Was the 1899 Menderes Valley Earthquake a double earthquake? Historical and paleosismological constraints

Abstract

September 20, 1899 Menderes Valley Earthquake is the last destroying event in the Büyük Menderes Graben, Western Turkey, just proceeding the installation of the earliest seismograms in the world. The previous geological studies revealed that this seismic event of estimated Mw=6.5–6.7 caused a surface rupture of 50km between Aydın and Kuyucak in the center of the graben. Based on the extremely different positioning of microseismic epicenter of this event in various historical documents, we re-evaluated the 1899 earthquake with a rather multidisciplinary perspective. Quantitative earthquake data including fatalities, damage and various types of aids were gathered from daily journals and special commission reports of the time. These data are corrected with the population statistics and georeferenced. For exact determination of the position of the surface rupture, we benefited the verbal description in historical documents and witnesses of elder local people. Furthermore, morphological and palaeoseismological investigations were realized in the eastern part of the graben where surface deformations were made mention in historical documents. Different ground shake-related indicators conformably revealed two distinct geographically-separated damage zone, one at Yenipazar and the other at Ortakçı, in the middle and east of the graben respectively. Trench studies in the west of the Ortakçı village indicated evidences of a seismic event, presumably the 1899 earthquake, with 180cm vertical displacement. Bimodal distribution damage unrelated to basement lithology and building types, and the incoherence between the total surface rupture length and the observed vertical offset and earthquake intensity lead us to conclude that the 1899 Menderes Valley Earthquake was an earthquake doublet occurred in two neighboring segments in very close temporal proximity.