa and Esencay faults were other barriers to
the rupture propagation of the 1939 event on the master strand and surface faulting jumped to
Ezinepazar splay. We also suggest that Karayaka and Esenay faults did not ruptured during the
earthquake sequence of 20th century along the NAFS should be taken into consideration as the most
important source faults for the seismic hazard assessment of the region.
Keywords: stepover, 1939 rupture, nafs