PALAEOSEISMOLOGICAL RESULTS on THE SOURCE OF 1970.03.28 GEDIZ EARTHQUAKE

The Akþehir-Simav Fault System (ASFS) is a 10-30 km wide, 500 km long, and NW-SE-trending discontinuous oblique-slip normal fault system. It is an intraplate seismogenic belt in western Turkey. The seismogenic characteristic of ASFS was proved several times by the occurrence of large historical and recent earthquakes such as the 94, 1766, 1873, 1876, 1896 1921, 1944, 1946, 1969, 1970, 2000 and 2002 earthquakes which caused heavy damages. All of these events occurred on the different segments of the ASFS. The detailed field geological mapping carried out in the study area revealed that the source of the 1970.03.28 Mw = 7.1 Gediz earthquake is the Erdogmuþ Fault.

Two cross trenches were excavated along the ground surface rupture of the 1970 Gediz earthquake in the flood plain deposits of the Kör Stream near west of Erdogmuþ Town, where the morphology of the surface rupture of the 1970 event is still well-exposed. In the first trench (KT–1), the stratigraphy consists of 6 different units. In general, it consists mainly of flood plain deposits at the top, fluvial sediments (colluvial wedge) and lake deposits (limestone-marl) at the bottom. A series of fault arrays were observed inside the trench. The most of displacement was measured along the master fault (source of 1970 event) but minor amount of displacement could also be observed along the second- and third-order faults. In the second trench (KT–2), five different stratigraphic units were identified. They contain more or less same units observed in the KT–1 without only the lake deposits at the bottom. According to structural, sedimentologic and stratigraphic relationships observed on the wall of trenches, at least 2 different palaeoevents, except for the1970 earthquake, were identified. Relative dating was used to determine the order of the events. Because, only one of samples taken for radiocarbon dating could be dated (1060 BP) owing to the pMC (high percentage of modern carbon) amount in them.

Based on the trenching studies, it was found out that the 1970 earthquake was preceded by at least two palaeoearthquakes. Due to the low slip rate (?) along the 12 km long Erdogmuþ Fault, the recurrence interval of large destructive earthquake such as the Mw = 7.1 Gediz earthquake is long. By using the timing of the 1060 and 1970 events sourced from the Erdogmuþ Fault, it can be concluded that the recurrence interval of large earthquake is an approximately 910 yrs.