Holocene activity of the Orhaneli Fault based on palaoseismological data, Bursa, NW Anatolia

Orhaneli Fault is 30 km long, right-lateral strike-slip fault with dominant reverse component located in the Biga Peninsula. This fault controls the southern margin of the Quaternary Orhaneli Basin. Additionally, it cuts the metamorphic rocks of the Tavşanlı Zone, Cretaceous ophiolitic units and Neogene/Quaternary sediments, and can be easily recognized on aerial photographs and in the fi eld due to linearity and fault scarps. Elongated ridges, offset valleys and fault saddles observed along the fault are important geomorphological features indicating fault activity. This study presents the results of the fi rst palaeoseismological study on the Orhaneli Fault. Two trenches were excavated along the fault. These trenches, named Serçeler and Kusumlar, exhibited that metamorphic rocks of the Tavşanlı Zone were thrust onto Quaternary sediments by the fault. The study encountered evidences of four large earthquakes accompanied by surface rupture in Quaternary. The results of Serçeler trench indicated the older earthquake occurred between 22,000±3,200 BC and 6,600±800 BC, and the most recent earthquake occurred before 770-415 BC. The Kusumlar trench data showed that the older earthquake occurred between 6,660 BC and 3,085 BC, and the most recent event in the period after 650 AD. Any interpretation could not be made about the recurrence period between palaeoearthquakes. In this study, Holocene activity on the Orhaneli Fault was proven. If this fault, with 30 km total length comprising two segments, ruptured as a single piece, it has the potential to produce an earthquake with Mw=6.9. http://dx.doi.org/10.19111/bulletinofmre.413248